livro irrigação asabe - capítulo 1

7/28/2019 Livro Irrigao ASABE - Captulo 1

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CHAPTER 1

INTRODUCTION

Glenn J. Hoffman (University of Nebraska,Lincoln, Nebraska)

Robert G. Evans (USDA-ARS, Sidney,Montana)

Abstract. This chapter introduces the second edition of Design and Operation ofFarm Irrigation Systems. The distribution and development of irrigation in the

United States and throughout the world is reviewed. Issues facing irrigated agricul-

ture including environmental concerns, sustainability, and changes in water policy arediscussed. The chapter concludes with a discussion of options for future directions

and some needs for irrigation research and education.

Keywords. Environmental concerns, History, Irrigation, Sustainability, Water pol-icy, Worldwide development.

1.1 OVERVIEW

This monograph updates and complements ASAE Monograph 3, Design and Op-eration of Farm Irrigation Systems, which was edited by M. E. Jensen and publishedby the American Society of Agricultural Engineers in 1980. The 1980 monograph hasbeen the most requested and most successful of all ASAE monographs over the inter-vening years. More than 8500 copies of the first edition have been distributed world-wide.

Significant developments and additions to our understanding and knowledge of ir-rigation along with major changes in the environmental, ecological, sociological, andpolitical scenes attest to the need for a second edition. This monograph provides cur-rent research results and state-of-the-art knowledge on all aspects of irrigation on thefarm. It is intended to provide design procedures and scientific guidance for the man-agement of water resources and irrigation systems.

This edition provides updated material for many of the chapters covered in the firstedition. Other chapters are significantly different or completely new. Chapters that arebuilt upon the subjects of the first edition are system selection, soil-water relation-ships, salinity control, water requirements, drainage systems, land forming, deliverysystems, pumps, hydraulics, design of surface systems, design of sprinkler and micro-irrigation systems, evaluation of performance, and irrigation management. New orsubstantially different subjects include efficiency and uniformity, environmental con-siderations, sustainability of irrigation, water table control, chemigation, wastewaterand reclaimed irrigation water, and site-specific management. By the number of new


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2 Chapter 1 Introduction

subjects alone, the reader should be convinced of the many new issues in irrigationthat need to be addressed today.

This monograph should benefit practicing professionals, farmers, researchers, andgoverning agencies. It provides technical data, design procedures, and managementstrategies to benefit all who make decisions about the design and operation of farmirrigation systems.

1.2 WORLDWIDE IRRIGATION DEVELOPMENT

At the beginning of a new millennium it is appropriate to review the history of irri-gation and its impact on civilization and the global food supply. Irrigation was devel-oped to counter both short-term and long-term drought on crop production in arid andsemiarid areas whenever a reliable supplemental water supply was nearby. The pru-dent use of water enables the consistent production of food, fiber, and landscaping atlevels and in locations where it would not otherwise be possible. An outstanding ac-count of the history of irrigation has been given by Postel (1999), and much of thefollowing historical discussion has been extracted from her writings.

It is important to note the impact irrigation has had on civilization from its earliestbeginnings to the present. Historians, through archeological evidence, credit early irri-gation with the assurance of dependable food supplies; thereby allowing some mem-bers of the society to pursue activities other than hunting, nomadic grazing, or farm-ing. Among the accomplishments of these early irrigation-based societies are writing,the wheel, water-lifting devices, yokes for draft animals, sailboats, palaces, pyramids,

temples, ceramics, fine textiles, handicrafts, the Hammurabi code of law, fired bricks,cities, and various forms of accounting, taxation, management, and government.

1.2.1 Historical DevelopmentSix thousand years ago, settlers in Mesopotamia (which is now Iraq, and part of

what is referred to as the Fertile Crescent) dug ditches and diverted water to theirfields from the Euphrates River and initiated the practice of irrigation. Irrigation trans-formed both the land and society like no other previous activity by producing a de-pendable food supply, frequently so abundant that many people were able to pursuenon-farm activities. The food surpluses had to be stored and distributed, which led tonew forms of centralized management. This need for management led to stratifiedsocieties and centralized control. With some of the populace not needed in farming,historians credit these early settlers, called Sumerians, with the development of writingand the wheel as well as the creation of sailboats, water-lifting devices, and yokes for

harnessing animals. With time, as the range of activities expanded, these early socie-ties increased in population, which led to the first true cities. The Sumerians were fol-lowed by other irrigation-based civilizations in Mesopotamia. Among these were theBabylonians, who are renowned for building magnificent palaces and developingHammurabis historic code of law which dealt, in part, with irrigation (Postel, 2000).

Irrigated agriculture endured in Mesopotamia through several civilizations. Archeo-logical evidence, however, indicates that around 2400 B.C. soil salination had reacheddetrimental levels. In those early civilizations, wheat was the preferred cereal for food,and it was grown in preference to barley. Wheat, however, is less salt tolerant thanbarley. Over time wheats share of the harvest dropped to less than two percent insome areas and barley production dominated, and by 1700 B.C. wheat was no longercultivated. By then, even barley yields were declining. This archeological evidence


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Design and Operation of Farm Irrigation Systems 3

has led historians to blame salination as the major contributor to the decline of civiliza-tion in the Fertile Crescent of present-day Iraq. To this day, the southern portions of theFertile Crescent have never fully recovered from the demise caused by salinity.

Elsewhere in the world, early societies dependent upon irrigation arose in the IndusRiver Valley in Pakistan, the Yellow River basin of China, and the Nile River Valleyof Egypt. Much later, irrigation-based cultures developed in the western hemisphere.Central Mexico, coastal Peru, and the American Southwest each saw the rise and fallof an advanced society built on irrigated agriculture.

The early civilization along the Indus River about 3500 B.C. also depended on irri-gation. The complex, hierarchical society that developed by about 2300 B.C. lastedless than 500 years. Although conquest by invaders was probably the direct cause ofthe collapse of this society, instability caused by salination, siltation, flooding, andpossibly climate change also contributed to its demise.

Ancient Chinese efforts to control and use the water resources of the Yellow Riverin the north China plain began about 4000 years ago. The Yellow, however, proved tobe a difficult river to tame. It emerges from the Loess Plateau carrying some 1.4 bil-lion Mg of silt each year, making channel alterations and flooding a common occur-rence. The Yellow has breached its dikes more than 1500 times over the last two mil-lennia. Heavy silt deposits have elevated the river above its surrounding plain necessi-tating the continual raising and reinforcing of dikes to prevent flooding. Ultimately abreach of the dike occurs and a change in the course of the river results. The river haschanged course about once every century. These unpredictable shifts have been devas-tating to irrigated agriculture and human settlement.

In sharp contrast to the other early civilizations built upon irrigation in the easternhemisphere, the Egyptian civilization in the Nile River Valley has endured for 5000years without interruption. Egypts civilization has survived warfare and conquestsand widespread disease. Only in recent times has the sustainability of Egyptian agri-culture come into question. In response to a 20-fold increase in population over thelast two centuries, Egypt replaced its time-tested agriculture based on the Niles natu-ral flow rhythms with more intensified irrigation and flood management that requirescomplete control of the river. The natural flow rhythm of the Nile was once dominatedby an annual flood that was relatively benign, predictable, and timely. With nearlyflawless predictability the river rose in southern Egypt in early July and reached floodstage in the vicinity of Aswan by mid-August. The flood continued to surge northwardand reached the northern end of the valley by the end of September. At its peak, the

flood covered the floodplain to a depth of 1.5 meters. By late November most of thevalley was drained. Egyptian farmers then had well-watered fields that had been fertil-ized by the rich silt carried from Ethiopia and deposited across the floodplain. Cropswere then planted as the mild winter began and harvested in the spring just in time forthe cycle to repeat. Egyptian irrigators did not experience many of the troublesomeproblems of other early civilizations. Fertility was renewed each year by the silt-ladenfloodwater and the inundation prior to planting pushed whatever salts had accumulateddown below the root zone.

In contrast to other ancient civilizations, early Egyptian society did not centrallymanage irrigation works. Irrigation was carried out on a local rather than a regional ornational scale. Despite the existence of many civil and criminal codes in ancientEgypt, no evidence exists of written water law. Apparently, water management was


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neither complex nor contentious, and oral traditions of common law withstood all testsover a considerable length of time. The many political disruptions at the state level,which included numerous conquests, did not greatly impact the systems operation ormaintenance, probably because the politicians had no control over the flows of the Nile.

Though later in time and smaller in size, the irrigation-based civilizations that de-veloped in the western hemisphere also shaped cultural developments. The geographiccradle of civilization in the west was Central America. Settled villages evolved about2000 B.C., when the productivity of domesticated corn reached a level that could sup-

port stable communities. Around 300 B.C., canals were used for irrigation throughoutthe Tehuacan Valley southeast of Mexico City. Along the Peruvian coast, an irrigatedcrop could be grown in four months leaving time to construct pyramids and templesand to develop fine textiles, ceramics and handicrafts. A centralized bureaucracy wasformed to manage the large irrigation system and to control the distribution of water.In North America, the Hohokam culture thrived for more than 1000 years along theGila and Salt rivers of south central Arizona but then the culture disappeared suddenlyaround 1400 A.D., most likely due to prolonged severe drought. Archeologists havedocumented more than 500 km of main irrigation canals, with many of them linked innetworks.

In the 16th century, Leonardo da Vinci studied the Amo watershed in northern It-aly. His studies led to a better understanding of the relationship between a riverscatchment area and its flow. This study helped establish the fundamentals of hydrol-ogy and river basin management (Newson, 1992). Several hundred years passed be-fore the principles of hydraulics and mechanized water control technologies were de-veloped that together transformed irrigation from an art to a science.

1.2.2 Recent Developments and TrendsOver many centuries, irrigated lands have become essential to the worlds food

supply. These lands now constitute approximately 20% of the worlds total cultivatedfarmland but produce about 40% of the food and fiber. Irrigated agricultural activitiesalso provide considerable food and foraging areas for migratory and local birds as wellas other wildlife. In short, irrigation underpins our modern world society and life-styles.

In 1800, the worldwide irrigated area totaled about 8 million ha. The irrigated areaincreased five-fold during the 19th century, mainly because during the latter half ofthe century much of the scientific and technical foundation for irrigation was devel-oped. In the 20th century, global irrigation grew from 40 to more than 270 million ha,an almost seven-fold increase (Figure 1.1).

Currently India and China, with nearly the same amount of irrigated cropland (57and 55 million ha, respectively), together account for about 40% of the worlds irri-gated land. Ten countries collectively account for two-thirds of the world total (Table1.1). It is estimated that from 36% to 47% of the worlds food is produced by irrigatedproduction (Gleick, 1998; Postel, 1999).

The total amount of irrigated land in a country depends upon its size, arable culti-vated land, and climatic conditions. Thus, it is interesting to note the proportion ofeach countrys arable land that is irrigated. The percent of arable land that is irrigatedwithin a country and by continent is given in Table 1.1. More than half of the arableland is under irrigation in Pakistan, Iraq, Japan, Bangladesh, and Iran; nearly or all of


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0

50

100

150

200

250

300

1800 1825 1850 1875 1900 1925 1950 1975 2000

Years

MillionsofHectares

Figure 1.1. Increase in irrigated area worldwide from 1800 to 2000

(adapted from Postel, 1999, and FAOSTAT, 2005).

the arable land in Uzbekistan and Egypt is irrigated. Thirty-eight percent of all thearable land in Asia is irrigated. Brazil has seen rapid growth in recent years. In con-trast, in Africa, a continent desperate for food, only 7% of the arable land is irrigated.On average, irrigation is practiced on 20% of the arable land in the world (Table 1.1).

Worldwide irrigated agriculture increased significantly during the second half ofthe 20th century with large government investments, major financial support frominternational donors and lenders, and the spread of improved pumping technologies.This period coincided with numerous, large-scale, government-sponsored surface wa-ter irrigation projects in many countries and the proliferation of both private and pub-lic groundwater wells in others.

In China, waterworks were constructed on major Huai, Hai, and Yellow River ba-sins, mostly for rice and small grain production. By 1990, more than 4 million ha were

irrigated from the Yellow River. With improved pumping and well-drilling technolo-gies and the availability of electricity, China turned to groundwater to irrigate theNorth China Plain. The number of wells increased from 110,000 in 1961 to more than2 million in the mid-1980s (Postel, 1999).

Likewise, irrigation by tubewells increased dramatically in India and Pakistan dur-ing the last half of the 20th century. Irrigation from tubewells expanded from 100,000ha in 1961 to 11.3 million ha in 1985 (World Bank, 1991). In this same time period,canal building for surface irrigation by the Indian government doubled the surfaceirrigated area. In the Indus River basin, two large storage dams and correspondingconstruction of irrigation canals transformed the basin into the worlds largest con-tiguous irrigation network, covering 14 million ha.


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Table 1.1. Irrigated areas in the top 25 countries, by continents, and in

the world in 2002 (adapted from FAOSTAT, 2005). These totals are

estimates because definitions of irrigated land vary by country.

Region of the World

Arable Land

Irrigated

(percent)

Irrigated Area

(million hectares)

Country

Egypt 100 3.4

Uzbekistan 96 4.3Pakistan 83 17.9

Iraq 60 3.5

Japan 59 2.6

Bangladesh 58 4.6

Iran 50 7.5

Viet Nam 45 3.0

China 38 54.9

India 35 57.2

Italy 34 2.8

Romania 33 3.1

Thailand 31 5.0

Afghanistan 30 2.4

Spain 28 3.8

Mexico 25 6.3Indonesia 23 4.8

Turkey 20 5.2

France 14 2.6

United States 13 22.5

Kazakhstan 11 2.4

Ukraine 7 2.3

Brazil 5 2.9

Australia 5 2.5

Russia 4 4.6

Continent

Asia 38 193.9

North & Central America 12 31.4

Europe 9 25.2

South America 9 10.5Africa 7 12.9

Oceania 6 2.8

World 20 276.7

In the former Soviet Union, leaders expanded irrigation in areas with otherwise fa-vorable climatic conditions that lacked adequate rainfall for crop production. Theyconcentrated on two key areasthe Central Asian republics, which accounted forabout 40% of the Soviet irrigated area before the nations breakup, and the southeast-ern European region, including parts of Russia and Ukraine. With irrigation waterdrawn from the rivers feeding into the Aral Sea, central Asia became a major cottongrowing region. However, the Aral Sea is a small fraction of its previous size because


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of declining inflows due to greatly increased upstream diversions for irrigation. Whilelarge scale irrigation in southeastern Europe protected this important grain-producingregion from drought there has been severe damage to the regions natural ecosystems.

Presently, chronic scarcity of water is experienced or expected in large parts of Af-rica and the Middle East, the northern part of China, the former Soviet Union and theCentral Asian republics, parts of India and Mexico, the western part of the U.S., andnortheast Brazil. A region is said to be water stressed when its renewable water sup-plies drop below about 1700 cubic meters per capita (Gleick, 1998; Postel, 1999).

With supplies below this level, it becomes difficult for a country to mobilize enoughwater to satisfy all the food, household, and industrial needs of its population. For ex-ample, about 1000 Megagrams (Mg) of water are required to produce 1 Mg of grain.With grain being the staple of the human diet, water-stressed countries import grain tosatisfy food requirements and reserve their water for other uses. Collectively, 32 of the34 water-stressed countries in Africa and Asia import about 50 million Mg of grainannually, about a quarter of the total traded internationally (Postel, 2000).

During the first quarter of the 21st century, the global irrigation base is expected togrow at less than 1% a year, down from the annual growth rate of more than 3% forthe last half of the 20th century. This has largely been the result of diminishing watersupplies and increased demands from other sectors. In most areas, the best and easiestsites are already developed. A 1995 study by the World Bank reported that irrigationdevelopment costs on more than 190 of their funded projects averaged about $4,800per hectare (Jones, 1995).

1.3 IRRIGATION DEVELOPMENT IN THE UNITED STATES

Irrigation has been practiced in the Southwest region of the U.S. for two millennia.Today, irrigation is practiced in every state of the union with the total irrigated agricul-tural cropland area exceeding 21 million ha (21.3 million ha using the USDA-NASS,2004, estimate; 22.5 million ha using a different methodology, as listed in Table 1.1)with a farm gate value exceeding $47 billion per year. This represents 49% of themarket value of all crops from 18% of all harvested U.S. croplands.

Nationally, per hectare sales from harvested irrigated lands average more than 4.5times the sales from nonirrigated land (Gollehon, 2002). However, this is a somewhatmisleading figure because much higher input, labor, and equipment costs often resultin net returns similar to that for rainfed or dryland producers, especially for fieldcrops. This is usually not the case for tree and vine crops, vegetables, and other high-

value crops.It is estimated that there are also approximately 16.4 ( 3.6) million ha of managed

turf in the U.S. (Milesi et al., 2005) that is not included in any of the estimates of crop-land referenced in this chapter (or elsewhere). Turfgrass areas include lawns, airports,institutional facilities, military bases, schools, parks, golf courses, athletic fields,churches, cemeteries, etc. These areas are expected to continue growing, and much ofthis land is irrigated to some degree, almost totally with sprinklers. The large area ofturfgrass makes it the largest irrigated crop in the U.S. Milesi et al. (2005) estimatedthat irrigated turfgrass amounted to three to four times the area of irrigated corn (about4.3 million ha), the largest irrigated agricultural crop. Applied water and other inputsare high because turf is generally managed for appearance rather than profitability.The amount of irrigated turfgrass is extremely significant because it indicates that ur-


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ban irrigators have a large potential for water and energy savings and pollution abate-ment that is comparable to irrigated agriculture. Furthermore, the political and eco-nomic base clearly lies with the urban users and they will have a major voice in futureland use and water policy issues.

Thus, including turf, there is a total of about 37 million ha of irrigated land in theUS. The impact of turfgrass is quite significant because urban users are often compet-ing for the same water supplies, especially in the 17 western states (not including Ha-waii and Alaska) that contain about 30% of the total turf area in the U.S. In addition,

urbanization often expands on to irrigated agricultural lands, and much of the associ-ated water is often lost to agriculture as it is then used for urban landscapes.

1.3.1 Historical Development of Irrigation in the U.S.Irrigation by indigenous people of the Southwest is known to have existed as early

as 100 B.C. Early irrigation was practiced in the Salt River Valley, on the ColoradoPlateau, and along several watercourses elsewhere in the West. Corn, beans, squash,milo, peaches, and other crops were grown through an intricate network of ditches andcanals.

Some of the early immigrants arriving in North America, particularly from theMediterranean area, brought with them a heritage of irrigation as part of farming. Forexample, Spanish colonists irrigated extensively at the missions established along thePacific Coast beginning in the 1760s. In the mid-1800s, irrigation development in theU.S. expanded along with the settlement of the West. Most of the early projects were

accomplished by private enterprise. In 1847, an advance party of Mormons settled inthe Salt Lake Valley of Utah and began diverting water to grow potatoes. Early Mor-mon activities set the stage for other private irrigation ventures, and by about 1875, thecenter for irrigation innovation and development had shifted to Colorado and Califor-nia. By 1890, irrigation was practiced on more than 400,000 ha in California, 350,000ha in Colorado, and more than 100,000 ha in Utah. About this time, irrigation becamea central theme of the federal governments strategy to encourage settlement of theWest. The Desert Land Act of 1877 and the Carey Act of 1894 were intended tostimulate private and state development of irrigated land. The federal government be-came involved in irrigation development with the passage of the Reclamation Act of1902. The Bureau of Reclamation formed by this act provided engineering expertiseand financial capital to develop large irrigation projects throughout the West.

The irrigated area in the U.S. increased from about 1 million ha in the 1880s toabout 8 million ha by the middle of the 20th century. At this time, the major irrigatedregions were the Southwest (2.2 million ha), the Mountain States (2.5 million ha), andthe Pacific Northwest (1.4 million ha) (U.S. Department of Commerce, 1983). Thedrought years of the 1950s stimulated irrigation development in the southern GreatPlains. Then, with the advent of center pivot sprinkler irrigation systems and withgroundwater readily available, irrigation expanded rapidly in the central Great Plainsduring the 1960s and 1970s. During the same time period, irrigation increased mark-edly in the southeastern states. The total irrigated area declined in the 1980s becauseof depressed farm commodity prices, increased energy costs, and declining water re-sources. In the 1990s the total irrigated area recovered and data for 2003 shows thetotal irrigated area to be about 21.3 million ha (USDA-NASS, 2004)

Recent growth in U.S. irrigation has been mostly in the southeastern areas of thecountry, primarily in the Mississippi River delta. The 1980s and 1990s saw growth in


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the lower Mississippi River areas of Missouri, Mississippi, and Louisiana; the 1960sand 1970s had significant growth in Georgia, South Carolina, Alabama, and Arkansas.Mississippi, Arkansas, and Missouri added about 1.2 million ha of irrigated land since1980. This essentially doubled irrigation in these states (USDA-NASS, 2004).

1.3.2 Current Status of U.S. IrrigationThe official survey of agricultural irrigation in the U.S. is typically taken as part

of the U.S. Census of Agriculture. The most recent results are in the 2003 USDA Farmand Ranch Irrigation Survey as part of the 2002 Census of Agriculture (USDA-NASS,

2004). The 2002 Census of Agriculture reported 21.3 million ha of irrigated land. TheUSGS (Hutson et al., 2004) estimated about 25 million ha of total irrigated farmlandin the U.S. in 2000. The differences between the two values are due to different esti-mating methodologies. Data reported in the figures and tables in this section are fromthe 2002 Census.

The approximate agricultural cropland area irrigated in each state is indicated inFigure 1.2. Except for Iowa and North Dakota, the states west of the Mississippi Riverall have in excess of 100,000 ha of irrigated land. In comparison, only seven stateseast of the Mississippi had more that 100,000 ha of irrigation.

Other interesting comparisons can be made from the U.S. agricultural census data.The source of water for farm irrigation is summarized in the 2003 survey and indicatesthat 26% of the water used for irrigation comes from surface sources off the farm,while about 60% of the irrigation water comes from groundwater delivered from

wells, and only 14% comes from on-farm surface sources (Table 1.2).Nationally, thermoelectric power accounts for 48%, irrigation 34%, and municipaland industrial uses 16%, of all water withdrawn in the U.S. from both fresh and salt-water sources (Hutson et al., 2004). However, irrigation is the largest user of freshwa-ter supplies, especially in the arid west, accounting for about 40% of the total U.S.freshwater withdrawals from both surface and subsurface supplies.

The methods of irrigation have changed over time. From early diversions of waterfrom streams by ditches dug by hand, irrigation technology has developed to include

Figure 1.2. Approximate irrigated farm land in the U.S. in 2003 by state

(adapted from USDA-NASS, 2004).


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Table 1.2. Amount of water used from various sources for U.S. farm irrigation

in 2003 (adapted from USDA-NASS, 2004).

Water Source

Area

Irrigated

(hectares)

Volume

Applied

(km3)

Average

Depth Applied

(cm)

Wells 13,094,000 53.6 41

On-farm surface water 2,946,000 14.5 49

Off-farm surface water 5,614,000 39.0 69

U.S. total 21,654,000[a] 107.1 49[a] Sum of land irrigated exceeds actual U.S. total (21,300,000 hectares) because some land receives water

from more than one source.

massive reservoirs and networks of canals to satisfy gravity irrigation systems, manu-ally and mechanically moved sprinkler systems, and a variety of low-flow systemsreferred to as microirrigation. Originally, irrigation was accomplished by methodsutilizing gravity to distribute and apply water. In the 20th century, sprinkler technol-ogy along with low-cost aluminum and later PVC pipe was developed, and currentlymore land is irrigated by sprinklers than by gravity. Similarly, low-flow microirriga-tion is based on plastic technologies evolved during the last quarter of the 20th centuryand is now used on about 5% of the irrigated area in the U.S. (USDA-NASS, 2004),but on less than 1% worldwide.

Table 1.3 summarizes the amount of land irrigated by the various irrigation meth-

ods. In 2003, 50.5% of the area being irrigated was with sprinklers, 43.4% by gravitymethods, 5.6% by microirrigation, and 0.5% by subirrigation. Of the 10.9 million hairrigated by sprinklers, 79% of this area utilized center pivots, which amounts to about40% of all the land irrigated in the U.S. To realize energy savings through low-pressure applications, 90% of center pivots reported in the 2002 Census use waterpressures below 400 kPa. Furrow irrigation is practiced on 51% of the land using grav-ity systems, while border or basin methods are utilized on 38% of the surface irrigatedlands.

The average depth of irrigation water applied per unit land area can also be calcu-lated from the Census results. Nationwide, an average depth of 49 cm of irrigation wateris applied annually to irrigated lands in the U.S. (Table 1.2). For land irrigated bypumped groundwater from wells, an average depth of only 41 cm of irrigation water isapplied each year. In comparison, an average depth of 69 cm of water is applied whenthe supply is surface water from off the farm. The USGS reported that more than 195billion cubic meters of water were diverted from off-farm surface sources annuallywith an estimated average application depth of about 75 cm in 2000 (Hutson et al.,2004). This is slightly higher than the 2002 Census of Agriculture estimates.

The differences in application depth estimates between the USGS and Census ofAgriculture reports are probably caused by the USGS analysis being more indicativeof gravity systems supplied from off-farm surface water sources. These tend to applymore water than other irrigation methods. The comparisons are also biased by howwater source was considered. Center pivots are generally more efficient than gravitymethods and are the most prevalent irrigation method in the subhumid regions of theGreat Plains and humid areas in the east which use well water to supplement precipita-tion. In contrast, gravity irrigation is dominant where the source of water is off-farmand where the climate is generally more arid, as in the Western States.


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Table 1.3 . Comparison of irrigation methods in the U.S.

in 2003 (adapted from 2002 Census of Agriculture, USDA-NASS, 2004).

Irrigation Method

Irrigated Area

(ha) % of Total

Gravity systems

Furrow 4,746,000

Border/basin 3,582,000

Uncontrolled flooding 930,000

Other 104,000

U.S. total, gravity systems 9,362,000 43.4

Sprinkler systems

Center pivot, pressures above 400 kPa 785,000

Center pivot, pressures 200 to 400 kPa 3,910,000

Center pivot, pressures below 200 kPa 3,926,000

Linear move 139,000

Side roll, wheel move, or other mechanized move 739,000

Traveler or big gun 256,000

Hand move 674,000

Solid set and permanent 477,000

U.S. total, sprinkler systems 10,906,000 50.5

Microirrigation systemsSurface drip 574,000

Subsurface drip 164,000

Microsprinklers 472,000

U.S. total, microirrigation systems 1,210,000 5.6

Subirrigation 113,000 0.5

Total U.S. irrigation[a] 21,590,000[a] The U.S. total irrigated area is larger than the 21.3 million ha quoted previously because more than one

irrigation method may be used on some lands.

1.3.3 Trends in U.S. IrrigationThe irrigated agricultural area in the U.S. from 1939 to 2003, based on national

census data, is given in Figure 1.3. Total irrigated area increased at an average rate of

3.6% during the last six decades of the 20th century. The rate of increase is relativelysteady except for the sudden increase in the 1970s followed by a decline in the 1980s.The surge in irrigated area in the 1970s can be attributed to the deployment of centerpivot systems and the installation of systems, particularly in the southeast, to augmentrainfall. The decline during the 1980s was created by a combination of depressed farmcommodity prices, increased energy costs, declining water resources, and some appar-ent changes in statistical procedures in determining irrigated areas. What is interestingis the return to a positive growth rate of irrigated land during the early 1990s, but aslight decline again during the last 5 years of census data (1997-2002). This slightdecline is expected to continue. It is also expected that the conversion from surfaceirrigation methods to self-propelled (center pivots and linear move) and microirriga-tion technologies will increase, but will have little effect on total irrigated acres.


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0

5

10

15

20

25

1935 1954 1974 1992

Years

Millio

nsofHectares

Figure 1.3. The total agricultural irrigated area in the U.S. from 1939 to 2002

(adapted from U.S. Department of Commerce, 1978, and

2002 Census of Agriculture, USDA-NASS, 2004).

1.4 ISSUES FACING IRRIGATED AGRICULTURE

Irrigation is the largest single consumer of water on the planet, accounting forabout 80% of the total freshwater consumed and about two thirds of the total divertedfor human uses, and it is responsible for more than 40% of all agricultural production.Irrigation has permanently changed the social fabric of many regions around theworld. It has provided major economic development of many semiarid and arid areas,stabilizing rural communities, increasing income, and providing new opportunities foreconomic advancement to many. However, the worlds supply of freshwater is basi-cally constant, and the amounts allocated to irrigated production will undoubtedly de-crease substantially because of increased consumption by non-agricultural users.

The development of irrigation early in the 20th century in the U.S. was created byan enormous level of governmental involvement, and despite the major contributionsto a stable and bountiful food supply, the commitment toward agriculture worldwide,and particularly in the U.S., has diminished in recent years. Many members of societyexhibit ambivalent feelings toward irrigated agriculture.

This phenomenal development has not been without controversy and not withoutproblems or critics. Opposition to irrigation has two primary themes. First, irrigation isincreasingly coming into direct competition with other water uses for scarce waterresources. There are escalating demands from other water user sectors, including pota-ble water, recreation, and tourism, that are looking to agriculture to supply the neededwater by conservation and other measures (Clemmens and Allen, 2005). Much of thisdevelopment in the Western U.S. has occurred during a long term, uncharacteristic,wet period causing unrealistic expectations and over-allocations of water supply. Sec-ondly, irrigated agriculture (although not alone) can have negative environmental im-pacts on water and soil quality over broad areas. Irrigation can degrade water quality,erode soils, reduce groundwater levels, deplete stream flow, and alter hydrologic re-gimes leading to unhealthy trends in aquatic and riparian ecosystems. The nations


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commitment to minimize such environmental issues continues to increase and irri-gated agriculture will receive more and more pressure to change existing practices toaddress these concerns.

It is expected that the current commodity surpluses will not last in the face of in-creasing population coupled with the worldwide decrease in ocean fisheries. Thesefactors will place large worldwide demands on agriculture to increase the global pro-duction of animal/fish protein, food, fiber, and livestock feed. Agriculture will also beasked to help supplement the worlds ever-increasing energy needs by producing more

bio-based fuels, lubricants, and chemical plant feedstocks that are now provided by thepetrochemical industry. Irrigated agriculture, including greatly expanded intensivegreenhouse culture, is expected to provide more than 70% of this increased future foodand fiber demand worldwide in the next 25 to 50 years. Aquaculture will also expandsignificantly, to offset the worldwide decline in fish catches, but the net amount of fishprotein will probably remain about the same.

However, most of the potential irrigation development has already occurred, and, infact, productivity of the irrigated land base around the world is declining due to soilsalination, waterlogging, and soil erosion. Despite phenomenal advancements in cropbreeding, genetic engineering, and other technologies, it is not known where or howall of this increased productivity, needed to satisfy the increasing population, will occur.

Further stresses are being imposed on existing water resources by endangered spe-cies regulations, international and interstate agreements on water allocations, energyavailability, and a suffering agricultural economy. Declining water tables in manyregions due to excessive pumping for irrigation are also a major concern. Temporaryand long-term water transfers between users, inter-basin diversions, and emergingeconomic water markets are also confounding the issues.

Water and land resources and their many competing uses must be considered in aregional and international framework. Withdrawals from a river deplete instreamflows that may have a large impact on downstream water users (municipalities, indus-tries, natural areas and wildlife, agriculture, livestock operations, recreation, naviga-tion, tourism, and hydropower production). Reservoir releases for power often conflictwith other competing demands for water, including irrigation. Withdrawals from aqui-fers by wells can negatively affect groundwater levels over large areas and reducerecharge to streams and other water bodies.

Global climate change may be further exacerbating the problems through changingtemperatures and probably by altering annual precipitation amounts and regional rain-

fall distribution patterns. The Intergovernmental Panel on Climate Change (2001) con-cluded that a global warming attributable to human activity is now evident in the his-toric record. Global mean surface temperature relative to 1990 is expected to increaseby about 2C by 2100. If global warming occurs, it is certain to have a major impacton water supplies, and the increased variability in precipitation will provide majorchallenges for the agricultural sector. A warmer climate would accelerate the hydro-logic cycle, increasing both the global rates of precipitation and evapotranspiration(ET). Timing of precipitation as well as runoff from mountain snowmelt may be dif-ferent from historical norms. There is evidence that some of these changes are alreadyoccurring, but regional impacts are uncertain at best. The hydrologic uncertainties arecompounded because relatively small changes in precipitation and temperature canhave sizable effects on the volume and timing of runoff as well as ET, especially in


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arid and semiarid areas. In short, the prospect of global warming introduces major newuncertainties and challenges for irrigators and society as a whole.

All of these factors have created a water demand crisis that only continues toworsen. Consequently, there is an urgent need to improve the management of irriga-tion worldwide to conserve water, soil, and energy as well as provide food, fiber andother critical needs. Adjustments will have to be made by all, but the major expecta-tions will likely be focused around agriculture, especially irrigated production in aridareas.

Equitable solutions to these issues must include irrigated agriculture as well as allof the other users in determining future programs and directions. Managing and devel-oping infrastructure and policies for water security to equitably satisfy the demands ofall users of this limited resource will be a difficult and lengthy task.

1.4.1 Environmental ConcernsThe major environmental issues relevant to irrigation are those concerned with the

protection and management of water resources and water quality. During the past fewdecades, society has become increasingly conscious of and concerned about the im-pacts of irrigated agriculture on environmental quality. The relative significance ofenvironmental issues varies among regions of the country, but the types of issues con-fronting irrigation generally are the same. However, with few exceptions, environ-mental laws and policies have not addressed irrigation-related concerns.

Irrigated agriculture has had profound positive and negative impacts on the envi-

ronment. Irrigation has contributed to losses and changes of aquatic and riparian habi-tats and the decline of some native species dependent on those habitats (Wilcove andBean, 1994). Irrigation runoff is a significant source for potential pollutants in surfacewaters (National Research Council, 1989). Federal and state responses to environ-mental concerns regarding irrigated agriculture include efforts to control soil salina-tion and agricultural nonpoint sources of water pollution, policies to protect streamflows and wetlands, and restrictions on the application of pesticides.

Environmental issues related to instream flow and wetland ecosystems arise wher-ever water is withdrawn for irrigation. Dams and diversions for surface water suppliesreduce stream flows, altering the natural hydrograph and changing water temperatureand flow regimes. These changes may degrade fish spawning and rearing habitats. Thedraining of wetlands for irrigated agriculture impacts waterfowl and other aquatic spe-cies that use these habitats. As an example, 92% of the historic wetland areas in theSan Joaquin Valley of California have been converted to irrigated lands (San JoaquinValley Drainage Program, 1990). Large-scale water resource development projectsincluding irrigation, flood control, recreation, hydropower, and navigation have alsoaltered aquatic and riparian habitat conditions of the Platte, Colorado, Columbia, andSnake rivers.

Irrigation development has greatly increased wetland areas in many arid areas. Thisis generally perceived as a social benefit because of increased land for migrating andnon-migratory birds, wildlife, and recreational activities. Streamflows tend to be stabi-lized due to buffering by increased subsurface return flows, thereby enhancing recrea-tion, fisheries, and waterfowl habitat over pre-irrigation conditions. However, effortsto improve irrigation efficiency may lead to a decrease in these artificially inducedhabitats because of reduced return flows to the area, thus leading to additional contro-versy with recreational interests, fishery and wildlife groups, and regulators.


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It should also be mentioned that urbanization is creating large pollution loads onfreshwater supplies and estuaries. The amount of pollution in the worlds waterways isincreasing while total water supply is decreasing due to increasing evaporation due tomore intensive use. Urban populations are concentrated in coastal areas where treatedand untreated sewage waters are discharged into the seas where the water is no longeravailable for reuse by agriculture and other uses, thereby contributing disproportiona-bly to water scarcity. Desalination of seawater is an option in these areas but this alsoraises significant environmental concerns related to salt disposal (Seckler and Amaras-

inghe, 2000).1.4.2 Sustainability of Irrigation

Sustainability is a broad concept with many different meanings and connotations.Some people define it solely in terms of environmental factors, such as soil and watersalinity, soil erosion, agrochemical use, and water pollution, that change the wildlifeand riparian ecologies. Others view sustainability only in terms of economics and thecontinued agronomic production of food and fiber and thus include commodity prices,infrastructure development, equipment costs, pest control, and energy. By either defi-nition, irrigated agriculture is not in fact economically or environmentally sustainableover the long term using existing technologies and policies.

Sustainability is an important concept, because irrigation underpins our modernworld society and lifestyles by providing at least 40% of our total food and fiber sup-ply. In reality, society cannot afford and will not allow the loss of this tremendous

asset. Despite the current problems and negative perceptions in many sectors of soci-ety, it is certain that irrigation will continue to be a necessary and important compo-nent of the worlds well-being and growth.

Agricultural water security is obviously a major part of sustaining irrigated agricul-ture. It is a term that is used to describe the need to maintain adequate water suppliesto sustain the food and fiber needs of the expanding world population. Factors thatmay impact water security include competition for water, environmental concerns,continued urbanization, government policy, and the globalization of the economy.

One of the biggest threats to the sustainability of irrigated agriculture is salinity.Surface and groundwaters contain dissolved salts that are picked up as the watermoves through various geologic formations and soils. When plants extract water fromthe soil for transpiration almost all of the salts in the soil water solution are left behind.Evaporation from the soil surface also deposits salts. (Irrigation without salinity prob-lems occurs in more humid regions such as the irrigated rice culture areas in Asia,where it has been practiced for thousands of years, and also where the timing andamount of rainfall combined with good natural drainage are sufficient to leach saltsfrom the system.) Excess accumulation of salts in the plant root zone can cause largeyield reductions or even total crop failure, and inability to remove these salts willmake agriculture unsustainable. Soil salination was probably the primary reason forthe failure of many ancient societies in irrigated arid areas (see Section 1.2.1 above).Presently, about 30% of the land in the conterminous U.S. has a moderate to severepotential for salinity problems (Tanji, 1990). Many areas in the West, such as theColorado River Basin, the northern Great Plains, and Californias San Joaquin andImperial valleys, suffer salinity problems, as do large irrigated areas in India, Pakistan,and elsewhere around the world.


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Improved irrigation efficiencies in arid areas often require additional water forleaching salts brought in with the irrigation water, which can cause a salinity concen-tration increase in the groundwater. Irrigation-induced soil salination can be avoidedby providing adequate drainage and appropriate water management, but drainage isexpensive and can degrade environmental water quality. In closed basins drainagefrom irrigated areas can render the water terminus biologically uninhabitable, as oc-curred in the Salton Sea in southern California. The Great Salt Lake in Utah is anotherdrainage catchment terminus that was naturally uninhabitable and is being further de-

graded by urban and agricultural water users.Subsurface drainage and surface return flows from irrigation are sources for chemi-cal and salt pollution in rivers, streams, lakes, and estuaries. Pollutants mobilized andtransported by irrigation return flows and drainage into streams and man-made wet-lands include trace elements (e.g. selenium, boron, and molybdenum), nitrogen, andsalts, as well as pesticides, herbicides, and other chemicals. In sufficient concentra-tions, these pollutants may be detrimental to wildlife and birds, as was the case withKesterson Reservoir in the San Joaquin Valley of California.

Field drainage systems are an essential part of controlling salination of agriculturallands due to irrigation activities. Desalination or other costly types of treatment ofsome drainage waters may be required to overcome these obstacles; however, disposalof these effluents may also be a problem. Human and environmental health implica-tions from reuse of degraded water must also be examined.

Thus, the real question facing the world today is how to make irrigation sustain-able, both environmentally and economically. The sustainability of irrigation dependson societys ability to find ways to use this technology so that important benefits con-tinue to be provided, but with less troublesome social, environmental, and economicconsequences. Society will need to improve agricultural productivity, change institu-tional structures, modify water policies, improve delivery and on-farm systems, im-prove management of degraded soils, enhance water reuse, improve crop water man-agement, and address rising energy prices. Greatly increased investments in irrigationinfrastructure and economic incentives will be required throughout the world to enablethese required increases in productivity that permit sustainable irrigated productionwhile addressing the changes in the priorities for water use.

A recent USDA agricultural water security listening session (Dobrowolski andONeill, 2005; ONeill and Dobrowolski, 2005) developed a list of research and actionareas over the next several years for maximizing the efficiency of water use by farm-

ers, ranchers, and communities. These water issues included: enhancing supplies with new storage facilities; expanding existing infrastructure; funding for water reclamation and reuse; lowering water consumption by rural and urban users with new technologies; developing new technologies and systems for recycling and reusing degraded

water; providing risk assessment and management for water scarcity; determining sociological and economic impacts of conservation technologies; evaluating the impacts of environmental degradation and subsidies for water and

crops;


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determining the role of physical and paper water banks and progressive waterrate structures and other market-based or incentive mechanisms;

applying biotechnological improvements in water use efficiency; measuring the consequences of water conservation and reuse; responding to public health and environmental concerns of reused water and wa-

ter treatment strategies to improve return flows from agriculture; and expanding decision making tools for water in agricultural, rural, and urbanizing

watersheds including forecasting supply and shortage.

1.4.3 Productivity ChallengesThe formidable pressures on water resources make it certain that water will be the

major natural resource issue of the 21st century (Seckler and Amarasinghe, 2000).There will be large economic and social pressures to reduce irrigation water use. Thismeans that productivity per unit of water consumed must be much higher than currentvalues to meet the high future demands for food, fiber, livestock feed, and biologicalalternatives to petroleum products. This is a major shift from the current emphasis onmaximizing yield per unit area, and it will require a significant re-thinking of how andwhy irrigation is practiced.

There may be small pockets of future growth, but the worlds irrigable land base isessentially developed. Some irrigated areas are actually declining in size and produc-tivity due to waterlogging and soil salination, urban encroachment, soil erosion, anddeclining water tables. Combining a fully exploited land base with the growing com-

petition for existing freshwater supplies along with the needs for increased agriculturalproduction and the concurrent need for energy conservation will require that irrigatorssubstantially increase efficiency and productivity per unit of water consumed. Postel(1999) estimated that water use efficiencies across the world were only about 40%,indicating that there are large opportunities for improvement. This applies to both ag-ricultural and urban crop production (i.e., turfgrass). The greatest potential may be indeveloping countries where improved water management strategies and practices havethe largest potential to increase production.

Productivity of a specific crop is a function of maximizing application efficiencies,whereas improving productivity, in general, implies maximizing both efficiency andcropland net return. Thus, to improve the productivity of crops with any irrigationsystem we must consider many factors including the crop variety, plant water re-quirements, quantity and quality of the water supply, soil characteristics, topography,field size and shape, local climate, and a large number of economic concerns, such aslabor requirements, available capital, and other resource costs. Many of these factorsare interdependent, and it will be necessary to custom design a synergistic mix ofstrategies that fit the delivery systems, farming culture, crop water use patterns, soils,regional hydrology, and other unique environmental characteristics of an area. Clem-mens and Allen (2005) present an excellent discussion of the environmental and eco-nomic trade-offs involved in implementing advanced irrigation practices that improveefficiencies.

A critical link in improved productivity is the implementation of advanced scien-tific irrigation scheduling techniques, especially under deficit management. However,most growers lack sufficient economic incentives to implement the techniques.

One of the greatest constraints to managing for enhanced productivity as well asprotecting water quality is the inability of agricultural producers to control inputs in


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ways that account for the variability in growing conditions across a field. Infiltrationrates can vary between irrigation events as well as with location within the field. Widevariations in soil types, soil chemical properties, subsurface conditions, topography,drainage, insect/weed/disease problems, soil compaction, weather patterns, irrigationsystem operation and maintenance, and wind distortion of sprinkler patterns, as well asexternal factors such as herbicide drift, can cause yields and crop water use to varyacross a field as well as across adjacent fields. These may also be impacted by tillagepractices and crop rotations. Thus, it may be more advantageous to apply water uni-

formly over smaller portions of a field to minimize production differences and envi-ronmental consequences due to the variability of these numerous factors rather thanmanaging the entire field as one management unit. The challenges and opportunitiesfor irrigation equipment manufacturers, designers, researchers, managers, and growerswill be immense and ultimately quite profitable.

1.4.4 Water Policy IssuesAs populations increase, competition for available water supplies will intensify. In

some areas of the Western U.S., agricultural water users have some of the most seniorwater rights. As cities have grown, the consumptive components of water rights havebeen transferred from agriculture to the municipal sector through the willing sale andpurchase of the water rights. Depending on the area and natural rainfall amounts, thefarmer selling his water rights reduces the area irrigated or converts the non-irrigatedland to dryland farming or pasture. Irrigated land could also be fallowed in alternate

years or as part of various irrigated-dryland cropping rotations.Worldwide, water policies involve many entities, each behaving according to its setof rules and incentives. These entities include irrigators, landowners, irrigation dis-tricts, water user organizations, state or provincial water agencies, national ministries,development banks and organizations, private voluntary groups, engineering and con-sulting firms, politicians, voters, and taxpayers. The policies and rules of these entities,in most cases, do not encourage improved irrigation efficiency through design or op-eration. An estimated $33 billion annually worldwide in government subsidies tend tokeep water prices artificially low and discouraging investments to conserve irrigationwater supplies (Myers, 1997). In addition, many laws and regulations have been a bar-rier to marketing or transferring water, leading to inefficient water allocation and use.Similarly, lack of policies to regulate groundwater use has led to over-pumping anddepletion of aquifers. There is a worldwide shortage of appropriate institutions to as-sist growers in managing water more effectively while reducing negative environ-mental impacts.

The following sections provide examples of current policy issues and introduce po-tential improvements in water policies. For those desiring more information, theseissues and more are covered in great detail in numerous publications by private andgovernmental groups, universities, and agencies that have conducted studies and de-veloped plans to address complex water-related issues in the 21st century. For exam-ple, a comprehensive series of reports was prepared for the Western Water Policy Re-view Commission (established under the Western Water Policy Review Act of 1992[PL 102-575, Title XXX]). These numerous reports present in-depth analyses of thewater resource issues and demographics of every major river system in the West, andare summarized in Water in the West: Challenge for the Next Century (WesternWater Policy Review Commission, 1998).


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1.4.4.1 Irrigation delivery management. More than 25 countries are changing theresponsibility for managing irrigation systems from the central government to localgroups (Vermillion, 1997). Most of these changes are driven as much by the need tocut government expenditures as by the desire to improve irrigation performance. Byreducing government subsidies and oversight, however, the hope is that such man-agement transfers will accomplish both goals.

The largest management shift of this type in recent times occurred in Mexico,where the management of 2.8 million ha of publicly irrigated land has been turned

over to farmer organizations. With the accompanying large reduction in governmentsubsidies, water fees have risen to cover costs. The irrigation districts are now about80% financially self-sufficient, up from 37% prior to the transfer. The cost of irriga-tion water, although higher, is still about 5% of total production costs, a typical valuefor irrigated agriculture. It has yet to be determined whether local management haspromoted greater equity in water distribution and more efficient water use (Johnson,1997).

1.4.4.2 Water pricing. The inability to agree on the economic and social values ofhealthy freshwater ecosystems and water quality, as well as flood control, recreation,irrigated agriculture, and tourism, has lead to conflicts in many areas of the world.There is a wide spectrum of options, each with its own range of consequences forvarious sectors and interests.

The price of water can be difficult to alter because of the many diverse viewpoints.In many scenarios, charging the full cost for water without changing other institutionaland economic structures would put most irrigators out of business. However, charginglittle or nothing, which is the situation in many irrigation projects, is a clear invitationto waste water and increases the potential for conflict.

One option gaining popularity is a tiered pricing structure, which provides strongincentives to conserve water to avoid higher rates. For example, irrigators might becharged the low rate they are accustomed to paying for up to, say, 60% of their aver-age past water use, a significantly higher price for the next 20%, and the full (high)cost for the last 20%. Any water used above the average use would be at full cost plusa penalty.

The Broadview Water District in California introduced a tiered water pricing struc-ture in the 1990s. The district determined the average amount of water used in previ-ous years for each crop. Irrigators were charged the customary rate for up to 90% ofthe average water need. Any water deliveries above 90% were 2.5 times higher in

price. Even though they were still paying much less than the real water cost, the irriga-tors had incentive to conserve. Depending upon the crop, water savings were from 9%to 31% while crop yields were unaffected or increased (California Department of Wa-ter Resources, 1998). Another example is Israel, which currently has a price structurewhere 65% of full evapotranspiration (ET) is provided at a reduced price, but the priceincreases exponentially for any additional water.

1.4.4.3 Water marketing. Removing barriers to water marketing can also lead to amore equitable distribution of water and improved water use efficiency. The ability tosell some water gives irrigators an incentive to conserve water so they can profit fromthe sale of the resulting saved water. Formal water markets work only where farmershave legally enforceable rights to their water, and where those rights can be sold. Aus-tralia, Chile, Spain, Mexico, and many states in the Western U.S. now have laws and


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policies that permit water markets. One important advantage is that this mechanism isvoluntary.

As with pricing, a variety of marketing options exist. Water rights owners can selltheir water on a seasonal basis or they could enter into a multi-year contract. Theycould also sell their water rights to another user, in which case the legal water entitle-ment is transferred permanently.

1.4.4.4 Water transfers. Traditionally, there have always been limited water trans-fers within irrigated agriculture at the farm and project level, normally from field

crops to horticultural crops of higher value. Economic decisions have also caused ag-ricultural water to be transferred to municipal and industrial uses in many areas. How-ever, as demands on the worlds scarce freshwater supplies increase, water transfersfrom agriculture to other sectors will become more and more common and, in fact,some may become mandated by judicial and legislative processes. Artificial ground-water recharge as a form of water banking, as well as the treatment of degraded aqui-fer and soil waters for later reuse, are all components in water transfer considerationsand policies.

Mutual agreements have been reached between municipal and agricultural usersthat allow an irrigation district or farmers to continue irrigating, but require they makeimprovements that will conserve water, i.e., reduce the consumptive use component ofwater diverted. The cost of the improvements is borne by the municipal user. Site con-ditions often will determine the effectiveness of improvements in reducing water con-sumption. For example, lining a canal whose seepage normally returns to the riverfrom which the water was originally diverted may not result in much, if any, reductionin water consumed in the river basin.

An example of a mutual agreement to conserve irrigation water that could be usedby a municipality was reached in 1988-1989 between the California Metropolitan Wa-ter District (MWD) and the Imperial Irrigation District (IID). The IID is located in theImperial Valley of Southern California and diverts water by gravity from the ColoradoRiver. Surface and subsurface flows from irrigated lands in the IID do not return to theColorado River from which water was diverted, but instead flow to the Salton Sea,which lies about 70 meters below sea level. Improvements were agreed upon thatwould reduce flows to the Salton Sea, but still enable irrigated farming to continue.The improvements were estimated to conserve about 123 million cubic meters of wa-ter annually within IID with the saved water available to the MWD for municipal use(MacDonnell and Rice, 1994; National Research Council, 1996). The IID-MWD con-

servation program involved lining canals within the IID, improved flow-monitoringstructures, installation of non-leak gates, prevention or recovery of canal spills, instal-lation of regulating reservoirs and seepage recovery systems, and system automation.Implied in this agreement is that the IID would reduce its net annual diversion ofColorado River water at Imperial Dam by the amount conserved and the MWD wouldincrease its diversion upstream at Parker Dam by a like amount. The MWD benefitsbecause the cost of the conserved water is less than $0.10 per cubic meter, much lowerthan its best option for a new supply. The IID benefits from the cash payments and anupgraded irrigation system, and no cropland is taken out of production because thewater transferred is generated through conservation (Gomez and Steding, 1998).

In another transfer agreement with the MWD, however, farmers were required toremove irrigated land from production. In 1992, the MWD entered into an agreement


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with the Palo Verde Irrigation District located on the west side of the Colorado Riverin Southern California. The agreement stipulated that farmers who laid fallow a por-tion of their cropland for two years would be paid $3,000 for each hectare left un-planted. More than 8000 ha were left fallow by 63 farmers. The equivalent of about10% of the MWDs yearly water deliveries were then transferred and stored in federalreservoirs on the Colorado River to be used anytime before 2000 (Loh and Steding,1996). There are also some associated environmental agreements being developedincluding a three-state, 50-year agreement covering restoration of 3300 ha of native

habitat along the Colorado River from Hoover Dam to the Mexican border.A third transfer agreement, entered into in 2002, provides mechanisms for transfer-ring water from agricultural to urban users, and serves as the basis for California tosettle nearly seven decades of disputes among its water agencies. It involves the IID,the MWD, the San Diego County Water Authority, and the Coachella Valley WaterDistrict. Implementation of this 75-year landmark agreement also requires Californiato meet specific benchmarks for receiving Colorado River water (surplus and other-wise) because the state has to reduce its annual Colorado River diversions from 6.4billion cubic meters to 5.4 billion cubic meters of water over a 15-year period. Theagreement combines temporary fallowing of irrigated lands and the transfer of about1.2 billion cubic meters while the IID implements on-farm and system water conserva-tion measures paid for by the urban water districts. San Diego agreed to a paymentschedule for the water transferred plus $20 million to help cover socioeconomic im-pacts to the local Imperial Valley communities and landowners over the 15-year pe-riod as a result of the transfers. This is in addition to the land management, crop rota-tion, and water supply transfer agreements with the Palo Verde Irrigation District men-tioned above.

So far, few countries have the necessary institutional and incentive structures toguide water competition. In Chile, where government policies encourage water mar-keting, negative impacts seem to be minimal, primarily because farmers have onlysold small portions of their water rights to cities that have funded upgrading of exist-ing irrigation systems and operational procedures.

1.4.4.5 Economics and incentives. Both rainfed and irrigated agriculture generallyprovide only a marginal rate of return to growers in todays world markets. Costs ofproduction are rising while crop prices are remaining static, and many growers needsubsidy payments to remain viable despite significant improvements in farming effi-ciencies. Thus, many producers currently cannot afford to make expensive improve-

ments to enhance productivity.From an economic perspective, many advanced irrigation methodologies have high

initial capital costs, which add increased production costs including energy, manage-ment, agrochemicals, and land preparation. Researchers and action agencies need tofind ways to reduce all inputs, including management time, for growers to remaincompetitive in a world market. Economists and politicians must find ways to moveworld crop prices upward to where they truly reflect the increased costs of production,including higher water prices.

It is expected that a considerable amount of the cost of any new agricultural infra-structure and field improvements would be privately, rather than publicly, financed.Private development of an intensive, irrigated greenhouse culture will no doubt ex-


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pand greatly. Individuals and farmer-owned irrigation districts will be asked to fundupgrades to existing facilities and systems, but it should not be a one-way street.

Recent years have seen governmental investments in irrigation research and infra-structure reduced significantly. However, it is clear that society will have to change itscurrent attitude of minimizing investments in irrigated agriculture, and instead makesubstantial additional investments towards improving irrigation infrastructure andmanagement to address increased water demands. Innovative policies must reflect theneed and mechanisms for equitably funding these investments, and urban users will

need to be much more involved in this process than has generally been the case. Thisis starting to become apparent in Southern California as they attempt to address com-plex regional water issues.

Economic and social policies that include incentives need to be implemented tomotivate farmers to reduce negative externalities and consider opportunity costs whenchoosing irrigation and drainage strategies. Incentives can be either negative or posi-tive, but should be directed both at inputs in agricultural production as well as efflu-ents such as salt, silt, nutrients, and other constituents in surface runoff and deep per-colation. They could include collaborative water markets, payments to irrigators forachieving higher efficiencies, higher costs for water per unit area, service charges foreach delivery time, reductions in water rights to more closely match ET, or other waysto encourage profitable deficit irrigation strategies. However, policies and incentiveshave to be economically and culturally acceptable to growers and be accompanied byrealistic programs to upgrade and support improved technologies and management.

Positive incentives could include payments to growers who meet or exceed targetedirrigation efficiency levels. Payments could be in the form of funds to improve irriga-tion systems and monitoring equipment, but must also include ways to offset the in-creased costs of management and labor associated with the improved efficiency. In-creasing the actual prices farmers receive for their products by changing governmentalpolicies would also provide incentives and the means for them to improve production,management, and environmentally beneficial practices in the face of declining wateravailability and rising energy costs.

Another incentive could allow landowners to move water from poorly producingsoils (whether due to salinity, erosion, or just natural causes) to more productive areasthat may be outside an irrigation districts boundaries. Some states, such as Washing-ton, are allowing water spreading in certain areas where water saved by converting tolow water use crops or deficit irrigation can be moved to previously nonirrigated lands

as long as total historical ET is not increased.Some grower and societal incentives to implement improved irrigation technologies

at the irrigation district and farm scales include: reduced labor requirements, lowercosts for treating water by reducing the volumes treated, lower costs for pumping wa-ter, reduced costs for added distribution capacity in an area of growth, less leaching offertilizers and chemicals and degradation of groundwater, and sustained flows in seg-ments of streams bypassed by irrigation diversions. These savings and potential envi-ronmental benefits accrue both to the irrigation manager and, ultimately, the generalpopulace. In the future, both groups will likely perceive these as necessary incentives.

Increasing crop productivity while reducing the amount of applied water dependson the ability of a particular type of irrigation system and the managerial skills of theoperator to correctly implement the water-saving practices and techniques. Frequently,


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the controlling factors are the knowledge base of the grower and the existence of in-centives to implement the improved practices. Thus, incentives should also rewardhigher management levels.

A concern is that skilled labor for irrigation is becoming more and more limitedand many are unwilling to perform the required activities and technological tasks. Inaddition, the average farming population age in the U.S. is in the 50s, and this agegroup is less likely than younger farmers to make major improvements or adopt newtechnologies without substantial incentives.

1.5 FUTURE DIRECTIONSThe discussion above describes standard practices that can be done with current in-

stitutional and technological systems. The next step is to look into the future with allof its uncertainty. To aid in that effort, the National Research Council (1996) devel-oped this list of likely future directions for irrigation in the U.S.:

Irrigation will continue to play an important role in the U.S. and the world forthe foreseeable future, although there will certainly be changes in its character,methods, and scope.

The total irrigated area will likely decline, but the value of irrigated agriculturewill remain about the same because of shifts to crops of higher value.

The amount of water dedicated to irrigated agriculture will decline as societalvalues change and competition for water increases.

A major factor in the sustainability of U.S. irrigation will be determined by our

ability to compete in global markets. Under-financed irrigation operations or those with less-skilled managers will

tend to decline in number. Previously, irrigation meant irrigation for agriculture. During the past 25 years

irrigation has become an important part of the turf industry, and irrigation forurban landscaping and golf courses is growing steadily as urban populations in-crease.

With time, increasing amounts of water will be removed from agriculture to sat-isfy environmental goals. In conjunction with this, there will be increasing pres-sures to reduce environmental degradation associated with irrigation.

Substantial research and educational challenges must still be addressed regardingwater availability, quantity, and quality, water use, and water institutions (NationalResearch Council, 2001, 2004). Changes in policy and incentives will clearly become

necessary. The following sections examine some potential issues and solutions to agri-cultural water security issues. Urban water users will have a similar set of challengesto reduce and modify water consumption.

1.5.1 Need for InnovationIrrigation has been practiced for more than 6000 years, but more innovation has oc-

curred in this arena in the last 100 years than in all of the preceding centuries. Almostevery aspect of irrigation has seen significant innovation: diversion works, pumping,filtration, conveyance, distribution, application methods, drainage, power sources,scheduling, fertigation, chemigation, erosion control, land grading, soil water meas-urement, and water conservation.

Major future improvements in water saving will be realized through innovative de-sign and operation of integrated irrigation systems for both agricultural and urban set-


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tings. It is obvious that all these technologies will have to continue to be improved andimplemented to better manage energy, water, and soil resources. Novel irrigationtechniques and management systems will be necessary to increase the cost-effectiveness of crop production, improve water quality, improve water reuse capabili-ties, reduce soil erosion, and reduce energy requirements while enhancing and sustain-ing crop production and water use efficiency. In addition, innovative water polices andinstitutional structures must evolve and foster emerging irrigation technologies.

Irrigation is a valuable technology, rooted in ancient tradition, and has proven to be

dynamic and flexible. However, new and improved strategies and practices are neededto reduce surface and groundwater contamination from agricultural lands, conservewater and energy, and sustain food production for strategic, economic, and socialbenefits. Systems must be designed and managed to minimize health hazards due tochemical applications of fertilizers and pesticides as well as to minimize insect infesta-tion and parasitic diseases, such as the West Nile virus and malaria. The effects ofwater conservation and reuse technologies on recreation, tourism, wetlands, andaquatic ecosystems must be assessed and balanced with other societal needs.

Future irrigators will often be operating under various managed crop water deficitscenarios. Increasing crop productivity while reducing the amount of applied waterimplies that producers will often be managing irrigations under severe to moderate soilwater deficit conditions during part or all of the growing season. Techniques such aspartial root zone drying and regulated deficit irrigation will be more and more com-mon on tree and vine crops as well as many annual crops (Chalmers et al., 1986; Fer-eres et al, 2003). Techniques such as fallowing of irrigated fields in alternate years toconserve water need to be investigated as to potential water savings and reduced agro-chemical use.

Water reuse and treatment of impaired waters will be part of agricultural water se-curity. Innovative approaches to groundwater recharge using treated and excess sur-face waters for later withdrawals by a multitude of users will be an essential part offuture water resource programs..

The following brief sections present more details on some of the issues that agricul-ture will have to implement to address water security issues. These measures will in-clude: modernizing irrigation delivery systems and on-farm systems, improved levelsof management, strategies for local water supply enhancement, and biotechnologicaladvances in crop breeding and selection.

1.5.2 Modernizing Delivery and On-Farm SystemsTraditional approaches to modernizing irrigation projects have focused on minimiz-

ing water loss during delivery and maximizing field application efficiencies. These arenecessary first steps, but future water delivery systems and application techniquesmust be modified to enhance grower flexibility in managing rates, irrigation frequen-cies, and durations, as well as reduce water evaporation and other losses. Small, dis-tributed internal regulation reservoirs, closed-conduit systems to reduce evaporationand leave unused water in the distribution system, extensive automated water-levelcontrols, accurate automated flow measurement, and improved ways to reduce weedgrowth on canal and lateral banks to minimize non-beneficial ET are all potentialmeans of improving water delivery efficiencies. Some of these features are just nowbeginning to be implemented in a few modernized irrigation projects.


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To maximize the potential of existing and emerging technologies, irrigators musthave the flexibility to manage rate, frequency, and duration of their water supplies.Thus, the delivery system and the farm must be considered as one integrated unit withtwo parts rather than two independent systems. With the imperative need to implementthe agreements and mandates discussed previously, the Imperial Valley in Californiawill be a proving ground for these concepts over the next couple of decades, and thereis much to learn. In this case, the delivery system will provide irrigation water to sat-isfy the specific field condition (i.e., rate, frequency, and duration) that is calibrated

for each crop and irrigation condition. This will require extensive canal automationand on-farm monitoring as well as economic incentives for achieving better water useproductivity. Positive payments for achieving a certain target efficiency or tailwaterlevels rather than solely raising water costs are anticipated.

The following identifies some of the potential areas where innovation is likely toimprove delivery and on-farm systems in the 21st century:

Computers and wireless control systems will play an ever-expanding role. Cellphone and satellite communications and internet technologies will likely play anincreasingly major part in management of irrigation systems. Feedback controltechnologies for automating canal operations, surface and pressurized systems,and drainage systems must be developed, tested, and supported by incentives.

On-farm systems will benefit from advanced technologies, such as precision ir-rigation, site-specific management, remote sensing, within-field real-time sensorsystems, and decision support systems, which collectively have great potential tofacilitate reduction of water quantity and quality problems in irrigated agriculture.

The use of real-time irrigation scheduling techniques (sensor-based) and site-specific precision applications of water through center pivot machines and mi-croirrigation are the next steps in the evolution of those technologies.

It will be necessary to expand modern crop production technologies to less pro-ductive rainfed and irrigated lands characterized by poor soils, low and unstablerainfall, steep slopes, and short growing seasons to increase food production andstimulate economic growth. Novel approaches will be needed to address theseareas.

Microirrigation with its many variations must be made less expensive beforemost growers will be able to adopt and utilize these technologies, especially indeveloping countries. Some localized efforts are ongoing and one company ismanufacturing tubing at relatively low cost, but these innovative efforts and

technologies need to be extended to other areas. For developing countries, innovative research and extension education is needed

to provide and implement simple but efficient low-cost methods of irrigation(e.g., pitcher irrigation) to make them easy to operate, suitable for the crop, andacceptable to growers. There is also a huge need for low-lift pumps that are in-expensive to buy and operate in these areas. Some of this is already being doneon relatively small scales, but there is much room for innovation.

Surface irrigation methods can be made more efficient using surge flow, dead-level basins, and other techniques for more uniform infiltration along the lengthof the field. Properly designed and operated level basins eliminate runoff, can bequite efficient and uniform, and are relatively inexpensive to construct and oper-ate. However, considerable investment for delivery system improvements, as


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well as sensor feedback controls and automation for both the delivery and appli-cation systems, is needed to fully realize the potential water savings.

Urban and agricultural irrigators will be the primary users of degraded waters.New approaches and techniques will be required to safely minimize detrimentaleffects while maintaining production goals.

Farm- and district-level drainage systems will require improved design, evalua-tion and simulation models defining the physical limits. Automated control sys-tems will assist in providing a more uniform soil water environment for plant

growth to improve productivity and minimize the volumes of drainage watersrequiring treatment, especially in arid areas. Water table elevations can be managed to permit subirrigation, if the groundwa-

ter is relatively shallow and of suitable quality, by controlling water tables or in-ducing water tables with irrigation applications. Subirrigation has been practicedsuccessfully in climate regions ranging from humid to arid. Using the effluentfrom deeper subsurface drainage systems as a source of irrigation water hasproven effective in many regions of the world. The biggest concern is the quality(i.e., salinity) of the drainage system effluent.

There is still no reliable, inexpensive electronic soil water sensor that matches orexceeds the accuracy and repeatability of neutron scattering devices. Innovativedevelopment of such sensors is essential for water management, particularly un-der deficit conditions.

Many of the needed and evolving technologies will require stand-alone, spatiallydistributed electrical power to be feasible. Controllers, monitoring equipment,and communications devices must be low power consumers. Photovoltaic, windturbine, and storage systems will need to be developed and implemented at lowcost at the farm or field level.

Economies of scale have led to large field sizes for irrigated production in manyareas, and engineers have been very successful in designing pressurized irriga-tion systems that apply water quite uniformly over these fields. However, thechallenge over the next 50 years for the irrigation industry and designers is todevelop highly efficient systems that are also suitable for small-scale farms andprovide the necessary extension education to equip the farmers with the skills torun them.

Although widely variable, it is estimated that 1% of the global water storage ca-pacity in reservoirs is lost each year to sedimentation (Palimeri, 1998), decreas-

ing the ability to store water. Innovative methods to reduce erosion at the water-shed and basin scale will be needed to increase the life of reservoirs for storage,flood control, and recreation uses.

Innovation will be required to enable adoption of more efficient irrigation methods.For example, high-frequency drip irrigation and other microirrigation methods havebeen shown to increase the yield and quality of fruit and vegetable crops through re-duced water and nutrient stresses. Tied to an effective soil water monitoring program,good design, and appropriate management practices, microirrigation can be 95% effi-cient or better. A modification of center pivot irrigation called low energy precisionapplication

livro irrigação asabe - capítulo 1

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