soil waternew
TRANSCRIPT
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Soil water
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Functions of water
Plant cells have 50 - 90% water
Keeps turgor
Seed germination
Transpiration
Photosynthesis
Moves products
Nutrients available
Lowers soil strength
Chemical reactions
Microbial activity
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Forces on Soil Water
Soil-water potential
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Forces on Soil Water
Capillarity
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1- Gravitational
2- Capillary
3- Hygroscopic
Types of soil water
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Forces on Soil Water
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Gravitational
Gravitational also called free water.
This is the water that drains out of the soil
after it has been wetted.
This water moves downward through the
soil because of the pull of gravity. This
water also feeds wells and springs.
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Capillary
Capillary water that moves into and is
held in the soil by capillary forces .
Plant roots can absorb or take up this
moisture.
The size of the soil pore will influence the
amount of water held by capillary forces.
Provides most of the moisture for plant growth.
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Hygroscopic
Hygroscopic - very thin water films around
the soil particles. These films are held by
extremely strong forces that cause the
water molecules to be arranged in a semi-
solid form. This water is unavailable to
plants.
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Types of Soil Water
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How is Soil Water Classified?
1) Hygroscopic Water is held so strongly bythe soil particles (adhesion), that it is not
available to the plants.(10.000 to 31 atm.)2) Capillary Water (31 to 0.33 atm) is held by
cohesive forces greater than gravity and issome of it is available to plants (0.33 to 15atm.) and some of it is not available ( 15 to 31atm.).
3) Gravitational Water ( 0.33 to Zero atm. ) is
that water which cannot be held against
gravity.
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Levels of Water in Soil
Saturation Point the moisture point at
which all of the pore spaces are filled with
water.
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Levels of Water in Soil
Field Capacity the maximum amount of
water left in the soil after losses of water to
the forces of gravity have ceased and before
surface evaporation begins. Water forced by
0.1 to 0.55 with average 0.33 atm. Or bar occurs when the soil contains the
maximum amount of capillary water.
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Field capacity : the amount of water that
remains in the soil after gravitational
drainage
F.C depends on soil type ;sands have a
lower field capacity than clays
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Levels of Water in Soil
Permanent Wilting Point the point at
which the plant can no longer obtain
sufficient water from the soil to meet its
transpiration needs. At this point the plant
enters permanent wilt and dies. The water
forced by 15 atm.
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Wilting point: the amount of water
that is so tightly held in soil that plant
can not utilize it.
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Plant available water = fc- wp
Soil moisture deficit SMD = TAW =
(fc- wp)* Z r (root depth zone)
Readily available water (RAW)
RAW = P(vfc- vwp)* Z r
Where P is the allowed depletion factor
which depend on type of plants
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Water Holding Capacities of Soils
The amount of water a soil can retain is
influenced by:
soil texture
soil structure
organic matter.
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Soil Texture
The smaller soil particles, the greater thesoils water holding capacity. Clay has
more water holding capacity than sand.
Small soil particles (clay) have more smallpores or capillary spaces, so they have a
higher water holding capacity. Large soil
particles (sand) have fewer capillaryspaces, therefore less ability to hold water.
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Some examples of field capacity and wilting
points for different soil textures
Field capacity
(m3/m3)
Wilting point
(m3/m3)
Textural class
0.400.25Clay
0.350.15Silt
0.300.10Loam
0.100.05Sand
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Soil Structure
A soil structure has a direct correlation to
the amount of water it can retain. Soils
which have a good structure such as
granular structure have a high water
holding capacity.
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Organic Matter
Organic matter aids in cementing
particles of clay, silt, and sand together
into aggregates which increases the
water holding capacity.
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Developing a Plan for Controlling Soil Water
- Cover Crop close growing crop planted toprotect the soil and prevent erosion
Crop Rotation planting of different crops
in a given field every year or every severalyears
Mulch material placed on soil to break
the fall of rain drops (preventing erosion),
prevent weeds from growing, or reduceevaporation from the soil surface.
Strip Cropping alternating strips of row
crops with strips of close growing crops
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Water Retention and Movement
Soil Texture and water movement
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Soil structure and water movement
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Practical Measuring Devices
Gravimetric method
Potentiometers (tensiometers)
Resistance Blocks (gypsum blocks)
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Crop WaterRequirements
Crop water use, consumptive use andevapo-transpiration (ET) are the terms that
are used to describe the water consumed
by a crop. Water requirement dependmainly on the nature and stage of growth
of the crop and environmental conditions.
Different crops have different water-userequirements under the same weather
conditions.
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Evapotranspiration ( ET ) the combination
of water that is lost from the soil
through evaporation and through
transpiration from plants as a part of their
metabolic processes
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Factors affecting evapotranspiration
Weather parameters
Crop factors
Management and environmental
conditions
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Weather parameters
The principal weather parameters affecting
evapotranspiration are radiation, air temperature,
humidity and wind speed. Several procedures have
been developed to assess the evaporation rate from
these parameters. The evaporation power of the
atmosphere is expressed by the reference crop
evapotranspiration (ETo).
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The reference crop evapotranspiration ETo
represents the evapotranspiration from a
standardized vegetated surface.
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Crop factors
The crop type, variety and development
stage should be considered when
assessing the evapotranspiration from
crops grown in large, well-managed fields.
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Crop evapotranspiration under standard
conditions (ETc)
refers to the evaporating demand from crops that
are grown in large fields under optimum soil
water, excellent management and environmental
conditions, and achieve full production under the
given climatic conditions.
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Management and environmental
conditions
Factors such as soil salinity, poor land
fertility, limited application of fertilizers,
the presence of hard or impenetrable soilhorizons, the absence of control of
diseases and pests and poor soil
management may limit the cropdevelopment and reduce the
evapotranspiration.
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Reference crop evapotranspiration
(ETo)The evapotranspiration rate from a
reference surface, not short of water, is
called the reference crop
evapotranspiration or reference
evapotranspiration and is denoted as
ETo. The reference surface is ahypothetical grass reference crop with
specific characteristics.
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The only factors affecting ETo are climatic
parameters. Consequently, ETo is aclimatic parameter and can be computed
from weather data. ETo expresses the
evaporating power of the atmosphere at a
specific location and time of the year and
does not consider the crop characteristics
and soil factors. The FAOPenman-
Monteith method is recommended as the
sole method for determining ETo.
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TABLE 2. Average ETo for different agroclimatic regions in
mm/day
Mean daily temperature
C0
Regions Warm30C >
Moderate20C
Cool~10C
Tropics and subtropics
5-73-52-3- humid and sub-humid
6-84-62-4-arid and semi-arid
Temperate region
4-72-41-2- humid and sub-humid
6-94-71-3-arid and semi-arid
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Calculation of ETo
Penman-Monteith equation
Blaney- Criddle equation
Turc
Modified penman
Radiation method
Pan evaporation
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2- Blaney-Criddle equation:
ETo = C {P (0.46 t + 8.13)}
Where:
ETo = reference evapotranspiration in mm for the period
considered.
t = mean daily temperature in c over the period
considered.
P = mean daily percentage of total annual daytime hours
C = adjustment factors which depends on minimum
relative humidity, sunshine hours and daytime wind
estimates, according to Doorenbos and Pruitt (1984).
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Crop water requirements
where
ETc = Kc x ETo
ETc crop evapotranspiration [mm d-1],
Kc crop coefficient [dimensionless],
ETo reference crop evapotranspiration [mm
d-1].
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maximum rate when soil water is at field
capacity. When soil moisture decreases,
crops
have to exert energy to extract water from
soil. Usually, the transpiration rate does
not
decrease significantly until the soil
moisture falls below 50% of field capacity.
The evapo-transpiration (Etc in mm) of acrop under irrigation is obtained by the
following equation:
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The growing period can be divided
into four distinct growth stages:
Initial stage
crop development stage,
mid-season stage
late season stage.
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Following four stages of crop growth:
i. Initialstage: Germination period and early
growth of crop when the soil cover by the crop is
less than 10%.
ii. Crop development stage: The end of initial
stage till the soil cover by the crop is about 70-80%.
iii. Mid-season stage: From the end of the crop
development stage to the start of maturing, for
most crops this shall be beyond flowering stage.
iv. Late-season stage: From the end of mid-
season stage to the full maturity or harvesting.
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Kc for maize
Late-
season
Mid -
season
developmentinitial
0.61.20.70.4
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Water requirement liter/ day / plant.Crops
ConventionalDrip
200-30075-100Coconut
90-10025-35Grapes
90-10030-40Mango
70-10022-30Guava
70-10020-30Sapota
60-10020-40Pomegranate
30-408-12Banana
20-6510-20Lemon
16-265-6Papaya
4-61-2Vegetables and Flowers
3-41.5-2Tapioca
3-51.5-2.5Cotton h brid
Water requirement of crops in drip and conventional system
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Irrigation scheduling
1 irrigation water quantity by irrigate
RAW= p(v fc - v wp) x zr
Where: p is the allowed depletion
2-irrigation intervals = RAW/ ETc
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*Calculate Crop water requirements for maize
crop which cultivated in Sand soil ( FC= 12%,WP= 4% ) and Clay soil ( FC= 42 % , WP = 22%),
and the effective roots of maize are 20,40,60,80
cm, ETo from May to August are 6.4,7.3, 6.8 and6.1 mm/day Crop coefficient are 0.4, 0.8,1.2,and
0.6 ,respectively calculate the depth of applied
water irrigation for the types of soil and theirrigation interval of each month .allowed
depletion is 50%
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RAW
mm
ZrPw.pF.CMonth
202000.50.220.42May
404000.50.220.42June
606000.50.220.42July
808000.50.220.42August
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Irrigation
interval
(day)
RAW
mm
ETc
mm/day
KcETo
mm/day
Month
10202.00.405.0May
8404.80.806.0June
66010.01.258.0July
20804.00.508.0August
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