CAN INTEGRATED WATERSHED MANAGEMENT BRING

GREATER FOOD SEC URITY IN ETH IOPIA?

Oloro V. McHugh, Amy S. Collick, Benjamin M . Liu,

Debele B ekele, Jim E. H aldeman and Tammo S. Steenhuis

Department of Biological and Environment Engineering

Cornell University, Ithaca NY 14853 USA

Abebe Yitayew

AMAREW Project, Bahir Dar, Ethiopia

Gete Zeleke

ARARI, Bahir Dar, Ethiopia

Abstract: In the food insecure regions, short annual droughts of 2-4 weeks with a

severe drought typically every 10 years are common. The moisture stress between

rainfall is responsible for most crop yield reductions. Field are prepared for sowing

with traditional animal drawn "Maresha" resulting in a root zone depth of less than

10 cm. In this paper, we show with the use of appropriate water balance models

given the limited data available, that increasing shallow tillage depth moisture

availab ility for the plant will increase. Experimental results confirm these finding

and as a result of the greater water availability yields increase significantly.

Strategies are d iscussed for implementation of these findings at a watershed scale

in order to increase food security. Copyright © IFAC

Keywords: Agriculture, Computer Simulation, Knowledge Representation,

Mathematical M odels

1. INTRODUCTION

Agriculture is the backbone of the Ethiopian economy.

It is responsible for approximately 50% of the Gross

Domestic Product, 90% of foreign exchange earnings,

and 85% of the livelihoods of the population.

Ethiopia's agricultural sector is driven by the

subsistence strategies of smallholder farmers and their

families. In the past due to insufficient knowledge base,

some misguided agricultural policies, coupled with a

rapidly growing population, chronic poverty, and

capricious rainfall, have caused severe food security

challenges for farm families and natural resource

degradation. Drastic new approaches that lead to

improvement of food security and a lessening of the

dependence on food aid are needed.

As part of a strategy to achieve food security while

protecting the environment through sustainable land use

development, integrated watershed management (IWM)

approaches are being developed The major advantages

of IWM approaches are involvement of those most

affected by the decisions (i.e. the stakeholders) in all

phases of the development of their watershed and

holistic planning that addresses issues which extend

across subject matter disciplines (b iophysical, social,

and economic sciences) and administrative boundaries

(village, woreda etc.).

It has been estimated that 2 million ha of Ethiopia’s

highlands have been degraded beyond rehabilitation,

and an additional 14 million ha severely degraded

(UNEP, 2002). Removal of vegetation cover (through

overgrazing and for charcoal production) exposes the

soil to wind and water erosion. Soil compaction occurs

in areas where there is excessive trampling by animals

and, in cultivated areas, soil fertility is declining, as a

result of the exhaustion of soils by mono-specific

cropping and reduction of fallow periods. Soil

degradation contributes to rising rural poverty and food

insecurity, because productivity is reduced, and

subsistence farmers are less and less able to accumulate

reserves of grain (UNEP, 2002).

The agro-pastoralists in Ethiopia living in semi arid

watershed with degraded soils are among the poorest

people in the world and depend totally on the renewable

natural resources for their livelihoods. According to

Hatibu (2003) of the Irrigation Water Management

Institute, their poverty is mainly caused by inadequate

availab ility of water for crop, livestock and other

enterprises. He then argues that the shortage of water is

not caused by low rainfall as normally perceived, but

rather by a lack of capacity for sustainable management

and use of the available rainwater on these degraded

soils. Hatibu (2003) states "the most critical

management challenge is how to deal with the poor

distribution of rainwater leading to short periods of too

much water and flooding, and long periods of too little

water. The question is: ‘can better management of the

availab le rainwater help to reduce the occurrence and

mitigate the impact of droughts during periods with low

rainfall’?"

Hatibu's viewpoint divergences from the many

traditional studies such as by Sonneveld and Keyzer

(2003) that consider soil erosion and its effect on soil

quality and water storage to be the main culprit in

securing food security. However, in strong support of

Hatibu's view, water scarcity was identified as the main

problem in formal and informal stakeholders surveys in

the one of the watersheds (M cHugh et al., 2004).

To examine if better water management of available

rainfall in semi-arid climates can mitigate the impact of

droughts and help to improve food security, this paper

uses a simulation model. Model results are tested with

number of experiments. The model selected has to be

appropriate for the limited data, available in the

developing world for watershed modeling. Two

watersheds were selected for the testing phase: the Yeku

and Lencha Dima watersheds. Both are located in

semi-arid mountainous areas that are severely eroded

and have unreliable rainfall. Mean annual rainfall

amounts are sufficient for most types of agriculture

provided appropriate water conservation measures are

in place. A large percentage of the people use food aid

to survive.

The research in the watersheds is carried out under the

watershed component of the USAID funded AMAREW

project. This component is designed to demonstrate

integrative approaches to research, extension,

community development, and micro-enterprise

development in two pilot watersheds in the eastern part

of the Amhara region. One of the aspects of the

program is to use food aid in a meaningful way in

watershed development.

2. THE PROBLEM AND PO TENTIAL

Even in the semi-arid watersheds, rainwater is available

in abundance during the rainy season and surpasses the

evapotranspiration during a few months (July, August

and September in most cases, and March and April for

selected Ethiopian conditions). The main reason is the

practical difficulty posed by the nature of rainfall. The

rain is very poorly distributed in both spatial and

temporal terms. Often there is too much water during a

few days of the year, while water supply is insufficient

during most of the year. It is estimated that in most

Semi-Arid Tropics the time when it is actually raining

is in total about 100 hours per year, out of the 8,760

hours of the year.

As a consequence, the moisture stress between rainfall

events (dry spells) is responsible for most crop yield

reductions and sometimes even for total crop failures

(Rockstrom et al., 2002). In their study, Rockstrom et

al. (2002) reported that dry spells in rain fed agriculture

of arid and semi-arid regions, which occur frequently,

are responsible for a decrease in yield by about 70% or

even sometimes a total crop failure. Hence, if one

conserves the excess water during heavy rains in the

rainy season so that plants can use it in the latter times

during dry-spells, it may be possible to avert the

majority of the production loss due to moisture stress.

Although well known in principle, the technologies

required overcoming the poor and extreme distribution

of water resources through storage and transfer are

usually not applied because of poor adaptation to the

local conditions and unavailability of capital. As a

consequence, there is critically low access to water for

agriculture, drinking and sanitation. Poor access to

water is, therefore, among the leading factors hindering

sustainable development in semi-arid watersheds.

Approaches to overcoming this problem include

technologies for enhancing the productivity of water in

rain-fed production, rainwater harvesting and precision

irrigation.

Rainwater harvesting is currently a high priority of the

Ethiopian government and this program is well on its

way. Precision irrigation is tried but often limited in the

semi arid areas due to the lack of baseflow in the rivers.

Therefore in this paper we are mainly concerned with

enhancing the productivity of the rainfall (i.e., more

crop per drop) by making more available to the plants

and less to surface runoff. The benefits are three fold:

less erosion because runoff is reduced; greater food

(1)

(3)

security by increased crop water availability and as a

byproduct increased ground water recharge leading to

higher baseflows and more precision irrigation during

the dry season.

Plowing depth, water retention and crop yield are

inter-related. The traditional oxen drawn plow

"Maresha" plows the soil only to a limited depth of 5-15

cm in the degraded soils and downward water

movement of water is restricted because of the tight

subsoil (Mwendera et al, 1997; Astatke and Saleem,

1998; Kaumbutho et al., 1999). Root depth is limited in

these soils (Seghieri, 1995). In this paper, we will

investigate if by increasing tillage depth water can be

stored in the soil such that plants can survive the dry

periods between rainstorms. We will use both computer

simulation and field evidence.

3. THE MODEL

Data requirements vary between models. In order for a

model to be useful, its data requirements must be

readily obtainable (Taylor et al., 2004). For example,

runoff models require detailed information on soil type,

moisture status, and vegetation characteristics. Often,

and in the case of Ethiopia highlands, these data are

extremely difficult to obtain. Lumped water balance

models have less stringent data requirements. With just

rainfall ®) and potential evapotranspiration (ETp) data,

discharge can be calculated with the lumped

Thornthwaite-Mather (TM) procedure for relatively

large watersheds using generalized soil and aquifer

characteristics (Thornthwaite and Mather, 1957;

Steenhuis and van der Molen 1986). The TM

procedure was developed in the early 1940s and has

successfully been applied in basins with southern

coordinates such as Mount Kilamanjaro in, Kenya

(Dunne and Leopold, 1978), Luancheng County in

Northern China (Kendy et al. 2003), Singkarark-

Ombilin in Indonesia (Peranginangin et al., 2004), and

northeastern Mexico (Mendoza et al. 2003).

Applying the T-M procedure requires several

assumptions. These are:

• The soil is divided in a root zone with a reasonable

high saturated conductivity and zone below that is

devoid of roots and has little or no connection with

the plowed soil above

• Percolation through the plow pan and lateral

subsurface flow is small and will be neglected.

• Overland flow is generated when the soil above the

plow pan becomes saturated. In other words daily

runoff is equal to the daily runoff minus the amount

of open pore space at the beginning of that day in

the soil above the plow layer.

• On days when the evaporation is greater than the

rainfall, actual evaporation is a linear function of

amount of water in the soil and the potential

evaporation.

• On days when the rainfall is greater than the

potential evaporation, the soil moisture content

increases equal to the difference between

precipitation and potential evaporation.

These assumptions results a simple but powerful

calculation method can be used to calculate daily fluxes

and moisture contents in the soils without the need of

arbitrary crop coefficients.

3.1 The Thornthwaite Mather Procedure

The T-M procedure uses rainfall, po tential

evapotranspiration, and as soil physical parameters the

availab le water capacity (AWC) of the roo t zone. With

these input data and the assumption mentioned above

the T-M model uses a spreadsheet to calculate the

actual evaporation and the moisture content in the soil.

The water balance for the root zone can be formulated

as:

where St is the soil moisture storage at time t, [L], R is

the rate of rainfall input, [L/T]; ET is the actual

evapotranspiration rate , [L/T]; ERF is the excess

rainfall rate during which will become runoff with the

assumptions made in the text [L]; is the soil

moisture storage at time, t - )t, [L]. The actual

evapotranspiration (ET) is in turn calculated by using

(2)

Where, ETp is the daily potential evapotranspiration,

[L/T]. The maximum soil moisture storage, Smax, is

calculated from:

Where, ms is the soil volumetric moisture content at

saturation; m l is the limiting volumetric moisture

content below which no evapotranspiration takes place,

and D is the soil depth of the root zone, [L]. In other

applications of the procedure, the upper moisture

content is taken as field capacity but, here, because of

the limited percolation in the subsoil, saturation is more

appropriate. In case Eq. 1 calculates on a particular day

a storage S t in excess of Smax the rainfall in excess of

saturation becomes runoff and St is set back to Smax.

The model was run with a daily time step with daily

precipitation and daily potential evaporation as input

and then calculates the amount of water stored in the

soil and actual evaporation as a function of the

maximum amount of soil moisture stored in the so il.

Figure 1:Average monthly rainfall (RF) and potential

evapotranspiration (PET) at Maybar and Mekele.

Figure 2:Daily rainfall and soil moisture storage in the

root zone for different root depths, Mekele, Tigray -

1992.

Figure 3: Daily rainfall and soil moisture storage in the

root zone for different root depths, Maybar, Wollo -

1992.

3.2 Input data

Among the major problems in hydrological studies

inputs is daily precipitation data. These are not

availab le for the Yeku and Lencha Dima watersheds.

We resorted, therefore, to two meteorological sites

within a 300 km radius. One climatological station is

Maybar (Wollo) that has daily rainfall records and

represents typical highland conditions with an average

annual rainfall of 1156mm. The other station is Mekele

(Tigray), which is located in a semi arid region with an

average rainfall of 600mm per year. This station has

only monthly records. Both stations report great

variation in annual rainfall. Over the 12-year period of

record the rainfall ranges for Maybar from

approximately 800 mm to 1500 mm and for Mekele

between 400 mm and 900 mm. Both stations have a

non distinct bimodal rainfall pattern (Fig. 1): light

rainfall from March to M ay (comprising about 24% of

the annual rainfall distribution), and heavy rainfall from

July to September comprising about 56% of the annual

rainfall distribution.

In running the model, rainfall a normal year, 1992 and

a wet year, 1993 was chosen for both Mekele and

Maybar. No daily rainfall data was available for

Mekele. Consequently, the daily rainfalls were obtained

by assuming that the ratios between monthly rainfall

amounts at Maybar and M ekele was the same as the

daily ratios. For example, if the ratio of the monthly

rainfalls between Maybar and Mekele was 1.5 for the

month of June, the daily rainfall amounts of Mekele

would be the daily rainfall readings at Maybar divided

by 1.5 for all the 30-days in June. For potential

evaporation we took 4.5 mm/day for all the months

except July and August for which 3mm/day was used.

In order to calculate the maximum storage in the soil we

multiplied the depth of the plow pan with the difference

of soil water contents between saturation (0.45 cm3/cm3)

and wilting point (0.10 cm3/cm3). Four depths of root

zones are examined: 10, 15, 20 and 30 cm. The

maximum water storages for these depths in the

rootzone are respectively 35, 43, 70 and 105 mm.

3.3 Results

Based on the Thornthwaite Mather method , the soil

moisture storage was calculated for so ils with a plow

pan at 10, 15, 20 and 30 cm (Figs 2, 3 and 4). As stated

above the roots did not extent below the rootzone Fig 2

is the simulation results for a normal year in Maybar

(1992) and both years are given for Mekele (Figs 3 and

4). In these figures the rainfall is "hanging" from the

top. As expected the amount of water stored at any time

in the soil is greater when the plow pan is deeper. In

these figures the storage for the 30 cm root zone is

always larger that for the root zone of 20 cm and larger

for 15 cm, etc. For the semi arid climate represented by

Mekele the moisture content builds up for the relatively

dry year is given (Fig 2). As soon as the potential

evaporation exceeds the rainfall on day 180 in the

beginning of July (Fig. 2). The maximum storage (i.e.,

the soil is saturated) is reached around day 210 at the

end of July and then remains so until the end of August

or beyond depending on the duration of the rains. This

also means that during August most of the precipitation

in excess of the potential evaporation becomes surface

runoff and will cause erosion.

For Maybar the annual rainfall is approximately twice

that of Mekele (Fig. 1) and consequently the soil is

much wetter throughout the year (Figs 3 and 4). This is

especially true for the period of March through May.

However the maximum soil water storage is only

reached for short periods of time during because

monthly average potential evaporation is higher than the

monthly rainfall(Fig. 2). The moisture storage during

July and August is the same for Mekele and Maybar

(compare Fig. 2 with Fig. 4) . In both cases the soils

become saturated. The soil remains wet longer in

Maybar than for Mekele.

Figure 4:Daily rainfall and soil moisture storage in the

root zone for different root depths, Maybar, Wollo -

1993.

Table 1: Number of days in a month with sufficient

(shaded) and insufficient moisture (not shaded) status in

the soil for different root depths (Mekele - 1992).

Table 1 provides another way to look at the impact root

depth on soil moisture storage and plant growth. It is

the same data as shown in Figs 3 . Mainly for

illustrative purposes, we decided that if on a particular

day when there is less than 10 mm of plant available

water in the soil , this is "insufficient". It is labeled as

such in the tables. Moreover, if there are more than 4

days in the month with insufficient storage, we assume

that the yield is impacted so that crop failure could

occur. Only months that have sufficient rainfall are

shaded gray (i.e., less than 4 days with insufficient

rain). The trend is obviously more significant than the

absolute numbers.

It is evident from Table 1 that the deeper the root depth,

the smaller the number of days that there is insufficient

soil moisture. Especially the root depth of 30 cm seems

to be most effective in reducing the stress days for the

normal rainfall year depicted in Table 1 . On the other

hand, when the roo t depth is 10 cm, a crop with growth

duration of 3 months will have difficulty surviving even

under normal rainfall conditions (Table 1). During the

wetter year, effect of root depth becomes less important

because the rains are usually spaced more closely

together.

3.4. Concluding Remarks Modeling

The T-M procedure demonstrated that integrated

watershed management plans that include deep plowing

or sub-soiling will likely increase food security. Deeper

plowing, which is being advocated among others by

GTZ, could make more water available to the crop by

infiltrating more of the rainfall. However, the results are

based on a model. In the next section, we will report on

an experiment in which the outcome of model is

checked. The “model recommended practice” of

subsoiling (also called deep tillage) is compared with

traditional plowing and other means of conserving

water such as by open ridges and tied ridges.

4. LENCHE DIMA WATERSHED

ON-FARM T RIALS

On-farm tillage and water harvesting experiments were

conducted during two years (2003-04) on a farmer's

field in the Lenche Dima W atershed (N 11o50.415', E

39o43 .871 ', 1540 m above sea level). The loamy clay

soil is classified as a vertic luvisol and has a bulk

density of 1.56 Mg m-3.

4.1 Experimental design

The experiment was setup as a randomized complete

block design with four treatments and three rep lications.

Each plot measures 6 m wide by 30 m long down slope

and is enclosed by 50 cm wide (20 cm high) soil bunds

on the top and two sides to prevent run-on water from

entering the plot. The treatments are subsoiling with an

ox-drawn subcultivator (DT), and in-situ rainwater

harvesting using tied-ridges (TR) and open ridges (OR).

These treatments are compared with the control tillage

using the traditional single tined-plow called maresha

(M).

All plots were plowed twice during the dry season (first

along the contour and second along the slope) with the

oxen-drawn traditional plow (maresha). The week

before sowing the open and tied-ridges were plowed

along the contour with the ARARI- (Amhara Region

Agricultural Research Institute) developed ox-drawn

ridger. The open and tied ridges were 50 cm apart and

with amplitude of 10-13 cm and average ridge width of

27 cm. The tied ridges were tied manually at an average

of 1m spacing. The traditional tillage (M) and subsoiled

(DT) plots were plowed along the contour with maresha

and the "tenkara kend" sub-cultivator, respectively,

before planting. The tenkara kend sub-cultivator was

developed by the German GT Z development

organization in Ethiopia. Similar to the traditional

maresha plow it turns the soil to a depth of about 8 -15

cm but the sub-cultivator has an blade extension which

cuts the soil an additional 6 -12 cm without turning the

soil.

No external nutrients inputs were applied to the plots

during experimentation or during the 10 preceding

years. Seeds of a local variety of red sorghum

(Djigourti) were manually sown at a rate of 10 kg per

hectare in rows 50 cm apart on all plots. Five weeks

Table 2:Monthly rainfall, evaporation, and temperature

for growing season in 2003 at Lenche Dima watershed,

Hara Town, Gubalafto district, Amhara region.

after sowing all sorghum plots were thinned to a

spacing of 25 cm between plants and 50 cm between

rows. Weeding was carried out manually twice at four

and eight weeks respectively after sowing.

Soil moisture was measured with TDR (time domain

reflectometry) soil moisture probes and the gravimetric

field technique. Ten measurements were taken with the

12-cm long soil moisture probes at each depth (0, 15,

and 30 cm) in random locations on the top and bottom

sides of each plot. The readings from the probes were

calibrated for the soil type using results from the

gravimetric measurements. Gravimetric measurements

were taken with 6-cm diameter soil cores at each dep th

(0-15, 15-30, 30-45 cm). Moist weight was measured

immediately in the field . The samples were sun and

air-dried for over two weeks before measuring dry

weight.

Plant height, total above-ground biomass, and root mass

were measured on six randomly selected plants

distributed throughout the plot. Root mass was

determined from the below ground part of the plants.

Grain yield was measured on 2 x 2 meter quadrats on

the top and bottom of each plot. The number of plants

within the quadrat was counted before harvesting.

Plants and grain were sun and air-dried for over two

weeks before taking dry weight.

4.2. Results

Only the results of the 2003 cropp ing season are

available. Table 2 presents the rainfall, evaporation, and

temperature during the growing season. Rainfall during

the cropping months totaled 516 mm and is comparable

to 1992 rainfall amount for Mekele used in the T-M

procedure. Evaporation rates exceed rainfall for a ll

months except August which received 50 % of the total

rainfall during the cropping season. Daily maximum

temperatures are hot (above 30oC for all months except

December).

Figure 5 shows the soil moisture for each treatment

during the cropping season at 0-15 cm, 15-30 cm, and

30-45 cm depth. The ridges for the open ridge (OR) and

tied-ridges (TR) plots remained dryer than the lower

than the furrows affecting negatively germination and

initial plant growth stages since the seeds were sown on

the ridges. Below the ridges in the 0-15 cm soil depth

the OR and TR treatments had consistently higher soil

moisture than the other treatments.

For the 15 - 30 cm soil depth all treatments had similar

soil moisture with slightly more for the subsoiling and

water harvesting (open and tied-ridges) treatments. At

the 30 -45 cm soil depth tied ridges had significantly

more moisture than the other treatments. The OR and

DT treatments also had more soil water content than M

for most of the season. During the season rills

developed in the OR plots reducing the capacity of

ridges to store water. This could be the reason why the

TR plot had considerably more moisture than the OR

plots. The tied-ridges had some breaks but the effects

on water retention were more localized due to the ties

compared with the open ridges. The relatively steep

slope (up to 9 %) of the plots and high intensity of the

first major storm (56 mm in 50 minutes on July 31, 2

weeks after sowing) destroyed several of the ridges of

the OR and TR plots creating in-plot rills and leveling

some parts of ridges reducing their efficiency of water

collection. This reduced the water harvesting function

of the ridge treatments considerably.

In accordance with the model results, sub-soiling

improved the soil moisture for most of the season

compared with the traditional tillage. The sub-cultivator

cut the soil an additional 6-12 cm below the depth of

soil turned by both the traditional plow and

subcultivator (8 - 15 cm). This add itional cutting

appears to have increased the soil moisture below the

30 cm dep th.

Figure 6 presents sorghum root growth during crop

development. During the first couple months (mid-July

through mid September) after sowing root growth was

similar for all treatments. After the initial phase of plant

germination and plant establishment, root growth in the

TR and OR plots excelled the DT and M plots. This

could be due to the roots extending to the high moisture

content deeper in the soil. During the second half of

crop growth the DT treatment had better root growth

than all other treatments. In the DT plots the soil is

plowed to greater depth softening the soil for root

growth. The final total root mass is higher for the water

harvesting (TR and O R) and subsoiled (DT )plots

compared with the traditional land preparation (M)

plots.

Table 3 shows the final sorghum biomass production

and grain yields. As expected from the simulation, the

DT plots produced the highest total above-ground

biomass and grain yield. The TR and OR treatments

produced similar grain yield. OR p lots produced less

root mass, but higher biomass compared with TR. The

DT, TR, and OR produced higher grain yield than M.

Germination and plant establishment rates for the TR

plots was significantly less than the other plots (see

Table 3) due to the low soil moisture content of the

Figure 5: Soil Moisture during 2003 cropping season

sorghum trials in the Lenche Dima watershed for depths

of 0-15 cm, 15-30 cm and 30-45 cm. M is traditional

single tined plow called Maresha; TR is tied ridges; OR

is open ridges and DT is deep tillage or sub-soiler.

Table 3: Plant biomass, root mass, plant density and,

grain yield from on-farm sorghum trials in Lenche

Dima watershed during 2003.

Figure 6: Sorghum root growth during 2003 cropping

season. Abbreviations are the same as in Figure 5.

ridges where the seeds were sown and numerous breaks

in the tied ridges washing the seeds and young plants

away. Grain yield for the TR plots might have been

higher if the plant density was not considerably less

than for the other plots.

5. CONCLUDING REMARKS

Both modeling and experimental evidence clearly

showed that by providing more storage for water on the

crop yield will increases and as a result there will be

less dependance on food aid without increasing the risk

to the farmer of crop failure.

Starting to manage rain water by sub soiling is only a

beginning for better overall watershed management.

Soils need to further improved so that all the rainwater

can be stored and not only a portion. Moreover by

better understanding the hydrology, it might be possible

to use interflow water for supplemental irrigation.

These improvements cannot be made without the input

of the farmers, extension personnel and local

researches. During the last two years, however, the

integrated watershed management approach in the

Lenche Dima watersheds has faced several difficulties.

Although there is more than reason for this, all the

conservation activities initially proposed were almost

all related to stopping so il erosion and to not increased

water storage. As clearly indicated by the farmers in

the survey water had a higher priority than soil. It will

be interesting to see if the farmers will more responsive

to subsoiling than to the erosion related conservation

measures.

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