沙漠研究 22-1, 203 -206 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－Evaluating the Consumptive Water Use of Teffin Consideration of Soil Water Movement in Central Ethiopia
Ayumi KUBO*1), Fumio WATANABE2), Shinji SUZUKI2) and Satoru TAKAHASHI2)
Abstract: Crop production in Central Ethiopia mostly depends on rainfall. Although the mean annual rainfall is adequate to growcrops, the crop production often suffers from severe drought stress resulting in unstable food security. Recently, several trials ofirrigation techniques have been practiced in Ethiopia to improve the crop production systems and to save water, such as spate irrigationand consecutive irrigation reservoir system with water harvesting, etc. On the other hand, basic irrigation designing data were still
limited (such as water consumption amount and crop coefficient, etc.). Especially, water consumption of teff (Eragrostis tef ), which
is a staple food in Ethiopia, is not clear. Incidentally, it has been reported that it is prone to overestimate consumptive water use by
Soil Moisture Depletion Method a conventional one, because it has not been able to distinguish with upward and downward watermovement. For this reason, we measured soil water content and matric potential in a teff cultivation field in order to identify upwardand downward of soil water movement and to determine an effective soil layer from water flow direction by using Zero Flux Method(ZFM). In this article, we proposed a method to evaluate the consumptive water use of teff in consideration of soil water movement
in order to make an effective irrigation plan for water conservation. The results of this study indicated that the ZFM caused reduce
30% of the amount of irrigation water in comparison with the conventional one.
Key Words: Ethiopia, Irrigation, Soil water movement, Teffplan for water conservation.

1. Introduction
2. Materials and Methods
Crop production systems in Central Ethiopia mostly
depend on rainfall. Although the mean annual rainfall isadequate to grow crops, the crop production systems often
suffer from severe drought stress resulting in unstable foodsecurity. Recently, several trials of irrigation techniques havebeen practiced in Ethiopia to improve the crop production
systems, such as spate irrigation and consecutive irrigation
reservoir system with water harvesting, etc. On the otherhand, basic irrigation designing data were still limited (such as
water consumption amount and crop coefficient, etc.).
Especially, water consumption of teff (Eragrostis tef ), which isa staple food in Ethiopia, is not clear. Incidentally, it has beenreported that it is prone to overestimate consumptive water use
by Soil Moisture Depletion Method a conventional one,because it has not been able to consider gravitational drainage.
For this reason, we measured soil water content and matricpotential in a teff cultivation field in order to identify upwardand downward of soil water movement and to determine aneffective soil layer from water flow direction by using Zero
Flux Method (ZFM). In this article, we proposed a method toevaluate the consumptive water use of teff in consideration ofsoil water movement in order to make an effective irrigation

2.1. Cultivation Experiment
The experimental site is located in Adami Tule Agricultural
Research Center (hereinafter referred to as “ATARC”), Oromiastate of Central Ethiopia (7˚51’ N, 38˚42’E, 1,650 m a.s.l.) (Fig.
1). In this region, the period during June throughout
September is usually classified as rainy season, and the other as
dry season. The soil texture in experimental site is classified
to be Sandy Loam according to the classification of theInternational Society of Soil Science.
In an upland cropping field, 1 m × 1 m cultivating plot wasprepared. In this plot, changes in soil water content wererecorded at a depth of 5, 15, 25 and 35 cm with soil moisturesensors (EC-5, Decagon Devices, Inc., Pullman, WA, USA)and data logger (Em50, Decagon Devices, Inc., Pullman, WA,USA). A broadcast sowing of teff seed was conducted on 4Sep 2009 with no fertilizer application, and the harvesting ofteff on 22 Dec 2009. The amount of irrigation was 8 mm per
day except rainy days. In order to estimate potential
evapotranspiration by Penman method (Allen et al., 1998),
temperature, relative humidity, wind speed, solar radiation, andrainfall were recorded by HOBO Weather Station (Onset*Corresponding Author: a.kubo@ntc-i.co.jp
1-1-1, Sakuragaoka, Setagaya-ku, Tokyo, 156-8502, Japan1)

Graduate School of Agriculture, Tokyo University of Agriculture, Japan (At present: NTC INTERNATIONAL Co., Ltd)

2)

Faculty of Regional Environment Science, Tokyo university of Agriculture, Japan

Fig. 2. Water retention of the soil in ATARC.

Fig. 1. Object site of investigation.

Computer Co., Bourne, MA, USA).
In this article, we estimated the consumptive water use ofteff by Soil Moisture Depletion Method and Zero Flux Method
in order to compare these methods.
In SMDM, a bottom of effective soil layer was defined assoil depth where soil water content remained at about the samelevel. The difference between consumptive water use by
SMDM and evapotranspiration is reported to be less than 10%(Tanner, 1967). In terms of that, an observation point wasdefined as the bottom of effective soil layer on condition thatthe decrement of soil water content was less than 10% of thecumulative consumptive water use in effective soil layer. The
equation is shown in (1)∆θ N< 0.10∑ ∆θ N(1)Where ⊿θ is decrement of soil water content (m3/m3), N
indicates soil depth (5, 15, 25 and 35 in the unit of cm). To
take a hypothetical example, when the ratio of sum total ofdecrement of soil water content at a depth 25 cm to at a depthof 5, 15 and 25 cm was less than 10%, the effective soil layer
was defined as from surface to 25 cm deep. The equations
are shown in (2), (3).

∆θ 25
< 0.10∆θ 5 + ∆θ15 + ∆θ 25

ET = ∆θ 5 + ∆θ15 + ∆θ 25( 2)(3)Where ET is evapotranspiration (mm).
On the other hand, in ZFM, soil water content wasconverted into matric potential in each depth, and then the soil
depth where the sign of total potential reversed was defined as

Fig. 3. Example of how to determine effective soil layer.

the bottom of effective soil layer (Fig. 2). Just after irrigation,
the negative hydraulic gradient in all layers resulted in
downward water movement. Thereafter the drier the soil
layer from surface, the smaller the total potential, and then soil
layer of zero hydraulic gradient appeared. This point wasdefined as the bottom of the effective soil layer. In this study,
the consumptive water use was defined as the sum total ofdecrement of soil water content from the time that thehydraulic gradient became zero until the next irrigation. Forinstance, Figure 3 showed that the soil depth of 25 cm waszero flux plane, namely, the bottom of the effective soil layer.
Therefore the consumptive water use was also shown in theequation (3).
2.2. Irrigation plan
In order to clarify how the difference of the estimated crop
coefficient by two methods has an influence on water
conservation, we made the irrigation plan of teff in dry season(from October to January) of Central Ethiopia using the crop
coefficients with Crop Wat 8.0 software developed by the Land

Table 1. Meteorological
dataandevapotranspiration in ATARC.
MonthJanFebMar
AprMayJunJulAug
Sep
OctNovDec
Aveestimated

potentialETo
Min Temp Max Temp Humidity Wind Sun°C°C%km/day hours mm/day
4.8
11.0
28.0
53
190
9.1
12.5
29.1
50
164
8.7
5.0
8.1
5.1
13.3
30.0
51
156
14.2
29.4
58
156
7.8
4.9
8.5
14.7
29.1
61
173
5.0
14.727.5
63
225
7.6
4.8
207
15.1
25.1
68
5.6
4.0
14.2
25.6
70
164
6.3
4.0
13.2
26.8
68
121
7.0
4.1
11.3
28.2
55
164
8.7
4.9
10.3
27.5
52
173
9.4
4.8
10.2
26.9
53
181
9.6
4.6
12.9
27.8
59
173
8.0
4.7

3. Results and Discussion.
3.1. Cultivation experiment.
The results of the temporal change of the consumptive
water use of teff by SMDM and ZFM showed that themaximum of value was 4 to 6 mm per day, and then the ears ofteff was appeared at approximately the same time (40th dayafter sowing) (Fig. 5). Compared with the estimated values
between two methods, the value estimated by SMDM waslarger than by ZFM by 1 to 2 mm throughout the entire
growing period (Fig. 6). The root system was distributed insoil layer from surface to 5 cm below. These results were dueto the slightly larger amount of irrigation (8 mm/day) promoted
the downward of soil water movement after irrigation. The
growing period was classified into four periods with the day whenthe ears of rice appeared and the growth situation (Table 3).
Consequently, there was also the same trend in respect ofcrop coefficient. However, compared with crop coefficient ofother miscellaneous crops, there was so validity in theestimated crop coefficient of teff (Table 4).

Fig. 4. Monthly rainfall corresponding to 10-year probable rainfall
amount in ATARC (Feb 1987-Jan 1988).
Table 2. Soil property in ATARC.
Parameter

ATARC

Total available soil moisture(Field capacity-Wilting point)(mm/m)

170.0

Maximum rain infiltration rate
(mm/day)

78.0

Maximum rooting depth(cm)80

Initial soil moisture depletion(%)

0.0

Initial available soil moisture(mm/m)

170.0

Fig. 5. Consumptive water use and daily rainfall.

and Water Development Division, FAO. The necessary datafor running the software were about climate for estimation ofpotential evapotranspiration, monthly rainfall, and crop andsoil property (Tables 1 and 2, Fig. 4). The irrigation timing
was set when soil moisture content reached wilting point.
The amount of each irrigation corresponded to refilling soil
moisture content 100% field capacity. On the supposition ofsurface irrigation, the efficiency was set to be 60%.

Fig. 6. Comparison of consumptive water use between SMDM andZFM

Table 3. Potential evaporation, consumptive water use and crop
coefficient per growing period.
Growing stage
Growing period (day)

Ⅰ
0-22

Ⅱ
23-47

Ⅲ
48-73

Ⅳ
74-109Potential evapotranspiration(mm/day)

3.0

3.7

3.4

3.2

ZFM
2.5
SMDM 3.9
ZFM
0.85
SMDM 1.29

4.2
5.8
1.13
1.57

3.0
4.5
0.89
1.33

2.1
3.2
0.66
1.00

Consumptive water use(mm/day)
Crop coefficient

Table 4. Crop coefficient of principal miscellaneous crop in middlestage (Doorenbos et al., 1977).

Crop coefficient (middle stage)
Maize

1.20

Millet1.00

Sorghum

1.00

Barley1.15

with the scale of a reservoir installed in ATARC (150 m3) (Fig. 8)
and the general yield per hectare of teff in Ethiopia (0.9 t/ha).

4. Conclusion
It was revealed that crop coefficient of teff was no different
from that of principal miscellaneous crops and using of ZFM
could save water owing to the discrimination of upward anddownward of soil water movement and to the exclusion ofgravitational drainage. A further direction of this study will
be to practice cultivation test of teff with the results in thisstudy and to verify irrigation plan from the view point ofgrowth and yield. Finally it will be possible to expand
cultivation area and make effective use of irrigation water byimprovement in water-harvesting technique and consecutive
irrigation reservoir system.

Acknowledgement
Authors are grateful to Mr. M. Hattori of Japan Overseas
Cooperative Volunteer, Ms. M. Sasaki of Hokkaido University,and Ms. K. Iiduka of Tokyo University of Agriculture for theirhelpful support to the research.
References

Fig. 7. Comparison of net irrigation amount with SMDM and ZFM.

Fig. 8. Water reservoir for irrigation in ATARC (150 m3).

3.2. Irrigation plan
Compared with the cumulative net irrigation amount by
SMDM and ZFM, it was revealed that using of ZFM could
save water more than SMDM by 30% (Fig. 7). Consequently,
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