沙漠研究 22-1, 287 -290 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－

The Prevention Methods of Leak and Evaporation
for Consecutive Water Reservoir
Satoru TAKAHASHI*1), Shinji SUZUKI1), Fumio WATANABE1), Akio GOTO2) and Ayumi KUBO2) 3)
Abstract: Authors have developed a practical use of the consecutive water reservoir system as a source of small scale irrigation forstable cropping in Semi-Arid Africa. Particularly in Semi-Arid Africa, it is important to prevent the losses of leak and evaporation atthe water reservoirs. In this study, the prevention methods of those losses from the water reservoir were considered in semi-arid areaof Central Ethiopia, where the irrigation system based on consecutive water reservoir is expected to be prevailed among small scale
farmers. The consecutive water reservoir system consists of the primary, the secondary and the tertiary reservoirs. This study
focuses on the tertiary reservoir since small scale farmers can afford to establish only small reservoirs sized as the tertiary reservoir.
For the prevention of leak loss, it is important to analyze soil characteristics in the area and to compact soil at the optimum watercontent based on the Proctor’s Principle of soil compaction. In this study it was revealed that it is the most effective to compact soil
when water content is 33%. For the prevention of evaporation loss, foam polystyrene was proposed as a material to cover the water
surface of reservoir and it was revealed that the coverage rate of 80% is the most effective covering rate in terms of both effectivenessand efficiency.
Key Words: Foam polystyrene, Reservoir, Semi-Arid Africa, Soil compaction, Water resource1. Introduction
In the arid and semi-arid area where rain-fed agriculture isrun, the shortage of water is the limiting factor of crop
cultivation. The water harvesting method has been proposed
as a solution against the above issue, however, there had been
little study done to determine stable, minimum, and timely
water supply in case of water shortage caused by rainfall
fluctuation in the area. Therefore, the consecutive water
reservoir system consists of the primary, the secondary and thetertiary reservoirs was proposed as the measure of stable watersupply against rainfall fluctuation (Fig. 1).
However, there are issues associated with water leakage

from the bottom or the side of reservoirs and evaporation fromwater surface. Those issues are notable particularly insemi-arid area of Africa.
Therefore authors selected an arid area in Central Ethiopia
where rainfall fluctuation is severe as it is represented with thetrend of annual rainfall at Adami Tulu Agricultural ResearchCentre (N7°51’44’’, E38°42’44’’, alt 1,650m.a.s.l.) (Fig. 2), tostudy on the current situation of arid lands in Ethiopia wherethe irrigation system based on consecutive water reservoir isexpected to be prevailed among small scale farmers.
Following this, the counter measurements of above problems
are discussed. This paper focused particularly upon the issuesof leaking water and surface evaporation at tertiary reservoirsof small scale farmers.

2. The Actual Condition of Leak and Evaporation atReservoirs

Fig. 1. Consecutive water reservoir system. ①the primary reservoir, ②
the secondary reservoirs, ③the tertiary reservoirs, ④cropping field.

There are some individual reservoirs sized like secondary
and tertiary reservoirs of consecutive water reservoir system inthe semi-arid area of Ethiopia for human consumption(drinking), feeding animal and small scale irrigation. Thereservoirs have the same chronic and mutual problems such as
leaking water and surface evaporation. In result, none ofthem retains and supplies adequate amount of water for usage.
Therefore, authors conducted surveys to reveal the actual* Corresponding Author: satoru@nodai.ac.jp
1-1-1, Sakuragaoka Setagaya Tokyo, JAPAN1) Faculty of Regional Environment Science, Tokyo university of Agriculture, Japan
2) Graduate School of Agriculture, Tokyo University of Agriculture, Japan

3) NTC INTERNATIONAL CO., LTD

Fig. 3. Monthly mean evaporation rates at ATARC for 24 years.
Fig. 2. The trend of annual rainfall between 1958 and 2006 observed at
Adami Tulu Agricultural Research Centre (ATARC) located in
the Central Ethiopia.

situation and the problem of leak and evaporation of existingwater reservoirs.
2.1. The actual situation of leak loss in the reservoirs
Out of investigation into the actual situation of existingwater reservoirs, it was revealed that the circumstances ofreservoirs vary in wide range from empty reservoirs towell-filled reservoirs. The differences among those reservoirs
were determined whether the intrusion of domesticated animals
into the reservoirs occurred or not. Reservoirs with theintrusion of domesticated animals retain water. On thecontrary, reservoirs without intrusion do not retain water.
In an interview survey, it was revealed that farmers believe
functional reservoirs which hold water can be constructed only
when materials such as polythene or cement are used, and thecost is not affordable to most farmers. Farmers are not awareof other methods to prevent leak loss for the construction ofreservoirs besides using the above materials. It is a major reason
why water harvesting technologies such as reservoirs are notprevailed among farmers.
Out of the above results, it can be pronounced that one of
the major problems in the construction of reservoirs is the lackof soil compaction in most cases. Therefore authors suggest
applying the Proctor’s Principle of soil compaction as a countermeasurement against the above problem particularly forchallenging farmers in terms of finance.
2.2. The actual condition of evaporation loss
To reveal the actual situation of evaporation loss, themonthly average evaporation rates were calculated from theevaporation data of Adami Tulu Agricultural Research Centre(ATARC) for 24 years (Fig. 3). The data was measured using
A-pan method.
Annual average cumulative evaporation rate isapproximately 6.5 mm/day although the cumulative
evaporation rate tends to be slightly lower in rainy season.
The high evaporation rate represents a characteristic ofsemi-arid land.
Furthermore, the evaporation rate isequivalent to the evapotranspiration rate of common crop in thearea which is approximately 6 mm/day. In other words, theamount of evaporation loss is almost the same as the amount ofwater which is supplied by irrigation.
In addition, it was reported that one fourth of flowed-inwater was lost by the evaporation at experiment reservoirs in
2008. This indicates that the prevention of evaporation loss
from the water surface of reservoirs in semi-arid area is asignificant issue to overcome.
To prevent the evaporation loss, it is common to cover thewater surface with materials. Authors proposed whitecolored foam polystyrene as the covering material since thematerial should be floatable-waterproof, and flexible to theshape of reservoirs and the movement of water levels. Itshould also insulate and reflect sunshine which promotes
evaporation. In addition to the above, foam polystyrene canbe piled up each other while water level goes down, which
promotes the effect of preventing evaporation. Therefore it isimportant to know how much foam polystyrene should beapplied in reservoirs to develop a practical method of usage.

3. Verification of Counter-measurement against theLosses of Leak and Evaporation in the Reservoirs
3.1. Materials and Methodologies
Experiments on soil compaction and saturated hydraulicconductivity were conducted by using a soil shown in Table 1at ATARC which is located in the central part of Ethiopia.
The evaluation on the prevention of evaporation using
foam polystyrene was examined at the experimental field of theTokyo University of Agriculture in Japan (Fig. 4). The
different covering ratios namely 40%, 60%, 80%, 100% and200% were tested comparing with control.

3.2. Results and Discussions
3.2.1. The prevention of leak loss
The compaction curve and saturated hydraulic conductivity
examined in ATARC are shown in Figure 5. When the watercontent was 33% and the bulk density was 1.26 g/cm3 (at point
S in Fig. 5), the saturated hydraulic conductivity decreased up
to minimum level (100 × 10-6 cm/s) (Fig. 5). That is to say, it
is effective to compact soil with 33 % of the water content for
preventing leaking water. A traditional technology ofcompacting soil in Japan which can be applied for the situationis shown in Figure 6. These technologies can be adopted byfinancially challenging farmers in Ethiopia since the investment
is quite affordable to small scale farmers. Besides, the several
layers of lining with clay soil can add more value as a countermeasurement on leak loss.

Fig. 6. A traditional construction technique of compacting soil forreservoirs in Japan.

Table 1. Physical characteristics of experimented soil.
Soil Texture

Soil Particle
Density3

Sandy loam

(g / cm )2.52

BulkDensity
3

Saturated Hydraulic
Conductivity(g / cm )(cm / s)1.01

1.15 x 10-13

Fig. 7. The relation between the cumulative evaporation andcovering ratio with foam polystyrene observed in the field ofTokyo University of Agriculture in Tokyo, Japan (5th August2009 - 23rd October 2009).
Fig. 4. A experiment on the prevention of evaporation loss using
different covering ratio with foam polystyrene to water
surface at Tokyo University of Agriculture in Tokyo, Japan.

3.2.2. Prevention of evaporation loss
The relationship between the cumulative evaporation fromwater surface and covering ratio with styrene foam floating onwater surface is shown in Figure 7. When the covering ratio
was 80%, the cumulative evaporation was approximately 65mm (the 60% decrease comparing to the control). However,
the increment of covering ratio above 80% did not giveefficient effects in terms of the decrement of the cumulative
evaporation from water surface. That is to say, 80% is themost effective cover rate in terms of both effectiveness andefficiency.

4. Conclusions

Fig. 5. Compaction curve and hydraulic conductivity.

It is effective to compact soil with the adequate watercontents based on the Proctor’s Principle of soil compaction toreduce the leak loss from the bottom or side of water reservoirs.
However one should study about the climates and soil
characteristics in the experimental field. This is the basic

principle for the development of technologies.
With respect to the prevention method of the evaporationfrom water surface, it is effective to cover water surface (with80% covering ratio) with a material such as foam polystyrene
which is floatable-waterproof, and flexible to the shape ofreservoirs and the movement of water levels.

References
Takahashi S. et al. (2011): Consecutive Irrigation Reservoir
System in Semi-Arid Area of East Africa. Journal of The

Japanese Society of Irrigation, Drainage and RuralEngineering, 79(2): 109-112
Takahashi S. et al. (2011): Improvement and Horizon ofRain-fed Agriculture in Africa from the Point of View ofWater Usage. Journal of Arid Land Studies, 20(4): 219-226
Takahashi S. et al. (2010): Final report for useful technology indeveloping countries and Feasibility study with university.
Japan International Cooperation Agency: 148-149