Water Saving in Tank Irrigation Systems in Sahel Region of Africa

Transcription

1 Paper Number: An ASAE Meeting Presentation Water Saving in Tank Irrigation Systems in Sahel Region of Africa Tahei Yamamoto, Professor Tottori University, Arid Land Research Center, 1390 Hamasaka, Tottori, Japan. F. Amu-Mensah, Research Scientist CSIR Water Research Institute of Ghana, P.O.Box M.32, Accra, Ghana. H. Fujimaki, Lecturer Tsukuba University, Tennoudai 1-1-1, Tsukuba , Japan. Hossein DehghaniSanij, PhD Student Tottori University, Arid Land Research Center, 1390 Hamasaka, Tottori, Japan. Velu Rasiah, Senior Soil Scientist Center for Tropical Agriculture, Dept. of Natural Resources & Mines P.O.Box 1054 Mareeba, QLD 4880, Australia. J. Utsunomiya, MS Student Tottori University, Arid Land Research Center, 1390 Hamasaka, Tottori , Japan. Written for presentation at the 2003 ASAE Annual International Meeting Sponsored by ASAE Riviera Hotel and Convention Center Las Vegas, Nevada, USA July 2003 Abstract. This study was aimed at sustainable irrigation development in the Sahel region. Climatic characteristics required for irrigation planning were initially analyzed. Statistical analysis showed that the three countries studied have relatively suitable rainfall for crop growth in normal year, which drastically decreased in draught years. A small-scale microirrigation system with rainwater storage The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural Engineers (ASAE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASAE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASAE meeting paper. EXAMPLE: Author's Last Name, Initials Title of Presentation. ASAE Meeting Paper No. 03xxxx. St. Joseph, Mich.: ASAE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASAE at hq@asae.org or (2950 Niles Road, St. Joseph, MI USA).

2 tank was then presented as suitable technology for the development of sustainable irrigation development in the Sahel region having had significant success in the South East Asian regions. The proper apron area, large enough to prevent drying-out of the initially filled tank over the simulated period ranged from 250 to 1,100m 2 when the tank capacity is 1,000m 3, with the ranking of Perkera > Accra > Tamale > Kumasi > Torodi. Also, water saving potential by introducing microirrigation was promoted for rainy regions. Keywords. Water harvest, Water saving system, Microirrigation, Apron area, Tank volume, Kenya, Ghana, Niger. The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural Engineers (ASAE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASAE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASAE meeting paper. EXAMPLE: Author's Last Name, Initials Title of Presentation. ASAE Meeting Paper No. 03xxxx. St. Joseph, Mich.: ASAE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASAE at hq@asae.org or (2950 Niles Road, St. Joseph, MI USA).

3 Introduction The continent of Africa is sometimes described as an arid continent, since the regions whose annual rainfall exceeds 1000 mm and 1600mm occupy only 27% and 9%, respectively. Most of the continent is warm or tropical land with long-term evapotranspiration always exceeding rainfall. The Sahel region (1.34 billion ha), located in the central portions of Africa, is a belt-like zone whose dimension is 7,000 km in the east to west direction and 1,000 to 2,000 km in the north to south direction. Steppe and savannah climate occupy 34% of the region. Annual rainfall of the region ranges from 600 to 1,200 mm with mono-modal or bi-modal rainy season. The objectives of this study were to investigate the process of desertification with irrigation farming in arid-lands and to present a sustainable irrigation plan. At first, several arid-lands in the region were selected, and analysed for those climatic conditions required for irrigation planning. Secondly, a small-scale irrigation system using a newly developed model that simulates rainwater-harvesting and microirrigation was discussed and designed in comparisons with those in Tottori sand fields (134 o 13'N, 35 o 32'W) under monsoon climate in Japan. Outlines of field study and small-scale irrigation system Data on desertification and irrigation farming in the region were collected. In Niger, Torodi research farm (13 o 30'N, 2 o E) located 50km south of Niamey the capital was selected. In Ghana, three locations, Accra (05 o 30'N, 00 o 10'W), Kumasi (06 o 43'N, 01 o 36'W), and Tamale (09 o 30'N, 00 o 51'W), were selected. Perkera irrigation project at Marigat (00 o 30'N, 36 o 00'E) in Kenya, was selected For each of the five areas, the basic data required for irrigation planning such as effective rainfall, evapotranspiration, irrigation efficiency, total readily available moisture (TRAM), and irrigation interval were determined. A small-scale irrigation system (Figure 1) was then designed. This system artificially harvests rainwater as overland flow from an apron, stores it in a tank, and uses it exclusively in the dry season or as supplemental irrigation during the rainy season. These kinds of irrigation systems using tanks have played an important role in agricultural development project in the south and Southeast Asian regions. Simulation of the small-scale irrigation system was conducted under the following conditions: Rainwater into the tank was achieved from a catchment apron constructed on a slope for efficient harvesting. Harvested amounts were calculated on an assumed daily runoff coefficient over the apron of 80%.The irrigated area was taken to range from 150 to 1000 m 2, with an irrigation efficiency of 80%.Two types of irrigation methods are examined: frequent and smallvolume (i.e. microirrigation) type and less-frequent and large-volume (i.e. conventional irrigation) type. Based on the Japanese Irrigation Guidelines, monthly evapotranspiration amounts are assumed for both methods to be equal (Japan. JIID, 1990). Effective rainfall, which actually contributes to crop growth, is calculated as 80% of daily rainfall. Although the guidelines set the lower limit of the effective rainfall at 5 mm, this was not employed in this study. The daily consumption of water for the irrigated field is determined from daily effective rainfall and net water requirement that are calculated from rainfall, evapotranspiration, and total readily available moisture (TRAM) etc. Figure 1 Microirrigation systems consisting of a catchment apron and a storage tank. 2

4 Water balance for the tank is calculated on daily basis. If the tank is depleted before the end of the simulation period, either apron area or tank volume is increased to ensure larger water storage. The authors are developing a simulation model that considers the above conditions. Amu-Mensah et al., 2000 investigated optimum tank volume and apron area for various combinations of two type of irrigation methods, three kinds of crops, and Ghanaian agroecological three regions (Amu-Mensah, 2000). To enhance the reality of the model, they set different evapotranspiration amounts for micro and conventional irrigation method, as well as incorporating evaporation and overflow losses. The model used in this study was basically the same as the previous one (Amu-Mensah et al., 2001). Main modifications were: Targeting intensive farming, irrigated area was assumed to be smaller. To evaluate net water requirement for the irrigation methods, the same runoff coefficient, irrigation efficiency, and evapotranspiration amounts were used. Evaporation from water surface (reservoir) was assumed to be zero. Since evaporation loss from the tank tends to be larger under hightemperature and dry condition, a device is required to minimize evaporation loss: a cover should be used in the dry season and windbreak trees should be planted to create a shady environment on the reservoir surface. Not only for Ghana, was the study area extended to Niger and Kenya. Characteristics of rainfall distribution and crop water consumption Main causes of desertification in the three countries are mainly artificial ones such as overexploitation due to population growth, deforestation, overgrazing, improper agricultural management of land and water. Salinization has not been serious, because the irrigated areas are still small and not intensively cultivated. Compared with Asian arid lands, the three countries Precipitation (mm) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Time (Months) Figure 2 Monthly precipitations in five locations Torodi (Ni) Accra (Gh) Tamale (Gh) Kumasi (Gh) Perkera (Ke) Penman Evapotranspiration (mm) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Time (Months) Torodi (Ni) Accra (Gh) Tamale (Gh) Kumasi (Gh) Perkera (Ke) Figure 3 Mean daily evapotranspiration for a return period of 1 in 10 years in five locations. 3

5 have longer growing seasons and plentiful rainfall (Yamamoto et al., 1999). Monthly rainfall, Penman value, and no-rain days for the five-research plots were studied. Figures 2 and 3 present rainfall and Penman values variation in each area, respectively. For Torodi and Perkera, observed values at the research plots were employed, while data from weather stations were used for the three regions in Ghana. The measurement periods were: for Torodi, for Accra and Kumasi, for Tamale, and for Perkera. In the statistical analysis, the Iwai method was used (Torodi data were not used for calculation of the 10-year-return period, because of lack of sufficient data.). While the mean annual rainfall ranges from 600 to 1,300 mm, rainfall in the draught year with an exceedence frequency of 1/10 reduces to between 62 and 76%, and that of 1/100 reduces to between 38 and 60 %. For norain days with mean value of 35 to 120 days, in the draught year with an exceedence frequency of 1/50, this increased by between 47 and 100%, and that of 1/100 increased by between 55 and 121 %. Also, while the mean Penman value ranges from 3.9 to 6.7 mm/day, in the draught year with an exceedence frequency of 1/10 it increases by between 7 and 35%. From annual rainfall and its distribution, we can estimate that the rainfall in a normal year is enough for crop growth. On the other hand, in a draught year, rainfall amount is scarce, especially for Perkera and Accra. As for the Tottori sandy fields, rainfall characteristics were estimated with meteorological data ( ) measured at Arid Land Research Center, Tottori University. In comparison with the mean rainfall of 989 mm within irrigation seasons, the rainfall decreased to 62~71% in the draught year with an exceedence frequency of 1/10 and to 53 ~ 54% in that of 1/50. In comparison with the mean continuous non-rainfall day of 13 days, the non-rainfall day increased to 1.65 times in that of 1/10 and to 2.2 times in that of 1/ 50. Although Tottori is considered to have relatively high rainfall, drought conditions seem to be appeared in irrigation seasons from May to September. An irrigation plan requires soil-water characteristics, soil moisture extraction pattern, and climatic data. These parameters for an irrigation project can be obtained from long-term basic studies conducted on the study area. In the Sahel region, these parameters were estimated from published data (Amu-Mensah, 2000, Yamamoto et al, 1999): i.e. Penman values at 1/2 and 1/10 year were used for the monthly evapotranspiration amounts. Based on soil water retention (pf) curves at Torodi and Accra research plots, the study was conducted for the cases in which TRAM was between 35 and 150mm. The soil moisture extraction pattern (SMEP) was assumed to be 40, 30, 20, 10 %, respectively, for each 10-cm layer from the soil surface. The TRAM and irrigation intervals measured in Tottori sandy fields were mm for three days interval (traditional irrigation method) and 3-7 mm for everyday interval (microirrigation method), respectively. Rainfall efficiency and irrigation ratio Irrigation was assumed to be year-round. Irrigation ratio (= net water requirement / evapotranspiration) and rainfall efficiency (= effective rainfall / total rainfall) was calculated for each year for Kumasi with the TRAM ranging from 111 to 150 (Figure 4). The results showed a tendency for the rainfall efficiency to increase and the irrigation ratio decrease as the TRAM increases. A soil with large TRAM has large available TRAM, which increases the effective rainfall and hence reduces the net water requirement. This tendency became more apparent as irrigation interval becomes shorter. At Kumasi, microirrigation with daily interval had 10 to 20 % lower irrigation ratio than that of the conventional method, indicating potential for water saving (Yamamoto et al., 1999). 4

6 For Torodi, the irrigation ratio and the rainfall efficiency were calculated with a TRAM of 35mm. Two irrigated periods were examined: from April till September (rainy season) and throughout the year. The net water requirements for year-round irrigation were 1,400 mm, nearly three times of the 470mm for rainy-season irrigation. Since rainfall in the dry season is scarce, yearround irrigation gave larger irrigation ratio ranging from 77 to 80 %, compared to that of rainyseason which ranged from 50 to 60%. Also for Torodi, which has smaller TRAM, difference in the irrigation ratio reduces to less than 5 % when micro irrigation was employed. Kumasi has two rainy seasons in a year and mean annual rainfall of 1,300 mm. Compared with Torodi that has about half the annual rainfall, Kumasi had 1/3-1/5 lower irrigation ratio for rainy-season irrigation, and about half the value for year-round irrigation. As for the Tottori sandy fields, the net water requirements and rainfall for May to September were mm and mm respectively, and irrigation ratio and rainfall efficiency were % and % under conditions of the draught year with an exceedence frequency of 1/10, respectively. Figure 4 Results of tank irrigation simulation; Comparison of tank design for three locations in Ghana; Tank volume=1000m 3 ; Apron size=accra(500m 2 ), Kumasi(100m 2 ), Tamale(300m 2 ). Small-scale irrigation system For the purposes of this study, the 'proper apron area' is defined as the minimum apron area that can prevent drying-out of the initially filled tank over the simulation period. Initially, the 5

7 proper apron area was determined for the three regions in Ghana. Penman values were used as the evapotranspiration amount, and irrigated area was set at 150m 2. Daily budget over the tank whose initial storage was 1,000 m 3 was simulated from 1985 till The resultant proper apron areas were: 500m 2 for Accra, 300m 2 for Tamale, and 100m 2 for Kumasi. Minimum storage occurred in 1995 for Accra, 1996 for Kumasi and Tamale. The final storage for micro irrigation was the largest for Kumasi, followed by Tamale, and the smallest for Accra. The region with larger rainfall had a larger effective rainfall and thus smaller net water requirement, indicating a saving effect on the water resources (Figure 5). The proper apron area was then estimated with tank capacity ranging from 500 to 1000 m 3. Simulated periods were: 4 years for Torodi, 22 years for Accra and Kumasi, 12 years for Tamale, and 8 years for Perkera. Results for 1000 m 3 tank capacity are listed in Table 1. Irrigated area was set at 700 m 2. When the capacity was 500 m 3, the proper apron area ranged from 300 to 10,000m 2, with the ranking of Accra > Perkera > Torodi > Tamale > Kumasi. Since Accra had an extremely large value, another proper apron area of 900 m 2 was obtained with the larger tank capacity of 750m 3. With tank capacity of 1,000m 3, proper apron area was m 2 Figure 5 Comparison of percentages of effective rainfall and irrigatin water requirement in Kumasi, Ghana 6

8 m 2. When the tank capacity was halved, the proper apron area was 250-1,100 m 2. This indicates doubling the tank capacity reduces the proper apron area by 10 to 50 %. The ranking of proper apron area was: Perkera > Accra > Tamale > Kumasi > Torodi. Yearly rainfall pattern and water consumption characteristics might affect the different ranking with different tank capacities. Water saving potential by replacing conventional irrigation method with micro irrigation was more apparent for rainy Kumasi and Tamale than Accra, Torodi, and Perkera. As for the Tottori sandy fields, simulated periods were for thirty years under tank capacity of 1000 m 3. The proper apron areas were m 2 under condition of irrigated areas of 1000 m 2. These results indicated smaller capacities of tank irrigation systems were recommended under monsoon climate in Japan, comparing with those of Kumasi and Torodi where located in the most wet areas in Sahel region. Table 1 Suitable apron sizes under tank volume of 1,000m 3 TRAM(mm) Irr.interval(day) Tank vol.(m 3 ) Suitable apron size(m 2 ) Min. water vol.(m 3 ) Date Accra ' * ' Tamale ' ' Kumasi ' ' Perkera ' ' Torodi ' ' *Date (Yr.Mo.Day) indicates the time the minimum water volume reserved in the tank. E.g represent minimum water volume m 3 on July 23rd, Conclusion Small tank based irrigation systems have played important roles in rice production, rural development, and the national economies of several South and Southeast Asian countries for centuries. Large potential and advantages, particularly in small-scale low input farming systems, exist for microirrigation use in small tank based agriculture, to improve the water use efficiency and long-term sustainability. For sustainable irrigated agriculture development and to prevent frequent famines in the savanna regions of Africa, we propose to increase the efficiency of small-scale irrigation system through appropriate catchment apron and storage tank size used for rainfall harvest and storage, respectively. Five sites were selected in Niger, Ghana and Kenya in savanna regions of Africa to investigate the (1) design capacity of tank volumes and apron sizes and (2) scheduling for microirrigation under tank irrigation system. Using limited field data in soil and soil-water characteristics available in the aforementioned countries different scenarios were explored employing simulation models. The simulations showed that: For a given tank volume of 500 m 3 the required apron size varied across sites and was in the order: Perkera (Kenya) > Accra (Ghana) > Torodi (Niger)> Tamale (Ghana)> Kumasi (Ghana). The suitable apron sizes for 1000 m 3 tank volume ranged from 250 to 1100 m 2. In Ghana, apron sizes in Kumasi were 1/3 smaller than in Tamale and 1/5 smaller than in Accra. The irrigation practices employing microirrigation indicated the highest amounts of effective rainfall in the crop root zones and most savings tank water in Kumasi, because the rainfall is highest at this site. The increasing order of water saving was: 7

9 Kumasi > Tamale > Accra in Ghana. Conversely, as for the Tottori sand fields under monsoon climate in Asia; capacities of tank irrigation system were smaller than those of Sahel region under savanna climate. Finally, almost parts of results in this paper were translated from Japanese original paper (Yamamoto et al., 2003). However, further discussions were carried out using new results obtained in the Tottori sandy field data. References Amu-Mensah, F.K Application of Tank Irrigation in the Promotion of Agriculture in Low Rainfall Areas of Ghana, PhD Thesis, Tottori University. Amu-Mensah, F.K., T. Yamamoto and M. Inoue Planning for Sustainable Agriculture in Ghana using Tank Irrigation, Irrigation Engineering and Rural Planning No. 40, pp. 79~92. Japan. JIID Engineering Manual for Irrigation and Drainage, Drip Irrigation Planning Guide, Japanese Institute of Irrigation and Drainage (JIID), pp.19~36. Yamamoto, T., M. Naruoka, S. Ito, Z. Yang, and J. Zhang Irrigation Schedules and Conservation Management for a Pilot Farm in the Mu Us Shamo Desert: Control of Desertification and Development of Agriculture in Arid Land Areas in China. Irrigation Engineering and Rural Planning No. 25, pp. 4~15. Yamamoto, T., H.L. Yao, A. Keshavarz and S.A. Agodzo Analysis of Soil Degradation Due to Irrigated Agriculture and Sustainable Water Management in Arid Land Areas, Proceedings of American Society of Agricultural Engineers, Paper No , Toronto, Ontario, Canada, July Yamamoto, T., F. Amu-Mensah, H. Fujimaki and J. Utsunomiya Characteristics of meteorology and sustainable irrigation schedules in the Saherian Region of Africa, Jour. Japanese Society Irrigation Drainage Reclamation Engineering, 70(11), pp (2003) (in Japanese) 8