ESTIMATION OF IRRIGATION WATER USING CROPWAT MODEL AT KM35 PROJECT SITE, IN SAVANNAKHET, LAO PDR: Toda, O. et al.ESTIMATION OF IRRIGATION WATER USING CROPWAT MODEL AT KM35PROJECT SITE, IN SAVANNAKHET, LAO PDR Osamu Toda1), Koshi Yoshida2) , Somura Hiroaki3) , Higuchi Katsuhiro3) and Hajime Tanji 3) Department of Global Agricultural Sciences Tokyo University, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-8657, Japan 2) Japan Science and Technology Agency, National Institute for Rural Engineering, 2-1-6 Kannondai, Tsukuba, Ibaraki, 305-8609, Japan 3) Department of Hydraulics National Institute for Rural Engineering, 2-1-6 Kannondai, Tsukuba, Ibaraki, 305-8609, Japan E-mail of Corresponding author:
[email protected] 1) ABSTRACT In Lao PDR, the population is growing rapidly and is expected to continue growing, which will inevitably lead to an increase in food demand. To maintain self-sufficiency in food supply, one option is to raise the unit yield. According to the Ministry of Agriculture-Forestry, Lao PDR, the total rice harvested area in 2002 was about 3% of the total land and the irrigated rice area was about 11% of the total rice harvested area. The rainfed yield was 3.5 ton/ha for lowland rice and 1.8 ton/ha for upland rice, and the irrigated yield was 4.5 ton/ha. Thus, the low proportion of irrigated rice area resulted in unsustainable food supply. For stable food supply, the key is determining the area of rainfed paddy that could be irrigated. Therefore, estimation of the water requirements and evaluation of the effects of installing irrigation are extremely important. The study area was the KM35 Irrigation Project Site at Savannakhet Province. The area was divided into 13 segments. Using the CROPWAT model, irrigated water into the paddy at each area of KM35 was estimated for the 2004 rainy season. In the estimation process, yield response coefficient ky was modified, and also, the partition height was taken into consideration. The estimated volume of irrigation water for the entire KM35 site was 7% of the active capacity of the reservoir. Irrigation days were from 26 to 43 (34 on average). From the relation between partition height, hardpan depth and actual yield, it was revealed that actual yield had a positive correlation to partition height, most likely due to the reserved water up to the partition height, and a negative correlation to hardpan depth. The relation to irrigation days indicated the opposite results to those of actual yield. In the case of KM35, total irrigation volume in the rainy season was minimal. When the supplementary irrigated rice yield is higher than the rainfed yield, this indicates that supplementary irrigation is available for raising the unit yield. In addition, even if part of the area is irrigated in the dry season, further increases can be expected within the year. Because this model did not take into consideration groundwater level or the effects of fertilizers, it is necessary to include these factors for improved accuracy when estimating dry season irrigation. INTRODUCTION In Lao PDR, evaluation of the food supply is very important from the perspective of the country’s rapid population growth (UN, 2004), and inevitable increase in food demand. On the other hand, water and land resources are not expected to change dramatically. In determining the methods for maintaining selfsufficiency in food supply, expansion of harvested areas is not possible because the land is almost fully used, but raising the unit yield is a viable option. A favorable method for raising the rice yield per unit is through irrigation. According to the Ministry of Agriculture-Forestry, Lao PDR, the total rice harvested area in 2002 was 738,104 ha, which is about 3% of Role of Water Sciences in Transboundary River Basin Management, Thailand, 2005 17 During the planning stage.0% Khammuane 7. demonstrating the importance of Savannakhet Province for rice production. However.9% 3.5% Attapeu 2. Fig. located 35 km from Savannakhet City. 2005 . Thus. 1: Location of the study area Xaysomboon Saravane Sekong Champasack 0. and 550 ha was allocated for paddy rice and 400 ha for upland crops with irrigation water in the dry season (JICA. the province’s rice harvested area was the largest in Lao PDR and its production accounted for about 20% of the total (Fig.3% Irrigated rice (Dry) 17. 2).5 ton/ha for lowland rice and 1. The field belongs to a branch of the Se Bang Hiang River. O.9% Phongsaly Luangnamtha 1. the area was not completely irrigated every dry season due to water shortage. Thailand.8% Lowland Rice (Rainy) 81.7% Huaphanh 3.8 ton/ha for upland rice.7% Vientiane Mun. IN SAVANNAKHET. 1). and the irrigated yield was 4. STUDY AREA The KM35 Irrigation Project Site (KM35) of Savannakhet Province in Lao PDR was selected as the study area (Fig. LAO PDR: Toda.3% 2. and the remaining 89% was rainfed paddy area. which is a tributary of the Mekong. the high cost of fertilizers and the breakdown of some 18 Role of Water Sciences in Transboundary River Basin Management. the key is determining the area of rainfed paddy that could be irrigated. Irrigated rice area was about 11% of the total rice harvested area.5% Luangprabang Bokea Oudomxay 2.4% Xayabury 4. was constructed under the Japan International Cooperation Agency (JICA) in 1996.1% Xiengkhuang 2. The rainfed yield was 3. the low proportion of irrigated rice area resulted in unsustainable food supply.1% SR 0. hua Vie ng ntia Bo ne rik ha Kh mxay am mu ane S av an Xa yso nakh et mb oon S S ar R av a ne Sek o Ch am n g pas ack Att ape u Irrigated (Dry) Upland (Rainy) Lowland (Rainy) Upland Rice (Rainy) 0. Total cultivating area is 950 ha with a reservoir having 8. the total land.ESTIMATION OF IRRIGATION WATER USING CROPWAT MODEL AT KM35 PROJECT SITE. For stable food supply. 1992).9x106 m3 of active capacity and 31.2% Borikhamxay 3. estimation of water requirements and evaluation of the effects of installing irrigation are extremely important.8% Fig.2% 9.9% 11.7% 140 120 Harvested area (1000･ha) 100 80 60 40 20 0 Pho ng Lu ang saly nam th Ou dom a xay B Lu ang okea pra ban g Hu aph anh Xa yab Vie ury ntia ne M Xie ngk un.5 ton/ha.3% Savannakhet 20. In 2002. 2: Harvest area and yield of Lao PDR KM35.1% 2.5% Vientiane 9. Therefore.0 km2 of catchment area. et al. 12. 950 ha was allocated for cultivating paddy rice with supplementary irrigated water in the rainy season. 6 11.65 2. 3: KM35 divided into 13 areas METHODOLOGY A model based on CROPWAT by the Food and Agriculture Organization of the United Nations (FAO) was used in this study.3 22. yield and area Number Hard Pan Depth (cm) Partition Height (cm) Actual Yield (t/ha) Area (ha) 1 2 3 4 5 6 7 8 9 10 9.46 2.6 24. All data was collected in the rainy season of 2004. In this study. Using this model. 3 shows the details of KM35. Crop Evapotranspiration Crop evapotranspiration can be calculated from the following equation (Richard.89 3. For the calculation.0 10. rainfed yields were estimated first. Average annual rainfall from 1993 to 2002 was about 1. Rainfall data was collected from the Kengkok Station closer to the field.ESTIMATION OF IRRIGATION WATER USING CROPWAT MODEL AT KM35 PROJECT SITE. the partition height. except for rainfall. four secondary canals on the right side.3 30. Then.6 11. meteorological data.7 21.5 11 12 13 Total 13.7 11. 1998): ETc = (Kc + Ke) ETo ETa = (KsgKc + Ke) ETo (1) (2) Role of Water Sciences in Transboundary River Basin Management.57 4.40 3.5 10.9 10. three secondary canals on the left side. Thailand. IN SAVANNAKHET.91 2.1 17.9 3.8 10. and several tertiary canals on each side.0 11.74 2.500 mm at Kengkok Station near KM35: approximately 85% was in the rainy season from May to September and the rest was in the dry season (MRC.9 18.3 16. O.0 22. There are two main canals. calculations were performed to determine how much supplemental water was irrigated to produce the actual yields in each area.9 14.96 3. was collected from the Savannakhet Weather Station. respectively. Table 1 shows each area.1 20.9 10.36 3.1 20. 1993-2002).79 5. canals. the field was divided into 13 areas: 5 areas on the left canal side and 8 areas on the right canal side.4 20.8 16.9 18. Table 1: Hardpan depth. Fig.2 10.17 39 62 71 109 87 47 18 83 89 50 99 144 52 950 Fig.95 3. rice cultivation was impossible without irrigation water. et al. Thus. hardpan depth and rainy-season rice yield in 2004. partition height. 2005 19 . Partition height was taken into consideration and the yield response coefficient was modified.86 2. LAO PDR: Toda. Water content is determined by the sum total of rainfall. Ks is calculated from water stress as explained below. These partitions can store rain or irrigated water in an amount up to its height. T is mean daily air temperature at 2-m height. which protects the rice plants against drought for a while. The water stress is determined by Ks estimated from the following equation: Ks = TAW − Dw TAW − RAW (4) D w. If the water content decreases across the surface. i = D e. 1. LAO PDR: Toda. Then.34 u 2 ) (3) where Rn is net radiation at the crop surface. respectively. when the water deficit exceeds the capacity of Readily Available Water. Paddy rice cultivated on lowland is generally framed in by partitions. evapotranspiration. irrigation water. Ks is water stress coefficient. For this reason. ∆ is slope vapor pressure curve and γ is psychometric constant. ETo is reference crop evapotranspiration. Evapotranspiration Rain Irrigation Run Off Partition Surface Deficit = Water Stress Water Content Hard Pan Readily Available Water Total Available Water Deep Percolation Fig. IN SAVANNAKHET. Fig. In Eq. some water deficit will occur. es and ea are saturation and actual vapor pressures. standard and adjusted for water stress. Each parameter is determined from meteorological data sets. 4 shows the concept of the model. and is calculated from the FAO Penman-Monteith equation as follows: ET o = 0. Kc is crop coefficient. and Ke is determined by the soil condition. runoff above the partition and deep percolation from the hardpan. i + DP i (5) 20 Role of Water Sciences in Transboundary River Basin Management. Thailand. However. et al. where ETc and ETa are crop evapotranspiration. and Ke is evaporation coefficient. but paddy rice differs in cultivating style. Yield Decrease CROPWAT is suitable for upland crops. O. G is soil heat flux density. u2 is wind speed at 2-m height. 2005 .ESTIMATION OF IRRIGATION WATER USING CROPWAT MODEL AT KM35 PROJECT SITE. water stress is generated.408∆ ( R n − G ) + γ 900 u (es − ea ) T + 273 2 ∆ + γ(1 + 0. ETo is evapotranspiration from the hypothetical reference surface. i -1 − Pi − I i + ET a. Kc is given from FAO. 4: Concept of model The rice can use water until the water deficit reaches the Total Available Water. the effect of the partition height was taken into consideration. respectively. Thailand. irrigation days were 26 to 43 (34 on average). 2005 21 . total daily irrigation water volume Role of Water Sciences in Transboundary River Basin Management.4. TAW and RAW are total and readily available water. The daily irrigation volume is constant during the entire period. DP is deep percolation.50 ton/ha ⎛ ⎝ ETa ⎞ ⎟ Yp ETc ⎠ (6) where Ya is actual yield. and was obtained from statistical data.0 and 0. respectively. I is irrigated water. In the estimation.15 to 3. irrigated water volume is adversely estimated from Eqs. The rainfed yield from each area is estimated under conditions of no rain and ranges between 2.24 ton/ha. RESULTS AND DISCUSSION The results of the estimation are shown in Fig. According to FAO (Doorenbos. LAO PDR: Toda.ESTIMATION OF IRRIGATION WATER USING CROPWAT MODEL AT KM35 PROJECT SITE. 5 and Table 3. the estimated yield of KM35 under rainfed conditions ranged from 3. Ky was adjusted to approximately equalize the yield between the actual and rainfed estimation at Area 2. the evapotranspiration must be decreased from non-stressed ones. flowering and yield formation. (1) through (6) and the following equation: Ya = ∑ Xi gYp n ⎛ ⎝ Xi = 1 – Ky ⎜1 − ETa.4 Ky from FAO 1.4 Stage Estimation of Irrigated Water From actual yield data. 4 and 10 was almost zero. Using these values. When the actual yield exceeded the potential yield such as in Areas 7 and 11. Ky value of rice plants is 1. The schedule depends on the water deficit. 3.4 for each stage. The yield decrease is estimated from the following equation: Yield Decrease = Yp – Ya = Kyg ⎜1 − Yp = 4. The FAO values were not suitable in the case of KM35.40 ton/ha at Area 2. Irrigation water was supplied to harvest the actual yield of each area individually.3 3 0. On the other hand. O.52 ton/ha. i ⎠ (7) where n is 150 days for the total rice planting period. vegetative. P is rainfall. therefore. respectively. which is only possible without water stress. Table 2: Ky value and growing stage EstablishYield Vegetative Flowering Ripening ment Formation Period (days) 15 65 15 35 20 Ky 2.3 4 1. et al. Irrigation water supplied to Areas 2. IN SAVANNAKHET. where Dw is the water deficit. The daily irrigation volume is determined to fulfill the above equations. i ⎞ ⎟ ETc. The value of Yp was the maximum rice yield of Savannakhet Province. This decrease means that the rice plant could not produce its potential yield. its irrigation water was supplied to harvest the entire potential yield. the measured minimum yield was 2. and i is the day number. and Ky is yield response coefficient. but is changed for each area. When water stress reaches the rice plant. Yp is potential yield under non-water-stress conditions. Determination of Ky The yield response coefficient Ky was adjusted to the measured actual yields (Table 2).23 and 2. The total irrigation water volume of each area is the sum of the estimated daily irrigation in this rainy season. 1986). 0 131 43 2 0.1 86 28 11 0.50 2. 2005 . 5: Results of estimation The relation between actual yield and partition height.09 2. The seasonal total was about 7% of the active capacity of the reservoir. 22 Role of Water Sciences in Transboundary River Basin Management.0 90 30 4 0.22 2.96 623.1 4 37 5 0.52 2.8 30 39 696 6 1.39 Fig.5 172 31 8 2. the relation of partition and hardpan to irrigation days is shown in Fig.1 4 42 3 3. O. and a negative correlation to hardpan depth. When the supplementary irrigated rice yield is higher than the rainfed yield.53 2. was about 20.48 2.000 m3. is shown in Fig.000 m3 per day. For the hardpan depth.23 2. However.48 63.40 2.44(avg) 1. It is assumed that the positive correlation to partition height is due to the partition reserving water up to its height.97 207.52 2.27 85.660 2. further increases can be expected within the year. It was revealed that actual yield had a positive correlation to partition height.44 43. this result indicates that supplementary irrigation is available for raising the unit yield. presumably because it was more difficult for deeper hardpan to maintain RAW.ESTIMATION OF IRRIGATION WATER USING CROPWAT MODEL AT KM35 PROJECT SITE.36 24.00 2.10 23. Furthermore.52 2.2 96 30 10 3.130 109 990 1.5 48 32 Total 29.2 50 43 564 7 5. even though it was able to save water under the soil as well as the partition. the reason is the same as explained above. In addition. a correlation between actual yield and partition height was shown. et al. it was revealed that total irrigation volume in the rainy season was minimal. and positive for hardpan depth.0 69 35 9 3.52 5. LAO PDR: Toda.1 3 29 12 2. which reveals that irrigation days were negative for partition height.35 2.5 144 26 13 1.52 2.96 57. the hardpan had the opposite result. On the other hand. hardpan depth. CONCLUSIONS In the case of KM35. 6. and the total volume of the whole rainy season was about 623. In terms of the partition height alone.550 99 7. IN SAVANNAKHET. only 7% of the active capacity.848 1. even if part of the area is irrigated in the dry season.36 26.920 780 Total Area Irrigation (10 m /season) 51. it was shown that a greater volume of irrigation water was required for deeper hardpan. 7.50 30. Thailand. Table 3: Results of estimation Number Rainfed Yield (t/ha) Irrigation (mm/day) Total Irrigation (mm/season) Irrigation Days Area Irrigation (m3/day) 3 3 1 3.90 4.578 2.36 2.1 927 34 (avg) 20.170 62 2. Core Research for Evolutional Science and Technology. K. H. et al. Mekong River Commission. G. Crop Evapotranspiration: Role of Water Sciences in Transboundary River Basin Management. REFERENCES J.ESTIMATION OF IRRIGATION WATER USING CROPWAT MODEL AT KM35 PROJECT SITE. A. it is necessary to include these factors for improved accuracy when estimating the dry season irrigation. FAO Irrigation and Drainage Paper 56. M. LAO PDR: Toda. 7: Relation between irrigation days.5 4. A. (1986). L. Uittenbogaard. C. Smith. The authors would also like to acknowledge the support from the Department of Meteo-Hydrology of Lao PDR for providing meteorological data. JICA. O. (1992) Master plan and feasibility study on the integrated agricultural rural development project in Savannakhet province.5 5. FAO. O. (1998). Plusje. (1993-2002) Lower Mekong Hydrologic Yearbook. ALEEN. Doorenbos. Kassam. IN SAVANNAKHET. J. Dirk RAES and Marthin SMITH. 6: Relation between actual yield.5 6. Japan International Cooperation Agency. partition and hardpan ACKNOWLEDGEMENTS This research was made possible by the fund from CREST.5 3.5 Depth Height Depth Height 2. Van Der Wal. 35 30 Partition Height and Hard Pan Depth (cm) 25 20 15 10 5 0 1.5 Actual Yield (t/ha) Fig. Branscheid. Bentvelsen. M. Vientiane. M. partition and hardpan 35 Partition Height and Hard Pan Depth (cm) 30 25 20 15 10 5 0 10 20 30 40 50 Times (days) Depth Height Depth Height Fig. 2005 23 . V. of Japan Science and Technology Agency. Yield Response to Water. Tokyo MRC. and H. Thailand. Luis S PEREIRA. G. Because this model did not take into consideration groundwater level and the effects of fertilizers. Richard G. Rome. un. New York. et al. UN. 2005 .org/unpp/. Thailand. IN SAVANNAKHET. guidelines for computing crop water requirements. LAO PDR: Toda. URL: http://esa. 24 Role of Water Sciences in Transboundary River Basin Management.ESTIMATION OF IRRIGATION WATER USING CROPWAT MODEL AT KM35 PROJECT SITE. Rome. FAO Irrigation and Drainage Paper 56. United Nation. FAO. World Population Prospects: The 2004 Revision Population Database. (2004). O.