Decadal fluctuations in the consumption of irrigation water during the rainy season, Lower Mekong River

The importance of proper water management means that the consumption of irrigation water should be estimated even in areas where data are scarce. The Mekong River, particularly the lower part, is one area where irrigation water has received little research attention. Studies that seek to estimate the use of irrigation water in this area are restricted by a lack of hydrological data; consequently, alternative methods are required to obtain these data. One solution is the use of model simulations. The main difference between our approach and existing irrigation-water estimation models is that we consider agricultural data in estimating the use of irrigation water, especially that used for rice production, as these data are relatively easy to collect. As a result, rice production during the rainy season in the target area was computed for 10 years with errors of less than 30%. The average values of calculated and observed rice yields per hectare were 1.3 and 1.7 t/ha in Thailand, 2.5 and 2.7 t/ha in Lao PDR, and 1.2 and 1.3 t/ha in Cambodia, respectively. In addition, we clarified decadal fluctuations in horizontal supplied (HS) water and irrigation water. The average values of HS water supplied to the paddy fields during the simulation period were approximately 17.2 km(3)/rainy season in Thailand (388 mm), 0-81 km(3)/rainy season in Lao PDR (245 mm), and 4.4 km(3)/rainy season in Cambodia (322 mm). Comparisons with HS water (7.4 km3/rainy season) and the existing requirements for irrigation water revealed that the simulation yielded acceptable values, thus verifying the reproducibility of the model results. Finally, we calculated the average water balances during the growth period for rice. The highest amount of irrigation water per unit irrigated area during the rainy season was used in Lao PDR, though the volume of irrigation water was greatest in Thailand. Copyright (C) 2008 John Wiley & Sons, Ltd.