A thesis submitted in partial fulfillment of the requirements for theWater Engineering and Management

Thesis (M.Eng.) - Asian Institute of Technology, 2009

This study aims to find out the optimal irrigation schedule and cropping pattern corresponding to drought condition which has an effect on limited available water supply. Huai Thein irrigation project, which is located in Lao Khwan district Kanchanaburi province, is concerned as the study area. The quantitative of rainfall in this region is very low and drought condition often occurs. The main analysis of this study separated into three parts. The first part is drought analysis which is used Standardize Precipitation Index (SPI) to classify the condition of drought in the study area. The second analysis is conducted corresponding to allocate irrigation water to each crop individually. Backward Dynamic Programming (BDP) technique takes its responsibility to determine the optimal way distributing water to each crop stage. The third part of analysis aims to finalize the optimal crop pattern with respect to maximize the net benefit of all crops. The drought analysis result shows that there will be the most possibility to be mild drought in all time scale of the year, especially in short period such as three month while moderate drought takes the highest opportunity to be occurred in nine month time series. Beside, the severe drought usually appears in three month time series and the extreme event unusually happen over the short time period however, it can be observed in long time scale. Moreover, this analysis also using rainfall deciles method in order to performs the classification of available annual rainfall in the period of 1978 to 2008. Its result can be explained that the amount of annual rainfall in 1979, 1989, 1997 and 2001 is much below the average and be classified as drought year. Furthermore, CROPWAT computer program is used to simulate the effects of different irrigation practice on crop yield. The analysis result of allocation to each crop presents that the reduction of yield is decreased when the additional irrigation water is adopted. Also, it can be seen that if the application water is reached to irrigation water requirement, yield reduction may not be appeared. In addition, the relation between net irrigation depth and yield presented the crop water production function. Finally, sugarcane and rice are combined together as the optimal pattern over the condition of adequate water capacity until the available water supply downs to 20 percents deficiency after that, rice is the suitable one appeared in crop pattern. Meanwhile, an increasing of sugarcane{u2019}s cultivated area with controlling rice and maize does not provide any increase of the maximum net benefit. The combination of all three crops in pattern shows in any condition which is less than 60 percent deficit of full available water supply.