Hydrologic Analysis of Rainfed Rice Areas Using a Simple Semi-distributed Water Balance Model

We analyzed the hydrologic characteristics of the Nong Saeng Basin (19.72 km2) in northeast Thailand. Because the land use in this basin is very complex, applying a fully distributed model would be extremely difficult. Therefore, we developed a semi-distributed hydrologic model for this basin. The hydrologic model comprised upland, paddy, and pond models. The new bucket model was applied for upland fields, and a modified tank model was used for the paddy and pond models. In addition, water movement between different land uses was considered. The results showed that the hydrologic model developed for the study basin performed extremely well if water movement between different land uses was considered. We simulated the water storage characteristics for two sub-catchments within the basin: sub-catchment 1 included few earthen weirs, whereas sub-catchment 2 included several earthen weirs. Owing to the earthen weirs, the maximum differences in ponding paddy water were 19,997 m3 in 2002 and 16,897 m3 in 2003, corresponding to 48% and 41% of the total volume of ponds in sub-catchment 1 (41,287 m3), respectively. If earthen weirs were to be constructed over the entire basin, the annual runoff from the basin would decrease by 2.0–3.2%. Although the decrease of the annual runoff is little, the maximum differences between the daily runoffs under real and simulated conditions in terms of percentage differences are −44.3% in September 2002 and −36.5% in September 2003, and it is found that the downstream impacts are quite large at the end of rainy season.