Assessment of Flow Timing in the Upper Prachin Buri Watershed by Using SWAT Model

This study aimed to analyze the effect of land use changes on flow regime in the Upper Prachin Buri Watershed (UPBW). The analysis was assessed by using the flow date (an amount of water passing through the measurement point) and the flow interval (the duration of the given amouth of water passing through the measurement point) data. Soil and Water Assessment Tool (SWAT) model was applied to analyze the amount of runoff and the amount of water flowing through the measurement point for  evaluating effects of  land used on runoff  during January 2008- March 2014 (Existing situation). Accuracy of model was determined by consideration of the Coefficient of Determination (R 2 ). Then, two scenarios of situations were initiated, (1) converting all forest areas to agricultural areas (Scenario 1), and (2) increasing forest areas by using the strategies of protecting the conservation and reserved forests (Scenario 2). Results obtained from the SWAT model showed that the UPBW area was 7,456.25 km 2 with 13 sub-watersheds and 135 hydrological response units (HRUs). Application of the SWAT model to Existing, Scenario 1, and Scenario 2 situation indicated the accumulated runoff compared with the flow timing from the flow interval by showing the shortest of flow timing at 25% of runoff volume during the flooding time was 21, 26 and 28 days respectively. During the dry season, the longest flow of water at 1% the last of Existing, Scenario 1, and Scenario 2 was 72, 18 and 38 days, respectively. The results from Scenario 1 and Scenario 2 can implied that forest conservation is one of the factors helping the flow timing, slow down the stream flow and regulating the amount of water during the flooding period, and keeping longer duration of water flow until the end of the dry season. Keywords : flow timing ;  SWAT model ; Upper Prachin Buri watershed