Determination of rainfall data for direct runoff prediction in monsoon region: a case study in the Upper Yom basin, Thailand

In a monsoon region, flooding is usually caused by heavy rainfall events in conjunction with saturated soil. A hydrograph in the wet season consists of multiple flood waves from periods of direct runoff lying on a wave of seasonal baseflow from groundwater. This study determined which rainfall duration and occurrence time were most correlated with direct runoff in the Upper Yom basin, Thailand. First, correlation coefficients (CCs) between basin discharge and rainfall under various durations throughout wet seasons during 2009–2018 were calculated. The discharges were correlated with 48 h rainfall ending 12 h before. However, the CCs were not very good (0.35–0.60, except in 2011). Therefore, the CCs only during saturated soil periods were calculated and this produced better results (0.50–0.79). Next, the baseflow was removed from the basin discharge to determine the direct runoff. The CCs between direct runoff and rainfall were even better (0.59–0.83). Therefore, after the removal of the unsaturated soil period and the baseflow, the direct runoff could be predicted from the 48 h rainfall that began 60 h before and ended 12 h before. This duration was associated with basin characteristics and overall spatial distribution pattern of rainfall.