Runoff Characteristics and Runoff Ratio in the Tropical Basin in Thailand by Filter Separation AR Method

Generally, it is difficult to estimate precisely the value of runoff ratio because it is influenced by land use situation / topography / soil / geological feature / rainfall in the river basin. About a runoff ratio, some calculation methods are suggested so far, but the values are different by calculation method. In this study, runoff analysis of the Pin River in northern part in Thailand is performed by applying filter separation auto-regressive method (Filter Separation AR method).This river basin has a few rainfall observation points in comparison with the size of the basin. And the observation precision of rainfall is not good, too. And authors investigated the runoff characteristic and the runoff ratio of this river basin. Filter Separation AR method has been carried out on the daily hydrologic data of watershed of the Pin River of A=3,850 km{2} in Thailand. Procedures of this analysis are as follows; (1) Daily runoff time series are separated into time series of interflow and groundwater flow by the coherence gap between rainfall data and runoff data. (2) Daily effective rainfalls are inversely estimated from only the daily runoff data. (3) By comparison with the effective rainfall calculated by inversely estimated from the runoff data and the effective one obtained from daily rainfall data, runoff ratio in monthly unit is estimated during wet season. (4) Lastly, the runoff characteristic in the tropical river basin of the Pin River is investigated. Consequently, the main results are as follows. (1) In this river basin, though groundwater flow is a little, it is thought that the cultivation effect by the forest is much. (2) Soakage to early stage of groundwater flow is constant approximately every year. (3) Runoff ratio increases almost from May to September during wet season.