Thornthwaite and Mather water balance method in Indonesian Tropical Area

The Thornthwaite and Mather water balance method has been widely applied in the world, one of which is in Indonesia. However, almost all studies using the Thornthwaite and Mather water balance method in Indonesia are not validated. Considering that the Thornthwaite and Mather water balance study makes it easier to study hydrology, especially in areas that do not yet have complete and evenly distributed hydrological and meteorological stations. The research locations are in the Wampu sub-watershed, Serayu sub-watershed, Ayung sub-watershed, and Tondano sub-watershed. The method used is the Thornthwaite and Mather water balance with input data on rainfall, temperature, land use, latitude, and soil texture. The validation test was performed using the RMSE equation from the runoff model compared to the observation runoff (river discharge data). The RMSE results of the Thornthwaite and Mather water balance method in the Ayung sub-watershed (42.42), Serayu sub-watershed (53.77) Wampu sub-watershed (65.58) and Tondano sub-watershed (83.68) were classified as weak correlation categories. The comparison results of the Thornthwaite method in tropical areas had greater rain, potential evapotranspiration, and runoff values than other climatic areas. Based on the result, we need to modify the Thornthwaite and Mather method for tropical areas and add a range of 25-30 years of climate data.