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This study presents the response of the Upper Mahakam sub-watershed in Indonesia to the estimated precipitation from the Tropical Rainfall Measuring Mission (TRMM) by applying the Hydrologic Engineering Center - Hydrologic Modeling System (HEC-HMS). With the absence of daily in-situ precipitation, we adjusted TRMM daily precipitation estimates based on the relationship between monthly TRMM and in-situ precipitation. We introduce the Thiessen polygon and area average method to match the spatial distribution of TRMM precipitation estimate with the meteorological stations. We developed the HEC-HMS model using Soil Conservation Service (SCS) Curve Number (CN), Clark unit hydrograph, Muskingum-Cunge and base flow recession methods. We calibrated flow ratio, recession initial flow, constant and ratio by comparing HEC-HMS simulated discharge with the in-situ discharge measured using Horizontal Acoustic Doppler Current Profiler (H-ADCP). Using the same discharge dataset for different period, we further optimize the HEC-HMS routing by fine tuning the storage coefficient. The HEC-HMS simulated discharge was sufficiently correlated with the observed ones, thus support our hypotheses that with a thorough evaluation, TRMM precipitation estimates can be used to drive a rainfall-runoff model in a poorly-gauged watershed.

hydrology; rainfall-runoff model; TRMM; HEC-HMS