Cost-effective raingauge deployment and rainfall heterogeneity effect on hydrograph simulation in mountainous watersheds

Abstract. To what extent hydrograph simulation was influenced by the representativeness of rainfall input were examined in meso-scale subtropical mountainous watersheds, accordingly, cost-effective raingauge deployment was suggested. Two nested watersheds in northern Taiwan and two extreme typhoons with torrential rains were undertaken as case studies. The input of radar rainfall estimates with high spatial resolution of 1.3 km2 served as a reference, which was applied onto hydrograph simulation in TOPMODEL. After calibration, optimal parameters were obtained and fixed to examine effect of deviated rainfall on hydrograph. To mimic possible raingauge networks we designed four raingauge number classes: very low (3 points/total pixels), low (10 points/total), medium (20 points/total), and high (50 points/total) based on radar rainfall for the two watersheds in different size, thus, creating wide spectrum of raingauge density. All the corresponding hydrographs were compared with the reference hydrograph to probe errors in event discharge induced by calculated rainfall input. Results showed that with the decreasing of raingauge density the biases (indicated by RMSE) of rainfall field estimates increase and the potential variability in rainfall field due to random sampling in raingauge location is exaggerated. By contrast, biases in model hydrographs are significantly smaller than that in rainfall field. When the raingauge governing area is <10 km2/gauge, the biased rainfall field shows no detectable effect on hydrographs. Incomparably lower RMSE in hydrograph indicates that surplus and deficit rainfalls at different locations were compensated in model simulation. In term of reliable hydrograph simulation, obviously, the criterion for raingauge density is not as high as that for rainfall estimate. When gauge governing is <20 km2/gauge, both the rainfall and discharge were successfully (±10% error) estimated in term of total volume. Accordingly, we suggested that covering area ~20 km2/gauge is acceptable for raingauge deployment to constrain the inherent variability in rainfall field and hydrograph simulation in mountainous watersheds.