Estimating flow duration curve in the humid tropics: a disaggregation approach in Hawaiian watersheds

The authors attempted to develop a simple methodology for estimating daily flow duration curves (FDCs) in island watersheds under humid tropic conditions, to realize better water management in data-limited island watersheds. This study disaggregated a FDC into three parts, namely high, middle and low flow parts and estimated each part independently. Based on Hawaiian watersheds, this study developed a methodology for estimating the high flow part using daily precipitation data and the Curve Number (CN) method, whereas we estimated the middle flow part directly from mean monthly flow (MMF) data. The results for the middle flow part showed the MMF data closely tracking the daily FDCs for nine of the eleven studied watersheds. For the high flow part, the results showed that only after calibration of the CN method, were there significant improvements in estimations. These results suggest it is possible to estimate the middle flow part of the FDC with MMF but the high flow part must use a calibrated CN in the CN method. This study also explored the relationship between calibrated CN and estimated absorption capacity of the studied watersheds to find the possibility of estimating suitable CN of a watershed from its geological characteristics.