Determination of natural stream flows on which to evaluate change is difficult because of human diversions, base-flows, and return flows from irrigation diversions even when hydrological modeling is coupled with a water right model. A hydrologic model requires an accurate calculation of evapotranspiration in order to accurately predict the natural flow of water. There have been many different ET estimation methods proposed in the literature, and selecting one is frequently contentious. In this paper, we assess the impact of Evapotranspiration estimation methods on the long-term natural flow of water estimation using the HEC-HMS hydrologic model. Natural flow of water was estimated using HEC-HMS employing the three different methods of ET estimation Hargreaves, Priestley Taylor, and Penman-Monteith in Walla Walla River basin. Our preliminary result shows that on an average natural flow of water estimated using Hargreaves and Priestley Taylor varies by 5% and 6% respectively compared with Penman-Monteith methods. This information will be valuable to the watershed managers and policy makers making decisions on further investment in data collection efforts related to ET for estimating natural flows. Our findings will also further highlight the possibility of using HEC-HMS as a supporting tool for long-term natural flow of water estimation.
