Performance evaluation of various evapotranspiration modeling scenarios based on METRIC method and climatic indexes.

Evapotranspiration (ET) is one of the most important factors controlling hydrologic, agricultural, and weather cycles. It also converts a large portion of rainfall into vapor, being known as the largest water flux from the earth into the atmosphere. Since ET is affected by many factors, such as land surface characteristics and climatic conditions, it undergoes considerable spatiotemporal variations, particularly at the watershed scale. Hence, to obtain a more accurate estimation of ET, it is required to identify homogenous and uniform regions, each represented by a meteorological station. In this study, three scenarios were proposed in order to identify homogenous regions to estimate ET based on METRIC method, and the scenarios were tested in Sefidrood Watershed in the north of Iran. The first scenario included only vegetation factor with one representative station for the entire case study watershed and ignored diverse conditions affecting ET across the watershed. The second scenario incorporated not only the vegetation factor but also the altitudinal variations of the watershed. In the second scenario, the watershed was divided into two distinct altitudinal sections, each with a representative station with a specific influenced area, with ET being estimated separately for each section. Finally, the third scenario incorporated the altitudinal and climatic variations. The results indicated that the second scenario performed better than two other scenarios in ET estimation. In other words, altitude and vegetation strongly influenced spatial and temporal distributions of ET, leading to considerable variations of it in the watershed.