Computation of potential evapotranspiration using a two-source method for the Xin'anjiang hydrological model

In this study, a physically based two-source potential evapotranspiration (PE) model was developed to calculate the spatiotemporal variation of PE over the Hanzhong watershed in China. The calculated PE was then integrated with the Xin'anjiang hydrological model to perform streamflow simulation. A formula similar to the Penman-Monteith equation was used in the PE model to explicitly calculate the PE components of potential canopy transpiration, interception evaporation, and potential soil evaporation. Based on the I km global land cover data, the related vegetation parameters were derived from the Land Data Assimilation System. The monthly maximum composite of NOAA-AVHRR NDVI data with a 1 km resolution was used to retrieve the spatio-temporal variation of leaf area index. Meteorological data from the China Meteorological Administration were employed for calculations. A single grid cell test shows that land cover exercises a significant impact on the PE amount, its temporal variation, and associated components. The replacement of different vegetation types at the same grid cell may lead to different PE results. The spatial distribution of mean annual PE over the watershed is remarkably uneven, ranging from 268.6 to 970.2 mm. The simulated daily streamflow by the Xin'anjiang model using the estimated PE matches well the observed daily streamflow at the watershed outlet during the period of 1980-1986.