backspace2062

Hi, I have a question I appreciate if somebody could help me. I need to estimate daily catchment scale evapotransporation. My catchment consist of irrigation farms ( varied vegetation type), Natural land , Small lakes and bare soil (almost 30 % of the catchment area). The study area located in semi arid region. I have already got ET-ref from weather stations in the area and prepared spline interpolated Et-ref map . I have got landsat R S image as well. Friend of mine suggested using average NDVI for the whole catchment and then use (Et =ET0*Kc, kc = 1.37 * NDVI-0086) equation. I am not sure if it is a good idea cause I have got large amount of negative value in my calculated Ndvi raster because of lakes and rivers . Furthermore I am not sure if NDVI is good choice for estimating ET in uncultivated area . Any advice? Cheers.