The effect of water storage change in ET estimation in humid catchments based on Budyko framework and water balance models

Abstract. An accurate estimation of evapotranspiration (ET) in humid catchments is essential in water-energy budget research and water resource management. While it remains a huge challenge and there is no well accepted explanation for the difficulty of annual ET estimation in humid region so far. Here we present the ET estimation in 102 humid catchments over China based on the Budyko framework and two hydrological models: abcd model and Xin'anjiang model, in comparison with ET calculated from the water balance equation (ET_wb) on the ground that the ΔS is approximately zero at multiannual and annual time scale. And we provide one possible explanation for the poorly annual ET estimation in humid catchments as well. The results show that the Budyko framework works fine in ET estimation in humid catchments at multi-annual timescale, while neither the Budyko framework nor the hydrological models can estimate ET well at annual timescale. One major cause for this poorly annual ET estimation is the neglecting of ΔS in ETwb since it enlarges the variability of real actual ET. Much improvement has been made when comparing estimated ET + ΔS with those ET_wb, and the bigger the catchment area is, the better this improvement can be. This provides an acceptable explanation for the poorly estimated annual ET and reveals the important role of annual ΔS in ET estimation and validation in humid catchments. We highlight that the annual ΔS shouldn’t be taken as zero in water balance equation in humid catchments.