Improvement of HIMS evapotranspiration module and its application in Haihe River Basin

Evapotranspiration (ET) is an vital element of the hydrological cycle and energy budget, and it is closely related to plant/crop growth. Accurate estimation and spatio-temporal distribution of ET in different vegetation types in river basins are critical for water resources research and sustainable water resources management. Distributed hydrological models are among the promising approaches to the simulation and estimation of actual ET in river basins. However, distributed models are limited in the estimation accuracy of the spatio-temporal distribution of ET in different vegetation types. The Hydro-Informatic Modelling System (HIMS), a modular framework for distributed hydrological models, has proven very efficient in simulating streamflow. It has a simple structure and has been successfully used in China and Australia. To improve the ability to estimate actual ET using HIMS, we modified daily ET module in HIMS by demanding detailed description of evapotranspiration processes and adopting recent findings about filed ET observations. The spatial distribution and seasonal variations in land cover in the investigated basin and the effects of irrigation on actual ET were considered in the modified module. In the modified ET module, vegetation types were classified and actual ET derived as the residual sum of each vegetation type. Then ET for each vegetation type was calculated using the single crop coefficient method. Next, leaf area index calculated by the crop model was applied to separately estimate soil evaporation and plant transpiration. Vadose zone was divided into two (root zone and transition zone) and root zone soil water used to calculate soil water stress coefficient. The relationship between root and transition zone soil water storage was modeled through a linear function. Seepage from the root zone to transition zone was considered in the soil moisture calculation module. The modified ET module was validated and applied in Haihe River Basin. The results suggested that the modified module improved the precise estimation of ET. The relative error of annual mean actual ET between the improved module simulation and water budget estimation was 3.4%. The relative error was 12.6% for the original HIMS model estimation, which was 3.4% for the modified HIMS model. Compared with the original HIMS module, the modified module improved capacities of simulation of actual ET of forest and grassland, total field ET for rational wheat-maize fields, and separation of soil evaporation and crop transpiration. The modified ET module quantified actual ET and accurately characterized temporal and spatial variations in ET. This was critical for the accurate management of ET in Haihe River Basin.