Calculation of reference evapotranspiration surfaces in distributed hydrological modelling at different temporal scales

Abstract

Abstract. Each individual process in the soil water balance affected by evaporation processes has a certain representative temporal scale (e.g. canopy evapotranspiration, snowmelt or soil water loss). In this study, the implementation in distributed hydrological modelling at cell scale of the ASCE-Peman-Monteith (ASCE-PM) equation is proposed at hourly time steps whilst the Hargreaves formulation was considered to be appropriate for the water and energy balance at a daily scale due to its simplicity of application once the distributed values of temperature are available at cell scale. However, the coefficient of Hargreaves equation must be previously calibrated. The interplay of different factors at different temporal scales became evident in the calibration process at the local scale of weather stations. However, the best fits against daily estimates by ASCE-PM were achieved when differentiating between the wet and the dry season. For the spatial distribution of Hargreaves coefficient at watershed scale, a regionalization in the area around each weather station was proposed in terms of areas of influence. The best results at watershed scale were obtained after a spatial correction for alpine areas, when the average of the difference cell by cell between ASCE-PM and Hargreaves distributed daily estimates were 0.02 and 0.15 mm day−1 for the wet and the dry seasons respectively. In all the cases, the best interpolation results were obtained using C–I (calculate and interpolate) procedures.