Topographic effects on solar radiation distribution in mountainous watersheds and their influence on reference evapotranspiration estimates at watershed scale

Abstract

Abstract. Distributed energy and water balance models require time-series surfaces of the climatological variables involved in hydrological processes. Among them, solar radiation plays an important role, especially in arid environments, as it is a key variable to the circulation of water in the atmosphere. The lack of reliable data for the assessment of solar radiation has led to the use of models. Most of the hydrological GIS-based models apply simple interpolation techniques to data measured at sparse meteorological stations disregarding topographic effects. Here, a topographic solar radiation algorithm is included for the generation of detailed time-series solar radiation surfaces using limited data and relatively simple methods, in order to quantify the effects of topography on the water losses through evapotranspiration estimates in a mountainous watershed in southern Spain. First, the comparison between the topographically corrected interpolated values of daily solar radiation and those obtained by a direct spatial interpolation technique (Inverse Distance Weighed, IDW) is provided. The results show the major role of topography in local values and differences of up to +60% and −90% in the estimated daily values. Besides, the results are compared to experimental data proving the usefulness of the model for the estimation of spatially distributed radiation values in complex terrain, with a good adjustment for daily values and the best fits under cloudless skies at hourly time steps. Finally, evapotranspiration fields estimated through the ASCE-Penman-Monteith equation using both corrected and non-corrected radiation values address the hydrologic importance of using topographically corrected solar radiation fields as inputs to the equation over uniform values with mean differences in the watershed of 62 mm/year and 142 mm/year of standard deviation. High speed computations in a 1300 km2 watershed in the south of Spain with up to a one-hour time scale in 30×30 m2 cells can be easily carried out on a desktop PC.