- Micro-catchment water harvesting systems are systems by which runoff is collected from a contributing area, stored in an adjacent infiltration basin, and support tree growth during the dry season. Increase of the efficiency of such systems can be achieved by minimizing non-productive water losses, which mainly occur from the impounding area during the period during which free water is available and from the wet soil surface thereafter. Designing the infiltration basin as a trench can potentially meet this goal. The main driver for water loss through evaporation is the flux of short wave radiation that reaches the wetted surface. The first step in developing a comprehensive model to this effect, and the objective of the work reported herein is to model the short wave radiative fluxes for different trench geometries. The model computes the radiation reaching the floor of a trench at each point across the trench width accounting for the direct, diffuse, and reflected components. Model validation indicated that the model accurately depicts the diurnal course of radiation reaching the trench floor in both north-south and east-west orientated trenches in both summer and winter. Simulations for the Negev Desert revealed distinct differences in the diurnal course of mean solar radiation reaching the trench floor between north-south and east-west orientations. During the rainy season (November through March) radiation load was found to be greater in the north-south orientation. Thus, aiming to reduce evaporative losses from trench-like runoff harvesting systems in the Negev, east-west orientated trenches are advantageous.