. At issue Drylands occupy about 41% of Earth’s land surface, and while eing home to about one third of the human population, they retain nly 8% of the world’s renewable water resources (Millennium cosystem Assessment, 2005). For thousands of years man has ried to skillfully manage this vital but scarce resource by collecting unoff water and utilizing it for irrigation (e.g., Evenari et al., 1971; roppelmann and Berliner, 2003). Water harvesting methods forerly developed for mere existence (Boers and Benasher, 1982) are owadays receiving renewed attention. Micro-catchment water harvesting systems, used in many dryand areas around the world (e.g., Critchley and Siegert, 1991) re systems by which runoff is collected from a contributng area, stored in an adjacent infiltration basin, and support ree growth during the dry season (Boers and Benasher, 1982). icro-catchment systems were developed and implemented in editerranean climates in which the dry season can be very long. In ∗ Corresponding author. E-mail address: agam@bgu.ac.il (N. Agam). ttp://dx.doi.org/10.1016/j.agrformet.2016.02.014 168-1923/© 2016 Elsevier B.V. All rights reserved. order to ensure tree production, non-productive water losses have to be minimized. Water losses mainly occur from the impounding area during the period during which free water is available and from the wet soil surface thereafter. The surface of the wetted area thus plays a major role (Zhang et al., 2013). Traditionally, the infiltration basins of micro-catchment systems were shallow (Fig. 1a) and deepening them significantly reduced evaporation from the soil surface (Zhang et al., 2013). It is, however, not practical to dig deep pits for each tree and the use of trenches has been proposed (Fig. 1b). The depth of the trench directly affects costs and it is therefore of interest to estimate the relation between trench geometry and water loss. The main driver for 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 (depth/width and orientation). Solar radiation intensity at a given point is the sum of the direct beam radiation, solar radiation scattered during its passage through the atmosphere (diffuse radiation), and the radiation reflected downwards by the walls of the trench. The relative contribution, and thus the relative importance of each of the above mentioned N. Agam et al. / Agricultural and Forest Meteorology 221 (2016) 152–163 153

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. (C) 2016 Elsevier B.V. All rights reserved.