Dimensions and Orientation of Runoff Trenches Designed for Minimizing Water Loss.

For thousands of years people in arid environments have tried to skillfully manage the scarce water resources by collecting runoff water and utilizing it for irrigation.  Water harvesting methods formerly developed for mere existence are nowadays receiving renewed attention.  Micro-catchment water harvesting systems, used in many dryland areas around the world, are systems by which runoff is being collected from a contributing area and stored for consumptive use in the root zone of an adjacent infiltration basin.  Conservation of harvested water by minimizing losses mainly involves reducing direct evaporation.  Traditionally, the infiltration basins of micro-catchment systems were at the surface level.  It is proposed that by replacing the shallow infiltration basin with a trench, the amount of harvested water will not change, but due to the reduced evaporating surface and the attenuation of solar radiation reaching down to the trench floor, the evaporative loss will largely decrease.  Under similar soil and meteorological conditions, the most dominant factor determining the amount of evaporation from the infiltration basin is solar radiation.  The overall objective of the research was to optimally design the trench dimensions and orientation for minimizing solar radiation load by modeling the radiation reaching trenches at various configurations of size and orientation.  A model developed to simulate solar radiation at the trench floor shows that the fraction of radiation at the trench floor relative to radiation outside the trench (RT) decrease from 0.8 for a width/depth ratio of 0.4 to 0.3 for a width/depth ratio of 2.5.  Orientation was found to significantly affect the RT ratio differently in different seasons.  Trench orientation preference is thus dependent on the time of year when water conservation is relevant (i.e., when is the rainy season).