Effect of variations in soil properties and precipitation on microcatchment water balance

The objective of the study was to investigate the effect of combined variabilities of rain and soil parameters as stochastic inputs for a runoff model on a microcatchment area of 250 m². This area was assumed to be a water harvesting element for a single almond tree and simulated a microcatchment water harvesting project in the Negev Desert of Israel. The average annual precipitation is about 120 mm. This can be described in terms of the number of storms per year, intensity and duration. In a 20-year simulation, the number of storms per year varied from 1 to 22. Annual rainfall ranged from 15 and 310 mm, and was strongly dependent upon the number of storms. The runoff from twenty microcatchment elements was calculated and used to predict the yield of water and almonds. Results indicated that the runoff probability density is strongly skewed toward the low values and is log-normally distributed for the time-space combinations. Twenty-five percent of the time a plot failed to yield the annual runoff required to produce a commercial almond yield. The average yield of almonds was reduced below the optimal relationship. However, results from the simulation indicate that planting the trees in clusters rather than a single tree per microcatchment element may be a way to address this problem.