Microphytic crusts, shrub patches and water harvesting in the Negev Desert:: the Shikim system

Human-made contour banks are a central component of the Shikim water harvesting system in Israel's Negev Desert. Efficient water capture depends on the presence of a stable microphytic crust which directs surplus surface runoff into the banks where it is stored. We used simulated rainfall to examine the impact of soil surface disturbance on runoff and sediment transport, and the effect of this on the efficiency of resource capture within the Shikim system. Two disturbance regimes: 1) removal of the microphytic crust only, and 2) removal of the crust and shrub patches by cultivation, were compared with an undisturbed control. In the undisturbed state, 32% of rainfall was redistributed as runoff. This runoff penetrated approximately 27% deeper under the shrub patches compared with the microphytic crust. When the microphytic crust was destroyed by simulated trampling, the runoff coefficient declined to 13%, and there was no significant difference in water penetration between shrub and crust patches. Complete destruction of the shrub hummocks and crust by cultivation resulted in a decline in the runoff coefficient to 6%. The result of sustained disturbance in these patchy Negev shrublands is a breakdown in spatial heterogeneity, a loss of ecosystem function, a reduction in ecosystem goods and services such as plant diversity and production, and ultimately a reduction in pastoral productivity. These results reinforce the view that microphytic crusts are critical for the efficient operation of the Shikim water harvesting system. Given that practices such as cultivation and trampling which disturb microphytic crusts result in enhanced infiltration, crusts should be left intact to maximise the water harvesting efficiency in these desert landscapes.