The water regime in the sandy dunal area of Nizzana, north-westem Negev Desert, Israel, is highly dependent on a fragile cryptogamic crust only several millimetres thick. This crust develops due to the presence of Cyanobacteria which agglomerate the sand grains and trap aeolian dust particles. Not only does this semi-permeable crust increase runoff but the water which does infiltrate the soil is protected from excessive evaporation. This study presents quantitative measurements of the physical properties of the crust which are important to the water regime in desert dunal areas-the granulometry, porosity and water retention capacity.
The grain size distribution shows a concentration of silt and clay in the crustcompared to the sands just beneath the crust. The microporosity (measured using a mercury pore sizer) shows that approximately 40% of the access pores can be blocked by the swelling of Cyanobacteria trichomes as they absorb water, which limits rainwater infiltration. These observations concur with rain simulation experiments made in the field. An evaporation phase was simulated in the laboratory in order to quantify the water retention capacity of the crust and compare it with that of other sediments, in which the algal mat is not intact, or absent. At the end of the cycle, the crust was found to contain approximately ten times more water than the other samples.