Characteristics and numeric simulation of soil evaporation in biological soil crusts

It is commonly believed that biological soil crusts may affect soil evaporation rates and consequently relieve or aggravate the shortage of soil water in arid and semiarid regions. However, the effects and mechanisms are largely unclear. In the “wind–water erosion crisscross” region on the Loess Plateau of China (so called because suffering serious water erosion in summer and serious wind erosion in winter) biologically crusted and uncrusted sandy and sand loam soils were sampled and then evaporation processes were measured in sunlight using micro-lysimeters. The results indicated: (1) The evaporation rate of biologically crusted soils was significantly (P < 0.01) decreasing at the beginning of evaporation process and thereafter it was constantly high for a long time. (2) The evaporation process of sandy soil was significantly (P < 0.03) affected by biological soil crusts, while that of sand loam soil was not significantly (P > 0.06) affected. This implied that the effects of biological soil crusts on evaporation were dependent on soil properties (mainly soil texture). (3) The evaporation rates of biologically crusted and uncrusted soil can be described by numeric functions (R > 0.88). As a whole, the amount of evaporation was slightly higher on sandy soils and lower on sand loam soils when biological soil crusts were present. It can be concluded that the presence of biological soil crusts would not greatly change soil evaporation in arid and semiarid environments due to the initially decreasing action, balanced by a final increasing effect.