Controlling effects of surface crusts on water infiltration in an arid desert area of Northwest China

PURPOSE: Surface crusts are important features in arid desert areas and are critical to hydrological processes and ecosystem development. This paper aims to understand the effects of crusts on water movement in the soil and the factors that affect this and to provide the soil parameters for estimation of saturated hydraulic conductivity (K ₛ) in ecohydrological models. MATERIALS AND METHODS: The study area was located in the middle and lower reaches of the Heihe River Basin, an arid desert area in Northwest China. There were three crust types in this region: physical soil crusts (PSCs, formed by water drop and erosion), biological soil crusts (BSCs, formed by microorganisms, moss, algae, lichen, and soil materials), and salt soil crusts (SSCs, formed by soluble salts). The infiltration rates of different soil and crust types and scalped soils were determined in situ in the field conditions using a disc infiltrometer with three repetitions. Crusts and soils were collected, and their properties were determined in the laboratory. RESULTS AND DISCUSSION: The K ₛ of crust were significantly lower than that of scalped soils with a decrease of 13–70 %. The K ₛ of crusts were related to the type of crust and the properties of soil beneath the crusts. In this region, the soil textures are similar throughout, due to ubiquitous loess sedimentation, so textural differences had no significant effect on K ₛ. Soil organic matter (SOM) played a weak negative role on K ₛ because most crusts had higher SOM than the underlying soil. However, both crust thickness and electrical conductivity (EC, an index of salt concentration) showed significantly negative exponential relationship with K ₛ. Therefore, the SSC with high EC and thick crust have the lowest K ₛ among all crust types. Because soil development is related to salt accumulation, structure, and crust formation, the K ₛ follows the order of Solonchaks < Cambisols < Regosols, from lowest to highest. CONCLUSIONS: Crusts have different characteristics compared with original soils and are the limiting layer of water infiltration in these arid soils. Therefore, the characteristics of crust must be considered in ecohydrological models. The main apparent controlling parameters of water infiltration rate in this area are crust thickness and EC.