Windblown Soil Crust Formation under Light Rainfall in a Semiarid Region

Surface crusting is a phenomenon that is common to many soils around the world. The soil crust includes both the skin of fine material on the immediate surface of the soil as well as the consolidated material immediately beneath the skin. Surface crusts play a critical role in conserving the soil resource base. Soil crusts are characterized by an interlocking network of particles that reduce the susceptibility of the soil to erosion when exposed to wind. This is particularly important in arid and semi-arid regions where vegetative cover may be insufficient to protect the soil from erosion.

Crust formation is influenced by soil physical and chemical properties as well as rainfall quantity, rate, intensity, duration, and energy. Although observations have been reported on the effect of rainfall characteristics on soil crust formation, wind erosion models suggest a significant rainfall is required before soil erodibility is affected by the formation of a soil crust. Knowledge of crust formation is important for evaluating management strategies aimed at mitigating wind erosion. The prediction of crust formation and strength based upon cumulative precipitation is very useful in simulating wind erosion processes, but there is a lack of information on crust formation of loessial soils in response to light rainfall events that are characteristic of the Columbia Plateau.

The objective of this study was to evaluate the effect of rainfall quantity on surface crust formation and to determine the relationship between rainfall amount and crust thickness and strength for five loessial soils of the Columbia Plateau. The five soils were Athena silt loam, Palouse silt loam, Ritzville silt loam, Walla Walla silt loam, and Warden sandy loam.

The results showed that soil crusts were formed in the laboratory using a rainfall simulator that applied water at a rate of 2.5 mm/h for various durations to achieve a total rainfall application of 0, 0.15, 0.30, 0.60 and 1.0 mm. Crust strength was measured by a penetrometer while crust thickness was measured by a ruler. Silt and clay content of the five soils ranged from 23% to 66% and from 9% to 17%, respectively. An increase in crust thickness and strength was observed with an increase in rainfall for all soils. Crust thickness was similar among the soils whereas crust strength varied among soils at any given level of rainfall. For example, crust strength of soils when subject to 0.15–0.60 mm of rainfall decreased in order of Walla Walla > Athena and Palouse > Ritzville and Warden. These results are consistent with a decreasing order of clay and silt contents. The strongest crust was formed on Palouse silt loam under the highest rainfall amount. A logarithm relationship adequately described the relationship between crust thickness and rainfall for the five soils (r²=1.00). This relationship over-predicted crust thickness based upon observations in the field, but performed better than the algorithm used by the Wind Erosion Prediction System.

The result was published in Soil & Tillage Research in April 2013, 128: 91-96.