THRESHOLD WIND VELOCITIES OF WET SOILS AS AFFECTED BY WIND BLOWN SAND

Soil moisture influences the susceptibility of a soil to wind erosion and has direct influence on the mind velocity required for initiation of soil movement (threshold wind velocity). Soil texture is another factor affecting threshold wind velocity. Threshold water content (TWC) and threshold friction velocity (U*(t)) of five soils, ranging from loamy fine sand to clay, under abrading and non-abrading conditions were measured using a suction-type wind tunnel. Each soil was wetted and subjected to 8.5, 10.5, and 13 ms(-1) free stream wind velocities at 80 mm above the wind tunnel surface, with and without abrading material in the wind tunnel. Soils with higher clay content had higher water holding capacity at -1.5 MPa (W'). Higher TWC for the same wind velocity was obtained for soils with higher W' or clay content under abrading and nonabrading conditions. All five sons required a similar wind velocity (U*(t) of 0.31 ms(-1) for nonabrading and 0.21 ms(-1) for abrading conditions) to initiate their movement at oven dried condition. However, the threshold friction velocity of soils increased about 20% for 0.1 unit of equivalent threshold moisture (TWC/W') increment. Because of the erosive impact of abrading materials on soil surface, less wind (U*(t)) was required to initiate soil erosion with abrasion compared with nonabrading conditions. A soil with a threshold water content of 4% at the surface and 5% at -1.5 MPa requires U*(t) of 0.646 ms(-1) under nonabrading and 0.596 ms(-1) under abrading conditions to start eroding. met, loose erodible materials at the son surface eroded when abrading materials were introduced in the wind tunnel. Predictive equations were developed from which TWC and U*(t) could be predicted, based on soil physical properties.