Modification of the wind erosion roughness index by rainfall

The cumulative shelter angle distribution (CSAD) is a soil surface roughness index used in the ''wind erosion prediction system'' to estimate the fraction of the soil surface susceptible to abrasion by saltating particles (FSA). However, little is known of the effect of rainfall amount and intensity on CSAD parameters. This study was conducted to determine how and to what degree simulated rainfall amount and intensity affect CSAD parameters and ridge height in a field with low (92 mm) ridges. Simulated rainfall was applied in cumulative amounts of 0, 6, 19, 32, 44, 57, and 83 mm at intensities of 13, 25, 51, and 76 mm h(-1) on duplicate plots of an Acuff sandy clay loam (fine-loamy, mixed, thermic Aridic Paleustoll). The CSAD parameters were estimated from elevations measured by a laser roughness meter in directions parallel and perpendicular to tillage after each cumulative rainfall amount. Laser roughness meter measurements were also used to calculate ridge height for each plot. We found the CSAD parameters varied in response to tillage direction. The mean FSA was 19% greater and the rate of change of FSA over rainfall amount was twice that when evaluated parallel to tillage compared with measurements made perpendicular to tillage. The CSAD was a more sensitive index than ridge height for describing the effects of rainfall and tillage on surface roughness. Analyses of variance of CSAD parameters and FSA revealed significant differences (P < 0.05) owing to rainfall amount and intensity for most parameters. No significant differences in ridge height (P < 0.05) were found to be due to rainfall amount and intensity. The 13 mm h(-1) rainfall intensity had no effect on the roughness parameters. Regressions of CSAD parameters and FSA over rainfall amount for each rainfall intensity showed that data for the 51 and 76 mm h(-1) rainfall intensities could be combined. This study clearly demonstrated the sensitivity of CSAD parameters to rainfall amount and intensity. The response of CSAD parameters to tillage direction confirmed and quantified the value of tillage as a wind erosion control practice. (C) 1997 Published by Elsevier Science B.V.