SOIL HYDROLOGY AND MANAGEMENT EFFECTS ON EROSION AND WATER QUALITY
Location: National Soil Erosion Research Lab
Title: Characterisation of soil microtopography effects on runoff and soil erosion rates under simulated rainfall
Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: July 6, 2011
Publication Date: September 18, 2011
Citation: Vermang, J., Norton, L.D., Huang, C., Gabriels, D. 2011. Characterisation of soil microtopography effects on runoff and soil erosion rates under simulated rainfall [abstract]. In: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE), September 18-21, 2011, Anchorage, Alaska. 2011 CD ROM.
Soil surface roughness is commonly identified as one of the dominant factors governing runoff and interrill erosion. Yet, because of difficulties in acquiring the data, most studies pay little attention to soil surface roughness. This is particularly true for soil erosion models which commonly don't include a roughness factor, or use a simplified, discrete roughness factor. This study aimed at analyzing the influence of surface roughness on runoff and soil erosion rates by simulated rainfall. Second, the implementation of a roughness factor in sediment transport models is investigated. Bulk samples of a loess derived silt loam soil were collected and sieved to 4 aggregate sizes: 0.003-0.012, 0.012-0.02, 0.02-0.045, 0.045-0.1 m. The aggregates were packed in a 0.60 by 1.2 m soil tray, which was set at a slope of 5%. Rainfall simulations using an oscillating nozzle simulator were executed for 90 min at an intensity of 50.2 mm.h-1. The surface microtopography was digitized by an instantaneous profile laser scanner before and after the rainfall application. From the laser scanner data, a digital elevation model was produced. Calculated roughness indices include random roughness and variogram parameters. Soil surface sealing was studied using image analysis of thin sections taken from undisturbed samples. The data revealed longer times to runoff with increasing soil surface roughness as surface depressions first had to be filled before runoff could take place. Once channels were interconnected, runoff velocity and runoff amount increased as aggregates were broken down and depressions were filled. Rough surfaces were smoothed throughout the rainfall event, diminishing the effect on runoff. Final runoff rates were highest for the smooth soil surface and lowest for the medium rough soil surface. The medium smooth and rough soil surface showed intermediary final runoff rates due to a higher storage capacity of the soil’s surface. The higher final runoff rate observed with the roughest soil surface is due to the creation of a thick depositional crust in the concentrated flow areas, thus lowering infiltration rates. Furthermore, the rougher the soil surface becomes, the more water will be directed to concentrated flow areas rather than allowing infiltration. Final wash rates were comparable for all applications, which was confirmed when applying stream power to predict the sediment transport. The simulations reveal that the significance of soil surface roughness is the delay in runoff for rougher surfaces and, up to a critical soil surface roughness, a decrease in runoff rate rather than the decrease of soil erosion amount. Apart from the random roughness, the sill and the proportion of sill over range as derived from the variogram proved to describe the soil surface roughness well. Measured random roughness and variogram parameters were introduced into the WEPP interrill sediment delivery factor and compared to the discrete random roughness parameter normally used in WEPP.