Estimation of depression storage and Manning's resistance coefficient from random roughness measurements

Abstract

Depression storage and values of Manning's n resistance coefficient of tilled soil surfaces were estimated from micro-topographic data measured on micro-catchments subjected to simulated rainfall. Manning's n values were calculated from a simplified momentum equation applied to overland flow combined with the depth discharge relationship. Random roughness was calculated as the standard deviation of micro-relief heights measured with a relief meter. Depression storage was measured for different surface roughness inclined at different slopes. Micro-relief height measurements were taken before and after individual rainfall applications in order to monitor changes in random roughness with cumulative rainfall. Regressing random roughness against rainfall showed that random roughness decreases exponentially with increase in water application. A regression equation which relates depression storage to slope steepness and random roughness was developed. Another regression equation was developed to predict Manning's resistance coefficient from random roughness. This approach of relating Manning's resistance coefficient to dynamic surface parameters can be applied to whole seasons as well as individual rainfall events. These relationships can potentially be incorporated into simulation models of soil water balance, overland flow and erosion on tilled soils.