Land Use and Runoff Uncertainty

Abstract

Data from about 160 neighborhoods in six Jefferson County, AL drainage areas were intensively investigated to determine the surface covers for each land use type. The data shows that the watersheds are highly impervious, with three of them having more than 50% of the watershed area composed of impervious cover. However, TR-55 (USDA 1986) guidance still shows that the impervious cover for all land uses investigated to be much greater than we observed for our area. It was also concluded that the variabilities of the surface covers within the different land uses for the investigated areas was small, especially for the impervious covers. The percent of directly connected impervious cover (DCIA) was determined by direct field observations and was also estimated by empirical equations. Equations for determining DCIA developed as part of several different studies were use to predict DCIA for our data, but did not give good estimates especially when analyzed at land use level. There was a similarity between "highly connected basins" Sutherland equation and the fitted equation for our overall data. However, the residual analysis for the regression model failed, suggesting that the power equation is not the proper equation to be used for those six drainage areas. Consequently, equations for each existing land use were developed, concluding that a single equation cannot accurately estimate DCIA for all regions and land uses.