Curve Number Estimation Accuracy on Disturbed and Undisturbed Soils

Estimates of the USDA-NRCS runoff curve number (CN) are generally based on a soil map and observed land cover. Because the CN method is increasingly applied to disturbed and urbanized land, the objective of this work was to collect effective saturated hydraulic conductivities, $K_{\mathrm{sat}}$, and sorptivities, $S_o$, from a range of land use types, use the results to estimate a CN, and compare these CNs with CN estimates made from soil survey information and corresponding land cover. A total of 331 double ring infiltration tests were conducted over the 15 sites. Based on land use and site history, the test sites were classified into categories of engineered, urban altered, rural altered, rural unaltered, and prairie. Measured $K_{\mathrm{sat}}$ values were skewed so the medians of these data were a better predictor of central tendency. The prairie and rural unaltered median $K_{\mathrm{sat}}$ values were closer to soil map estimates than the other categories (between 0.0 and 91% different from soil survey). Two empirical methods developed in Hawaii using sprinkle infiltration tests were used to estimate CN values from infiltration data; method 1 used only $K_{\mathrm{sat}}$ as the predictor and method 2 used $K_{\mathrm{sat}}$ and $S_o$. Results from these two methods were not statistically different at 12 of the 15 sites ($\alpha =0.05$). When comparing these methods to CN values developed from soils data and land cover (method 3), better overall agreement existed between method 1 and method 3. The median CN value from method 1 was the best predictor for the mean CN based on measured runoff data for an urban altered land use site (0.6% different), whereas method 3 was the best predictor for the mean CN based on measured runoff data for a prairie land use (0.0% different).