Impacts of Land Cover, Rainfall Intensity, Subsoil Hydraulic Conductivity and Depth to Groundwater Table on Initial Abstraction, Infiltration, Runoff and SCS-Curve Number

To quantify the SCS/NRCS Runoff Curve Numbers for various land covers, a laboratory study was conducted and the test data was analyzed. During the lab tests, rates of runoff and infiltration were measured from four (4) types of land cover with ten (10) subsoil hydraulic conductivities under rainfalls of four (4) intensities. Durations of the test rainfalls were at least two hours. The four land covers tested were bare soil, vegetated area, gravel and porous hot mix asphalt. The rainfall intensities ranged from small to extreme values. The subsoils had very low to very high hydraulic conductivities covering all the four hydrologic soil groups: A, B, C, and D. All the tests were done using soil columns of the same height representing the same depth to groundwater table. Additional tests were conducted with taller soil columns as well. The starting soil moisture was at the field capacity, that is, at the completion of drainage by gravity. The NRCS/SCS rainfall-runoff function, with the initial abstraction set to be twenty percent (0.2) of the potential maximum retention after runoff begins (S), was used to fit the curve numbers from the obtained laboratory data. Reasonably good fits were obtained with all the coefficients of determination larger than 0.5, and thus the SCS runoff curve numbers were quantified for all the four land covers on top of the subsoils of all the four hydrologic soil groups. Better fits of the curve numbers (larger coefficients of determination or higher levels of confidence) were achieved by using the directly measured initial abstraction instead of the preset relation. However, applications of the better-fitted curve numbers would require the input of additional information beyond the current practices, such as the rainfall intensities.