Evaluation of the Constant Slope Baseflow Separation and Empirical Infection point Method

Baseflow separation of storm events is performed using the constant slope method, which requires finding the point on the hydrograph recession curve where runoff dominated flow switches to baseflow. This inflection point can be found a number of ways. One empirical method developed by Linsley et al. (1975) uses the empirical equation: D = 0.827A^0.2 where D is the number of days between the storm peak and the end of runoff, and A is the watershed area in square kilometers. In southeastern Wisconsin we examined 4-8 storms from five USGS gauged watersheds ranging in size from 18.2 to 872 sq. miles. Inflection points were determined using MATLAB’s curve fitting tools and compared to the inflection point determined from the Linsley empirical equation. Our objective was to determine if the 0.2 exponential value characterized the inflection point in watersheds in southeastern WI. We found the exponential values were dependent upon storm size. For small storms (10-800 cfs) an exponent of 0.26 averaged among the sites. For larger storms, (>1000 cfs) a value of 0.36 was a better fit for the exponent. We hypothesize that due to sandy soils, the exponent is higher than those found in other watersheds.