The Boussinesq equation is used to show that base flow recession is an exponential decay function. For a single peaked hydrograph, the storage delay factor K_b, the time for base flow to decrease by a factor of 10, is estimated. The parameter K_b is adjusted by trial so that surface flow, or the observed total flow minus base flow, is also an exponential decay function after the point of inflection on the hydrograph recession. Surface flow K_s is so determined. For 22 basins in Illinois, dormant season K_b ranges from 15–100 days and is associated with physiography. K_s varies from 1.8–6.0 days.