Low-streamflow hydrographs from 22 subbasins in the U.S. Department of Agriculture--Agricultural Research Service (USDA-ARS) Washita River Experimental Watershed complex in central Oklahoma were subjected to recession slope analysis; this method, after that of Brutsaert and Nieber <1977>, was derived from a Dupuit-Boussinesq formulation for the groundwater outflows from the adjoining phreatic aquifers. The long-time aquifer response characteristics were generally found to be close to linear, and the short-time response characteristics were consistent with Boltzmann similarity. Representative values of the resulting basin-scale effective groundwater parameters were (35 days)-1 for the low-flow extinction coefficient (i.e., a storage half-life of 25 days); 0.021 m2 s-1 for the hydraulic diffusivity, Dh; 0.0035 m2 s-1/2 for the hydraulic desorptivity, Deh; 8¿10-4 ms-1 for the hydraulic conductivity k; and 0.018 for the drainable porosity (or specific yield), f. The variabilities of Dh, Deh, and k from basin to basin could be better represented by the log-normal than by the normal distribution; f could be described nearly equally well by both. The storage half-life is moderately and positively correlated with basin size; in the case of k the correlation is negative but weaker. Any scale dependence of Dh, Deh, and f appears to be negligible. ¿ 1998 American Geophysical Union