A method is suggested whereby two or more watersheds may be calibrated for hydrologic studies within a period of 2 to 3 years. The method is based on comparative reactions of the watersheds to the same storm. The comparison is made by regression analysis; the reaction is measured in two ways: (1) by noting the change in stage or flow in each watershed from the time each hydrograph starts rising until it reaches its peak, and (2) by noting the elapsed time for the period of hydrograph rise. Since in any one year the number of storms may be rather large and the size of storms quite varied, the technique yields a regression line with a standard error of great precision and capable of being used to detect even small differences resulting from minor treatments. The technique is illustrated by an example in which increases in streamflow are caused primarily by rainfall and is used to detect hydrologic effects of road construction in a watershed.