The relationship between catchment scale and climatological variability of surface‐runoff volume is evaluated through theoretical and empirical analyses. Using a point description of surface‐runoff volume following infiltration excess under the time compression approximation, climatological mean and variance of areal runoff volume over heterogeneous soil are evaluated by integrating second‐order statistics of point runoff volume. Second‐order statistics of rainfall duration and intensity, required for evaluation of those of point runoff volume, are obtained from mean and variance of areal rainfall volume under fractional coverage considerations. Hourly Weather Surveillance Radar‐1988 Doppler rainfall data were used to estimate rainfall parameters following a set of simplifying assumptions on the space‐time correlation structure. Two contrasting climate‐soil systems were considered in specifying soil and soil moisture parameters following a set of assumptions on spatial variability and interdependency of the soil parameters.