On Radar Rainfall, Catchment Runoff and the Response Scale

The general research hypothesis is that: "a rainfall event, extreme at a specific scale, has the potential to generate an extreme runoff event in a catchment, which characterized by this response scale". In the presented study, which is a first step in testing this hypothesis, we examine if catchments have a stable response scale in the above context. For that purpose, we compare maximum storm rainfall intensities at different time and space scales with runoff peak discharges in order to determine at what scale these two variables are best related to each other. Three types of rainfall variable are tested: 1) gage rainfall intensity, 2) radar rainfall intensity, and 3) radar reflectivity. Initial results are available for the Walnut Gulch Experimental Catchment, a 150-km² semi-arid catchment, located in southern Arizona. The catchment is well equipped with dense networks of rainfall and runoff gages. Radar data are also available for the catchment from the Tucson NEXRAD system. Preliminary results indicate a response scale in the order of 6-km and 2-hours for the 150-km² catchment and for the 126- and 94-km² sub-catchments. The response scale of a 25-km² sub-catchment is reduced to 1-km and 20-minutes. The three types of rainfall variable tested point to the same response scale. As mentioned, the above results are initial and based on a limited number of events. We are investigating this hypothesis on a larger number of events as well as additional catchments.