National Weather Hydrology Laboratory

The gridded threshold runoff values can be compared with current county runoff values which in most cases have been used for many years. Assuming the current county runoffs are valid, gridded runoff values within the counties should range from below to above current county runoff values. The averages of gridded runoff values in the respective counties should approximate current county runoff values. For those current county runoff values known to be of questionable quality and less indicative of past flood events, the gridded runoffs can be evaluated indirectly by comparing the county FFG with rain events. If the average rainfall over a sub-basin equals county FFG and the small streams slightly overflow the banks, the gridded runoff values can be assumed to be valid. It is assumed the rainfall-runoff model closely approximates current soil moisture conditions and several rain events are included in the evaluation.

The WFO is in the best position to monitor and compare areal rainfall with stream response. Information needed to compare county FFG and stream response should include the county FFG, average rainfall over the sub-basins in the county, and some indication of stream response such as at bankfull, a foot below bankfull, or only half bankfull, etc.

Ideally, site visits to numerous locations on small streams to measure bankfull widths and depths and to determine the shapes of the streams' cross-sections would provide localized stream information to compute threshold runoff values. The time required to collect this information for all the small streams makes it prohibitive. However, with minimal effort over a few weeks it should be possible to collect the information from a few-or more--representative streams in the WFO's area. The RFCs can recompute and update the gridded runoff values using the localized stream information collected by the WFOs, .