RUNOFF MODEL SENSITIVITY TO RADAR RAINFALL RESOLUTION

Rainfall rates estimated from polarimetric weather radar measurements of a convective rainstorm are used as input to a two-dimensional physically based rainfall-runoff model. The correlation length of the input rainfall field L(S) is 2.3 km. Runoff simulations are performed on two semi-arid watersheds covering 32 km2 and 121 km2 at basin data grid sizes L(M) of 125 m and 200 m, respectively. The characteristic basin length scale L(W) is taken as the square root of the watershed area. Rainfall data at resolutions L(R) of 1, 2, 3, 4, 6 and 8 km serve as model input to determine the effect of precipitation data spatial resolution on computed outflow hydrographs. Two dimensionless length parameters are identified which describe the similarity of the effect of rainfall data aggregation on both basins. The first parameter, L(R)/L(S), describes 'storm smearing', and the second parameter, L(R)/L(W), describes 'watershed smearing'. Results from simulations without infiltration show storm smearing occurring as L(R) --> L(S). Watershed smearing causes more significant deviations from simulations using the finest-resolution data when L(R)/L(W) exceeds 0.4. Results with infiltration reveal that excess rainfall volumes decrease with increasing L(R)/L(W). Additionally, excess rainfall volumes do not converge as L(R)/L(W) is decreased to the practical lower limit provided by contemporary weather radars, which is of the order of 1 km.