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 physi cally based rainfall-runoff model. The correlation length of the input rainfall field L(S) is 2.3 km. Runoff simulations are performed on tw o 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. R ainfall data at resolutions L(R) of 1, 2, 3, 4, 6 and 8 km serve as mo del input to determine the effect of precipitation data spatial resolu tion on computed outflow hydrographs. Two dimensionless length paramet ers are identified which describe the similarity of the effect of rain fall data aggregation on both basins. The first parameter, L(R)/L(S), describes 'storm smearing', and the second parameter, L(R)/L(W), descr ibes 'watershed smearing'. Results from simulations without infiltrati on show storm smearing occurring as L(R) --> L(S). Watershed smearing causes more significant deviations from simulations using the finest-r esolution data when L(R)/L(W) exceeds 0.4. Results with infiltration r eveal 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 weath er radars, which is of the order of 1 km.