Rainfall Accuracy Considerations Using Radar and Rain Gauge Networks for Rainfall-Runoff Monitoring
Components of urban drainage during wet weather affecting water quality in receiving waters are stormwater and overflows from sanitary or combined sewers. A common element affecting each of these components is the spatial distribution of rainfall over contributing areas. Knowing quantities of stormwater arriving at inlets, infiltrating into sanitary sewers, and the inflow into combined sewers is critical to successful hydraulic model calibration and sewer system design. Accuracy and representativeness of the spatial and temporal distribution of rainfall over contributing areas is an important determinant of model accuracy. It is not always feasible to install sufficient rain gauges to measure spatially representative rainfall over a metropolitan sewer district at the scale of sewersheds. Nor is it feasible to install streamflow monitoring stations or sample priority pollutants in every impacted watershed. Thus the combination of radar and rain gauges to characterize the distribution of rainfall offers technical advantages for monitoring both rainfall and runoff in urban areas.
Evaluation of a 55-event series, the median accuracy, as measured by gauge-radar comparison, has a median average difference of ±8%. Gauge network density requirements should take into account the variability of precipitation, distribution over sewershed areas, and local or climatological trends caused by terrain or large water bodies. Runoff measured by streamflow is used to validate the radar to gauge correction and to test the influence of random and systematic error in the radar input. Because simulated runoff is dependent on the rainfall input uncertainty, runoff simulated using gauge-corrected radar is evaluated for a series of five storms composed of both tropical storms and convective events.
For the five storm events over Brays Bayou in Houston Texas, the predicted hydrograph volume depends on the uncertainty of the radar input. Using the gauge-corrected radar as input, the rainfall-runoff model is able to predict volume to within ± 7.8 mm, which is nearly identical to the uncertainty of the input, ± 7.98 mm as measured by radar-gauge comparison. The influence of uncorrected radar (bias) is much greater than the random errors that remain in the gauge-corrected radar inputs as demonstrated by a distributed runoff model. Accurate rainfall derived from a combined system of radar and gauges reduces input- and model-output errors associated with rainfall that is not representative over the drainage areas modeled.
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