Use of NEXRAD and Rain Gauge Precipitation Data for Hydrologic Studies in the Cedar Creek Watershed.

Precipitation is a major driving force variable behind all hydrologic processes needed for watershed modeling studies. The use of precipitation data recorded from rain gauges in hydrologic water quality models may introduce significant variability in the rainfall data; thereby, directly affecting the water balance, and introducing large uncertainty in the modeling outcome. Rain gauge physically measures the depth of precipitation within the sampling area of the rain gauge itself (100 cm²). Although it provides good quality data, a dense rain gauge network must be established to capture the spatial variability of precipitation in an area. Spatially distributed precipitation, such as radar precipitation estimation from the Next Generation Weather Radar (NEXRAD) of the National Weather Service, may provide better input data for modeling the overall watershed hydrologic response. However, NEXRAD estimates can suffer from errors due to drop size distributions of rainfall and properties inherent in the radar measurement system. Consequently, there is a need to evaluate NEXRAD Stage III precipitation data against rain gauge precipitation data that are not included in the processing algorithm, as they become available before being used in hydrologic studies. Thus, the objective of this study was to examine the possible error sources in the Stage III product through radar-gauge intercomparison with a 3-yr record (2005-2007) of precipitation data from the Agricultural Research Service National Soil Erosion Laboratory in northern Indiana. The results indicate that Stage III estimated an average of 1035.5 mm of precipitation while the rain gauges recorded 955.1 mm. Difference in the total precipitation depth and the percent bias between Stage III and rain gauge were 80.4 mm and 8.4 percent, respectively. An overestimation was observed at four rain gauges, while only one rain gauge was underestimated.