GARR uses two independent sensors that each indirectly measure rainfall. It provides better maps of rainfall than either sensor could produce alone. A tipping bucket rain gauge measures rainfall over a 6-12” diameter area using bucket tips to interpret a time series of rainfall values. Weather radar reflectivity, in this case NEXRAD, detects the relative intensities of rainfall in a polar coordinate grid (1 degree by 1-km). Each system can be used to improve the other, resulting in better characterization of rainfall over catchments and basins.
Both NEXRAD and gauges have strengths and weaknesses. Rain gauges can provide excellent point estimates of rainfall but can also report erroneous data if they are clogged, off-level, intermittently or failed communications, are shadowed by structures or vegetation, or are standardized. In addition, raingauge networks provide no information about how much rainfall fell between rain gauges. NEXRAD is excellent at detecting the relative intensity of rainfall, but is limited in ability to quantify rainfall amounts.
GARR can provide representative rainfall even when some rain gauges in a rain gauge network do not provide useable data due to mechanical or communications failure, or lost data. Rain gauge networks can provide data if there are short outages of NEXRAD data, providing more reliable and complete data during monitoring periods.