Flash-flood and urban hydrologic forecast systems required accurate quantitative measurements of precipitation at temporal scales of minutes and spatial scales on the order of one square kilometer. We propose to explore the potential of X-band polarization diversity radar to provide such high-resolution rainfall products. Polarimetric radar measurements add information about rainfall system and have better sensitivity to rainfall rate variability. However, a major drawback of using high frequency radar is that both beam power attenuation and backscattering phase shift can be significant at high rainfall intensities and thus negatively affect the accuracy of rainfall products. The goal of the proposed research is to develop a methodology, which would combine the advantages of X-band radar systems with those of polarization diversity techniques. We aim at demonstrating that such a methodology can lead to reliable quantitative rainfall estimates at spatio-temporal scales suitable for small-scale hydrologic predictions. Our investigation will be facilitated through two field experiments to obtain high-resolution polarimetric radar data at well-instrumented sites. The first experiment will take place in Southeast Iowa at the hydrometeorological monitoring facilities of the Iowa Institute of Hydraulic Research. These facilities include a two-dimensional video disdrometer, optical rain gauges, a mobile vertically pointing Doppler radar, and a very dense network of tipping bucket rain gauges. We will collect the polarimetric radar measurements using the National Observatory of Athens. (NOA) mobile dual-polarization X-band radar (X-POL). We will use the data in the development of a rainfall estimation scheme. We will assess the error structure of the scheme's estimates a various spatial and temporal scales of aggregation using high-resolution rain gauge cluster data. The second experiment will take place at Goodwin Creek basin in Mississippi. A high-density rain gauge and streamflow network there will be complemented with several disdrometers, and the NOA X-POL radar will join another X-band mobile radar (Doppler on Wheels) operated by researchers from Princeton University. Successful completion of this research may provide a new remote sensing tool that, if proven reliable, would be an attractive and economically affordable solution to the problem of monitoring remote basins and urban areas that are poorly covered by the NEXRAD radar network.