Figure 3.Rainfall and runoff for the March 2002 storm event.The reference rainfall dataset for calibrating hydrologic models comprised the WSR-88D Stage III data that were calibrated to the gages in and around the basin (Fulton et al., 1998). Using the gage-adjusted WSR Stage-III radar rainfall data as reference for ground validation (GV) data, it was possible derive very accurate stream flow simulations for various model configurations (see Figure 4). Table 1 also summarizes the various error statistics in hydrograph simulation obtained with WSR-88D rainfall estimates. Due to the very negligible difference observed between WSR-88D-simulated and observed stream flow (Table 1 and Figure 2), all subsequent assessment of satellite derived stream flow simulation was therefore performed with respect to the observed stream flow hydrograph.Figure 4.Observed versus simulated stream flow for four hydrologic model configurations in the UC river basin (taken from Harris, 2007). Note: Other than TOPMODEL, all other configurations shown above can be set up in HEC-HMS.Table 1.Runoff error statistics for calibrated models based on WSR-88D Stage-III radar rainfall data. Error is defined as the difference in observed and modeled value and expressed as a % relative to the observed value.For satellite rainfall data, we use the TRMM Multi-satellite Precipitation Analysis (TMPA) product. The TMPA provides a calibration-based sequential scheme for combining rainfall estimates from various satellites, at fine scales (0.25���0.25�� and 3-hourly) (Huffman et al., 2007). It is available both after and in real time, based on calibration by the TRMM Combined Instrument and TRMM Microwave Imager precipitation products, respectively. In this study we assessed the IR-based product known as 3B41RT (0.25�� and hourly). The subscript ��RT�� to each product name refers to real time, which in reality refers to a pseudo real time where data is available to the use via the internet with a 8-16 hour latency for the end user. Because our analysis was intended for assessing the reliability of satellite products for now casting applications (such as flood forecasting and sequential data assimilation), we have chosen to use this ��RT�� product. The 3B41RT data set covers the latitude band 50��N-S for the period 2002 to the present.3.?Hydrologic Model ConfigurationsWe selected the Hydrologic Engineering Center’s (HEC) Hydrologic Modeling System (HMS) to perform our assessment on three hydrologic model configurations. We also used the topographic index based model called TOPMODEL, first developed by Beven and Kirkby (1979), as our fourth model configuration. The use of multiple (four) conceptual model configurations thereby helped us to remove any potential bias of our findings for a particular model choice. USACE’s on-going work involving HEC-HMS over the Upper Cumberland basin allowed us access to various calibration and input databases that were found to be already quality-controlled.