The study investigates the identification of suitable sites for the construction of water storage structure and perform water availability analysis in the drought prone upper Falgu catchment in Bihar. Given the seasonal nature of Falgu river, excess monsoon rainfall needs to be conserved for use in the lean season. Site selection was performed using remote sensing and geographic information system followed by physical site survey, leading to identification of three potential sites. Hydrological modelling using HEC-HMS model and rainfall-runoff coefficient methods were employed to estimate runoff at the selected sites. Despite model calibration using 2021 streamflow data at the Gaya gauge discharge site, its performance was unsatisfactory with daily data, likely due to multiple water abstraction in the catchment, prompting a shift to the rainfall-runoff coefficient method for runoff estimation. The monthly rainfall and monthly streamflow at the watershed outlet was used to estimate the monthly runoff coefficient. The runoff coefficients derived were applied to estimate water availability at the identified sites for 80% dependable rainfall. Storage capacities and water spread areas were assessed using ALOS PALSAR DEM data for various depth of embankment. Site-specific depth-capacity curves were generated, and effective water availability was estimated by accounting for evaporation and seepage losses. The estimated storage capacities of site1 and site2 at 4 m height of embankment are 0.273 MCM and 0.439 MCM respectively whereas the corresponding water spread areas are 7.57 ha and 18.45 ha. Site3 exhibited the highest storage potential of 1.042 MCM at 5 m embankment height, followed by site2 and site1. For sites 1, 2, and 3, the corresponding median water availability values are 0.239 MCM, 0.352 MCM, and 0.694 MCM. These findings support the strategic planning of water storage infrastructure in the monsoon-dependent semi-arid regions.