Inundation impact assessment is a critical component in reservoir site selection and capacity expansion planning. However

during the early planning stages

the high cost of high-precision topographic surveys and comprehensive field investigations often limits the availability of detailed data

making traditional assessment methods time-consuming and data-deficient. This study proposes a rapid inundation impact assessment method by integrating Geographic Information System(GIS) spatial analysis with Digital Elevation Models(DEM) and remotely sensed land cover classification data.Using Hedi Reservoir as a case study

the method calculates

at one-meter water level intervals above the flood-limited level

the corresponding storage capacity and the affected population

cultivated land

roads

and other key factors. The results show that the calculated water level–storage capacity curve closely matches historical records

with a coefficient of determination(R2) of 0.99.The analysis of the trade-offs between increased storage benefits and inundation-related costs associated with higher water levels is found to be reasonable.Compared with traditional approaches

the proposed method offers lower cost and faster response time. It provides valuable data support and decision-making assistance for the early-stage planning of water conservancy projects.