A cell-based model for the Las Vegas Wash Watershed in Clark County, Nevada, USA, was developed by combining the Thornthwaite water balance model and the Soil Conservation Survey's Curve Number method with pixel-based computing technology. After the model was validated, it was used to predict the 2030 and 2050 hydrologic conditions under future scenarios of climate and land-use changes. The future climate projections were based on the Intergovernmental Panel on Climate Change (IPCC) B1 climate scenario, and the land-use scenarios were derived from a CA-Markov land-use model. Results indicate that under these hypothetical conditions, the future surface runoff in the watershed will significantly decrease in winters but increase in summers. Climate change will be the primary controlling factor over runoff. Urban development is projected to increase runoff and may contribute 1.1–18.7% of the changes. This finding may be useful in devising future urban development plans and water management policies.