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A potential ungauged water-harvesting site was chosen in the central Riyadh Region of Saudi Arabia. A hydrological study was carried out on the catchment area from which runoff water will be diverted to the chosen site. Rainfall depth records from three neighboring rain gauges were used. Runoff volumes and peak discharges for the 2-, 5-, and 10-year storms were estimated using three methods, namely, (a) Soil Conservation Service (SCS) Dimensionless Unit Hydrograph (DUH) method assuming Gumbel distribution for rainfall depth analysis, (b) HEC-HMS modeling, and (c) the modified Talbot formula. The results show that the modified Talbot formula yields an order of magnitude higher peak discharge values for all return periods. The SCS-DUH method and HEC-HMS modeling provide comparable estimates for the peak discharges and runoff volumes. The peak discharges obtained through the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) model for the 2-, 5-, and 10-year storms are 0.17, 0.83, and 1.34 times than those obtained by the SCS-DUH, respectively. The HEC-HMS runoff volume estimates are 0.18, 0.85, and 1.36 times than those estimated by the SCS-DUH for the 2-, 5-, and 10-year storms, respectively.