The distributed-parameter hydrologic models GSSHA (a version of CASC2D) and Vflo™ are being tested in the case of a heavy rainfall over west-central Arizona. The heavy rain was produced by Tropical Storm Nora, 25-26 September 1997. The primary test area is the Santa Maria, an unregulated headwater basin flowing from 2100 m elevations west of Prescott toward the Bill Williams River in the lowland desert of western Arizona. The Bill Williams discharges into the mainstem of the Lower Colorado River near Lake Havasu City. The area of the Santa Maria basin is 3,727 square kilometers.

The primary objective of the test is to assess the performance of the two hydrologic models with radar quantitative precipitation estimate (QPE) input from the WSR-88D Doppler radar network. Of principal interest are not only streamflow output accuracy but also the time required to run model simulations. The latter is crucial to determine whether it is feasible to run the models in near-real-time for decision assistance to the Bureau of Reclamation’s water management officials along the Lower Colorado River. This study has been supported by Reclamation with a goal of accurately assessing in near-real-time the inflows from numerous sidewashes of the Lower Colorado. These inflows are problematic to reservoir operations in the rare event of a heavy widespread rainfall into sidewash basins. If either model is suitable for this task, it or similar models will be recommended for operational use as a decision assistance tool for water operations managers.

The radar QPEs are from the WSR-88D near Flagstaff, Arizona, about 150 km from the Santa Maria headwaters. The QPEs were derived from Reclamation’s new Precipitation Accumulation Algorithm (PAA), which uses a reflectivity-rain rate (Z-R) relationship tailored for the 25-26 September rain event. Preliminary results of hydrologic model performance with this QPE input will be presented.

These efforts are part of the Agricultural Water Resources Decision Support (AWARDS) system, which has been supplying web-based radar QPEs to water managers and users in the Lower Colorado region. Reclamation wishes to integrate AWARDS into Riverware, an object-oriented generalized river basin modeling environment that integrates the multi-purposes of reservoir systems. Riverware is already in use by Reclamation’s water operations managers along the Lower Colorado.