WMOD00425

• Title: Demonstration of Improved Operational Water Resources Management Through Use of Better Snow Water Equivalent Information
• Date: 9/1/2002
• Weather Modification - Doc Type: Report

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EXECUTIVE SUMMARY

The U.S. Bureau of Reclamation (Reclamation) operates over 350 dams and reservoirs
throughout the western United States, managing critical water resources for the benefit of the
American people. This research is designed to aid Reclamation's water managers by providing
them with decision assistance in the form of enhanced precipitation estimation over watersheds
affecting the reservoir system. The overall goal is more efficient use of water for agriculture,
hydroelectric power generation, Municipal and Industrial (M&I), recreation, and endangered
species protection. To achieve this goal, the foll~wing project objectives were defined: (1) to
demonstrate a significant improvement in the technology used to estimate snow water equivalent
(SWE) distributions without requiring instrumentation in addition to that already deployed and
(2) to demonstrate benefits to Reclamation from improved streamflow forecasting resulting from
better SWE information and emerging new NOAA hydrologic prediction schemes.

During the period 1991-1997, approximately 160 WSR-88D (Weather Surveillance Radar-
1988 Doppler) radars were installed in the United States and abroad. The WSR-88D's
operational precipitation algorithms, collectively known as the Precipitation Processing
Subsystem (PPS), were designed for rain rather than snow. In 1995, the Bureau of Reclamation
was tasked by the WSR-88D Operational Support Facility (OSF) to develop a snow
accumulation algorithm (SAA) and did so in the succeeding 3 years. The SAA was targeted for
incorporation into the operational WSR-88D software suite. The SAA was designed solely for
use with dry (not melting) snow.

Under the auspices of the Global Energy and Water Cycles Experiment (GEWEX) Continental-
Scale International Program (GCIP) and Reclamation's Science and Technology Program, a
version of the SAA was developed to use real-time Level ill radar data. This version was
deployed in the north-central United States as a demonstration project during cool seasons from
1998-2001, providing near-real-time graphical distributions of snow water equivalent (SWE) and
snow depth (SD) estimates on the Internet.

The SAA underwent several major modifications in the final year of the project. These
modifications extend its accuracy and applicability to water management operations. The
enhancements were so far reaching that the SAA now accommodates several more precipitation
types. Accordingly, the algorithm is now called the Precipitation Accumulation Algorithm
(PAA). The modifications are briefly described as follows.

First, the SAA correction for range degradation was replaced by a fixed adjustment for the
vertical profile of reflectivity (VPR). This adjustment is widely acknowledged as the most
important one in radar precipitation estimation. The adjustment is indispensable for accurate
estimates in mountainous terrain, where there is severe radar beam blockage.

Second, the new P AA uses temperature and relative humidity from numerical (Eta) model
soundings and temperature altitude thresholds to classify precipitation type. Areas of dry snow,
melting snow (bright band precipitation), rain, and virga contamination are distinguished. For all
these areas except virga, a liquid-equivalent precipitation at the surface is estimated.

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