Integration of Climate Information and Forecasts into Western US Water Supply Forecasts

Abstract

Since the early 1900's, the Natural Resources Conservation Service and cooperating agencies have produced long-lead seasonal volumetric water supply forecasts throughout the western US. These statistical regression-based forecasts primarily rely on measurements of current snowpack and proxies of soil moisture such as antecedent streamflow and autumn precipitation. It has long been recognized that the largest source of forecast uncertainty and error is the amount of precipitation falling between the forecast issue date (e.g., January 1st) and the end of the target season (e.g., September). Seasonal climate information and forecasts offer the potential to improve the skill and lead time of water supply forecasts by reducing uncertainty about future precipitation. This chapter reviews the use of climate information in operational water supply forecasts at the Natural Resources Conservation Service's National Water and Climate Center. The use of climate information in other operational hydrologic forecasting agencies, such as the National Weather Service and the Salt River Project, are also highlighted. While there are benefits gained from using climate information in water supply outlooks, there are also potential hazards arising from climate forecast use and abuse. In particular, there is an acute need to manage risk and expectations about forecast skill among forecasters and users alike. OPERATIONAL WATER SUPPLY FORECASTING Snowmelt provides approximately 80 percent of the streamflow in the West. Streamflow forecasting in the West, therefore, is strongly dependent on snow measurements. The Natural Resources Conservation Service (NRCS), in partnership with other federal and state agencies, conducts snow surveys in 12 western states and Alaska. The NRCS integrates hydrometeorological data from 1100 manual snow courses, 670 automated SNOTEL (SNOwpack TELemetry) sites, 575 stream gages, 310 major reservoirs, and 3200 climatological observing stations to create basin and watershed analyses and water supply forecasts. These forecasts are produced monthly, January through June, in partnership with the National Weather Service (NWS) and local cooperating agencies, such as the Salt River Project in central Arizona. The forecasts discussed here are unrelated to water