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INTRODUCTION 1. Introduction The National Operational Hydrologic Remote Sensing Center (NOHRSC) developed the Snow Data Assimilation System (SNODAS) to generate estimates of snowpack states and fluxes on a I-hr temporal and a I-km spatial scale for the United States (Ban-ett 2003). Two major components within SNODAS are a spatially distributed snow model that estimates snowpack characteristics and a data assimilation algorithm that adjusts snowpack characteristics based on observations from the Natural Resources Conservation Service (NRCS) Snowpack Telemetry (SNOTEL) network, snow courses, satellites, and radar. Outputs from SNODAS include snow depth, snow water equivalent (SWE), snowmelt, snowpack temperature, blowing snow sublimation, and snow surface sublimation and condensation. The purpose of this project was to assess whether information from SNODAS could aid in snowpack assessments and water supply forecasts for the Upper Rio Grande basin. The SNOTEL network cun-ently serves as the primary source for assessing snowpack conditions in the Upper Rio Grande basin. However, point observations at SNOTEL sites often do not indicate the true distribution, depth, and water content of snow for an area. The SNODAS outputs offer several potential benefits over SNOTEL observations, including spatially continuous estimates of snow cover characteristics as well as estimates of transport processes (i.e., snowmelt, sublimation) that affect whether snow ablation is available for runoff. However, the limitations of SNODAS include a short historical period and limited validation against other data sources. The project steering committee identified six key index gages in the Upper Rio Grande that constitute the project area (see Table I-I). These locations are significant for water supply forecasting and water administration to meet the terms of the Rio Grande Compact. Table 1-1. Key Index Gages Location DWR ID USGSID Rio Grande near Del Norte CO RIODELCO 08220000 Pinos Creek near Del Norte CO PINDELCO 08220500 Conejos River below Platoro Reservoir CO CONPLACO 08245000 Conejos River near Mogote CO CONMOGCO 08246500 San Antonio River at Ortiz CO SANORTCO 08247500 Los Pinos Ri ver near Ortiz CO LOSORTCO 08248000 RTi performed two major activities during the course of the project. First, RTi processed historical and current SNODAS data for the project area and disseminated an updated map product to the steering committee on a weekly basis. Second, RTi performed a limited investigation of NRCS seasonal water supply forecasts for the RIODELCO forecast point. In particular, the investigation focused on water year (WY) 2005 and WY 2007 due to large discrepancies between the forecast and observed runoff values. ~Riverside Technology, inc.