This Major Research Instrumentation grant supports development of a basin-scale (4,500 km2) integrated hydrologic observation system in the Sierra Nevada Mountains spanning the rain-snow elevation transition in the American River basin. The basin-scale instrument will include 23 local sensor clusters each with 10-20 snow depth sensors and meteorological sensors to include temperature, precipitation and humidity. A subset of the clusters will have sensors for the measurement of incident and reflected solar radiation, soil moisture and temperature and tree sap flow sensors, the latter to elucidate the elevation dependent timing of the onset of evapotranspiration in the spring. The basin-scale instrument design takes into consideration the needs for integrated measurement of key water cycle reservoirs, fluxes and their controlling factors in the high Sierras with sensor cluster sitings across variable elevation, aspect, slope and vegetation cover. All of the sensor observations will be tied with a wireless communications strategy delivering real-time observations to a central information management system that will be open for public access via the web. The need for quantification of key water cycle reservoirs and the balances between them at increased spatiotemporal scales is a long standing goal of the hydrologic science community. The proposed hydrologic observation system focuses on a snowpack dominated mountain ecosystem that supports the water needs of California's Central Valley agribusiness empire and the needs of tens of millions of California residents. The American River basin hydrologic observatory will be unprecedented in scale and leveraged on existing NSF investments in smaller prototype snow hydrology observations in the southern Sierras and more widely distributed and focused ecohydrologic observations funded through the Division of Earth Sciences Critical Zone Observatories (CZO) program as part of the Southern Sierra CZO. Partnerships with relevant state agencies including the California Department of Water Resources and the Sacramento Municipal Utility District will foster and support the development. Data generated from this observation system will inform new classes of modeling tools to produce quantitative climate assessments, influence hydrologic forecasting, probe system response to climate and land-cover perturbations, increase process understanding of basin-scale water cycles, and provide defensible scenarios for infrastructure planning at a scale currently not possible. Scientific questions that will be addressable using collected observations will include: 1)) How do physiographic and ecological patterns influence the variability of mountain water fluxes?; 2) How do hydrologic systems that are subjected to multiple perturbations respond?; 3) How do pulses and changes propagate through the hydrologic system?; and 4) How can the predictive ability for these responses be improved? The PI team includes faculty at UC-Merced, a non-Ph.D. granting institution, with expertise in hydrology and environmental engineering, and a USDA agency hydrologist. The development will engage undergraduate students in construction and the wireless telecommunication strategy and development will be accomplished through subaward to support acquisition of wireless radio and miniaturized computing architectures and for a collaborating computer scientist at UC-Berkeley who specializes in wireless communications development to design and complete the network topology.