Water Budgets for the Delta Watershed: Putting Together the Many Disparate Pieces

Abstract

Water budgets integrate and summarize the water inputs and outputs that are essential for effective water resources management. Using water data collected from different sources, we constructed three water budgets (a 12-year annual average, a wet year, and a critically dry year) for the Sacramento-San Joaquin Delta (Delta), the Sacramento River (SR) watershed, and the San Joaquin River (SJR) watershed. Although multiple water budgets for the Delta exist, the water budgets presented here are the first to provide all three of the following: (1) water budgets for the entire Delta watershed, divided into management-relevant components, (2) comparisons between wet and dry years and between different regions of the watershed, and (3) discussion of major gaps and uncertainties in the available water data to guide and inform future data collection and water management. Results show that, from 1998 to 2009, the Delta received 24.2 million acre feet (maf) of water each year on average, which primarily exited the Delta as river outflow (71%), water exports (22%), and evapotranspiration (ET; 6%). The SR watershed received 56.9 maf of water (95% as precipitation). The major outputs from the SR watershed were ET (63%) and flows to the Delta (34%). In the SJR watershed, total water input was 28.7 maf composed of precipitation (74%), water imported from the Delta (18%), and storage depletion (7%). The major outputs from the SJR watershed were ET (65%), water exports (19%), and flows to the Delta (14%). Most values varied greatly from year to year. Although streamflows, water exports, and valley precipitation are relatively well measured and estimated, uncertainties are higher for groundwater storage change as well as for ET and precipitation in montane regions. Improvement in data collection and synthesis in these components is necessary to build a more detailed and accurate water budget.

Supporting Restoration Decisions through Integration of Tree-Ring and Modeling Data: Reconstructing Flow and Salinity in the San Francisco Estuary over the Past Millennium

“…Due to the complexity of direct observation of Delta outflow, these estimates are not based on tidal flow measurements. Rather, these estimates are computed from a budget of inflows and diversions from the Delta [27,82] As discussed below under Modeling Approach, we used calibrated runoff-outflow relationships (rather than historical data) to reconstruct contemporary Delta outflow conditions. We decided to use reconstructed flows for the contemporary period to provide a homogeneous time series for comparison with predevelopment conditions.…”

“…For internal consistency, rather than using historical data, we followed a similar methodology to generate salinity data necessary to calibrate contemporary relationships between annual Delta outflow and seasonal average X2. Specifically, we used a subset of the daily contemporary Delta outflow time series [27,82] spanning WYs 1920-2018 and a contemporary flow-salinity relationship reported by [26]. The initial years of the contemporary period (WYs 1912-1919) were excluded from model calibration due to lack of historical Delta outflow data at a daily resolution.…”

“…We define the terminus of the pre-development period as water year (WY) 1850, which roughly aligns with the California Gold Rush and follows previous work [24][25][26] California water years run from 1 October through 30 September. Furthermore, we define the start of the contemporary period as WY 1912, a date that aligns with availability of Delta outflow estimates [27] but pre-dates the availability of systematic estuarine salinity measurements by about a decade [14]. By default, the intervening early development period spans six decades between WYs 1851 and 1911.…”

Supporting Restoration Decisions through Integration of Tree-Ring and Modeling Data: Reconstructing Flow and Salinity in the San Francisco Estuary over the Past Millennium

“…For internal consistency, rather than use historical data, we followed a similar methodology to generate salinity data necessary to calibrate contemporary relationships between annual Delta outflow and seasonal average X2. Specifically, we used a subset of the daily contemporary Delta outflow time series (DAYFLOW, 2019;Hutton and Roy, 2019) spanning WYs 1920-2018 and a contemporary flow-salinity relationship reported by Andrews et al (2017). The initial years of the contemporary period (WYs 1912(WYs -1919 were excluded from model calibration due to lack of historical Delta outflow data at a daily resolution.…”

“…Furthermore, we define the start of the contemporary period as WY 1912, a date that aligns with availability of Delta outflow estimates (Hutton and Roy, 2019) but pre-dates the availability of systematic estuarine salinity measurements by about a decade (Hutton et al, 2015). By default, the intervening early development period spans six decades between WYs 1851 and 1911.…”