Modeling nitrogen runoff from Sacramento and San Joaquin river basins to Bay Delta Estuary: Current status and ecological implications

Authors: WANG, RUOYU - University Of California, Davis; CHEN, HUAJIN - University Of California, Davis; BUBENHEIM, DAVID - National Aeronautics And Space Administration (NASA); Moran, Patrick; ZHANG, MINGHUA - University Of California, Davis

Submitted to: Journal of Aquatic Plant Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2020
Publication Date: 8/31/2021
Citation: Wang, R., Chen, H., Bubenheim, D., Moran, P.J., Zhang, M. 2021. Modeling nitrogen runoff from Sacramento and San Joaquin river basins to Bay Delta Estuary: Current status and ecological implications. Journal of Aquatic Plant Management. 59s:107-111. https://apms.org/journal/.

Interpretive Summary: The Sacramento-San Joaquin Delta in northern California is the confluence of the Sacramento and San Joaquin Rivers, and forms the nexus of California's water supply. Water pumped from the Delta irrigates over 4 million acres of farmland in the Central Valley producing over $30 billion in crops per year, and provides drinking water for over 25 million people. Non-native, invasive aquatic weeds such as floating water hyacinth, water primrose, and spongeplant, and submersed Brazilian waterweed, curly-leafed pondweed, and Eurasian watermilfoil threaten movement of water resources in the Delta, as well as recreational boating, commercial shipping, and natural ecosystem preservation. The Sacramento and San Joaquin Rivers receive runoff from millions of acres of farmland and ranchland, and this runoff contains the nitrate form of nitrogen and other nutrients that could enhance growth of aquatic weeds in the Delta. In this paper, we used a computer model called SWAT (Soil and Water Assessment Tool) developed by USDA to predict nitrate runoff based on historical trends in rainfall, land use patterns, elevation, and irrigation runoff infrastructure, and we compared predicted runoff to known nitrate concentrations from various Delta water quality monitoring programs. For a simulation run between 2003 and 2016, the Sacramento River had water flow levels five to ten times higher than that of the San Joaquin, and annual tons of nitrate loading into the Delta was about six-fold higher from the Sacramento River into the Delta than from the San Joaquin River. Flow and nitrate loading into the Delta from both rivers were highest in winter and spring due to winter rains and snowmelt in spring. Simulated nitrate concentrations were predicted to exceed a threshold needed for healthy aquatic weed growth (1.5 mg per L nitrate) 25% of the year at the point where the San Joaquin River enters the Delta, compared to a 9% annual exceedance rate for the Sacramento River entry point. This result indicates that nitrate concentrations are likely to be much higher in the San Joaquin River than in the Sacramento River. The San Joaquin River is thus more likely than the Sacramento River to be a source of invasive aquatic weed populations in the Delta. The results have implications for strategies to treat 'nursery' populations of aquatic weeds that float into the Delta from the upstream watersheds.

Technical Abstract: Sacramento and San Joaquin River basin are different in drainage area, crop types and climate, which are expected to exhibit discrepancies in hydrology and water quality runoff patterns. Therefore, this paper aims to model nitrogen runoff from the Sacramento and San Joaquin River basin to the Bay-Delta using the USDA Soil and Water Assessment Tool (SWAT) model. We utilize SWAT model to understand current nitrogen runoff exporting status from both upstream watersheds, and then discuss their ecological implications on the growth of invasive aquatic weeds in downstream Bay-Delta waterways. The simulation period was set from 1/1/2003 to 12/31/2016, with two years (2001 and 2002) as model initialization period, allowing SWAT model to fully adjust to the hydrological cycle for the watersheds. For both watersheds, monthly peak flow for each year is usually detected between December and April. 2006 and 2011 are two extreme wet years, causing the highest and second highest monthly flow in the entire simulation period (2595 and 2472 m3/s for Sacramento, 832 and 651 m3/s for San Joaquin). The monthly distribution of NO3-N loadings from both watersheds exhibits quite similar pattern like flow, with annual peaks found in winter and spring months. Since NO3-N is highly dissolved, and easily transported with water, it is not surprising to find similar monthly pattern of flow and NO3-N. Compared to San Joaquin River basin, Sacramento River basin generates substantially higher flow and NO3-N loading. For the entire simulation period, Sacramento contributed 629 m3/s flow and 14.81 tons NO3-N/yr to the Bay-Delta, which is five times higher than San Joaquin river basin (95 m3/s for flow and 2.72 tons/yr for nitrate). The seasonal patterns of NO3-N concentration are different for Sacramento and San Joaquin. For San Joaquin River watershed, higher concentration months were usually accompanied with lower streamflows and lower NO3-N loadings. For wet year 2006 and 2011, lowest NO3-N concentration are found in April. For other years, higher NO3-N concentration was usually found after May. Although the San Joaquin River watershed provided less nitrogen loadings to the Bay-Delta, nitrate concentration at San Joaquin River outlet was substantially higher than that from Sacramento River for almost the entire concentration range. The output of nitrate nitrogen from San Joaquin River watershed to the Bay-Delta estuary would be more beneficial to the rapid growth of invasive aquatic weeds. Future studies should integrate our land surface loading model results with aquatic weed growth models to evaluate the impact of agricultural nitrogen loading on the phenology of invasive aquatic vegetation, leading to proactive management strategies for aquatic weed control.