Nitrate runoff contributing from the agriculturally intensive San Joaquin River Watershed to Bay-Delta in California

Authors: WANG, RUOYU - University Of California, Davis; HUACHIN, CHEN - University Of California, Davis; LUO, YUZHOU - University Of California, Davis; Moran, Patrick; GRIENEISEN, MICHAEL - University Of California, Davis; ZHANG, MINGHUA - University Of California, Davis

Submitted to: Sustainability
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2019
Publication Date: 5/18/2019
Citation: Wang, R., Huachin, C., Luo, Y., Moran, P.J., Grieneisen, M.L., Zhang, M. 2019. Nitrate runoff contributing from the agriculturally intensive San Joaquin River Watershed to Bay-Delta in California. Sustainability. 11(10):2845. https://doi.org/10.3390/su11102845.
DOI: https://doi.org/10.3390/su11102845

Interpretive Summary: The Sacramento-San Joaquin River Delta, also known as "The Delta", of northern California is the place where two major rivers-the Sacramento and the San Joaquin-come together, forming a complex web of canals, sloughs, wetlands and islands, and supporting $2 billion in local agriculture. In addition, water pumped out of the Delta supports over $30 billion in irrigated agriculture in the arid Central Valley of California and provides part of the drinking water supply for 25 million people. The Delta faces major environmental challenges, including water that is high in nitrate and other nutrients due to agricultural and domestic runoff, and invasions by non-native floating and submersed aquatic weeds, such as water hyacinth and Brazilian waterweed, that form dense mats on and under the water surface. Aquatic weed growth in the Delta may be increased by high nutrient content in the water. The State of California and other Federal and state agencies spend millions of dollars each year to control aquatic weeds in the Delta to protect water resources, native ecosystems, human recreational use of the Delta, and human health. To improve aquatic weed control, it is necessary to understand and mathematically model seasonal patterns of aquatic weed growth. To better understand the influence of upstream runoff of nitrate from the San Joaquin River watershed into the Delta on nutrient availability in the Delta, the Soil Water Assessment Tool (SWAT) developed by the USDA-ARS was applied and modified to predict nitrate runoff from a complex system of tile drains in the western San Joaquin River watershed. These tile drains have been installed to improve drainage from irrigated soils that do not drain well naturally. This study applied a new method to simulate tile drainage in SWAT, and found that the improved model accurately predicted that 40% of all of the nitrate in the San Joaquin River came from the system of tile drains from 2003 to 2014. Characteristics of the soil and depth to tile drains influenced model accuracy. Importantly, the study found that nitrate loads in the San Joaquin River decreased by up to 50% between 2003 and 2014, possibly reflective of improved management of crop fertilization regimes and nitrate runoff in the watershed. Nitrate runoff where the San Joaquin River empties into the Delta peaks annually in spring and early summer, coinciding with crop fertilization upstream and potentially favoring rapid aquatic weed growth. However, many factors influence nitrate concentrations and aquatic weed growth in the Delta, and additional modeling is needed to predict where and when aquatic weed populations will be at their worst.

Technical Abstract: Nitrogen loading from agricultural landscapes can trigger a cascade of detrimental effects on aquatic ecosystems. Recently, spread of aquatic weed infestations in the Sacramento-San Joaquin Delta of northern California has raised concerns and nitrogen loading from California’s intensive farming regions are considered as one of the major contributors. In this study, we used the Soil and Water Assessment Tool (SWAT) to model nitrogen exports from the agriculturally intensive San Joaquin River watershed to the Delta. The alternate tile drainage routine in SWAT was tested against monitoring data in the tile-drained area of the watershed to examine the suitability of the new routine for tile nitrate simulation. We found that the physically based Hooghoudt and Kirkham tile drain improved model performance in representing tile nitrate runoff, which contributed to 40% of nitrate loading to the San Joaquin River. Calibration results show that the simulated riverine nitrate loads matched the observed data fairly well, According to model simulation, the San Joaquin River plays an important role in exporting nitrogen to the Delta by exporting 3,135 tons of nitrate-nitrogen annually, which has a strong ecological implication in supporting the growth of aquatic weed, which has impeded water flow, impairs commercial navigation and recreational activities, and degrades water quality in Bay-Delta waterways. Since nitrate loadings contributed by upstream runoff is one important factor to facilitate weed development, our study results should be seen as a prerequisite to evaluate the potential growth impact of aquatic weeds, and scientific evidence for area-wide weed control decisions.