2017 OFR demonstration site monitoring and analyses: Effects on soil hydrology and salinity, and potential implications on soil oxygen

On-farm recharge (OFR) is a practice that uses surface water to alleviate demand on and replenish groundwater supplies. It can take on two forms: in lieu recharge and direct recharge. In lieu recharge utilizes surface water supplies instead of groundwater to irrigate crops. Direct recharge applies water beyond the needs of the crop and replenishes the groundwater supply. ... The present study examined OFR with grapes, walnuts, and pistachios at six sites in the San Joaquin Valley, plus one additional site from a previous study, also in the San Joaquin Valley. Each site was comprised of a recharge plot that received direct recharge paired with a control plot with the same crop and soil characteristics, but meant to receive in lieu recharge (via the flood system) or drip application with groundwater. At the end of the 2017 recharge demonstration, however, three control plots had also received direct recharge from water applications that exceeded the crop’s water demand. At another site, both control and test plots had only received in lieu recharge due to limited surface water amounts or the host growers’ more conservative volume of water application. ... The present study only covers one season of recharge. Long-term effects of recharge are not described by the present study and will require further monitoring. Further study is needed of the dynamics of soil oxygen during and after recharge events. Similarly, the fate of the water after it infiltrates past the root zone is not always known and the rate at which recharged water will reach an aquifer is seldom known for deep aquifers. A method to predict the fate of water quickly and broadly would be quite helpful in developing an on-farm recharge strategy. The present study does not look at the effects of recharge on soil biological processes, such as microbial respiration and plant oxygen demand. Further study of the recharge tolerance of specific species and rootstocks, as well as the impact on plant disease, is crucial

On-Farm Flood Capture and Recharge (OFFCR) at an Organic Almond Orchard, Recharge Rates and Soil Profile Responses Groundwater Recharge Project, 2016

Groundwater in much of California’s Central Valley (CV) has been critically over-drafted resulting in the implementation of the 2014 Sustainable Groundwater Management Act (SGMA). As Groundwater Sustainability Agencies (GSAs) work to comply with SGMA requirements and timelines, On-Farm Floodwater Capture and Recharge (OFFCR) is being studied to help increase recharge capacity. We implemented an OFFCR test on an organic almond orchard in the CV to assess achievable recharge rates attained through over-irrigation, and potential soil and water quality impacts. Irrigation water was applied via flood irrigation. We developed study sites and installed soil sensors for moisture and salinity monitoring, took post-irrigation deep cores to assess changes in soil and porewater nitrogen and salt concentrations through the vadose zone, and monitored agronomic practices, recharge loading and crop yields. These studies were conducted on three recharge treatments with three replicated stations for each: 1) Control at about 6 inches of flooded water to meet ET as typical for irrigation (Control treatment), 2) Low Flooding of about 12 inches per irrigation application (Mid treatment), and 3) High Flooding of about 24 inches per irrigation application (High treatment)

Technical report: Modeling nitrate leaching risk from specialty crop fields during on-farm managed floodwater recharge in the Kings Groundwater Basin and the potential for its management

This project has focused on better understanding the potential impact of On-Farm Flood Capture and Recharge (OFFCR) on groundwater quality pertaining to salts and nitrate and on assessing potential management opportunities. To achieve these goals, we used a combination of field and modeling studies. For the field study, soil cores were taken to a depth of 30 feet in replicate across fields with three different specialty crops identified as important to the San Joaquin Valley (tomatoes, almonds, vineyards) and with potential suitability for OFFCR. A prime goal of the field study was to provide data for parameterizing two models developed to assess nitrate, salt and water transport through the vadose zone, prior to percolating into the groundwater aquifer. However, the field study also resulted in key findings that show its value as a stand-alone study: 1) Nitrate concentrations are highest in the upper vadose zone and affected by texture. Those effects are not evident in the deeper vadose zone. 2) Vadose zone nitrate concentrations are affected by the crop grown. These results suggest an opportunity for lower legacy mass transport for grapes and higher legacy mass transport for both tomatoes and almonds. 3) Variability in individual farmers’ past and present fertilizer and water management practices contributes to different legacy salt and nitrate loads in the vadose zone. Data from the field study and other related and concurrent OFFCR field efforts were used during model development. The overall modeling approach was designed to model nitrate and salt transport for lands under OFFCR operation for different crop types, vadose zone characteristics and groundwater characteristics. The defined goals of this design and modeling approach were to: 1) model nitrate and salt movement through the vadose zone and into groundwater; 2) test the model against scenarios that consider different recharge rates, cultural practices, soil types, and depths to groundwater, assessing the timing and magnitude of loading through the vadose zone and the effects on underlying groundwater; and 3) recommend management practices to mitigate potential groundwater impacts. To achieve these goals, two models were integrated to simulate nitrate and salt transport through the vadose zone to groundwater under different scenarios: a 1D Hydrus model and an analytical groundwater model (AGM)

Groundwater relationships to pumping, precipitation and geology in high-elevation basin, Sierra Valley, CA

Sierra Valley, located in the northern Sierra Nevada, California, serves as the Middle Fork Feather River headwaters and provides surface water to Oroville Dam of the California State Water Project (SWP). Under California’s Sustainable Groundwater Management Act (SGMA), the Sierra Valley sub-basin has been designated a medium-priority basin, due to chronic groundwater declines and the valley’s high ecological value as the largest freshwater marsh and meadow system in the Sierra Nevada. The Sierra Valley Groundwater Management District (SVGMD) serves as the Groundwater Sustainability Agency (GSA) for the Sierra Valley sub-basin. As such, SVGMD is tasked through SMGA with achieving sustainable groundwater management over an approximate 20-y timeframe. The first step is the development of a Groundwater Sustainability Plan (GSP) (to be completed by January 2022) that 1) hydrologically assesses the basin, 2) identifies methods and protocols to track groundwater trends, and 3) develops an initial suite of actions to move the basin towards groundwater sustainability. ... Our investigation builds on previous watershed studies and further establishes the Sierra Valley watershed as a highly complex hydrologic system. These complexities include: large variation in precipitation phase and quantity throughout the watershed; geologic features that restrict both vertical and lateral groundwater flow; many water inflow pathways, both surface and sub-surface, that are logistically impossible to quantify by conventional monitoring means. Prior attempts at developing accurate water budgets and numerical models of the watershed have been hindered by the uncertainty these factors present. Thus, though a hydrologic budget is required by SGMA for the development of the GSP, numerical models will be of limited utility as either tools to derive hydrologic budgets or to help determine the efficacy management actions to achieve sustainable groundwater conditions. In developing strategies to address undesirable groundwater conditions, we recommend an adaptive management approach paired with targeted and defensible data collection with standardized data collection, management and quality control procedures

Sierra Valley, CA – A white paper on the opportunities and challenges for management of groundwater under SGMA

This paper discusses groundwater sustainability in California’s Sierra Valley based upon review of various hydrologic and geologic data sets and publications and presents our findings in the context of the 2014 Sustainable Groundwater Management Act (SGMA). The discussion related to SGMA is based upon our current understanding of the legislation. As this legislation is implemented, its interpretation may evolve. The paper provides potential next steps and mitigation strategies as Sierra Valley works to move toward sustainable groundwater management