Cover crops can play a vital role in protecting and enhancing soil during the off-season or between rows of trees and vines. They can offer numerous benefits, including improving infiltration and water storage, particularly critical for the San Joaquin Valley, where SGMA implementation pushes landowners to optimize every drop of water. However, the management actions of groundwater sustainability agencies (GSAs) could unintentionally hinder the adoption of cover crops.
The report, Cover Cropping in the SGMA Era, assembled by Sustainable Conservation, includes an analysis of the impacts of SGMA and GSA management on cover crop implementation and recommendations for water planners and managers to ensure the viability of cover cropping as a multi-benefit management tool.
The report is the product of a group of 100 multidisciplinary experts convened to generate practical insights for water planners, managers, and users around the water-related impacts of cover cropping and the implications of current water management within SGMA.
WHAT ARE COVER CROPS?
Cover crops are non-cash crops cultivated to protect and improve the soil in the off-season or between rows of permanent crops, such as orchards or vineyards. Cover crops generally are winter-season annuals or managed resident vegetation that is minimally irrigated or unirrigated and terminated before the growing season begins.
COVER CROPS VS. BARE GROUND
Cover crops offer a wide range of valuable benefits. During the off-season, they shield soil from wind erosion, minimizing dust and particulate matter. Cover crops reduce runoff of sediment, fertilizers, pesticides, and organic matter, helping to protect nearby waterways. By incorporating excess nutrients like nitrogen into their biomass, cover crops can prevent nitrate leaching into groundwater, safeguarding water quality for local communities.
Effective cover crop management can also decrease growers’ reliance on synthetic fertilizers, pesticides, and herbicides, resulting in lower chemical residues on crops and reducing potential health risks.
The alternative is to manage the soil so there is no vegetative growth, only bare ground, and in fact, for some GSAs, this is a requirement to receive credit for fallowing land. However, contrary to the assumption that bare ground evaporation is negligible, bare ground can evaporate significant amounts of water. Leaving the ground unvegetated can also come with unintended side effects, such as air and water quality degradation, and can be expensive to maintain through chemical or mechanical means.
COVER CROPS AND WATER USE
Concerns over consumptive water use may hinder cover crop adoption rates; however, existing research suggests winter cover crop water use can be negligible compared to bare ground. Additionally, cover crops can increase groundwater through increases in infiltration, soil water storage, and reductions in evaporation, although the extent to which the positive benefits outweigh the water consumption is still uncertain.
Despite the varying factors that influence the water consumption of cover crops, the potential benefits are significant. The authors conservatively estimate that California growers could see substantial water benefits, including increased infiltration and reduced runoff, often greater than 40% as compared to bare ground management. In wetter years, these benefits are likely to be even greater.
“Given the current available information, it is clear that cover crops are more likely to have net positive water impacts during years with significant and intense precipitation; in fields where soils demonstrate poor infiltration, are prone to compaction or crusting, and where fields are sloped; and when cover crops are managed for reduced water use,” the report states.
COVER CROPS AND SGMA
There is growing concern that the water supply pressures tied to SGMA implementation could hinder the adoption of cover crops in California. This is mainly because GSAs, which are responsible for managing groundwater resources, often account for the water use of cover crops but overlook their water-related benefits. GSAs make these decisions when determining and tracking elements of their water budgets, which is required under SGMA.
GSAs often rely on assumptions to streamline processes and address knowledge gaps, but certain assumptions can pose challenges for cover cropping and even compromise the overall accuracy of water budgets. For instance, assumptions such as negligible evaporation from bare ground, negligible runoff, and a fixed percentage of precipitation percolating into groundwater may not fully reflect real-world conditions. Additionally, some crediting programs require growers to maintain bare ground in annual cropping systems to qualify for water-use savings credits from fallowing, further complicating the adoption of cover crops.
Estimating water use via satellite ET for winter cover crops is problematic. While much is known about the margins of error of satellite ET during the growing season, much less is known about how increased cloud cover during the winter impacts the accuracy of ET estimates. Furthermore, GSA methodologies for converting satellite ET data or flowmeter data into estimates of consumptive use of groundwater are variable and not always rigorous. Approaches that can capture both the water impacts and the water benefits of cover crops are vital for managing California’s groundwater, especially in the face of increasingly extreme weather events.
RECOMMENDATIONS
The report makes several recommendations to support SGMA implementation and promote cover cropping as a sustainable agricultural practice. Key actions include funding and coordinating research to better understand cover crops’ net water impacts, collecting grower data, and developing guidance for water-efficient cover cropping practices.
GSAs should receive technical assistance and best practice methodologies for converting evapotranspiration (ET) and precipitation data into accurate groundwater use estimates. Improved data quality and distribution, such as expanding CIMIS stations or creating an eddy covariance tower network, are essential for effective water management.
Additionally, tracking cover crop adoption through spatial datasets and applying strategies used for natural lands can help integrate cover crops into GSA plans.
To ensure successful implementation, short-term funding should support initiatives like guidance development, while long-term funding should provide technical assistance and data provision. Addressing structural limitations in GSA fee structures is critical for raising adequate funds.
These recommendations aim to remove barriers to cover crop adoption, accurately account for their water impacts, and improve GSA water management in California’s changing climate.
NEW GUIDANCE FOR GROWERS
Sustainable Conservation and the Community Alliance with Family Farmers (CAFF), in partnership with UC ANR and other collaborators, have introduced a new guide to help farmers maximize the benefits of cool-season cover crops while minimizing water use. As growers face an evolving water landscape, this guide offers practical advice on choosing the right cover crop species, optimizing planting and management timing, and employing practices to reduce runoff, improve infiltration, retain water, and enhance soil health.
This resource is part of a broader collaboration among agricultural, research, and conservation groups to support California farmers in addressing the challenges of climate change and water shortages. The guide covers key aspects of managing winter cover crops, including selecting drought-tolerant species, effective planting and establishment techniques, timing strategic terminations, managing residues, and promoting overall soil health for multiple benefits.
CONCLUSION
Cover cropping has many benefits vital to California’s sustainable agricultural future; however, current patterns in the implementation of SGMA may create unintended barriers to realizing those benefits. The net impacts of cover crops on water budgets are highly context-dependent, and research is active, but there are clear cases demonstrating that even in California’s San Joaquin and Sacramento Valleys, it is possible to implement them with negligible impacts on water use.
“The water-related benefits of cover crops, such as their ability to slow down and capture more water during precipitation events, are only becoming more important as California faces more extreme weather events. Our understanding of the conditions under which they have the lowest net water use (and even potential water savings) is rapidly increasing. As we reduce barriers to the implementation of cover crops, this management tool can be utilized to its true potential: as one of many in a toolkit supporting the health and sustainability of California’s agriculture, environment, and communities in a rapidly changing future.”
