Selecting and Managing Vineyard Cover Crops to Reduce Consumption of Net Basin Water

View the project final report

• 2015 annual report

Commodities

• Fruits: grapes

Practices

• Crop Production: cover crops, irrigation, no-till, water management
• Production Systems: agroecosystems

A decline in the California Paso Robles Groundwater Basin has increased awareness of water conservation practices in vineyards. This project investigated whether and to what degree the depletion of soil moisture over the winter by certain cover cropping practices might affect the quantity of groundwater pumped for irrigation to replace these losses. While cover crops have many benefits, the cost of those cover crops in terms of water use and associated pumping costs to replace soil moisture that may be depleted due to the cover crop is not well understood on the Central Coast. This study consisted of two experiments conducted over two seasons. One experiment investigated whether the timing and method of terminating a grass cover crop affected the depletion of soil moisture over the winter. The second experiment evaluated the effects of cover crop selection (species) on the depletion of soil moisture over the winter. Neither the timing and method of terminating a grass cover crop nor the selection of cover crop species differed consistently in terms of soil moisture depletion compared to clean cultivation or fallow/no-till controls. These outcomes suggest that growers have the freedom to choose cover cropping management practices based on factors other than potential soil moisture depletion in this area during low-rainfall years.

Introduction

Declines in the California Paso Robles Groundwater Basin and several years with very little rain have increased awareness of water conservation practices in vineyards. The planting of winter cover crops is a widely used management practice in vineyards in the Paso Robles area. Cover crops can improve water infiltration from precipitation while also preventing erosion and building organic matter in the soil. However, the cost of those cover crops in terms of water use is not well understood on the Central Coast. If cover crops substantially deplete soil moisture during the winter and early spring, then more water has to be applied through irrigation to make up for that loss.

In 2014, grant funding from Western Sustainable Agriculture Research and Education (Western SARE) was awarded to the Vineyard Team for a two-year study to evaluate cover crop species and management of grass cover crops to reduce net water consumption. The assumptions behind this study were:

1. Different species of cover crops would deplete soil moisture at different rates during the winter and early spring. This would mean some types of cover crops are more desirable to use than others in terms of maintaining soil moisture before the beginning of the growing season.
2. The timing and manner of terminating a grass cover crop would impact how much water was lost from the soil. This would mean some timings or techniques are preferable to others in terms of conserving soil moisture prior to the start of the growing season. This would be important as grasses, such as Blando Brome or cover crop seed mixes that contain a high proportion of grass seed, are popular cover crops in the Paso Robles area.

The outcomes of this study would allow growers to make more informed decisions about what cover crops to plant if they desire the benefits of cover crops while simultaneously conserving soil moisture. The same is true for when and how to terminate a cover crop. These outcomes could then be disseminated to the larger grower community through the existing education and outreach mechanisms and activities of the Vineyard Team.

Two experiments were designed and executed to investigate those assumptions and produce information about what may or may not be advisable cover cropping practices during years of low rainfall and limited irrigation water. The first experiment compared five species of popular cover crops against a clean cultivated control. This experiment was repeated at three different sites in the same general area. The experimental design was a randomized complete block design with three replications each of five treatments and a control. The experiment was executed twice, in consecutive years, using the same design - i.e. the same cover crop was grown in the same plot each year.

The second experiment compared various combinations of timings and methods for terminating a Blando Brome cover crop to an unplanted control. This experiment was also repeated at three different sites in the same general area. The experimental design was again a randomized complete block design with three replications each of the five treatments and a control. The experiment was executed twice, in consecutive years, using the same design- i.e. the method/timing was used in the same plot each year.

1. Evaluate five cover crop species for their impact on water infiltration, soil moisture retention, and use of plant available water in the vineyard.
2. Evaluate treatments to suppress and terminate cover crops and their impact on water infiltration, soil moisture retention, and use of plant available water in the vineyard.
3. Perform economic evaluation of vineyard floor management practices and document financial return on investment in terms of potential water savings.
4. Promote and maximize the adoption of beneficial practices by producers from the findings of this cover crop project and extend through a well-established portfolio of outreach mechanisms including tailgate meetings, newsletters, email blast, website, social media, fact sheets, presentations, and trade publications.
5. Measure adoption of water conservation practices identified in this project by surveys of the grower audience who participate in tailgate meetings, a webinar, or an online educational module.