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

Recent reports show a decline in most of the Paso Robles Groundwater Basin since 1997, with levels decreasing greater than 70 feet in sub regions. In addition to the increased water demand in the basin, winter rainfall in 2012 and 2013 was 54% and 72% less than the 60 year average respectively, exacerbating the existing strain on basin capacity. Almost all of the annual rainfall in the Paso Robles AVA falls during the winter and is essential for recharging basin reserves, contributing to plant available water and leaching salts in soil. Emergency ordinances and regulation of water use in the Paso Robles basin in 2013 have reinforced the need for water conservation efforts. Grape producers represent the largest consumer of water in the Paso Robles Groundwater Basin. There is a strong desire for agricultural
producers in the basin to improve rainwater retention and reduce water consumption overall.

Due to the high susceptibility to soil erosion of farmed hillsides during rain events, cover crops are often planted during the winter in vineyards in the Paso Robles AVA. The benefits of cover crops during the dormant season are well-documented, including reduced erosion and nonpoint source pollution, increased soil organic matter and improved infiltration of rainwater. Interviews with the five producer collaborators in this project indicated that erosion control, water infiltration and addition of organic matter to the soil were the top reasons for planting cover crops in their vineyards.

Despite the benefits of cover cropping in vineyards, grape producers are increasingly concerned about net loss of soil moisture through evapotranspiration by cover crops. A Vineyard Team survey of 50 Central Coast grape producers in 2013 indicated that quantity of water available for irrigation of grapes is a top issue over the next five years. Conversations with producers in 2013 suggest that the drop in groundwater levels, recent drought and public perception of agricultural water use has influenced them to consider alternatives to planting cover crops. Consequentially there is a risk of losing cover crop benefits.

Water loss through evapotranspiration by cover crops has been investigated in other winegrowing regions. For example, mowing of inter-row fescue cover crop reduced evapotranspiration by 35-49% immediately after mowing, with a 20% reduction still in effect 19 days after mowing. In another study, vineyard cover crops allowed to complete their growth cycle (vetch & ryegrass) consumed more water (75mm in 36 days) than those stunted with herbicide just before bud break. The residue remaining from a dormant cover crop has also been shown to reduce evaporative losses of soil water in the spring. Cultivated soil between vine rows lost 21.6% more water than those with cover crop mulch residues and water conservation was observed in the surface soil, as well as in the subsoil.

Data driven protocols for cover crop management with the goal of water conservation have not been developed in the Paso Robles AVA. In working with producers in this project, we will evaluate cover crop species for their potential water use and identify those with potential for lower water use and greater infiltration and retention of soil moisture from winter rain. Additionally, we will evaluate several methods for terminating cover crops in the spring to reduce the net loss of basin water to evapotranspiration. In this project we will work with five producers to determine how to best implement cover cropping practices in vineyards while minimizing consumption of stored winter rainwater and improving moisture available to vines, ultimately conserving water in the Paso Robles Groundwater Basin.

1. Evaluate five cover crop species for their impact on water infiltration, soil moisture retention and use of plant available water in the vineyard. November – May, 2015-2016

a. Cover crop species will be compared to a non-cover cropped control in three of five sites using a split-block design with three replicate treatments per block on a 0.5 acre experimental block per site. (November – May)

b. Soil moisture will be logged with sensors at three depths in three replicate treatments per site.

c. Rooting depth and biomass will be measured for each cover crop. (March)

d. Water stress of grapevines will be measured by leaf porometer (stomatal conductance) and pressure bomb (leaf water potential) during pre-irrigation growth period to determine cover crop treatment effects on irrigation commencement date. (April – May)

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. November – May, 2015-2016

a. Treatments to suppress or terminate a cover crop including mowing, chemical mowing (OMRI approved herbicide and a standard systemic herbicide as a control). A cultivation soil treatment, undisturbed cover crop treatment and a non-cropped “fallow” treatment will serve as controls.

b. Suppression and termination of the cover crops will be tested at two of five sites, split-block design with three replicate treatments per block using a split-block design with three replicate treatments per block on a 0.5 acre experimental block per site. (November – May)

c. Additional data will be collected according to methods described in Objective 1, b-d.

3. Perform economic evaluation of vineyard floor management practices and document financial return on investment in terms of potential water savings. May – June, 2016

4. Promote and maximize the adoption of beneficial practices by producers from the findings of this cover crop project and extended through a well-established portfolio of Vineyard Team outreach mechanisms, including tailgate meetings, newsletters, email blast, website, social media, fact sheets, presentations and trade publications. January 2015 – December 2016

5. Measure adoption of water conservation practices identified in this project by survey of the producer audience who participate in tailgate meetings, webinar and online educational module. April 2016 – April 2017