Water Management

Profitable California walnut culture requires supplemental irrigation.

The California Walnut Board has long provided significant financial support, on behalf of the California walnut industry, for research that provides tools for both efficient use of our limited water resources through scientific irrigation management and improves both surface and ground water quality due to reduced pesticide use within the IPM program.

Efficient Water Use: California walnut growers now have access to crop water use data based upon evapotranspiration and historical climatology information. Those data, combined with a crop coefficient for walnuts, also developed within the California Walnut Board’s research program, are being used, along with cultural improvements that avoid runoff, such as laser levelling and tail water recovery systems, to apply precise amounts of irrigation water walnuts require for best productivity and quality. The research technology growers are implementing include: 

  • Using Pressure Chambers: The pressure chamber (see Fig.6) is a plant based tool that measures tree water status in real time. Pressure chamber guidelines, developed through California Walnut Board research funding, are now widely used by California growers to determine optimal irrigation timing avoiding wasted water applications (see Fig. 6). http://cetehama.ucanr.edu/files/20516.pdf and https://anrcatalog.ucdavis.edu/pdf/8503.pdf


Fig. 6. Pressure chamber diagram and use in the field to determine tree water status.


  • Irrigation Scheduling Using “water budgets”: A water budget combines day by day tree water use data and soil moisture depletion information based upon tree water use (from California Walnut Board’s  research program) to develop irrigation management strategies for those walnut growers with traditional irrigation systems (flood, furrow, etc.). Throughout the season, a water budget provides growers with information: 1) when to apply water and 2) how much to apply to avoid inadequate or wasteful irrigation and runoff.
  • Using Low Volume Irrigation (Fig. 7): Low volume irrigation systems (see Fig. 8) are designed to frequently apply the water walnut trees require in amounts not exceeding a soil’s infiltration rate. Low volume irrigation systems avoid runoff and water waste. Low volume irrigation systems, very popular with California walnut growers, are efficient, reduce  water costs while being environmentally friendly, and, when combined with pressure bomb use and evapotranspiration data, provide optimal water management for best productivity and quality.
Fig. 7. Walnut orchards being irrigated with buried drip and low volume irrigation systems.


  • Re-use of Water: The vast majority of walnut growers laser level orchard sites and/or use tail water return systems to mitigate runoff of irrigation waters. Not only does this make efficient use of limited supplies, it avoids runoff that may potentially contain fertilizers or pesticides to contaminate our water ways.
  • Water Quality: California walnut growers are concerned about environmental degradation of surface and ground water quality, particularly nitrate leaching and pesticide contamination, as they farm.

Walnut farmers are reducing pesticide use: The California Walnut Board’s research funding commitment to IPM has developed pesticide alternatives: pheromone mating disruption of codling moth, less disease prone cultivars and resistant rootstocks, refined application technologies for blight, biological control of aphids, alternatives to fumigation, and harvest management technologies. These practices significantly reduce contamination of our water ways.

  • Walnut Blight Management: Walnut blight (see Fig. 8), a major foliar and fruit bacterial disease of walnut, has traditionally required multiple copper/Mancozeb bactericidal sprays in spring for control. California Walnut Board funded IPM research has provided growers with an effective program that reduces spray numbers required for control. Reduced sprays mean less potential for runoff into our water ways.
Fig. 8. Walnuts infected with walnut blight


  • Walnut Breeding Program:  Blight control is also achieved by a breeding program that has developed very productive varieties (e.g. “Chandler”) that leaf out in late spring which, coincidently, avoid rain exposure that encourages blight infection. The result is significantly less copper applications required of Chandler than traditional varieties reducing potential for water contamination. Chandler is now the leading walnut cultivar grown in California.
  • Using Nitrogen (N) budgeting: The nitrogen budget process accounts for non-fertilizer sources of N and adjusts supplemental fertilizer applications accordingly. (Univ. of Calif. Ag. And Natural Resource Publication 21623). Using leaf tissue analyses to determine N needs, California walnut growers determine how much supplemental nitrogen is required, accounting for N removed by the crop, contributions from irrigation water (using well/surface water analyses), contributions from composts and manures, and contributions from cover crops within the orchard. This technology avoids leaching excess N into the ground water and movement off the farm into waterways.
  • Respecting Ground Water Protection Areas (GWPA – Bianchi, M., Ground Water Protection Areas and Wellhead Protection Draft Regulations for California Agriculture. University of California Agriculture and Natural Resources publication # 8063.): http://www.cdpr.ca.gov/docs/emon/grndwtr/gwpa_locations.htm. Walnuts are largely grown on lighter alluvial soils that potentially facilitate pesticide leaching. Many are classified as GWPAs. Certain pesticides require special permits if applied in these areas. The walnut IPM program funded in part by the California Walnut Board, has developed alternative pest control technologies that negate leaching issues. Unused wells are capped to protect accidental pesticide movement into ground water.
  • Integrated Pest Management (IPM): Pesticides are known to runoff land to contaminate water. The California Walnut Board’s research program focuses on a pesticide reduction program, enhancing biological control and use of low environmental risk pesticides that are environmentally sound posing less risk to our water resources.