Evaluation of Crop Coefficients and Evapotranspiration in English Walnut

Abstract. In 1998, crop coefficient (K_c) values for an early-leafing, walnut cultivar grown in the San Joaquin Valley were published. They have been used in combination with short canopy reference evapotranspiration (ET_o) to estimate crop evapotranspiration (ET_c) for irrigation scheduling. Land planted in late-leafing English walnut varieties now surpass early-leafing varieties. There is a need to update walnut K_c, especially in the Sacramento Valley, which accounts for 50% of the statewide production and has about double the rainfall of the San Joaquin Valley where the earlier K_c curves were developed. Research was undertaken to evaluate walnut K_c values for modern cultivars and production systems in the Sacramento Valley. A residual of the energy balance method was used to measure real-time ET_c and K_c over multiple years in two separate orchards with different designs and growing conditions. Net radiation and ground heat flux were measured, and both eddy covariance and surface renewal methods were used to measure sensible heat flux. The residual of the energy balance equation was used to calculate latent heat flux density, which was converted to mass flux density giving ET_c. K_c values were slightly higher in the first half of the irrigation season, but slightly lower in the latter half relative to the older K_c curves. Regional walnut ET_c averages about 5 percent lower with the updated K_c values. In the Sacramento Valley, dormant-season soil moisture storage and in-season rainfall reduces the need for irrigation. Since about 105,000 ha of English walnuts are grown in California, this reduction in seasonal ET_c and irrigation can contribute to higher water productivity in California.