Water Use Requirements of High and Conve~tional Density Walnuts under Localized Irrigation

Crop water use (ET) of third year Chico walnuts under low volume sprinkler irrigation in both high and conventional density plantings were measured at the Kearney Agricultural Center in 1984. Crop yield was also evaluated. Additionally, the yield response to varying levels of pruning of selected high density guard row trees was assessed. On a per tree basis, high density tree peak ET (25 gal/tree/day) was lower than that of conventionally-spaced trees (41 gal/tree/day). Maximum daily ET took place in later July. Although canopy cover, reflected by orchard floor shading, was lower with the conventional density, advective energy transport from the sunlit areas of the dry orchard floor to the tree canopies presumably enhanced the transpiration component of ET. Additionally, the greater percentage of the wetted surface area exposed to direct sunlight undoubtedly increased surface evaporation rates. Both of these processes apparently resulted in the relatively high rate of ET in the conventionally-spaced trees. However, on a per acre basis, ET was lower in the conventional planting due to the lower number of trees. This was reflected by peak crop coefficient (Kp) values of approximately 0.60 and 0.45 for the high and low densities, respectively. Nut yields were 241 and 36 lbs dry in shell/acre in conventional sections, respectively. This was due greater number of fruiting positions and the number the high density planting. the high and to both the of trees in The high density guard row pruning trial showed tha tthe standard practice (main uprights pruned 2 ft) yielded 323 lbs dry in shell/ acre, partially pruned trees (6-8 in off main uprights) yielded 754 lbs dry in shell/acre, and 877 lbs dry in shell/acre were obtained from unpruned trees. Since partially pruned trees had only modestly lower yields, it suggests that this practice, on a long term basis, may be an attractive alternative for sustaining both high yields and tree growth rates.