Linking Plant Water Use to Field-Level Water Efficiency Under Surplus and Deficit Irrigation: A Case Study in Almond Orchards.

Water-use efficiency (WUE), defined at the plant level as the amount of carbon assimilated per amount of water transpired, and depth of water uptake in almond trees under varying degrees of deficit and surplus irrigation was examined. Three different orchards, spanning a latitudinal gradient (35^o to 39^o N), were sampled during two growing seasons in the central valley of CA.  The orchards, located in Kern, Merced, and Tehama counties, encompass a variety of climatic and edaphic conditions, providing an opportunity for comparisons of WUE under contrasting environmental conditions. In each orchard, the control treatment received 100% replacement of water lost to evapotranspiration (ET), while the water deficit treatment received ~70% and surplus treatment received ~110% replenishment of ET, since 2013. Preliminary results based on the analysis of carbon isotope ratios (δ13C) in leaves throughout the 2015 and 2016 growing seasons, revealed a significant change in WUE in all three orchard sites, increasing up to ~20% on average in the deficit irrigation treatment relative to controls. In contrast, trees growing under surplus irrigation had the lowest WUE across all orchard sites. However, the difference in WUE between surplus irrigated trees and control irrigated trees was not always statistically significant. These physiological responses to irrigation level were not reflected in crop yield. Over several years of experimental irrigation, yield was highly variable across sites with no consistent response to treatments. Additionally, isotopic analysis of oxygen (δ18O) and hydrogen (δ2H) isotope ratios of stem, leaf, and soil water has been undertaken to determine the effect of irrigation treatments on the depth of root water uptake. The hypothesis that almond trees can effectively acclimate to water-stress through higher WUE and deeper water uptake than well watered trees will be tested. Ongoing work will elucidate the impact of drought on orchards and other tree-dominated systems.