Establishing an almond water production function for California using long-term yield response to variable irrigation

The consumptive use of water by almond trees in California has become a controversial topic due to a worsening drought that began in 2011. The research reported herein was designed to provide information that could be used in irrigation decision-making including how to evaluate the risks/rewards of seeking/acquiring additional supplies in a drought. Ten irrigation levels ranging from 1000 to 1350 mm per season were applied in a mature southern San Joaquin Valley 'Nonpareil' almond orchard over a 5-year period. Nut load was invariant over the range of irrigation regimes with no evidence of any sustained negative impacts. Individual kernel weight was reduced commensurate with the level of deficit irrigation; the lowest irrigation resulted in a 15% lighter kernel. Maximum yield (3900 kg/ha) was achieved with 1250 mm of applied water. There was statistically significant tree stress in the 1250-mm application regime with midday shaded leaf water potential about 0.2 MPa more negative than trees receiving additional water. Maximum marginal water productivity (WPM) was 0.30 kg/m(3) at 1080 mm of applied water, falling to zero at 1290 mm. With almond prices of $8/kg, this is equivalent to water valued at $2/m(3). Estimated annual statewide almond orchard evapotranspiration based on this study is 1100-1350 mm which is about 25% higher than estimates made four decades ago. However, yields have increased in the 250% range since that time. This tenfold difference between yield and evapotranspiration means that WPM is much higher now than in the past, and at current prices, it demonstrates that almond trees are suited for high water cost areas.