Available water and wheat grain yield relations in a Mediterranean climate

Wheat ( Triticum aestivum L.) is the principle crop grown in many Mediterranean climate zones around the world, including the 3.35 million hectare dryland cropping region of the Inland Pacific Northwest (PNW) of the United States. Farmers in the low- and intermediate-precipitation areas of the region are often reluctant to plant spring wheat (SW) because grain yields are highly variable compared to winter wheat (WW) after summer fallow (SF). Our objectives were to: (i) assess available water and wheat grain yield relations from well-fertilized dryland field experiments conducted from 1953 to 1957 versus related studies from 1993 to 2005, (ii) compare and compartmentalize available water-use efficiency of WW compared to SW during the 1993–2005 period, and (iii) provide a tool to allow farmers to predict SW grain yield based on stored soil water at time of planting plus expected spring (April, May, June) rainfall. Simple linear regression showed that 10.1 cm of available water was required just for vegetative growth (before wheat reproductive development begins) in the 1953–1957 study ( n = 90 replicated treatments), whereas only 5.9 cm of available water was needed in the 1993–2005 experiment ( n = 175 replicated treatments). In addition to water required for vegetative growth, multiple regression analysis showed that from 1953 to 1957 each centimeter of available stored soil water and spring rainfall (SR) produced 140 and 183 kg grain ha −1, respectively, compared to 150 and 174 kg grain ha −1, respectively, for the 1993–2005 study. Multiple regression further demonstrated in the 1993–2005 studies that April rainfall contributed much less to grain yield than rainfall in May and June for both SW and WW. Winter wheat always produced more grain per unit of available water compared to SW. Data reveal that modern semi-dwarf wheat cultivars begin grain production with 4.2 cm less available water than standard-height cultivars of the 1950s. This, along with improved agronomic management, is a major contributor to ever increasing wheat grain yields during the past 50 years.