Regional Alfalfa Yield, ETc, and Water Value in Western States

Alfalfa hay yield, yield-consumptive use ratio (Y/ETc), and hay price across a range of rainfall and evapotranspiration conditions of the western states is evaluated to determine alfalfa hay water value or benefit. Included is a determination of long-term mean values and variability of yield, Y/ETc ratios, and associated irrigation water values. These are compared with published hay water-use efficiencies, production, and water costs. Available rainfall, reference evapotranspiration ET0, hay yields, and prices for counties in Arizona (1987–1999), California (1998–1999), and Idaho (1993–1999) were used. Alfalfa hay Y/ETc ratios decrease with increasing ETc, although their variability increases with increasing ETc. The greatest Y/ETc ratios (16–17 kg/hamm) and irrigation water values IW$ (IW$ = $2,800–$3,000/ha-m), with relatively moderate variability, are associated with an irrigation water IW requirement of ~800 mm, reflecting a combination of relatively high hay values, ETc, and beneficial rain. Although this IW$ is twice that of water delivery prices below the California delta and is comparable with average municipal water costs of $4,000/ha-m for large western cities, the average is nearly 1/3.