## 1 Introduction
The review of nine studies conducted in the west and Midwest of the U.S. showed that averagely 188 mm is required to produce 1 Mg of alfalfa [22]. Some of these studies used older alfalfa varieties and had less frequent cutting schedules than present practices. Although alfalfa has a high-water demand footprint, it has a range of water advantages during times of water scarcity [23]. These include its deep rooting patterns to utilize residual moisture during drought, high water use efficiency compared with other crops, and perenniality which enable quick re-growth without stand establishment. However, alfalfa is particularly valuable for its flexibility to be 'deficit irrigated' when water is unavailable, or partial-season water applications. The fact that multiple harvests of alfalfa are possible enables partial season irrigations to produce some harvest, while conserving water for other purposes, unlike annual crops with a single harvest. The ability of alfalfa to sustain temporary droughts, maintain yields and survive severe irrigation deficits is somewhat unique among productive crops. Three basic deficit irrigation strategies have been proposed for drought mitigation in alfalfa fields [24], including: (1) triage (cease irrigation of some alfalfa fields while fully irrigating others), (2) starvation diet (deficit-irrigate the entire alfalfa field during the whole crop season), and (3) partial-season irrigation (fully irrigate fields the early cuttings, then cease irrigation pathway through the rest of the season). Moderate deficit irrigation (MDI), skipping some of the irrigation events over the summer period, has been proposed as a water conservation tool in California low desert [1]. However, estimates of yield impacts are often lacking for various of these strategies. Yield impact tools, and water conservation estimates are necessary for recommending various strategies for farmers. Deficit irrigation was investigated as a sustainable crop production strategy to maximize water productivity and to stabilize alfalfa yields while conserving irrigation water [24; 25]. A recent study reported that deficit irrigation could be applied to alfalfa at the bud stage when it less affects the yield than deficit irrigations at the regrowth or branching stage in the arid and semi-arid areas [25]. The result of a study conducted in the arid area of Northwest China suggested that the partial root-zone drying could be a promising technique for alfalfa production with improved water productivity and positive effect on quality characteristics [26]. At the field scale, alfalfa crop water use, yield, and water productivity are variable. These variabilities may be attributable to inappropriate irrigation management due to the lack of sufficient irrigation scheduling knowledge and tools, spatial and temporal variations in water availability, agronomic constraints, and irrigation systems and infrastructure limitations. Therefore, knowledge of alfalfa crop water-use, crop coefficients and water deficit impacts may assist in promoting effective irrigation management. Since agriculture is the major water consumer in many regions, adjustments in cropping patterns and practices are likely to be necessary when confronted with genuine water scarcity, as is currently the case in the Colorado watershed. Although wholesale changes in crops and extensive fallowing have been proposed, partial-season deficit irrigation of alfalfa is an important strategy to consider, which could maintain forage production and farm profitability while conserving scarce water resources. The tools that are needed to understand the ability of agriculture to respond to water scarcity are (1) accurate on-farm estimates of total water use and water application practices, (2) yield estimates across soil types that are sensitive to seasonal yield patterns and potential yield penalties from water deficits, and (3) value of improved water application methods that will further improve water productivity under water scarcity.