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Soil water shortage is a critical limiting factor that threaten the growth of vegetation and their sustainability in semi-arid areas. Grasses directly respond to the soil water availability (SWA), and the soil water shortage degree can be studied through plant physiological response thresholds under soil drought stress. In this study, the lower-and-optimal limitation of SWA was evaluated with pots along the soil water gradient by quantifying plant gas exchange parameters. These parameters showed a strong cubic relationship with the relative soil water content (RWC) in four forage species. The lower limitation of RWC was obtained for Onobrychis viciifolia (4.0%–20.2%), followed by Astragalus adsurgens (15.5%–25.5%), Elymus nutans (16.1%–27.2%), and Lolium multiflorum (24.6%–35.8%). The average RWC was of 74.83%, and the water saturation point of water use efficiency (WSPWUE) showed a similar critical scope in the four species: Onobrychis viciifolia (71.2%–82.8%), Astragalus adsurgens (72.1%–84.2%), Elymus nutans (79.3%–85.1%), and Lolium multiflorum (76.7%–85.6%). The highest productivity with the highest availability of water (HPHAW) for Onobrychis viciifolia, Astragalus adsurgens, Elymus nutans and Lolium multiflorum was of 71.2%–82.8%, 72.1%–84.2%, 79.3%–85.1% and 76.7%–85.6%, respectively. The lower limitation of RWC, the maximum water use efficiency (WUEmax) and HPHAW in legumes were lower than those values obtained in gramineae, suggesting that legumes were more susceptible to soil drought stress. We concluded that plant gas exchange parameters acted as indicators for SWA under drought conditions, and represented a valuable tool for identifying SWA threshold response in environmental changes.