Abstract: A field experiment was conducted in Hexi oasis of Gansu Province to study the effects of different irrigation amount (conventional irrigation, 330 mm; 20% watersaving irrigation, 264 mm; and 40% water-saving irrigation, 198 mm) and N application rate (0, 40, 80, and 120 kg N·hm^-2) on the plant height, hay yield, and water use efficiency (WUE) of Medicago sativa. Under the experimental condition, both irrigation and N fertilization had no significant effects on the plant height of M. sativa. However, irrigation had significant effects on the hay yield, which was increased with increasing irrigation amount, being 7232, 7603, and 7796 kg·hm^-2 under 40% water-saving irrigation, 20% water-saving irrigation, and conventional irrigation, respectively, and there existed significant differences between the treatments. Irrigation also had significant effects on the WUE of M. sativa, which was decreased with increasing irrigation amount, being 15.56, 13.86, and 12.60 kg·hm^-2·mm^-1under 40% water-saving irrigation, 20% water-saving irrigation, and conventional irrigation, respectively, and the difference between the treatments was significant. When the N application rate was 40 kg N·hm^-2, the total hay yield (8223 kg·hm^-2) and the WUE of M. sativa(15.18 kg·mm^-1·hm^-2) reached the maximum, and the increment of hay yield and WUE was 15% and 14%, 16% and 14%, and 7% and 8% higher than that when the N application rate was 0, 80, and 120 kg N·hm^-2, respectively. From the viewpoints of economic return, ecology, and environment, it was considered that 40% water-saving irrigation combined with 40 kg N·hm^-2 N supply would be the best choice for the first year of M. sativa  planting in Hexi oasis of Gansu Province.

Key words: Taihu Lake, Non-point sources pollution, Phosphorus, Quantification