Determination of an optimum irrigation amount and buried depth of subsurface drip irrigation laterals is significantly important in improving crop yield and irrigation water using efficiency in arid regions. In this study, the effects of three irrigation volumes (15.0, 22.5 and 30.0 mm) and three burial depths (10, 20 and 30 cm) of drip laterals on water consumption, yield and water productivity of alfalfa were investigated in a field trial. The water balance equation and FAO-56 crop coefficient methods were applied to determine the evapotranspiration of alfalfa. The results showed that the alfalfa evapotranspiration estimated by the dual crop coefficient method was closer to the actual measured values. The alfalfa water consumption, yield and water productivity increased significantly (p < 0.01) with the increase in irrigation amount, but the increasing trend turned to decrease (p < 0.05) with the increase of the buried depth of the drip irrigation laterals. During the whole growing period of alfalfa, the total water consumption ranged from 400 to 500 mm, the total yield ranged from 7500 to 12,000 kg/hm2 and the water productivity was 1.80 to 2.50 kg/m3. An optimized irrigation amount of 22.5 to 30.0 mm for an irrigation event with a frequency of 5 to 7 days using buried drip irrigation with an irrigation lateral depth of 20 cm was recommended for alfalfa in the study area.

Crop reduction caused by cryogenesis and drought is a serious and global problem. The environmental stress caused by low temperature and drought during the overwintering stage of forage is the key factor leading to this low yield. In cold and arid grassland, winter irrigation can effectively alleviate the stress of alfalfa during overwintering, improve the survival rate of alfalfa, and significantly increase the yield. However, the water uptake patterns of alfalfa under winter irrigation are not clear, which are important to explore the mechanism of alleviating environmental stress by winter irrigation. In this research, the stable isotope compositions of all probable water sources and alfalfa xylem water were measured after winter irrigation. A graphical method was applied to identify the main soil layers with water uptake by the alfalfa roots. The contribution rate of available water sources to alfalfa xylem water was quantified by the MixSIAR (Bayesian isotope analysis mixing model in R) model. The results indicated that alfalfa absorbed soil water when the soil water content was high enough in the root layer when under high water volume freezing irrigation (irrigation in early winter when soil is freezing) but not under low and medium water volume freezing irrigation. Alfalfa gradually began to absorb soil water on the third day after thawing irrigation (irrigation in late winter when the soil is thawing) and showed different water uptake characteristics under low, medium, and high water volume. Thawing irrigation also accelerated the regeneration of alfalfa.

A field experiment was conducted for the purpose of examining the effects of different combinations of water and fertilizer applications on the water consumption and yields of alfalfa under subsurface drip irrigation (SDI). The results showed that the jointing and branching stages were the key stages for alfalfa water requirement. The water consumption had varied greatly (from 130 to 170 mm) during the growth period of each alfalfa crop. The water consumption during the whole growth period was approximately 500 mm, and the maximum water consumption intensity was 3.64 mm·d-1. The overall changes in water consumption and yields during the growth period of the alfalfa displayed trends of first increasing and then decreasing. The sensitivities of the yields to water changes were much higher than that of fertilizer. The water use efficiency (WUE) of the alfalfa was determined to range from 1.68 to 3.20 kg·m-3, and the rate of growth had ranged from 4.85% to 51.77%. The WUE and rate of growth of the alfalfa indicated the following trend: second crop > third crop > first crop. The results of frequency analysis based on the water-nitrogen-yield regression equation are the following: irrigation amounts of 142~165 mm and nitrogen application of 61~80 kg·hm-2 have a 95% probability of obtaining a hay yield of alfalfa of more than 11903 kg·hm-2. These results suggest that SDI is a promising irrigation method, which can increase the WUE and hay yield of alfalfa under the condition of SDI within an appropriate amount of water and nitrogen fertilizer, and too low or too high water and nitrogen fertilizer will adversely affect the WUE and hay yield of alfalfa.

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