Evapotranspiration characteristics and soil water balance of alfalfa grasslands under regulated deficit irrigation in the inland arid area of Midwestern China

Abstract

Drought has been a major limiting factor affecting agricultural production in the world. Accurately quantitative analysis of the dynamic changes of evapotranspiration and soil water balance is very important for effectively managing water resources and improving water use efficiency under water scarcity. Soil water balance for perennial forage is more complex than the annuals due to stronger roots and periodic evapotranspiration. The objective was to explore the evapotranspiration characteristics and soil water balance of alfalfa (Medicago sativa L.) grasslands under border irrigation at different stand ages and irrigation treatments. A 3-yr field experiment with a full irrigation and 6 regulated deficit irrigation treatments was conducted, and growth-related indicators, soil evaporation and soil water dynamics were observed regularly. Daily evapotranspiration and soil water balance were computed using a modified dual-Kc model which is able to simulate evaporation and transpiration separately. The results showed that soil water simulated by the model and the measured value were in good agreement, and regression coefficients were higher than 0.6 and close to 1.0 in most cuts. The change trend of root zone water under deficit irrigation at single growth stages was similar to that under full irrigation. Under irrigation, the transpiration was the main water loss of alfalfa grasslands. The daily actual evapotranspiration of alfalfa in the 1st cut was higher than 8.0 mm d−1 and even over 15.0 mm d−1 at the middle and later growth stages. In contrast, in the 2nd and 3rd cut, it was always less than 10.0 mm d−1. The daily actual evapotranspiration decreased significantly and immediately after cutting, and daily actual transpiration was reduced to less than 1.0 mmd−1. The change trend of daily actual evapotranspiration under deficit irrigation at single growth stages was also similar to that under full irrigation. The seasonal evapotranspiration could reach 800 mm and was reduced by deficit irrigation. The average proportion of soil evaporation to total evapotranspiration was 18%. With the decrease of irrigation amount, the evapotranspiration, transpiration and irrigation water compensation decreased, while evaporation remained relatively stable and the ratio of evaporation to evapotranspiration increased. A lower relative evapotranspiration deficit (0.041) was obtained under moderate deficit irrigation at the budding stage than those in other deficit cases (0.086 or over). In conclusion, in the arid and semi-arid areas, deficit irrigation could be applied to alfalfa grassland at the budding stage in alfalfa production.