Evolution of soil salinity and the critical ratio of drainage to irrigation (CRDI) in the Weigan Oasis in the Tarim Basin

Soil salt accumulation and salt leaching are very important issues for a long-term irrigation-dominated oasis with shallow groundwater in arid land. The critical ratio of drainage to irrigation (CRDI) was defined and determined to help the guide drainage and irrigation practices and maintain sustainable oasis agricultural development. This study aimed to identify the soil salinity evolution under long-term irrigation and to determine the threshold of irrigation to drainage with land-use change and groundwater depth by regression, water and salt balance, and geostatistical analysis. The results showed that the soil salinity in 0–20 cm soil depth has decreased greatly in the Weigan Oasis over the past 60 years, with an annual average decrease of 0.68 g·kg−1 in the Weigan Oasis. The soil salt content generally increases from inside to outside of the region and varies from the upperstream to downstream. In the past 30 years, land-use change has caused a sharp reduction in the salinity in the region, and the process of irrigation and drainage has made a decrease in the total salt of the irrigated district of 207.38 × 105 t. Meanwhile, shallow groundwater depth and salinity are the main factors influencing soil salinization in this fluvial oasis. A CRDI is determined to be 8.20% in the Weigan Oasis. In conclusion, our study provides scientific support for the prevention and control of regional soil salinization, as well as a managerial basis for the rational allocation of water resources in long-term irrigation-dominated oasis.