Effect of water table depth on soil water and salt dynamics and soil salt accumulation characteristics under mulched drip irrigation

Abstract: Water scarcity and soil salinization are 2 of the most serious problems in the arid areas of Northwest China with extremely low precipitation and very high evapotranspiration. Mulched drip irrigation (MDI) has been introduced to save water and promote crop yield and has made significant benefits on local economy and ecology in recent 20 years. But the effect of MDI on soil salinization is still unclear and people even got contrary results about the effect of long-term MDI use on soil salinization. The situation becomes more serious considering the increase of water table depth as the large-area expansion of MDI in recent years. In order to explore the dynamics and accumulating characteristics of soil water and salt in MDI fields under deep water table depth, in situ observation was carried out in a cotton field in Korla, Xinjiang from 2012 to 2016. The water and salt profiles of 150 cm, the variation of soil water and salt storage with water table depth and exchange water flux at the depth of 90 cm calculated with water balance method were analyzed and compared with other studies. The results showed that the soil moisture profile presented the opposite S shaped distribution with an increase trend in the depth of 0-50 cm and a decrease trend in the depth of 50-150 cm. The soil salinity profile presented the "red wine cup" distribution with soil salt accumulated in the surface layer of 0-40 cm and it didn't change much below 40 cm, which means that the soil salinity was in a new state of equilibrium and wasn't affected by groundwater. We divided the whole study period into 4 periods, i.e. seedling period, drip irrigation period, harvesting period and non-growing period according to the main affecting factor on soil salinization. During the experiment period, the water table depth increased from 2-3 to 5-6 m, and the returning of salt to the surface was significantly reduced at the seedling and non-growing periods. More salinity was flushed in the drip irrigation period. And the phenomenon that soil salt decreased at the harvesting period still existed. The average salinity at 150 cm depth decreased from 6.5 to 1 g/kg in the 5 years. The soil salinity showed a negative exponential relationship with the water table depth with the correlation coefficient of 0.8. It indicates that the groundwater level is the main factor affecting soil salinity. But other studies found that water table depth and groundwater salinity both altered soil salinity in shallow groundwater conditions, which indicates that the relationship between soil salt and groundwater exhibited a new relationship under deep water table condition. The exchange water flux showed that the relationship between soil water and the groundwater was significantly weakened with the increase of water table depth. It is suggested that the water table depth in similar areas should be controlled at about 3.5 m according to the relationship of soil salinity and groundwater and phreatic water evaporation. At this water table depth, MDI combined with winter and spring irrigation can effectively inhibit salt accumulation in the soil layer and ensure the ecological water demand of natural vegetation.