Mechanisms and feedbacks for evapotranspiration-induced salt accumulation and precipitation in an arid wetland of China

Abstract

Salt dissolved in soil water is transported upward to the soil surface through capillary rise from shallow groundwater, leading to salt accumulation near the surface and salinization of wetlands in arid regions. However, it is not clear how the mechanism and feedback for evapotranspiration induces salt accumulation and precipitation. Here, we developed a model to simulate the transport of water, solutes, and heat, and measured the meteorological, hydrological, and hydraulic parameters of soil using field experiments to calibrate the model for riparian and saltmarsh wetlands in northwestern China. The results showed that the annual atmospheric precipitation averaged 125.3 ± 10.2 mm in two types of wetlands, the evapotranspiration, depth to the groundwater and soil salinity averaged 587.7 mm yr−1, 85.4 ± 5.4 cm and 29.80 g kg−1 in the riparian wetland, while 637.2 mmyr−1, 129.7 ± 15.1 cm and 63.64 g kg−1 in the saltmarsh wetland, respectively. We found that the flux of liquid water, flux of water vapor, salinity, and efflorescence had maximum values of 1.23 mm day−1, 0.22 mm day−1, 104.16 g kg−1, and 2.10 cm in the riparian wetland, respectively, versus 1.28 mm day−1, 0.32 mm day−1, 202.02 g kg−1, and 3.70 cm in the saltmarsh wetland. Our simulations show that the salinity increases significantly with increasing evapotranspiration, soil temperature, saturated water content, and groundwater depth in the saltmarsh wetland, but that the effects of saturated water content and groundwater depth were small in the riparian wetland. As a result of these factors, the efflorescence pattern exhibited characteristic seasonal and inter-annual variability in which complex interactions among atmospheric precipitation, evapotranspiration, groundwater, and river water provided the long-term driving forces for water flow and salt transport. We found that the increasing efflorescence disrupted evaporation more than subflorescence, which reduced the soil porosity and possibly affected water vapor transport.