Spatio-temporal dynamics and eco-hydrological controls of water and salt migration within and among different land uses in an oasis-desert system.

Identifying the eco-hydrological processes associated with water-salt dynamics is important for the sustainable management of water resources and eco-environmental systems in groundwater-dependent ecosystems, especially across different land use types in salt-affected oasis-desert ecosystems. In this study, a typical cropland-shelterbelt-desert site at the oasis-desert system in the Sangong River watershed of northwestern China was selected to investigate the spatio-temporal variations of water-salt dynamics using the Spearman rank correlation analysis and water/mass balance analysis, and to identify the response of vegetation dynamics to water-salt variations based on a model framework for vegetation-salinity-groundwater interactions, within and among these land uses during crop growth period (CGP: April 1-June 28, 2018) and non-crop-growth period (Non-CGP: June 29-October 31, 2018). Results showed that the soil water content (SWC) and soil electrical conductivity (SEC) had clear vertical stratification, horizontal transition and seasonal fluctuation characteristics during both CGP and Non-CGP. Significant differences in groundwater depth and salinity were exhibited between both study periods. The water exchange flux (WEF) and salt exchange flux (SEF) in both the cropland and shelterbelt were closely related to irrigation events and evidently higher than that in desert. The cropland maintained a salt accumulation state (especially at the >60-80 cm soil layer) during CGP. Hydrological links and salt transport processes among adjacent land uses have been weakened owing to the application of water-saving irrigation in cropland and the significantly declined of regional groundwater tables. Groundwater pumping and lateral groundwater flow (LGWF) were the most important media for water-salt exchange in the site. The interactions of vegetation with both the soil water-salt balance and groundwater dynamics may cause a discontinuous and irreversible ecosystem response to changes in land use or environmental conditions. Anthropogenic processes, especially the development of modern water-saving irrigation agriculture with groundwater-fed, are dominating the vegetation-salinity-groundwater interactions and its ecohydrological consequences in this ecosystem. Adaptive management of water and salt migration in soil and groundwater is essential for maintaining the coexistence of oasis-desert ecosystems in arid areas.