Assessing the impacts of irrigated agriculture on hydrological regimes in an oasis-desert system

Abstract

Irrigated agriculture greatly affects the hydrological systems and thereby eco-environment and food security in arid and semi-arid regions. However, a comprehensive assessment of the hydrological system dynamics in response to irrigated agriculture is still scarce, especially in oasis-desert systems of arid inland regions. Thus, we investigated the impact of irrigated agriculture on hydrological regimes in an oasis-desert system of northwest China. Temporal variations in water balance and lateral groundwater flow in oasis and desert experimental fields (i.e. oasis plot and desert plot) were quantified using the water balance analysis method. Soil water variations, groundwater dynamics and surface water and groundwater interactions in the oasis-desert system were determined by combining line transect survey methods and stable isotope (18O, 2H) techniques using the USGS MODFLOW software. The results showed that water exchange fluxes and groundwater dynamics were altered by the application of water-saving irrigation techniques and the increase in total precipitation. In particular, the downward total exchanging flux above groundwater table and the outward total lateral groundwater flow reduced significantly in the oasis plot. Spatial heterogeneity and profile variability of soil moisture along the soil water content (SWC) transect significantly reduced. Simultaneously, the depth dependence of SWC in oasis cropland and the deep soil moisture at the edge of desert were improved under water-saving irrigation scenario. Temporal variations in groundwater table depths (GTDs) exhibited three different patterns (i.e. stable, metastable and fluctuating) and maintained a continuously downward trend except for the oasis-desert ecotone along the GTD transect, and an abrupt change was found to have occurred during 2006–2012, prompting changes in the seasonality of groundwater resource availability. Groundwater recharge and discharge processes changed dramatically due to large-scale land reclamation and groundwater pumping. Groundwater depletion is approaching hydrological limits although the process is largely compensated by precipitation and streamflow variability. Water-saving irrigation in conjunction with restriction of land reclamation and adaptation of water management practices is, at least for the present, the most effective way to maintain the coexistence of oasis and desert ecosystems in arid inland regions.