Abstract:Shallow groundwater is an important source of water for the maintenance and restoration of ecosystems in arid environments, which necessitates a deeper understanding of its complex spatial and temporal dynamics driven by hydrological processes. This study explores the dominant hydrological processes that control the shallow groundwater dynamics in the Gobi Desert-riparian-oasis system of the lower Heihe River, a typical arid inland river basin located in northwestern China. The groundwater level and temperature were monitored in 14 shallow wells at 30-min intervals during the 2010-2012 period. After combining this information with meteorological and hydrological data, a comprehensive analysis was conducted to understand the dynamic behaviour of the shallow groundwater system and to determine the dominant factors that control the groundwater flow processes. The results of the study indicate notably large temporal and spatial variations in both the groundwater level and temperature. Noticeable fluctuations in the groundwater level (0.5-1 m) and temperature (4-8 C) were observed in the riparian zone, evidencing a clear river influence. In comparison, the groundwater fluctuations in the Gobi Desert were more stable (the annual variations of the water table were less than 0.5 m, and the water temperature varied by no more than 2 C). Strong variations in the groundwater table (1.5-5.0 m/year) and temperature (1.5-6.5 C), mainly caused by surface flood irrigation and groundwater pumping, were observed in the oasis area. The investigated sites were categorized into three types that reflect the dominant hydrological processes: (1) the riparian zone, dominated by riverbank filtration and groundwater evapotranspiration; (2) the Gobi Desert area, controlled by groundwater evaporation and lateral recharge; and (3) the oasis area, dominated by groundwater evapotranspiration as well as surfacegroundwater interactions caused by human activities.

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Groundwater runoff is an important component of river runoff and is sensitive to climate change. However, it is difficult to differentiate groundwater runoff from total river runoff by current base flow separation methods. Here, we propose a physically based model with a two‐pass digital filter for separating groundwater runoff from streamflow time series. The proposed method is based on the fundamental theory of surface–groundwater interactions under a variable groundwater recharge process that is highly associated with river stages. Our results show that the groundwater flow recession constant and the base flow recession constant can be determined separately by performing variogram analysis on long‐term streamflow time series data. For watersheds with areas of 103–104 km2, the groundwater flow recession constant differs from the typical values of the base flow recession constant by an order of magnitude or more. Therefore, the developed method can be used to quantify groundwater runoff dynamics on a temporal scale spanning tens of years. Unlike other approaches to base flow separation, our proposed method can reveal the effects of climate change on groundwater runoff.

The balance structure of the pumpage sourses of riverside water-intakes, developing a subsoil aquifer or intermediate water that hydraulically interacts with it, can show the effect of the processes of water balance adjustment in the unsaturated zone to the accompanying subsoil water level drawdown. In this case, because of the shallow depth to subsoil water, its level drop due to water withdrawal causes a decrease in evapotranspiration and an increase in groundwater infiltration recharge. These processes have their effect on the balance structure of usable water resources as components of natural and involved resources and reduce the impact of groundwater pumping on river flow. Analysis of the operational data of the Sudogda waterintake in Vladimir oblast and geohydrological modeling were used to evaluate variations of the groundwater evaportanspiration losses and infiltration recharge and their role in the water balance structure of reserves of a field and in the impact of groundwater withdrawal on river flow.