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In semiarid and arid regions, the evaporation from bare soil is highly sensitive to changes in the depth to the water table. This study quantifies the relation between water-table depth and the groundwater contribution to evaporation in the Ordos Basin in China. In-situ field experiments were combined with numerical simulations of heat, vapor and liquid water flow. Based on lysimeter experiments and a calibrated numerical model, a relation between depth to groundwater and evaporation rate was established for the lysimeter site. In addition, a sensitivity analysis considering the hydraulic conductivity and the inverse of the air-entry pressure (van Genuchten α) was established. For the field site, the results showed that for the water-table depths less than 52 cm below the ground, evaporation is independent of the water-table depth. For water-table depths exceeding 52 cm, an exponential relation between depth to groundwater and evaporation is observed. No phreatic evaporation occurs for water tables deeper than 105 cm, which is nearly two times the capillary fringe height. The sensitivity analysis showed that the extinction depth decreased with decreasing hydraulic conductivity and increased with α. The field-specific results and the sensitivity analysis provide valuable information to understand the dynamic processes of soil evaporation in the Ordos Basin. From a methodological point of view, the proposed modelling approach and the integration of lysimeter data proved to be a highly efficient combination to study evaporation dynamics in semi-arid and arid environments.

The interaction of surface water (SW) and groundwater (GW) is becoming more and more complex under the effects of climate change and human activity. It is of great significance to fully understand the characteristics of regional SW&ndash, GW circulation to reveal the water circulation system and the effect of its evolution mechanism to improve the rational allocation of water resources, especially in arid and semi-arid areas. In this paper, Yinchuan Plain is selected as the study area, where the SW&ndash, GW interaction is intensive. Three typical profiles are selected to build two-dimensional hydrogeological structure models, using an integrated approach involving field investigation, numerical simulation, hydrogeochemistry and isotope analysis. The SW&ndash, GW transformation characteristics are analyzed with these models, showing that geological structure controls the SW&ndash, GW interaction in Yinchuan Plain. The SW&ndash, GW flow system presents a multi-level nested system including local, intermediate and regional flow systems. The runoff intensity and renewal rate of different flow systems are evidently different, motivating evolution of the hydro-chemical field, human activities (well mining, agricultural irrigation, ditch drainage, etc.) change the local water flow system with a certain impacting width and depth, resulting in a variation of the hydrological and hydro-chemical fields. This study presents the efficacy of an integrated approach combining numerical simulation, hydrogeochemistry and isotope data, as well as an analysis for the determination of GW-SW interactions in Yinchuan Plain.

Vegetation plays an active role in soil water dynamics and water balance in groundwater-soil-plant-atmosphere continuum systems. Therefore, we characterized soil water transport at depths of 0–4.0 m induced by root water uptake (RWU) at a groundwater-dependent Salix site during the whole growth stage and compared the results with those from a non-vegetated soil site, in the semiarid Ordos basin of China. The results showed that: during the whole experimental period, evapotranspiration at the vegetated site was more than twice evaporation at the bare site, indicating that the transpiration exceeded the evaporation and was the main output of soil water. For the bare site, the increase of soil water storage in the lysimeter was 97.9 mm in total, accounting for 27.5% of the precipitation, with 16.8% of rainfall as retention in the shallow vadose zone and 10.7% of rainfall as seepage below 150 cm depth. Conversely, in the vegetated site, rainfall-driven soil water was mainly consumed by vegetation, and the decrease of soil water storage was 254.4 mm in total, with no seepage being observed. Although the root system as the preferred channel improved the soil infiltration capacity, it did not mean that the deep soil water recharge increased. The infiltrated rainfwater could not offset the water consumption of the root system. An exponential normal composite RWU model obtained by the reverse method showed that the water deficit soil layers caused by RWU changed the soil water flow field and blocked soil water interchange. Consequently, deep soil water and groundwater could not be replenished by precipitation. Our results emphasize ascertaining the hydrodynamic processes of vegetation in semiarid regions and help to strike a balance between vegetation restoration and groundwater management.

In regulated rivers, many studies have found that ecological water conveyance was conducive to protection and restoration of riparian plants. However, most of the studies about the influence of water conveyance on plants did not consider river-groundwater relationship in regional scale. In this study, field investigation, dynamic observation, Canonical Correspondence Analysis and ANOSIM analysis were mainly used to explore the lateral zonation of plant composition and diversity among different river-groundwater relationship types and reveal its reasons. The obtained results showed that: Spatio-temporal change of river-groundwater relationship controlled the difference of lateral zonation of species composition and diversity. Within the groundwater fluctuation zone, the water-table depth and salinity decreased during flood season. The proportion of hygrophyte herb and mesophyte shrub species is large with the highest diversity. If the river was gaining river during dry season, the riparian water table depth was less than 4–5 m, the strong evaporation-crystallization process lead to Cl-, Na+, and SO42- enriched. The total dissolved solids (TDS) was more than 6–10 g/L and the plants degenerated to halophyte herbs and shrubs with low diversity outside the groundwater fluctuation zone. If the river dry out during dry season, the riparian water table depth was more than 6–10 m. Plants were under water stress and degenerated to xerophyte herbs and mesophyte and xerophyte shrubs with low diversity outside the groundwater fluctuation zone. The study can enrich the understanding of riparian plant distribution pattern and provide guidance for making appropriate water conveyance scheme.

Soil water and groundwater are important water resources for vegetation survival and growth in arid and semiarid regions, and their contributions to water use over a whole growth period are difficult to quantify with limited observations. Additionally, the root water uptake (RWU) processes of groundwater-dependent xeric vegetation are specific and aggravate the complexity of these issues. In this paper, Salix is selected to study the interaction among atmosphere, soil water, groundwater, and RWU from the sprout to withered stage based on an observation within the ‘In situ Monitoring Lysimeter System of the Atmosphere-Plant-Unsaturated Zone-Groundwater Continuum’. Two lysimeters with a diameter of 2 m and depths of 1.2 m and 4.2 m were implemented with Salix. The initial water table depth was 0.7 m in case 1 and 2.2 m in case 2. The results show that: Water use of Salix is the main contribution to actual evapotranspiration (ETa), no less than 80%. ETa was 462.75 mm in case 1 with a shallower soil layer and limited groundwater, during the entire growth period, 2016. Contrastively, it was 619.70 mm in case 2 with relatively abundant soil water and groundwater. The soil water and groundwater contribution were 89.6% and 10.4%, 74.3% and 25.7% for both cases. The selective water use by Salix in different soil layers was subject to temporal moisture availability. Salix extended to utilize groundwater in drought period and shifted back to wetter upper layers in case of rainfall. The access to groundwater significantly alleviates water stress, although rainfall replenishment can also play a part. Salix can not only intercept rainfall infiltration in the root zone, but also cause the decrease of water level, implying that the replanting of Salix should be carried out in a reasonable mode to ensure rainfall infiltration and reduce the ineffective loss of groundwater. Our results provide a practical reference for groundwater management and ecology restoration in semiarid desert regions.