Based on the field survey, this paper analyzes the relationship between groundwater and ecological environment in arid and semiarid areas in northwest China. Groundwater in the arid and semiarid areas is a natural resource with economic and ecological values. The concept of the ecological value of groundwater is firstly presented. This ecological value can be reflected in four aspects: (1) the maintenance of base flow in rivers and areas of lakes and wetland; (2) the supply of physiological water demanded by vegetations; (3) the regulation of soil moisture and salt content; and (4) the stability of the geological environment (such as land subsidence etc.). In addition, the threshold system between the ecological environment and multi-dimensional indices as variations in levels of the groundwater table, groundwater quality, water and salt content in the vadose zone as well as groundwater exploitation, are proposed in the arid and semiarid areas. It is expected that the system could provide a scientific basis and technological support for the regulation of the groundwater status and the maintenance of sustainable development of the ecological environment and utilization of water resources.
Ground soil heat flux, G 0 , is a difficult-to-measure but important component of the surface energy budget. Over the past years, many methods were proposed to estimate G 0 ; however, the application of these methods was seldom validated and assessed under different weather conditions. In this study, three popular models (force-restore, conduction-convection, and harmonic) and one widely used method (plate calorimetric), which had well performance in publications, were investigated using field data to estimate daily G 0 on clear, cloudy, and rainy days, while the gradient calorimetric method was regarded as the reference for assessing the accuracy. The results showed that harmonic model was well reproducing the G 0 curve for clear days, but it yielded large errors on cloudy and rainy days. The force-restore model worked well only under rainfall condition, but it was poor to estimate G 0 under rain-free conditions. On the contrary, the conduction-convection model was acceptable to determine G 0 under rain-free conditions, but it generated large errors on rainfall days. More importantly, the plate calorimetric method was the best to estimate G 0 under different weather conditions compared with the three models, but the performance of this method is affected by the placement depth of the heat flux plate. As a result, the heat flux plate was recommended to be buried as close as possible to the surface under clear condition. But under cloudy and rainy conditions, the plate placed at depth of around 0.075 m yielded G 0 well. Overall, the findings of this paper provide guidelines to acquire more accurate estimation of G 0 under different weather conditions, which could improve the surface energy balance in field.
Groundwater is often the main source of available water, and precipitation is one of the main recharge sources of groundwater in arid and semi-arid regions. This paper studies a fixed dune in Mu Us Sandy Land in China, establishes a numerical model, acquires hydraulic parameters and heat parameters of the vadose zone, and calculates the recharge coefficient based on field observation data and numerical modelling. These measurement results show that the response depths of storm rain are more than 90 cm, while those of small rain events are less than 10 cm. The numerical results show that infiltration depths are 10 cm for small rain and more than 90 cm for middle rain respectively. The lag time of the water content at 90 cm below the surface was 25 h following a middle rain, 18–19 h following a heavy rain, and 16–18 h following a storm rainfall. Groundwater recharge lag times (matrix flow) varied from 11 h to 48 h. Excluded precipitation for groundwater recharge was 11.25–11.75 mm in 1 h when groundwater depth was 120 cm and 15–15.5 mm when 140 cm, showing significant influence in groundwater resource evaluation.
In arid and semi-arid regions, water is a major limited factor to vegetation and environment. To study the influence of membranes on water retention, three homogeneous sand columns with membranes were installed. Keep the height of free water at Scm above the soil surface of those columns for 3 hours. Experiments results showed that membranes have strong influence on water retention in saturated sand columns. The membranes could slower water movements through the columns and reduce water dynamic parameters such as hydraulic conductivity. So it has an important role in water retention in sand soils, especially in arid and semi-arid regions.
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