Global Isotope Hydrogeology―Review

Summary In order to predict future hydrological trends, it is important to understand those that have occurred in the past. In this study, the entire area of China was divided into 10 basins to analyze the country’s hydrological trends. 317 weather stations with data from 1956 to 2005 were selected for trend analysis. Based on the observations, we found that precipitation has decreased in the north but has increased in the south due to weakening monsoon winds over the past 50 years. In the same period, the pan evaporation decreased as a whole but it was not uniform in either timing or spatial dimension. With the stepwise regression method, we concluded that precipitation had a greater impact on pan evaporation in the north than in the south. Furthermore, sun duration had a more significant impact on hydrological processes in the south than in the north. On the assumption that water storage and water use for irrigation has not changed, which is reasonable when the actual evaporation is decreasing, the actual evaporation estimated by the water balance equation showed a decreasing trend in both the north and south. Using complementary relation curves, we concluded that the decrease in the north was caused by the decrease in precipitation while the decrease in the south was caused by a decrease in potential evaporation.

Understanding the Hydrological Trends of River Basins in China

In this study, we incorporated a groundwater exploitation scheme into the land surface model CLM3.5 to investigate the effects of the anthropogenic exploitation of groundwater on land surface processes in a river basin. Simulations of the Haihe River Basin in northern China were conducted for the years 1965-.2000 using the model. A control Simulation without exploitation and three exploitation simulations with different water demands derived from socioeconomic data related to the Basin were conducted. The results showed that groundwater exploitation for human activities resulted in increased wetting and cooling effects at the land surface and reduced groundwater storage. A lowering of the groundwater table, increased upper soil moisture, reduced 2 m air temperature, and enhanced latent heat flux were detected by the end of the simulated period, and the changes at the land surface were related linearly to the water demands. To determine the possible responses of the land surface processes in extreme cases (i.e., in which the exploitation process either continued or ceased), additional hypothetical simulations for the coming 200 years with constant climate forcing were conducted, regardless of changes in climate. The simulations revealed that the local groundwater storage on the plains could not contend with high-intensity exploitation for long if the exploitation process continues at the current rate. Changes attributable to groundwater exploitation reached extreme values and then weakened within decades with the depletion of groundwater resources and the exploitation process will therefore cease. However, if exploitation is stopped completely to allow groundwater to recover, drying and warming effects, such as increased temperature, reduced soil moisture, and reduced total runoff, would occur in the Basin within the early decades of the simulation period. The effects of exploitation will then gradually disappear, and the variables will approach the natural state and stabilize at different rates. Simulations were also conducted for cases in which exploitation either continues or ceases using future climate scenario outputs from a general circulation model. The resulting trends were almost the same as those of the simulations with constant climate forcing, despite differences in the climate data input. Therefore, a balance between slow groundwater restoration and rapid human development of the land must be achieved to maintain a sustainable water resource. (C) 2015 Elsevier B.V. All rights reserved.

Effects of anthropogenic groundwater exploitation on land surface processes: A case study of the Haihe River Basin, Northern China

Investigating groundwater and surface water interactions using stable isotopes and hydrochemistry in the Notwane River Catchment, South East Botswana.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018JD028413

Groundwater Isotopes Biased Toward Heavy Rainfall Events and Implications on the Local Meteoric Water Line

Remote sensing information acquisition technology based on UAV has been widely used in the agricultural field. UAV remote sensing platform to obtain crop information technology has the characteristics of high timeliness, high resolution, low cost, rapidity and accuracy, and is one of the important means of obtaining farmland information in precision agriculture. Using UAV remote sensing technology to obtain multispectral images, taking wheat, barley and Chinese cabbage as the research objects, selecting different vegetation indices for image threshold segmentation, and combining the results of the machine learning classification method of the images in the experimental area to determine the crop extraction fractional vegetation coverage (FVC) method. The experimental results show that the FVC of crops can be quickly and effectively extracted by using the threshold classification method of vegetation index (VI) of UAV multispectral images. This method has high accuracy in extracting FVC information for three crops: wheat, barley, and Chinese cabbage.

Field Monitoring of Fractional Vegetation Cover Based on UAV Low-altitude Remote Sensing and Machine Learning

The deeply buried river-connected Xishan karst aquifer (XKA) in western Beijing, China, has been suffering from diminishing recharge for several decades, which in turn leads to the disappearing of spring water outflows and continuously lowering of groundwater level in the area. Thus, it is important to correctly recognize the groundwater recharge and flow paths for the sustainable development of the XKA. To investigate these issues, the hydrochemical and isotopic compositions are analysed for both surface water and groundwater samples collected over an area of about 280 km2. Results show that (a) the river water is characterized by high Na contents; (b) the δ2H and δ18O values in the river water are distinctively higher than those of groundwater samples, after experiencing the long-time evaporative enrichment in the upstream reservoir; (c) the Sr concentrations and 87Sr/86Sr ratios of groundwater clearly indicated the interaction between water and carbonate minerals but excluded the water–silicate interaction; and (d) the groundwater samples in the direct recharge area of the XKA have the lowest Na concentrations and the δ2H and δ18O values. Based on the large differences in the Na contents and 18O values of groundwater and surface water, a simple two-component mixing model is developed for the study area and the fractions of the river water are estimated for groundwater samples. We find that the distribution pattern of the river water fractions in the XKA clearly shows a change of directions in the preferential flow path of the groundwater from its source zone to the discharge area. Overall, our results suggest that the recharged surface water can be a useful evidence for delineating the groundwater flow path in river-connected karst aquifer. This study improves our understanding of the heterogeneity in karst groundwater systems.

Using hydrochemical, stable isotope, and river water recharge data to identify groundwater flow paths in a deeply buried karst system

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Field Monitoring of Fractional Vegetation Cover Based on UAV Low-altitude Remote Sensing and Machine Learning

Using hydrochemical, stable isotope, and river water recharge data to identify groundwater flow paths in a deeply buried karst system