Study on the retention effect of condensed water at different soil levels in exposed areas of Hoh Xil lake upon water recession Open Access

In September 2011, the natural outburst of Zhuonai Lake in the Hoh Xil hinterland of the Qinghai-Tibet Plateau after the water level had risen continuously for years (Li 1998; Zhang & Xie 2017) became a typical example of lake changes in the Qinghai-Tibet Plateau region in the context of global warming and wetting. The outburst of Zhuonai Lake led to the interconnecting of four lakes downstream, including Kuse Lake, Haidinuoer Lake, and Salt Lake. The interconnecting of these lakes resulted in a sudden reduction of more than 100 km² in the size of Zhuonai Lake (Wang et al. 2008; Yao et al. 2012; Liu et al. 2016), as well as a yearly expansion of the lake surface area after the water level of Salt Lake rose sharply (Xiaopeng et al. 2010; Yao et al. 2018). In 2019, according to an analysis of the meteorological conditions of past years, Salt Lake was found to be possibly at risk of overflowing under extreme rainfall conditions (Liu et al. 2019). In order to avoid the natural overflow of Salt Lake, which could affect six major transportation routes including the Qinghai-Tibet Highway and Qinghai-Tibet Railway, as well as a number of transmission lines and optical cables, the China Geological Survey conducted several expert discussions on the rising water level of Salt Lake. Several special reports were submitted to the Ministry of Natural Resources following the discussions, which attracted the attention of government leaders. In August 2019, after making multi-period predictions on the equilibrium of Salt Lake water level, a diversion project was carried out. After the water was released from the lake, the water level of Salt Lake was effectively controlled and the risk of outburst has been mitigated.

The outbrust of Zhuonai Lake led to the exposure of a large area at the lake bottom, as well as the formation of a new sandy area. Coupled with the abnormal climate and frequent windy weather in Hoh Xil, this has led to the continuous development of natural disasters such as wind and sand storms in the Zhuonai Lake area (Xie et al. 2019). Years of continuous erosion by the mobile cover of wind and sand will cause hazards such as grass degradation. Since the outburst of Zhuonai Lake and the implementation of the diversion project, the surface area of Salt Lake has undergone a process of rapid expansion, to slow expansion, to slow contraction. This process will gradually form a new exposed area of the lake bottom. In order to avoid the formation of a large sandy area in the Salt Lake area similar to that of Zhuonai Lake, it is of great importance to take effective measures to stop further sanding in the early stages of desertification. Based on the principle of ‘natural restoration as the mainstay, artificial restoration as a supplement’, members of the research team conducted a study on soil condensation, water absorption and retention in the exposed sandy bottom of the receding Salt Lake in Hoh Xil in order to support the local ecological restoration and rehabilitation of the Salt Lake area.

Condensed water refers to the liquid water formed by the gaseous to liquid phase change of water vapor in the atmosphere, soil pores, and in the ground and surface soil. It occurs when the surface ground temperature reaches the dew point, a necessary prerequisite for the accumulation of soil water. Natural conditions such as severe day-night temperature differences and the relative humidity of air are other important factors which affect its generation. In regions where water is extremely scarce and experience arid or even hyper-arid conditions, any supplemental water resources may have a positive impact on the local ecosystem. As a stable and continuous water resource, although the amount is low (Kidron et al. 2002; Yan-xia et al. 2010; Hanisch et al. 2015), condensed water plays a paramount role in maintaining the stability of ecosystems in these dry regions. Condensed water is a crucial source of water for some plants, insects, small animals, and biological soil crusts in arid environments. As such, it helps improve the germination rate of plant seeds and effectively reduces water loss due to soil evaporation. So far, many scholars worldwide have carried out a great deal of research on the subject. However, though such work has addressed the mechanisms of formation and their influencing factors (Malek et al. 1999; Escolar et al. 2012), the issue of how to efficiently utilize condensed water is rarely reported.

As one of many water-saving materials, a ‘water-retaining agent’ can absorb hundreds of times or more of its own weight in water, and has continuous water absorption capacity, allowing it to be used in soil for a long time without any polluting effect. The water-retaining agent fixes water through absorption and expansion and when the soil becomes dry, the absorbed water can be released in time by osmotic pressure to maintain a stable level of soil water which can then be taken up and utilized by crops. After being applied, the water-retaining agent aggregates discrete soil particles, which increases the soil's porosity, reduces its bulk density, and adjusts its three-phase balance. Since the molecules of the water-retaining agent contain active groups which can adsorb and exchange ions, the ions it has adsorbed can fix fertilizer nutrients and achieve the effect of a slow and controlled release of said nutrients. Mineral composite water-retaining agents have gradually begun to be used in flower breeding, fruit tree transplanting, agriculture and forestry, and other fields due to their low cost, good gel strength, and environmental friendliness (Woodhouse & Johnson 1991; Nnadi & Brave 2011).

Based on this, this research selected the exposed sandy area of the Hoh Xil Lake where the water recedes in Qinghai to observe condensation generation patterns and absorption effects, thereby enriching the research related to condensation hydrology in sandy areas of different origins in China while also aiming to provide basic data for the rational management, utilization, and ecological restoration of fragile ecosystems.

The Hoh Xil Salt Lake area is deep inland, with high terrain, thin air, and high mountains to the north and south. It is located in a semi-enclosed terrain, causing oceanic winds to have little influence. The climate is characterized as dry, cold, and windy with low rainfall, showing significant continental climate characteristics.

The average annual temperature of Hoh Xil is −10.0 to 4.1 °C, and the minimum temperature is −46.2 °C. The distribution trend decreases gradually from southeast to northwest. The coldest month is January and the warmest month is July. The average annual precipitation is 173–495 mm, decreasing gradually from southeast to northwest. Precipitation is mainly concentrated in May to September, accounting for more than 90% of the annual precipitation. The average annual wind speed distribution increases gradually from the southeast and northeast to the hinterland and the west, and the contour line is basically in the shape of a ‘trumpet mouth’. The wind speed is between 8.0 and 3.5 m/s. Modern glaciers are widespread in Hoh Xil, with 255 glaciers covering 750.7 km² and ice reserves of 81.6489 billion (81.6489 × 10⁹) m³. The reserve is located in the permafrost zone, which accounts for more than 90% of the area of the reserve and is up to 400 meters thick. Glaciers and frozen soil are huge reservoirs of water.

Salt Lake, also known as 68 Daoban Salt Lake (Hu 1992), is located in the north-eastern part of the Hoh Xil National Nature Reserve on the Qinghai-Tibet Plateau. This location is approximately 220 km from Golmud in the North and 8 km from the Qinghai-Tibet Railway in the East and is under the administrative division of Zhiduo County, Yushu Tibetan Autonomous Prefecture, Qinghai Province. It is also part of the Yangtze River Source Park of the Sanjiangyuan National Park. A large part of the Salt Lake basin makes up the core conservation area of the Sanjiangyuan National Park, while Kusai Lake, Haidinuoer Lake, and the northern shore of the Salt Lake to both sides of the Qinghai-Tibet Railway are the zones traditionally used.

Haidinuoer, Kusai, and Zhuonai lakes are located in order from east to west on the northwest side of Salt Lake. Prior to the outburst of Zhuonai lake in September 2011, all the other three lakes were closed lakes (Luo et al. 2010). Among them, the main tributary of Zhuonai Lake was Zhuonai River, which originated from the meltwater of the Five Snow Peaks glacier. The main tributary of Kusai Lake was Kusai River from the Kunlun Mountains’ Great Snow Peaks and Xueyue Mountain, while Haidinuoer Lake is mainly recharged by seasonal rivers and surface runoff. Following the outburst of Zhuonai Lake, the Kusai-Haidinuoer chain of outflows rapidly expanded the surface area of Salt Lake and created rivers between them. According to the results of the survey in March 2020, the areas of the lake bottom that were exposed after the Zhuonai Lake outburst were mainly located in the shallow water areas in the West and South of the original lake, in addition to a relatively small patch of exposed area in the northern region where the topography slopes to a large receding body of water. The exposed areas of the lake bottom were mostly the areas that were inundated by the rapid rise of the lake water before the outburst and the areas inundated by the stable water of the lake for a long time before the outburst. A fine particle sediment layer of terrestrial origin that was formed in this area is dominated by lithological powder soil and powder-fine sand. These fine particle materials can easily form sand and dust storms when exposed to wind erosion. After the outburst of Zhuonai Lake, the area of exposed lake bottom in the West and Southwest of the lake, which has been inundated for a long time, became the most important source area for sand in the region, covering an area of about 87 km² and accounting for 75.6% of the entire exposed lake bottom area. The Salt Lake area, with the rapid expansion of the Zhuonai Lake outburst and the gradual reduction of artificial diversions, will form a new exposed lake bottom area in the receding water area. Soil and water conservation of the top soil is particularly important in the early stages of desertification, and one of the most important procedures of preventing desertification (Amiraslani & Dragovich 2011). The strong evaporation, as well as the low and scattered rainfall, in the Hoh Xil region severely hinder soil and water conservation and revegetation of the surface soil in the receding lake bottom. At the same time, the strong light and large diurnal temperature differences in the Hoh Xil region provide favorable conditions for condensation formation (Zhang et al. 2009; Pan et al. 2010; Temina & Kidron 2011; Tomaszkiewicz et al. 2017; Groh et al. 2018; et al. 2019; Kool et al. 2021). To investigate the effect of rainfall and condensate absorption capacity in the Hoh Xil region, the authors added a mineral composite water retention agent developed by the Laboratory of Pollution Mechanisms and Remediation of the China Geological Survey to the surface soil of the receding area and used the artificial water retention layer it formed as the main research subject to conduct a study on rainfall and condensate absorption capacity in the receding area of the lake bottom. This will provide a research direction for the efficient use of soil condensate in the local desert area of Hoh Xil.

The mineral composite water-retaining agent is a high molecular polymer compound containing a large number of strong hydrophilic groups such as -OH and -COOH. It is synthesized by polymerizing acrylate in situ between bentonite mineral layers to obtain a milky white translucent water-absorbing resin. The addition of minerals mitigates the defects of synthesizing water-absorbing materials using small molecular organic compounds, while greatly enhancing the gel strength of the product and reducing the production cost. This agent was developed by the author at the Pollution Mechanism and Restoration Laboratory of the Chinese Geological Survey, and has been approved for patenting as a Chinese invention as well as being in extensive use in many site restoration projects. The mineral composite water-retaining agent has a strong associative effect on water, capable of achieving a water absorption ratio of 1/150–700. It is characterized by its abilities in several functions: to repeatedly absorb and release water; absorb, retain, and slowly release fertilizer; enhance fertilizer efficiency and drug efficacy; and loosen soil (Woodhouse & Johnson 1991). Therefore, it is of great significance to add this mineral composite water-retaining agent to the soil surface layer as a means of holding water and study the mechanisms of soil water retention on the surface in the desert areas exposed when water recedes.

For the experiment, a control group and artificial water retention layer group were set up to represent bare ground (that is, undisturbed soil) and surface soil where mineral composite water-retaining agent was applied to different layers, respectively. The latter group was divided into a series of artificial water retention layers which comprised evenly distributed mineral composite water-retaining agent within the undisturbed soil of the respective layer (with an application ratio of 30 kg/mu; 1mu = 0.067ha), placed 0–5 cm, 5–10 cm, and 0–10 cm from the surface (see the experimental arrangement in Table 1). When taking samples in the test tubes, care was taken to ensure the inner tube was flush with the surface, and the outer tube was slightly higher than the surface to prevent additional soil from entering. The inner tube was removed each hour and weighed on an electronic balance with an accuracy of 0.01 g, while the temperature and humidity data at the corresponding time were recorded.

During the experiment, weather station data was used to monitor indicators such as ground temperature, air temperature, relative humidity, and wind speed in real time.

The research area was located at an altitude of 4,500 m above sea level in an alpine anoxic zone and there are often fierce animals such as coyotes in the uninhabited area. Thus, due to these hostile working conditions in the research area, this field observation on condensation was carried out in July when the ultraviolet light is strong and evaporation is strong. Preliminary meteorological data collection, data calculation, test tube preparation, equipment burial and other preparatory work took about 15 days. The experiment was carried out over a period of five days from 16:00 on July 4th to 14:00 on July 9th, taking into account the necessity of continuous operation at high altitude.

The authors set up meteorological stations to monitor air temperature, humidity, rainfall and snow melt in the experimental research area, and buried soil temperature and humidity sensors 5, 10, 20, 30 and 50 cm away from the ground surface. Synchronous air and ground temperature monitoring data showed that the surface cooling rate was higher than the deep soil cooling rate, and when the surface temperature was lower than the deep soil temperature, a dissipative zero-flux surface was formed below the surface. In other words, the temperature at a certain depth below the ground presents the highest value in the vertical profile. The soil temperature above and below this zero-flux surface was lower than the ground temperature at that depth, which was the dissipative zero-flux surface (Zi-Yong et al. 2008; Rong-yi 2012; Wei et al. 2017; Luchen et al. 2021). Driven by the temperature gradient, water vapor in the soil pore space above the heat zero-flux surface was transported towards the surface and became a source of water vapor for surface soil condensation (Wang et al. 2017). The location of the zero-flux surface changed with time during diurnal alternation (Hu et al. 2014; Wang et al. 2017).

According to analysis provided by the principles of condensed water generation and how soil water vapor migrates, evaporation and condensation always alternate, with the former starting immediately once the latter ends. The water which is continuously condensed at night accumulates on the soil surface, and quickly dissipates into the air upon exposure to ultraviolet radiation after sunrise. Although the proportion of water affected is small, it is of great significance for the restoration of grasslands in areas afflicted by desertification which form in the early stages of water recession from a lake's edge. During the observation period, the ratios of total evaporation and total condensation of the six test tubes of the bottom-unsealed and bottom-sealed tubes with artificial water retention layers applied at different positions varied widely (see Table 2). In particular, there is an order-of-magnitude difference among the results obtained from the near-surface air unidirectional condensation test tube. According to the analytical data, the total evaporation from the 0–5 cm, 5–10 cm, and 0–10 cm unsealed test tubes under the artificial water retention layer was 13.17, 20.59, and 13.8 times the total amounts of condensation, respectively. Such results indicate that in the bidirectional condensation state, the water-retaining layer was applied to the surface and played a significant role in maintaining it. By contrast, the total evaporation of the 0–5 cm, 5–10 cm, and 0–10 cm test tubes sealed under the artificial water-retention layer was 38.91, 137.67, and 124.60 times the total condensation, respectively. This shows the substantial adsorption effect of the artificial water retention layer on the surface under the condition of unidirectional condensation of water vapor from the near-surface air.

A micropermeability meter method was used to study the variation characteristics, formation time, yield and absorbency of soil condensed water under the conditions of bottom-unsealed treatment and bottom-sealed treatment of an artificial water-retaining layer in the bare sandy area of a lake bottom under the influence of warm humidification in the salt lake region of Qinghai-Tibet Plateau. The characteristics of condensation water retention in different soil layers in the bare sandy area of the backwater lake bottom of Hoh Xil Lake in Qinghai Province were investigated. The following conclusions can be drawn:

• (1)

  Condensate exists in the Hoh Xil Salt Lake area, and there are two sources of soil condensation: condensation from atmospheric water vapor and condensation from deep soil pores. The amount of water vapor condensation in the lower part of the soil in the artificial water retention layer conditions was 2.588 times the amount of condensation in the air near the ground, while the amount of water vapor condensation in the lower part of the bare ground was 1.783 times the amount of condensation in the air near the ground.

• (2)

  The artificial water retention layer in the Salt Lake area of Hoh Xil had little influence on the time of condensation generation, as it ended only slightly earlier than the bare ground did, and the trend of the condensation rate remained the same. However, the unidirectional condensation of water vapor in the near-surface air in the artificial water retention layer was significantly better than that of the bare ground, and the condensation rate was three times higher than that of the bare ground.

• (3)

  The total amount of bidirectional condensation in the artificial water retention layer was slightly less than that of the bare ground, while the total amount of unidirectional condensation in the artificial water retention layer was significantly higher than that of the bare ground. Together, this data indicates that the artificial water retention layer helped to promote the absorption capacity of water vapor in the near-surface air.

• (4)

  Due to the existence of permafrost in the Qinghai-Tibet Plateau region, the zero-flux surface transport of heat dissipation in the Salt Lake area of Hoh Xil lies approximately within 30 cm of the surface.

• (5)

  The analogous humidity coefficient, which characterizes the coefficient of condensate absorption as the storage capacity of condensate in the surface layer of soil in a certain area, strongly demonstrated that the condensate absorption capacity of the artificial water retention layer in the Hoh Xil Salt Lake area was higher than that of bare ground, regardless of whether it was bidirectional or unidirectional condensation, with the absorption multipliers being 1.29 and 2.74 times, respectively.

This study was supported and funded by the Basic Scientific Research Operations Fund of the Chinese Academy of Geological Sciences (No. SK202116), the Hydrogeological Investigation of the China Geological Survey (No. D2021504003), and the Hebei Province Outstanding Youth Science Fund (No. DD20190331).

We would like to thank the anonymous reviewers for their valuable comments and guidance on the manuscript. We are grateful to researcher Zhang Yuxi for his guidance throughout the process and to the Suonan Dajie Protection Station of the Sanjiangyuan National Park administration bureau for their help.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict of interest.