New measures of deep soil water recharge during vegetation restoration process in semi-arid regions of northern China

New measures of deep soil water recharge during vegetation restoration process in semi-arid regions of northern China Yiben Cheng1,4*, Wenbin Yang2, Hongbin Zhan3, Qunou Jiang1,4, Yunqi Wang1,4*, Mingchang Shi1 5 1School of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China 2 Institute of desertification control, Chinese Academy of Forestry, Beijing, 100093, China 3 Department of Geology & Geophysics, Texas A&M University, College Station, TX 778433115, USA. 4Jinyun Forest Ecosystem Research Station, School of Soil and Water Conservation, Beijing 10 Forestry University, Beijing 100083, PR China


Introduction 35
Desertification is currently a global environmental and societal concern (Reynolds et al., 2007b). Arid region covers about 41% of the Earth's surface, and supports more than 38% of the world's population. 20% of these areas have experienced serious land degradation, which is expected to affect the survival of 250 million people (Reynolds et al., 2007a;Dregne and Chou, 1992;D'Odorico et al., 2013). In 1992, the United Nations adopted the International 40 Convention to Combat Desertification in order to focus on desertification issues (Bestelmeyer et al., 2015). With no exception, China is also facing severe desertification problems (Liu and Diamond, 2005). Up to 2010, the total desertification area in China is 2,623,700 km 2 , which is 27.33% of the country's entire land area. Among this, the arid region desertification area is 1,158,600 km 2 (44.16% of the total desertification area of China), the semi-arid region 45 desertification area is 971,600 km 2 (37.03% of the total desertification area of China), and the sub-humid arid region desertification area is 493,500 km 2 (18.81% of the total desertification area of China). To battle desertification, an effective prevention and control measure is to build shelterbelts, using artificial sand-fixing vegetation (Tao, 2014). It is unquestionable that the implementation of 3NSP in China has reduced aeolian erosion, and improved the overall living environment in the impacted regions (Hanjie and Hao, 2003). 60 However, it is undeniable that the poor choices of vegetation species in some areas of 3NSP has resulted in consumption of a large amount of water resources, causing shortage of water supply to meet other needs, thus threatening the sustainable development of the regions (Wang et al., 2010a). Furthermore, a high planting density in some areas resulted in the death and/or malfunction of a large number of trees (Duan et al., 2011). In contrast, shrubs and herb sand-65 fixing vegetation appear to grow healthily, thus receive great interests to become proper choices of vegetation species for desertification prevention (Tao, 2014).
To understand the impact of afforestation to the ecohydrological system, thus to assess the long-term sustainability, especially after vegetation reconstruction, we need to know how the soil moisture changes in these area. We select a typical semi-arid area in 3NSP for this 70 research. Artamisia sphaerocephala Krasch (ASK) is a unique Chinese native sand-fixing shrub plant with strong adaptability . ASK sand-fixing land developed on top of bare sandy land has increased evapotranspiration. Meanwhile, because of the form of an organic-rick biofilm commonly seen in ASK forest, the near surface soil permeability has been reduced (Su and lin Zhao, 2003). This will reduce the soil infiltration capacity, resulting in the 75 concentration of soil moisture in shallow soils, and reducing the replenishment of soil moisture in deep soils. In order to understand the soil moisture variation and deep soil recharge (DSR) changes resulted from ASK sand-fixing forest, this research choose a 40-year-old ASK sandfixing land as the experimental site. This research focuses on monitoring precipitation-induced infiltration, soil moisture distribution, and DSR changes in ASK sand-fixing land. We have 80 also conducted a comparative research using a bare sandy land 400 m away from the ASK sand-fixing land site.

2.1
Research area description Figure 1 shows the research site which is located in Ejin Horo Banner, on the Eastern 85 margin of Mu Us Sandy Land in the Ordos basin of China, with a geographic location of 39°05'02.8 N, 109°35'37.9 E, and an altitude of 1303 m above mean sea level (m.s.l.). The groundwater table between sand dunes are 5.3-6.8 m below ground surface. The climate is within the semi-arid continental monsoon climate zone. Annual precipitation concentrates from July to September and is highly sporadic. The average annual precipitation from 1960 to 2010 90 is 358.2 mm. The average annual temperature of this area is 6.5℃, with about 151 days of frost-free season, and the lowest temperature is -31.4 ℃ . The average annual potential evapotranspiration is 1809 mm, the average annual sunshine is 2900 hours, and the average annual wind speed is 3.24 m/s. The research area is located in relatively gentle mobile dunes, and the soil type is aeolian sandy soil (Liu et al., 2015). 95   This research chooses five ASK plants with similar heights and crown widths, in which 100 the heights are around 60 cm above the ground. Using the whole root system excavation method, the plant soil is excavated layer by layer with a 20 cm vertical interval, until there are no observable roots. As the deepest root is at a depth of 120 cm (the root system will be discussed in details later in this section), thus the deepest soil moisture that the plant can utilize is 180 cm (120 cm root depth plus 60 cm capillary rise, where the capillary rise is calculated 105 based on the soil texture from experimental plot) (Cheng et al., 2017). The 180 cm depth can be regarded as the maximal depth of evapotranspiration. A new lysimeter is used to measure the deep infiltration, or deep soil recharge (DSR) at a depth of 220 cm (to avoid root water absorption), 40 cm below the maximal depth of surface evapotranspiration. The newly designed lysimeter is improved on the basis of the traditional lysimeter, but it has a reduced 110 size and a new water balance part to improve the measurement accuracy. As shown in the Figure 2, the measurement surface is transferred from the soil surface to soil layer at any designated depth. The detailed explanation of such a lysimeter has been documented in a previous research of Cheng et al. (2017) and will not be repeated here. To understand the soil condition in the research site, the sandy soil samples are collected using a ring cut method, 115 layer by layer with a 20 cm vertical interval, until reaching a 220 cm depth. Soil particle size distribution measurements are conducted using a laser particle size analyzer (Mastersizer 2000, Malver, U.K.). We use EC-5 soil moisture probe to measure every 20 cm soil layer of the first 100 cm depth, and every 40 cm soil after the first 100 cm depth until reaching 220 cm depth.
The reason of doing so is because the shallow soil layer has roots thus is monitored more 120 closely while the deep soil is relatively uniform and has less roots, thus can be monitored more sparsely.
To study the soil water dynamics of ASK, we selected a typical ASK plot in the Mu Us Sandy Land and an adjacent bare sandy plot as a comparison study to quantify the differences https://doi.org/10.5194/hess-2020-200 Preprint. Discussion started: 29 June 2020 c Author(s) 2020. CC BY 4.0 License.
in the characteristics of soil water dynamics in bare sandy plot and ASK plot. The experimental 125 design is shown in Fig.2 and explained sequentially as follows. Firstly, in order to minimize the disturbance of the original soil structure, we need to water both plots in advance before installing the instruments. Watering the soil in the test area makes the relatively dry sandy soil stable and easy to excavate, as the native dry sandy soil is relatively loose. Secondly, after watering the ASK plot, we start to excavate a soil profile vertically downward at a distance of 130 1 meter from the main branch of ASK, reaching a depth of 3.2 meters. After this, at the depth of 3.2 meters, we excavate horizontally toward the location of the main branch of ASK to a distance about 1.3 m. Eventually, a body with a height of 1 m, a length and width both of 0.3 m is excavated to install the lysimeter right below the main branch of ASK. By doing so, the distance from the ground surface to the top of lysimeter is 220 cm, and the root system (which 135 is less than 220 cm deep) will not be disturbed. Meanwhile, as the plot has been watered to make the soil stable, no collapse of soil has occurred during the installation of the lysimeter.
Thirdly, after putting the lysimeter in place, we use in-situ soil to backfill. During this process, one needs to continuously water each layer of backfill to ensure that the soil is relatively compact. For the installation of lysimeter in the bare sand plot, it is straightforward as one does 140 not need to worry about the disturbance of integrity of the root system. For such a plot, one can water the soil first, then excavate a square of 1 meter by 0.3 meters to a depth of 3.2 meters to install the lysimeter. After the installation of lysimeter, one can backfill using native soil, making sure to continuously water each layer of backfill to ensure the soil compaction. Soil moisture probes are installed at different depths for both plots. Finally, wait for the watered 145 plots to stabilize to its pre-excavation status, since pre-watered sandy plot and excavated sand layer will take six months to settle down and meet the requirements of the experiment. Then one can start the experiments.

Water balance of rain-fed ASK forest land
When precipitation reaches ground surface in semi-arid sandy land, the infiltration rate is usually unpredictable, it may evaporate or run away, or infiltrate. Years of observation records in the area show no occurrence of surface runoff. The water infiltrating into the soil goes through a redistribution process. Part of it is absorbed and utilized by plants' root 155 system, and part of it is stored in soils as soil moisture. The rest will infiltrate passing the maximal depth of evapotranspiration depth and eventually recharges the groundwater system. This research uses the following water balance method to calculate moisture distribution at different depths: where P is annual precipitation (mm) measured by a rain gauge as the volume per unit square meters, Cm is soil volumetric moisture content (m 3 /m 3 ), d is soil column depth to be measured (mm), DSR is annual deep soil recharge (mm), measured by the newly designed lysimeter as the volume per unit square meters, E is annual evapotranspiration (mm) which is the volume of water lost to the atmosphere due to evapotranspiration per square meters, and △ 165 W is the annual soil moisture storage change per unit square meters (mm).

Root system distribution
This research selects representative plants and excavate the soil profile to research the ASK root system growth range. The results show that the ASK root system distribution is umbrella-170 shaped, as shown in Table 1 surface to a depth of 40 cm, the root system gradually increases, and reaches the maximum density at the 40 cm depth. The dry weight of root between 20-40 cm layer is 51.77% of the weight of the entire root system. The root system gradually decreases after depths of 40 cm, with the deepest root system depth of 120 cm. The results show that the ASK root system in this area is mainly developed in the horizontal direction, which confirms that rainfall is the 180 main water supply for plants in the Mu Us Sandy Land. This conclusion is based on the following reasons. The root development of plans is closely dependent on the source of water supply for the root system, and there are generally two sources of supply: a) rainfall-induced downward infiltration and b) uptake of groundwater from the underneath soil and aquifer. If the primary source of supply for the ASK root system comes from the deep groundwater table, 185 then the root prefers to grow vertically in order to access the underneath groundwater. On the other hand, if the primary source of supply for the root system comes from the rainfall-induced infiltration, the root system prefers to grow horizontally to maximize the intercept of such infiltrated water, and the field observation results confirm that this is the case in Mu Us Sandy

Effect of ASK on soil development
There are many factors that affect the soil particle size, including soil crust, vegetation root is not easy to see the main affecting factors. In this research, to understand the impact of ASK on the local soil, the ASK soil samples and bare land soil samples are collected and sorted based on U.S. Department of Agriculture's soil particle size grading scheme, we collected 200 samples of every 20 cm depth and mixed them together, treated the entire 220 cm thick soil layer each as a homogenized system.
The soil particle size distribution was measured using the MS2000 soil particle size analyzer produced by Malvern, UK. Samples need to be pretreated before the experiment. All soil samples have passed through a 2 mm soil sieve, added 30% H2O2 solution to remove 205 organic matter (including biological crust) from the sample, then add NaHMP solution to fully dissolved, and shake 30 seconds to destroy the microaggregate structure of the soil particles.
Overall, in ASK plot, the medium sand is 19.26%, the fine sand is 68.53%, the very fine sand (or powder sand) is 9.35%, and silt is 2.86%. The soil particle size distributions of the bare 210 sandy plot are as follows. The coarse sand is 3.23%, the medium sand is 50.53%, the fine sand is 36.06%, the very fine soil is 7.19%, and the silt is 2.99%. Comparing the results in ASK plot and bare sandy plot, one can see that the main soil type in the ASK plot is fine sand (68.53%), and the main soil types in bare sandy plot is medium (50.53%) and fine sands (36.06%).
Another notable point is that there is 3.23% of coarse sand in the bare sandy plot, but no coarse 215 sand in the ASK plot.
There are clear evidences that the sand-fixing vegetation changes the particle size distribution of the soil (Fearnehough et al., 1998;Pei et al., 2008). A few possible reasons may be responsible for such a change. First, the fine-sand in the 220 cm-thick soil of the bare sandy land is easily removed or eroded from its original position under the force of wind, which 220 initiates sand movement both horizontally and vertically (as suspended particles carrying away by wind), consequently the content of fine sand in the bare sandy land decreases, and the soil structure continuously coarsens. In contrast to this, the content of fine particle in the ASK plot is significantly higher than that in the bare sand. This is largely due to the presence of vegetation in the ASK plot which has substantially subdued the eroding force of wind. In another word, 225 ASK essentially protects the fine sands in the soil to be removed or eroded by wind force. This observation is direct evidence showing that vegetation has a positive role in improving soil particle size composition by maintaining the fine sand particles in the plot. However, one must also be aware that such a change of particle size distribution is a consequence of a complex interplay of aerodynamic force, sand mass movement mechanics, and root-soil interaction force, 230 which are not completely understood up to now and needs further investigation.

Annual soil moisture variation of rainfed ASK plot
The experimental area is located in Northern China, with more than three months of 235  and the minimum precipitation is 0.2 mm. However, these precipitation events did not change the decreasing trend of soil moisture. This means that during the germination and early growth periods, the moisture absorbing capacity of the ASK root system is extremely high. There is a 9.4 mm precipitation event on June 28 th , and the infiltration associated with this event can reach 255 a depth of 20 cm. This means that the growth of ASK starts to slow down around this time, and the shallow soil moisture starts to increase. In October, temperature drops and ASK starts to enter winter dormancy. There is a 4.2 mm precipitation event on October 4 th , and the infiltration associated with this event can reach a depth of 60 cm. There is a 24.6 mm precipitation event on November 7 th , and the infiltration associated with this event can reach a depth of 140 cm. 260 Soil moisture at 220 cm depth changes very mildly. The results show that though DSR occurs in all seasons, especially during freeze-thaw period, due to vegetation consumption, the amount of DSR is relatively small.

3.4
Effects of annual precipitation on soil moisture and DSR

Comparison of DSR on rain-feed ASK land and bare sandy land 265
For deep soil moisture variation and distribution, this research uses a newly designed lysimeter to measure DSR on-site (Cheng et al., 2017). The soil layer may be disturbed after the instrument is installed in 2015, so the 2015 precipitation-infiltration data are not used in this study. Results are shown in Table 3. The ratios of DSR to annual precipitation are 60.02%, 21.57%, 21.04%, respectively. The experimental plot of Artamisia is less than 100 m away from the bare sandy land plot, the annual precipitation is basically the same, and DSR values are 90.6 mm, 31.2 mm, 2 mm, respectively. The ratios of DSR to annual precipitation are 19.49%, 9.96%, 0.82%, respectively.
According to above data, DSR of the bare sandy land is obviously higher than the Artamisia 275 plot. On Artamisia plot, the interception of the aboveground vegetation, root absorption, evapotranspiration consumes a large amount of water resources, which affects the production of DSR.  Table 3, 2016 is a wet year, 2017 is a normal year, and 2018 is a dry year. In 280 the wet year, the deep soil moistures of the two experimental sites were greatly supplemented, and the effect of bare sand was more obvious. The amount of DSR in the dry years is significantly reduced on both plots, especially in the Artamisia plot, from 90.6 mm in wet years to 2 mm in dry year. Based on these, one can conclude that in semi-arid areas, though vegetation cover can fix mobile sand dunes, it consumes a lot of water resource. Bare sandy land can 285 transport large amounts of water resource to shallow groundwater. Heavy precipitation completely wets the entire soil layer and forming a moisture transport 300 channel that facilitates the transport of moisture throughout the soil layer. In bare sandy land, as the entire soil layer is wet, the subsequent small precipitation can also replenish the deep soil layer moisture, as shown in Figure 3A. In the experimental area of Artamisia plot, heavy rainfall wets the entire soil layer, but for the root system soil water consumption, the subsequent small precipitation cannot significantly replenish the deep soil moisture, as shown in Figure  305 4D.

Research on rain-feed ASK land water distribution 355
There are many methods of measure surface layer evapotranspiration, but all have poor precise, because there are many factors that affect surface layer evapotranspiration and one cannot consider all impact factors, these factors including vegetation coverage, environmental and temperature factors. This study treats shallow soil as a whole layer and measures the amount of surface rainfall recharge, soil water storage and DSR directly. Based on the directly 360 measured DSR and precipitation, the soil moisture storage change can be calculated using  Table 4.
In 2016, the soil moisture reserve in the 220 cm soil layer of bare sand increased by 47.15 370 mm, and the annual evaporation was 134.04 mm, while the soil water storage of Artamisia plot increased by 31.95 mm, and the evapotranspiration was 342.25 mm. In 2017, the soil water storage of bare sandy plot increased by 13.77 mm, and the annual evaporation was 232.03 mm, while the soil water storage of Artamisia plot was reduced by 83.7 mm, and the evapotranspiration was 365.9 mm. In 2018, the soil water storage of bare sandy plot increased 375 by 72.14 mm, and the annual evaporation was 121.46 mm, while the soil water storage of Artamisia plot increased by 2 mm, and the evapotranspiration was 202.63 mm. One should be noted that the change in soil water storage only represents the distribution of soil moisture from April to November, rather than the net increase of the whole year, because the water in the soil will continue to infiltrate to deep soil layer when the surface soil layer is frozen. As shown in 380 Figure 3, there is no significant precipitation from January to June 2017, but deep infiltration has been occurring. Comparing the data from 2016 to 2018 in Table 5, it can be found that when there is sufficient precipitation, for example, in 2016, soil water storage increases and evapotranspiration increases as well. When the precipitation is low, the soil water storage decreases and the evapotranspiration decreases as well. The results show that after vegetation 385 reconstruction in this area, the amount of DSR is significantly reduced, which may threaten the https://doi.org/10.5194/hess-2020-200 Preprint. Discussion started: 29 June 2020 c Author(s) 2020. CC BY 4.0 License. safety of groundwater recharge; The precipitation water resource is concentrated in the shallow soil layer, vegetation gets sufficient moisture, then evaporation increases, and the regional microclimate environment will be improved. Evapotranspiration of plants in drought years is significantly reduced, which shows that vegetation will adapt to the environment by increasing 390 or decreasing water consumption according to the amount of precipitation.

Influence of vegetation coverage on infiltration rate
In many aspects one can find the influence of vegetation on infiltration, the interception 395 of precipitation by the aboveground part of vegetation, the interception and absorption of surface soil layer moisture by vegetation, the absorption and utilization of soil water by vegetation roots, the root system occupying soil voids to reduce infiltration speed, and the conduction effect of the catheter formed by death root on the infiltration ability. In this study, we consider the above-ground and underground parts of vegetation as a whole system, and 400 compare the bare sand plot and ASK plot on the infiltration speed. During the observation period, the Precipitation-DSR interaction occurred alternatively. In order to show the characteristics of the two types of infiltration, we selected a typical infiltration process, and the result is shown in Figure 7. A precipitation of 90.2 mm/d was generated at 23:00 on July 7, https://doi.org/10.5194/hess-2020-200 Preprint. Discussion started: 29 June 2020 c Author(s) 2020. CC BY 4.0 License. 2016, and a DSR event was observed at 21:00 on July 9 on the bare sand plot. From the surface 405 soil layer to 220 cm depth soil layer, the infiltrate process took 46 hours. The DSR of the ASK plot was observed at 8:00 on July 12, and the infiltrate process from the surface layer to the depth of 220 cm soil layer took 107 hours. The infiltration rate of ASK plot is 2.33 times of bare sandy land. One can see that vegetation cover significantly affects the infiltration rate.
However, under natural conditions, multiple precipitation processes occur in a short period, so 410 it is difficult to distinguish the DSR event caused by a certain precipitation of different land coverage types under sufficient precipitation.
The results show that the characteristics of precipitation-induced DSR in the sandy land plot and the ASK plot are different. The two precipitation events leave marks on the bare sandy plot, leading to two spikes of DSR. In contrast, such spikes do not appear in the ASK plot, 415 because water is utilized by the root system mostly and only a very small portion of the precipitation-induced infiltration can reach as far as 220 cm to be detected by the lysimeter.
ASK not only delays the infiltration rate but also reduces the total amount of DSR. ecological elements (such as ASK root systems in this study) are always changing, thus any monitoring methods that cannot continuously accommodate the ecological elements will miss a significant piece of the machinery of understanding the precipitation-recharge relationship.
Our research here is an attempt to utilize a low-cost, field-based lysimeter method to monitor DSR for four years in Mu Us sandy land, a task has never reported before. 440 In semi-arid areas, Mu Us sandy land as an example, the main limiting factor for trees is available water resources (Gao et al., 2014;Skarpe, 1991). Therefore, the key to understand the vegetation ecosystem in semi-arid areas is to study the supply of water resources (Cheng et al., 2018;Cheng et al., 2017). The ASK has been in existence in the study area for more than 40 years, so the purpose of this study is to find out whether there is sufficient water resource 445 available in the region to support vegetation ecosystem, through the measurement of DSR. The "sustainable" growth of plants in this study means that water resource from precipitation can meet the growth needs of ASK, and can still have an excess amount of water to replenish deep https://doi.org/10.5194/hess-2020-200 Preprint. Discussion started: 29 June 2020 c Author(s) 2020. CC BY 4.0 License. soil layer. In this study, the soil moisture distribution has been studied by using the newly designed lysimeter to measure whether the soil layer below the root layer could produce DSR 450 or not.
In the dry years, the differences in soil water storage and DSR between the two plots are significant, taking 2018 as an example. At the beginning of the experiment, the soil moisture storage in the ASK plot is 126.16 mm, and the soil moisture storage of bare sandy land is 147.22 mm. At the end of the experiment, the soil moisture storage in the ASK plot is 166.72 455 mm, which is 40.56 mm less than that at the beginning of the experiment. The soil moisture storage of the bare sandy plot at the end of the experiment is 219.37 mm, which is 72.15 mm more than its counterpart at the beginning of the experiment. There is no significant difference in soil water storage, but the DSR difference is obvious. The DSR of bare sand is 51.6 mm, and that of ASK plot is only 2 mm. Although the DSR is significantly reduced, even in the dry 460 years, there is still a small amount of DSR, indicating that the selection of ASK as sand-fixing vegetation in this area is a suitable plant species. Another interesting point to note is that ASK is capable of adjusting their own growth conditions based on the available moisture recharge, and a larger moisture recharge will result in a faster growth rate of such plants. When the rainfall is insufficient, the evapotranspiration of ASK is reduced from 342.25 mm in 2016 to 465 202.63 mm in 2018.
As surface soil is frozen and ASK enters dormancy during winter in the research site, snow can only accumulate on the surface and cannot recharge soil moisture. However, moisture in deep soil continues to infiltrate downwards because of the driving force of gravity. This is particularly true in bare sandy land as a large amount of soil moisture has been accumulated at 470 the start of the frozen period. A portion of those accumulated soil moisture will slowly infiltrate downwards and recharge groundwater reservoir. Because the amount of snowfall in winter is https://doi.org/10.5194/hess-2020-200 Preprint. Discussion started: 29 June 2020 c Author(s) 2020. CC BY 4.0 License. difficult to calculate, the amount of frozen water accumulated in winter cannot be obtained.
How to accurately obtain the details of winter infiltration requires further research.

Conclusions 475
This research uses a newly designed lysimeter to monitor shallow soil layer infiltration, and results show that in order to absorb more precipitation moisture, ASK develops a horizontal root system and retains more water in the shallow soil layer. ASK has shown to be effective in fixing the mobile sand and increasing the proportion of fine particles in the sandy land. ASK changes its own evapotranspiration mount to adapt to the annual precipitation changes. Under 480 the existing precipitation conditions, the ASK community can develop healthily, as a small amount of precipitation can recharge the groundwater, even in dry year. This indicates that precipitation in the area is sufficient to meet the needs of vegetation water consumption.
However, with the unforeseeable global warming and abnormal precipitation events, semi-arid region may become drier and the ASK community may be seriously affected. Therefore, 485 continuously monitoring the key controlling factors associated with the ecological system in the semi-arid region is needed.
The following conclusions can be drawn from this research: 1) In Mu Us sandy land, the ASK root system develops horizontally to absorb more precipitation-induced infiltration. The root system mainly concentrates within the upper 490 40 cm deep soil layer.
2) After 40 years of vegetation reconstruction, the soil particle size distribution has been significantly changed. Specifically, the sandy soil mainly consisting of medium sand (50.53%) grows into a sandy soil mainly consisting of fine sand (60.53%). Vegetation is particularly important in semi-arid areas since it directly changes the composition of 495 soil.
3) The yearly DSR in the ASK sand-fixing experimental plot is from 2 mm to 90.6 mm.
In contrast, the yearly DSR in the bare sand plot is from 51.6 mm to 283.6 mm. This shows that the rainfed vegetation has reduced DSR substantially but there is still a small amount of recharge left to replenish the deep soil moisture, implying that the current 500 ASK community is still hydrologically self-sustainable because it does not consume all the water moisture replenished by precipitation and the DSR has not been reduced to zero.
4) Under the conditions of sufficient precipitation, the infiltration rate of bare sandy land is 2.33 times of ASK land. 505