The dynamics of soil moisture balance components and their relations with the productivity of natural vegetation in an arid region of northwestern China

Ecological Engineering 143 (2020) 105672

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The dynamics of soil moisture balance components and their relations with the productivity of natural vegetation in an arid region of northwestern China

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Danfeng Lia, , Mingan Shaob, Shuaipu Zhangc, Kun Zhangd a

Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China b Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China c Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guilin University of Technology, Guilin 541004, China d Nanjing Institute of Environmental Sciences, Ministry of Ecology and Environment of China, Nanjing 210042, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Soil moisture Soil water balance Evapotranspiration Ecological water requirement

Precipitation and groundwater are important water source in soil and are vital for the survival and growth of natural vegetation in arid regions. Analyses on the dynamics of soil water balance components and their relations with plant growth are usually restricted by the lack of continuous measurement of deep soil moisture at a local or regional scale. By monitoring monthly soil water contents in the 0–3 m soil profiles, we analyzed the contributions of precipitation and groundwater to the growth of natural vegetation in an arid region of northwestern China from 2011 to 2013. The results showed that the 0–3 m desert soil and the 0–1.8 m grassland soil exhibited moderate spatial variability in the soil water contents (θ). Temporal variability of spatial mean θ decreased with depth in the soils of the desert and grassland. The profile-average of spatial mean θ maintained 0.11 cm3 cm−3 in the desert and varied from 0.37 to 0.44 cm3 cm−3 in the grassland over time. The evapotranspiration after calibration gave accurate estimation of water available for plants (402.0 and 532.4 mm year−1 in the desert and grassland, respectively). Annual mean net primary productivity of the grassland (233.4 g C m−2) was 3.8 times that of the desert. Water exchange in the desert soils was dominated by recharge into the 0–1 m soil from layers deeper than 3 m, with annual mean value of 303.4 mm. Recharge alternated with discharge events in grassland soils. Annual mean recharge and discharge amounts were 494.2 and 50.1 mm for the 0–1 m layer and 641.1 and 228.9 mm for the 0–3 m soil profile, respectively. Water supply into soil was totally consumed by evapotranspiration. Precipitation use efficiency increased when groundwater level became shallow (0.04 and 0.11 g C m−2 mm−1 in the desert and grassland, respectively). Groundwater supplied about 75% of water requirement for natural vegetation, and its use efficiencies were 0.12 and 0.33 g C m−2 mm−1 in the desert and grassland, respectively. Seedling of herbaceous species with trichomes at the interspace and under the canopy of shrubs can guarantee great utilization of precipitation in the desert. Transplanting of shrubs with large amounts of shallow lateral roots and shrubs with deep rooting systems can effectively utilize the shallow and deep soil water or groundwater. Reducing the irrigation consumption of groundwater and controlling the expansion of oasis cropland are foremost to guarantee the ecological water requirement and maintain the sustainability of natural ecosystems. Keep or increase current area and layout of the grassland may benefit the water cycle and carbon sequestration in the arid regions.

1. Introduction In arid regions, plant activities are tightly coupled to water availability, and the primary productivity and community structure of the ecosystem will be profoundly affected by water shortage. Soil moisture

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integrates the effects of climate, vegetation, topography, and landscape patterning during soil formation and resulting soil profile properties (Duniway et al., 2010a). Soil moisture controls the photosynthesis, growth and mortality of plants and profoundly affects the net primary productivity (NPP) of water-limited ecosystems (Lozano-Parra et al.,

Corresponding author. E-mail address: [email protected] (D. Li).

https://doi.org/10.1016/j.ecoleng.2019.105672 Received 4 March 2019; Received in revised form 15 September 2019; Accepted 16 November 2019 0925-8574/ © 2019 Elsevier B.V. All rights reserved.

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growth of phreatophytes and desert vegetation in arid and semiarid regions (Padilla and Pugnaire, 2007; Miller et al., 2010; Dai et al., 2015; Liu et al., 2017; Liu et al., 2018). For example, the Tamarix chinensis in the lowlands has taproot and most lateral roots extending to depths near to groundwater table and can effectively utilize groundwater. Liu et al. (2017) found that groundwater use efficiency decreased along a gradient of increasing groundwater level. Depths from which plants extract water shift throughout the growing season in response to changing environmental conditions and soil moisture availability (Asbjornsen et al., 2008). For example, Halaxylon ammodendron mainly used shallow soil water in early spring when the upper soil water was abundant, and used groundwater in summer after the depletion of upper soil water in the Gurbantonggut Desert (Dai et al., 2015). In the arid inland river basins of northwestern China such as the middle reaches of the Heihe River basin, grassland, oasis cropland and desert locates successively with the increasing distances from the river banks. The grassland has shallow groundwater level and soils with more fine particles, whereas soils in the desert are coarse-textured and loosely-structured. Plants in these two ecosystems profoundly rely on precipitation and groundwater. Oasis cropland relied heavily on irrigation and consumed 96% of water exploited from the Heihe River for agricultural and domestic usage (Chen et al., 2003). Agricultural irrigation after implementing the water diversion scheme from 2000 becomes increasingly dependent on groundwater. The expansion of artificial oases leads to the decline of groundwater level and in turn the deterioration of natural vegetation. Studies on water use strategy of natural vegetation usually focused on individual species in the desert by investigating shallow soil moisture for short-term periods during the growing season (Xu and Li, 2006; Zhao et al., 2016; Zhuang and Zhao, 2017; Liu et al., 2018). Few studies investigated the importance of precipitation and groundwater to natural ecosystems along a gradient of increasing groundwater level (Liu et al., 2017). However, these studies were not able to reflect the response of soil moisture to groundwater recharge due to the absence of soil water content monitoring either in the root-active zones or in deep profiles. Continuous measurement of deep soil moisture is indispensable to recognize the responses of soil moisture to precipitation and groundwater, and to estimate water utilization by plants with deep-rooting systems in arid regions. The relation of soil moisture balance components to plant growth is vital for taking ecological measures to sustain the groundwater resources and ecosystem functions in arid regions. The objectives of this study were: 1) to characterize the spatial and temporal variations of soil water balance components; and 2) to analyze their relations with the productivity of natural vegetation in an inland river basin of northwestern China.

2018). For example, Xu et al. (2017) found that the transpiration of desert shrubs estimated by different models was greatly improved by incorporating root-zone soil moisture in arid region of northwestern China. Knowledge of the spatial and temporal variations of soil moisture is crucial for understanding the patterns and processes in the critical zones of dryland. Characterized as episodic events or pulses, precipitation is a key determinant of soil water availability. The vertical distribution of soil moisture exerts overwhelming control over the near-surface water and carbon cycles in arid ecosystems. The horizontal distribution of soil moisture may be equally important in determining ecosystem carbon fluxes (Huxman et al., 2004). Arid vegetation preferentially use precipitation-derived soil moisture rather than soil moisture derived from groundwater (McLendon et al., 2008). The rates and depths of soil infiltration varied greatly with the quantity, intensity and intermittency of individual rainfalls in the deserts and sandy lands (Xu and Li, 2006; Wang et al., 2008; Raz-Yaseef et al., 2012; Zhang et al., 2016; Cheng et al., 2018). Surface soil texture, subsoil horizon development, and other aspects of the soil-geomorphic template determine how the combined effects of precipitation, temperature, and evaporative demand shape soil water availability (Duniway et al., 2010b; Cleverly et al., 2016; Duniway et al., 2018). Dry sandy soils are interrupted by transitory wetting during rainfall pulses, enabling the fast infiltration and soon evaporation in the topsoil. Accumulation of clay fraction decreases the infiltration, improves the capacity of soil to retain infiltrated water for longer periods, and possibly maintains more plant-available water during dry periods (Hamerlynck et al., 2000; Kochendorfer and Ramírez, 2008). Duniway et al. (2018) demonstrated the varying responses of soil moisture to rainfall pulses among soil textures, landscape settings, and ecosystem types based on 27-year of monthly measurements in the northern Chihuahuan Desert. Pulsed water inputs in dryland ecosystems control ecosystem carbon exchange through a series of drying and rewetting cycles in soil. Small events favor ecosystem carbon loss through microbial respiration in surface soil, and larger events are necessary to elicit net carbon gain through autotrophic components in the ecosystem (Huxman et al., 2004). Wu et al. (2015) reported that soil moisture alone explained 71–74% of variation in net CO2 exchange, and 2-mm precipitation pulses resulted in carbon efflux, whereas increasing carbon uptake occurred after ≥ 5 mm precipitation events in moss crusted soil in the Gurbantonggut desert. Annual precipitation amount is thought a good predictor of annual variations in NPP, net ecosystem productivity, or gross ecosystem production (Huxman et al., 2004; Biederman et al., 2016). However, seasonal pattern of precipitation may be more important in determining the annual carbon balance (Thomey et al., 2011; Liu et al., 2016). Pulse patterns and soil hydrological properties determine in part the ability of plants to utilize rainfall (Bansal et al., 2013), thus impact the physiological activity, individual morphology and community structure of plants (Reynolds et al., 2004; Xu and Li, 2006; Liu et al., 2016). Heavy pre-growing season precipitation increases annual net ecosystem CO2 exchange by facilitating the growth and carbon assimilation of shallow-rooted herbs, and the aboveground biomass production will increase with high frequency growing-season rainfall events (Knapp et al., 2008). Analyzing the pulse and spatial patterns of precipitation and its relation to soil water variation and plant growth are essential for recognizing the role of rare precipitation in arid regions. Plant communities in arid ecosystems are commonly structured in two distinct layers of woody and herbaceous plants (Lloyd et al., 2008; Vourlitis et al., 2015; De Arruda et al., 2016). The root morphology or architecture is another important determinant of soil water availability. Optimal phenotypes with predominantly shallow root system and high leaf conductance tend towards adaptations maximizing pulse water use; whereas phenotypes of deep root system and lower leaf conductance adapt to maximize usage of deeper soil water or groundwater (Schwinning and Ehleringer, 2001). Studies exhibited the importance of deep soil moisture and groundwater in maintaining the

2. Materials and methods 2.1. Study area This study was conducted around the Linze Inland River Basin Comprehensive Research Station of the Northwest Institute of EcoEnvironment and Resources, Chinese Academy of Sciences. This region locates in the middle reaches of the Heihe River basin of northwestern China, and has a continental arid climate with annual mean air temperature of 7.6 °C. The highest and lowest air temperatures were 39.1 °C in August and − 27.3 °C in January. The annual mean precipitation approximates to 120 mm, and the drying index is 15.9. Each year has about 105 frost-free days, and the growing seasons last from April to September for seed maize and from April to October for natural vegetation. A rectangular transect covering an area of 100 km2 with a length of 20 km from north to south and a width of 5 km from east to west (39°12′30″–39°23′28″N, 100°05′32″–100°10′01″E, 1372–1417 m a.s.l.) was chosen in this study (Fig. 1). The Heihe River flows across the area from east to west. The northern part of this transect includes the 2

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Fig. 1. Location of the monitoring points and the study area in the middle reaches of the Heihe River, and pictures of the grassland and Halaxylon ammodendron with different planting years in the desert.

southern margin of the Badain Jaran Desert. The desert with fixed and semi-fixed sand dunes, cropland with different cultivation histories, and the grassland with high groundwater levels are dominant landscapes. Cropland for maize seeding expanded from the river banks approaching the desert and grassland and relied heavily on irrigation by pumping groundwater (Fig. 1). Main soil types in the desert are aeolian sandy soil from the long-term encroachment and deposition of drift sand. The grassland is derived from alluvial materials and has high levels of fine particles. Desert vegetation consists of shrubs including Halaxylon ammodendron (C. A. Mey.) Bunge, Reaumuria soongorica (Pall.) Maxim, Calligonum mongolicum Turcz., and Nitraria sphaerocarpa Maxim, and herbaceous plants such as Bassia dasyphylla (Fisch. & C. A. Mey.) Kuntze, Agriophyllum squarrosum (Linn.) Moq., Artemisia scoparia Waldst. et Kit., and Agropyron cristatum (L.) Gaertn. Predominant species in the grassland are Tamarix chinensis Lour., Phragmites australis (Cav.) Trin. ex Steud., Leymus secalinus (Georgi) Tzvel., Achnatherum splendens (Trin.) Nevski, and Kalidium foliatum (Pall.) Moq. (Li and Shao, 2014).

measured at the 70 points using a neutron probe at piecewise constant intervals (0.1 and 0.2 m for the 0–1 and 1–3 m soil layers, respectively). The measurement lasted for 3 days by the end of each month from May to October in 2011, in December 2011, from February to December in 2012, and from April to October in 2013, forming a dataset with 25 occasions. The neutron probe was calibrated in both rainy and dry seasons of each year, and the details of the calibration have been reported in Zhang et al. (2017). Before installing the neutron-probe access tubes, disturbed soil samples were collected at the same depth intervals using a hand auger (5-cm in diameter) at each point. After having been air-dried and crushed, each soil sample was passed through a 2-mm mesh to determine the particle size distribution by laser diffraction with a Mastersizer 2000 analyzer (Malvern Instruments, Malvern, England) and the organic carbon concentration by dichromate oxidation (Nelson and Sommers, 1982).

2.2.2. Monitoring of meteorological and vegetative features The meteorological variables including precipitation, air temperature, humidity, vapor pressure, solar radiation and wind speed were recorded hourly by a standard automated weather station installed at the Linze Inland River Basin Comprehensive Research Station (between locations 12 and 13 in Fig. 1) from January 2011 to December 2013. Annual mean precipitation was 104.7 mm, and the amounts of rainfall during the growing season occupied 95.9%, 86.6% and 99.2% of the total precipitation amounts in 2011, 2012, and 2013, respectively. Furthermore, approximately 89% of the growing-season rainfall occurred from June to August. Annual mean air temperature was 8.79 °C, with the highest and lowest mean daily air temperature in August

2.2. Field sampling and investigation 2.2.1. Measurement of soil moisture Forty-seven and 23 aluminum neutron-probe access tubes were installed in a grid size of about 1 × 1 km in the desert and grassland of the transect in April 2011, respectively (Fig. 1). Due to loose structure of the desert soil and high groundwater level (up to 1.5 m, Liu et al., 2017) in grassland, forty-six tubes for the desert and 15 tubes for the grassland reached 2 m, among which, only 39 and 8 tubes reached 3 m, respectively. Soil volumetric water contents (θ, cm3 cm−3) were 3

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Fig. 2. Daily air temperature (a), precipitation (a), wind speed (b), and saturation vapor pressure deficit (b), and the distribution of precipitation events in each month (c) from 2011 to 2013.

2.3. Data analysis

(28.5 °C) and January (−19.1 °C), respectively (Fig. 2a). Annual mean wind speed was 1.73 m s−1, and the strongest wind activity occurred in April. The vapor pressure deficit generally increased from January (0.2 kPa), peaked in June (2.1 kPa), and then decreased (Fig. 2b). Groundwater levels at the weather station and the banks of the Heihe River were continuously monitored at a ten-day time interval from groundwater wells using water-level sensors (HOBO, Onset Computer Corporation, Bourne, MA, USA) (Fig. 1). The 0–1.5 m soil water contents at a depth interval of 0.1-m were also measured at a time interval of ten days using the neutron probe at the weather station. The normalized difference vegetation index (NDVI) is strongly related to the green biomass level of landscapes. The monthly cumulative ET and monthly NDVI data with a spatial resolution of 30 m, and the monthly NPP data with a spatial resolution of 250 m were used to give information about the vegetation growth from January 2011 to December 2013. The former two datasets were provided by “Heihe Plan Science Data Center, National Natural Science Foundation of China” (https://www.heihedata.org). The NPP data were determined by the Moderate Resolution Imaging Spectro-radiometer (MODIS). Vegetation survey was conducted in late August 2013. Five quadrats at sizes of 10 × 10 m for shrubs and 1 × 1 m for herbs were designed around each monitoring point. In each quadrat, plants were separated, and properties including species, number, height, and coverage were recorded. Dry weight of aboveground parts was determined after clipping and oven-drying at 65 °C for 72 h.

2.3.1. Validation of evapotranspiration Remote sensing models are feasible tools for ET estimation and partitioning at larger spatial scales, and can provide an integrated assessment of ET, which is less affected by inevitable heterogeneity in point measurements (Kool et al., 2014). However, remotely sensed ET data require validation under sparsely vegetated conditions and drought stress. For the maize cropland in the transect, the ET values can be determined by combining the FAO Penman–Monteith equation and single crop coefficient to validate the remotely sensed ET values during the three years. Using daily meteorological variables, the daily crop evapotranspiration of maize cropland, ETc, from January 1st, 2011 to December 31st, 2013 is given by (Allen et al., 1998): 900

ET0 =

0.408Δ (Rn − G ) + γ T + 273 u2 (es − ea )

ETc = K c ET0

Δ + γ (1 + 0.34u2 ) (1)

where ET0 is the reference evapotranspiration (mm), Rn is the net radiation at the crop surface (MJ m−2 day−1), G is the soil heat flux density (MJ m−2 day−1), T and u2 are the mean daily air temperature (°C) and wind speed (m s−1) at a height of 2 m, respectively, es and ea are the saturation vapor pressure and actual vapor pressure (kPa), respectively, es − ea is the vapor pressure deficit (kPa), △ and γ are the slope of the vapor pressure curve and the psychrometric constant (kPa °C −1), respectively, and Kc is crop coefficient (dimensionless), and its value for each growth stage of maize was obtained during field experiments. Readers for more details of the calculation are referred to Li 4

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multiplying the slope (1.582) of the linear fitting, and those of the rest months remained unchanged. The calibration improved the ET estimation (Slope = 0.99, Intercept = 0, R2 = 0.98, RMSE = 10.59 mm). The predicted monthly ETc values of the desert and grassland were therefore validated using above method and employed in following analysis. Net radiation and vapor pressure deficient play important roles in determining canopy transpiration and soil evaporation (Zhao et al., 2016). The ET values increased from January, reached the peak value in July (84.3 mm in the desert and 121.7 mm for the grassland), and then decreased to the lowest value in December (2.93 and 3.92 mm for the desert and grassland, respectively) in the three years (Fig. 5a). The largest ET in July was due to the high net radiation, temperature and vegetation coverage and longer daylight hours, and then the ET reduced with the decrease in net radiation and the senescence of plants (Fig. 2). Annual mean ET values were 402.0 mm in the desert and 532.4 mm in the grassland, and 90.7% and 92.4% of which occurred during the growing season, respectively. The similarity in low ET values during non-growing seasons between the desert and grassland suggests the controlling effects of meteorological factors on soil evaporation (Fig. 5a). The NDVI value is favorable in reflecting the relative abundance and activities of vegetation. The temporal variations of monthly NDVI and NPP synchronized with that of monthly ET (Fig. 5). The monthly NDVI and NPP of the grassland were remarkably higher than those of the desert during the growing seasons (Fig. 5b). Monthly NDVI values increased from February (0.10 for the desert and grassland), reached the highest value in July (0.26 for the desert and 0.44 for the grassland), and then decreased until December (0.10 and 0.12 for the desert and grassland, respectively) (Fig. 5b). The highest and lowest values of monthly NPP were in July (13.0 and 64.3 g C m−2 for the desert and grassland, respectively) and January (0.29 g C m−2 for the desert and 0.31 g C m−2 for the grassland), respectively (Fig. 5b). Annual mean NPP value of the grassland (233.4 g C m−2) was 3.8 times that of the desert. By adopting the ratio of NPP to aboveground biomass for herbs (1.69, Zhao et al., 2010b), aboveground biomass values estimated by annual mean NPP values were 36.7 and 137.8 g m−2 for the desert and grassland, respectively. These values agreed with those in vegetation survey (44.4 g m−2 for the desert and 145.3 g m−2 for the grassland) (Table 1), suggesting the accuracy of the NPP data.

and Shao (2014). The daily ETc values were summed up to obtain cumulative monthly values (named calculated ETc), and compared with the remotely sensed monthly values (named predicted ETc) of maize cropland. 2.3.2. Discharge or recharge of soil water Soil water budgets in the desert and grassland represent the balance between moisture input (precipitation and/or groundwater) and output (evapotranspiration, runoff, and net drainage). Soil water storage for location i at time j, SWSi,j (mm), in the 0–1, 1–2, and 2–3 m soil layers was the summation of the product of θ(i,j,k) (where k is soil depth, m) and depth intervals. Downward drainage out of the considered soil layers or recharge from deeper layers can be calculated as (Allen et al., 1998):

DP = P + ΔSWS − ET − R

(2)

where DP is the net drainage (mm), positive values of DP indicate deep percolation, whereas negative values indicate upward capillary action from lower layers. P is precipitation (mm), △SWS is SWS change in a given soil layer (mm), and R is the surface runoff (mm), which was neglected due to low rainfall and flat topography in this study. 3. Results 3.1. Spatio-temporal variations of soil moisture Temporal series of the spatial variability (indicated by coefficient of variation over space, CVs) and spatial mean of θ values among locations at a depth interval of 0.2 m in the desert and grassland are presented in Fig. 3. Moderate spatial variability of θ was observed along the 0–3 m soil profiles of the desert, with the weakest spatial variability in the 0–0.2 m layer (temporal mean CVs = 0.39) due to water loss by strong atmospheric evaporative demand (Fig. 3a). The strongest spatial variability of θ in the 1.6–2.0 m desert soil (temporal mean CVs = 0.84) can be attributed to the occurrence of clayey impermeable layers at this depth of some locations (Xiao et al., 2014). Spatial mean θ values increased from 0.05 cm3 cm−3 in the topsoil to 0.17 cm3 cm−3 in the 2.8–3.0 m soil of the desert (Fig. 3b). The grassland exhibited moderate spatial variability of θ in the above 1.8 m soil and weak spatial variability in underlying layers, with relatively strong spatial variability in the topsoil (Fig. 3c). Spatial mean θ values increased from surface (0.28 cm3 cm−3), reached the maximum in the 1.2–1.4 m (0.46 cm3 cm−3), and remained in below layers of the grassland over time (Fig. 3d). The temporal variability (in terms of CVt) of spatial mean θ generally decreased with depth in the 0–3 m soil of the desert (CVt decreased from 0.28 to 0.06) and the grassland (CVt decreased from 0.14 to 0.04). The variability of spatial mean θ among layers was relatively high during the non-growing season in the desert and the growing season in the grassland. The profile-average of spatial mean θ maintained about 0.11 cm3 cm−3 in the desert and varied from 0.37 to 0.44 cm3 cm−3 in the grassland over time. Over the 25 measurement occasions, the spatial mean SWSs in the 0–1, 1–2 and 2–3 m soil layers of the desert had temporal averages of 60.1, 111.6, and 142.4 mm, respectively. The spatial mean SWSs averaged 355.1, 451.3, and 438.8 mm in the 0–1, 1–2, and 2–3 m soil layers of the grassland, respectively.

3.3. Water exchange in soil profiles The temporal series of discharge or recharge amounts of water (blue diamonds) in the 0–1 and 0–3 m soil profiles of the desert and grassland are shown in Fig. 6. The dark red strips referred to the 95% confidence intervals, and the width of the stripes provided information about the dispersion of estimations among locations. Water stored in the soil profiles of the desert and grassland was dominantly maintained by recharge from the deeper layers, especially in the growing seasons. Recharge amounts into the 0–1 and 0–3 m layers of the desert during the growing seasons were 281.7 and 290.0 mm, among which, 71.8% and 80.8% occurred from June to September, respectively (Fig. 6a and b). Compared to all recharge into the first 1-m soil, there were 11.9 and 6.0 mm of soil water leakage from the 0–3 m soil profiles of the desert in October and November of 2012, respectively (Fig. 6b). Recharge amounts into the upper 0–1 and 0–3 m soil layers of the grassland from June to September occupied on average 76.3% of those during the growing seasons (446.0 and 506.1 mm, respectively) (Fig. 6c and d). Discharge events alternated with recharge events from the 0–1 and 0–3 m soil layers of the grassland, and a total of 50.1 and 228.9 mm of soil water discharged annually from these two layers, respectively (Fig. 6c and d). The narrow confidence intervals suggest weaker spatial variability in the estimations of drainage or recharge amounts, especially in the 0–1 m desert soil.

3.2. Vegetation growth in the desert and grassland The predicted ETc values agreed well with the calculated values, except the periods from June to August, when the predicted values were distinctly smaller than the calculated values (Fig. 4). However, the calculated ETc values of maize cropland in 2011 and 2012 agreed satisfactorily with the outcome in Zhao et al., (2010a) in this study area. The predicted ETc values from June to August were adjusted by 5

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Fig. 3. The temporal pattern in the vertical distribution of both spatial variability (in terms of coefficient of variation over space, CVs) and mean soil volumetric water content (θ, cm3 cm−3) over space in the 0–3 m profiles of (a and b) the desert and (c and d) grassland. Numbers from 1 to 25 refer to the sequence of measurement occasions from May to October in 2011, in December 2011, from February to December in 2012, and from April to October in 2013.

4. Discussion

Overall, annual mean net recharge amounts into the 0–1 and 0–3 m soil layers of the desert were 303.4 and 305.8 mm, respectively, indicating that water exchange in the desert soils was dominated by recharge into the 0–1 m soil from underlying layers deeper than 3 m. In grassland, annual mean recharge and discharge amounts were 494.2 and 50.1 mm for the 0–1 m layer and 641.1 and 228.9 mm for the 0–3 m soil profile, respectively. Annual mean values of net recharge into these two layers were 444.1 and 412.1 mm, respectively.

4.1. Characteristics of rainfall and its importance In arid regions, the amount and seasonality of precipitation exert an important control on the structure, composition and productivity of the plant community (Zhao and Liu, 2010). High pre-growing season precipitation results in greater fractions of spring annuals, which can effectively use the spring and early summer rainfall (Peng et al., 2013; 6

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Gurbantonggut Desert of China (Huang et al., 2015; Liu et al., 2018). Total rainfall in growing season occupied 96.2% in amount and 83.1% in frequency of annual mean precipitation. Rainfall pulses during the growing season serve as important trigger for biological activity and determine to some extent the gross ecosystem productivity of natural ecosystem. The maximum net photosynthetic rates of both Nitraria sphaerocarpa and Calligonum mongolicum after rainfall were about two times the pre-rainfall values in the desert (Liu et al., 2012). Significantly positive correlation between cumulative precipitation and cumulative ET (R2 > 0.96, P < .001) demonstrates the importance of growing-season rainfall. Annual mean precipitation was 104.7 mm from 2011 to 2013, and 33.8% in cumulative amount and 83.1% in frequency of which occurred as events < 5 mm. Rainfall < 5 mm during the growing season can easily be intercepted and evaporated, and is usually regarded as ineffective in arid regions (Zhao et al., 2010b). However, high-frequency small rainfall events may also make sense for plant growth. Sandy plants can absorb the raindrop adhering to their leaves and branches to survive or maintain leaf area, which increases their capacity to respond to larger rainfall events (Huxman et al., 2004). The leaf, stem or spikelet of some herbs such as Bassia dasyphylla, Tribulus terrester, Artemisia scoparia, Agropyron cristatum, and Cirsium japonicum in the study area had sparse or dense trichomes. Small rainfall can provide a relatively wet micro-environment for surrounding photosynthetic cells, causing a change in water vapor pressure of the air inside and outside the leaf (Asbjornsen et al., 2011; Wu et al., 2016). The trichomes can improve the overall area available for water vapor condensation, and water retained on hairy leaf surfaces would quickly penetrate into mesophyll to prevent herbs in part from severe water loss under stress conditions (Grammatikopoulos and Manetas, 1994). Zhuang and Zhao (2010, 2017) found that the dewdrop significantly improved the aboveground biomass of Bassia dasyphylla, and was a frequent and stable water resource in Halaxylon ammodendron plantations in the desert of this study. Wu et al. (2016) found that C. discoidea, covered with dense trichomes, had remarkably higher maximum water absorption and leaf water content than smooth-leaf species. Wet micro-environment from small rainfall can also benefit the non-hairy shrubs. The sap velocity in the branches of shrubs increased rapidly after rainfall of 3.8 mm, and the lower responding thresholds of stem and branch to rainfall were 5.2 and 1 mm for Nitraria sphaerocarpa and Elaeagnus angustifolia in this study area, respectively (Zhao and Liu, 2010).

Fig. 4. Comparison of the calculated monthly evapotranspiration (ETc) and the remotely sensing based monthly ETc values of the maize cropland in the study area.

Huang and Li, 2014). Precipitation events in the non-growing season (from November of last year to March of next year) were all < 5-mm and summed up to 2.8, 4.8 and 5.8 mm in 2011, 2012, and 2013, respectively. There was 8.8-mm snowfall during March and April in 2012. The θ values in the 0–1 m soil of the desert and grassland in April 2012 was higher than in February 2012 (Fig. 7a and b), indicating the positive effect of snowfall on soil moisture. Infiltration water after 10-mm rainfall reached to a maximum depth of 0.5 m in the desert (Yang, 2014). Soils froze from early November, reached the maximum frozen depth of about 1 m in next February, and thaw completely by mid-April in the study area. Thawing of frozen water in soil functions like a rainfall pulse and fills soil pores. Liquid water content in the first 40-cm desert soil in April 2012 was nearly seven times that in February 2012 (Yi et al., 2014). The snowmelt and thawing of frozen soil thus can provide the bulk of water requirement for most ephemeral or annual herbs scattered in the interspace of shrubs in early spring. This agreed with the report that the stable snow cover is a key determinant to the germination and rapid growth of herbaceous plants in the

Fig. 5. Temporal series of the monthly (a) ET, and (b) the normalized difference vegetation index (NDVI) and net primary productivity (NPP) of the desert and grassland from January 2011 to December 2013. 7

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Table 1 Characteristics of plants in the desert and grassland obtained from vegetation survey in this study. Habitat

Description

Species

Canopy (m2)

Height (cm)

Coverage

Aboveground biomass (g m−2)

Desert

Shrub

Haloxylon ammodendron (C. A. Mey.) Bunge Nitraria sphaerocarpa Maxim Calligonum mongolicum Reaumuria songarica (Pall.) Maxim. Nitraria tangutorum Bor. Sarcozygium xanthoxylon Bunge Alhagi sparsifolia Shap Artemisia scoparia Waldst. et Kit. Peganum harmala L. Agriophyllum squarrosum (Linn.) Moq. Bassia dasyphylla (Fisch. & C. A. Mey.) Kuntze Eragrostis pilosa (L.)Beauv. Chloris virgata Sw. Halogeton arachnoideus Moq. Suaeda glauca (Bunge) Bunge eganum nigellastrum Bunge Tamarix chinensis Lour. Kalidium foliatum (Pall.) Moq. Comarum salesovianum (Stepn.) Asch. et Gr. Agropyron cristatum (L.) Gaertn. Leymus secalinus (Georgi) Tzvel. Phragmites australis (Cav.) Trin. ex Steud. Scorzonera mongolica Maxim. Sonchus arvensis Linn. Glaux maritima L. Taraxacum mongolicum Hand.Mazz. Halerpestes tricuspis (Maxim.) Hand.Mazz. Halerpestes cymbalaria (Pursh) Green Juncus effusus L. Oxytropis kansuensis Bunge Thermopsis lanceolata R.Br. Achnatherum splendens (Trin.) Nevski Iris lactea Pall. var.chinensis (Fisch.) Koidz

3.768 0.773 1.805 0.226 0.513 0.006 0.130 0.309 0.004 0.001 0.001 0.002 0.001 0.002 0.001 0.067 7.923 0.005 0.002 0.001 0.007 0.003 0.001 0.005 0.001 0.001 0.001 0.001 0.001 0.002 0.001 0.150 0.043

151.2 26.95 93.00 22.90 31.26 3.667 28.13 22.42 6.667 6.000 13.71 6.667 5.600 16.57 11.17 15.78 210.3 12.50 12.00 14.86 28.80 13.65 12.92 24.36 12.18 11.88 11.50 13.00 10.00 14.14 13.83 35.33 28.50

0.434 0.025 0.158 0.094 0.028 0.001 0.184 0.246 0.007 0.011 0.030 0.002 0.013 0.044 0.121 0.244 0.333 0.044 0.008 0.168 0.153 0.101 0.037 0.039 0.029 0.032 0.066 0.090 0.175 0.025 0.036 0.092 0.097

160.8 4.201 29.95 29.85 3.986 0.247 78.06 44.37

Perennial herb Annual herb

Grassland

Shrub

Perennial herb

113.9 – – 145.3

(p < .01). High clay content can enhance the adsorptive capacity of soil to water and favor the capillary rise of water through micro-pores (Hamerlynck et al., 2000; Botula et al., 2012). Organic matter (OM) is another important factor influencing soil water condition. The lateral or fibrous roots of herbs in the grassland will be rapidly decomposed by microbes when exposed to aerobic condition. The fast turnover of roots contributes to the OM accumulation in root zone, which can improve the infiltration capacity and retention of soil water by affecting the inter-aggregation and structural porosity. The OM concentration of the grassland soil was significantly higher than that of the desert soil (Li and Shao, 2014). High contents of clay, silt and OM fractions favor higher soil water contents in the grassland. This concurs with the finding that clay and OM contents are the chief controlling factors of water adsorption in fine-textured soils (Rawls et al., 2004; Naveed et al., 2012). Variability in water availability imparted by soils is linked to plant community characteristics in water-limited ecosystems. Perennial shrubby plants are dominant species and major primary producer in the desert of this study. Halaxylon ammodendron has been transplanted to form shelterbelt in the southern edge of the desert since 1975. These plants currently had a mean height of 3.4 m and a mean canopy of 8.6 m2. With the implementation of ecological restoration, following plantation of Halaxylon ammodendron scattered towards the desert. Mean height and canopy of these plants were 2.2 m and 4.5 m2, respectively. In the middle sandy land of the study area, Halaxylon ammodendron was planted within 3 years by 2011, and had a mean height of 1.2 m and a mean canopy of 1.4 m2. In the 0–0.1, 0.1–0.2 and 0.2–0.3 m soils, clay and silt contents and θ values increased with the planting years, suggesting the windbreak and soil improvement effects of Halaxylon ammodendron (Fig. 8). Ephemeral or annual herbs, and small shrubs and herbs scattered in the interspaces of the old and middle-aged Halaxylon ammodendron, respectively. Greater canopy shading of the soil surface and increased plant litter under shrubs may

Rainfall events larger than 5 mm happened fully in the growing season, with rainfalls of 5–10 and 10–20 mm occupying 25% and 31% in amount and 10% and 6% in frequency, respectively (Fig. 2c). Large rainfall events lead to infiltration deep enough. Rainfall of 30.6 mm on July 14 in 2013 was the sole event larger than 20 mm in the three years. The higher soil water content in the first 1-m desert soil during July and August and reduced recharge into the 0–1 m soil from underlying layers of desert reflect the replenishment of this large rainfall to soil moisture. Large rainfall events increase soil water availability and strongly impact the sap velocity of the stems of N. sphaerocarpa and E. angustifolia during rainfall periods (Liu et al., 2011). Infrequent large rainfall events make more sense for the survival of vegetation in the habitats where groundwater is unavailable. In the stony desert positioned northwest of this study area, groundwater level was below 11.5 m and was far beyond the maximum rooting depths of sparsely distributed small shrubs. Investigating the response of soil water content and vegetation activity to these infrequent large rainfall events is vital for the survival of plants there. 4.2. Effects of soil properties and vegetation on soil moisture Soil properties translate precipitation pulses into biologically available water in the soil and are the foremost factors impacting the spatial variability of soil moisture at local scale (Cho and Choi, 2014). Soils of the desert were developed from aeolian sand and consist mainly of well-sorted medium-fine sand and lack fine particles (Su et al., 2010). The grassland soils are derived from alluvial materials, exhibiting relatively high pedological development. Clay contents and silt contents in soils of the grassland were 30.8% and 43.7%, respectively, being significantly higher than those of the desert (11.4% and 13.4%, Fig. 7c). Clay and silt contents of the grassland soil exhibited weaker spatial variability than the desert soil (Fig. 7d). Both clay content and silt content were positively correlated with temporal mean θ values 8

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Fig. 6. Temporal series of the discharge amount from the 0–1 and 0–3 m soil layers of (a and b) the desert and (c and d) grassland from June 2011 to October 2013. Positive values refer to discharge, and negative values refer to recharge from the underlying layers.

deeper soils increased through the taproots (Scott et al., 2008). In the desert, more than 64% of roots of Halaxylon ammodendron were in the 0.2–1.2 m soil, and its tap root extended to 4.2 m (Xu et al., 2017), and taproot of the young Halaxylon ammodendron and one-year Alhagi sparsifolia Shap reached to depths of 2.8 and 2.5 m, respectively (Yang, 2014). Groundwater table was around 4.5 m in the desert during the study period. These deep-rooted species are capable of using groundwater to maintain their growth and facilitate the productivity of understory herbs through hydraulic lift during inter-pulse periods (Ludwig et al., 2004). The Tamarix chinensis in the grassland has taproot and most lateral roots extending to depths near to groundwater table and can effectively utilize groundwater. Its flexible rooting system enables it to reduce gas exchange and remain vigorous at deep groundwater levels. The root channels, macropores due to woody root decomposition, and the cracks formed during the freezing and thawing cycles of soil act as preferential pathway favoring water movement (Devitt and Smith, 2002). This may explain the frequent recharge of groundwater into soils of the desert and grassland. Decreasing soil evaporation and plant transpiration resulted in the drainage of abundant soil water in the

decrease soil evaporation by reducing soil temperature, although canopy interception of small rainfall events can reduce soil infiltration. This may explain the non-significant increase in θ values of the topsoil with the planting years (Fig. 8a). The phenotype or architecture of root system is the main feature employed in classifying root functional types, and one of the most important determinants of soil water availability (Sperry and Hacke, 2002). More than 80% of the absorbing roots of the young Halaxylon ammodendron concentrated in the 0–0.2 m desert soil (Yang, 2014). The lateral roots of Reaumuria soongorica were in the 0–0.3 m soil (Xu and Li, 2006). About 54% and 33% of roots of Calligonum mongolicum and N. tangutorum distributed in the 0–0.2 m soil and their tap roots extended to 1.2 and 1.6 m, respectively (Xu et al., 2017). Most roots of the herbaceous plants distributed in the 0.2 m topsoil. Coexistence of these species may improve the utilization of precipitation and upper soil water in the desert. This may explain the lowest θ values in the 0.1–0.2 m desert soil under Halaxylon ammodendron no matter seedling years (Fig. 8a). After the drying out of upper layers, sap flow moving towards canopy from the lateral roots reduced and water use from 9

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Fig. 7. The profile distributions of (a) soil water content of desert in February and April in 2012, (b) soil water content of grassland in February and April in 2012, (c) soil particle contents, and (d) the coefficients of variation (CV) of soil particles over space in the desert and grassland.

plants. Approximating saturated surface soil restricts the sap flow thus the transpiration of plants after this large rainfall event (Šimůnek et al., 2018). Stable rainfall thus stimulates more water occupied by ET and water supply into soil was totally consumed by plants in the desert and grassland. Water use efficiency (WUE), the ratio of NPP to ET, is effective to assess the response of ecosystem productivity to water availability. Annual mean WUE were 0.16 and 0.44 g C m−2 mm−1 in the desert and grassland, respectively, indicating that the WUE generally increased when the groundwater level became shallow. These values were smaller than those in Liu et al. (2017), who reported the WUE values of 0.26 and 0.82 g C m−2 mm−1 for the desert and grassland around locations 9 and 55 in this study, respectively. With the increasing distance from the oasis, coverage and biomass of the herbaceous plants and shrubs become lower. The desert in this study was classified as landscape with loosely-structured sandy soil and natural desert vegetation, covering the sandy desert, oasis-desert ecotone, oasis edge, and sandy land areas between the riparian forest with groundwater level of 0.85 m and wetland with groundwater level about 1.30 m in Liu et al. (2017). High water availability from either soil or groundwater at most locations contributed to the larger ET and annual NPP values in this study compared to those (103 mm and 26.8 g C m−2, respectively) of the sandy desert in Liu et al. (2017). The ET value (505 mm) in wetland of Liu et al. (2017) approximated to the ET value (532.4 mm) in the grassland of this study. However, its annual NPP value (414.1 g C m−2) was remarkably larger than that (233.4 g C m−2) of the grassland in this study. Easy access to groundwater expanded the functional growing season of plant species and increased ET and net ecosystem production

grassland during the non-growing seasons. 4.3. Contribution of groundwater and precipitation to plant growth The ET can be used as a proxy for the amount of water available to ecosystem production at an annual basis (Biederman et al., 2016). The monthly NDVI values were positively correlated with monthly ET values (R2 = 0.739, P < .001 in the desert; R2 = 0.867, P < .001 in the grassland) (Fig. 9a). Significantly positive correlations were observed between cumulative NPP and cumulative ET values in the desert (R2 = 0.985, P < .001) and grassland (R2 = 0.990, P < .001) (Fig. 9b). The ET values decreased with the deepening of groundwater level. This concurs with the report that increasing groundwater depth beyond 2 m corresponded with decreased ET in a riparian forest in California's central Valley (Kochendorfer et al., 2011). Temporal mean ratios of cumulative ET to the summation of cumulative precipitation and cumulative net recharge into the 0–1 m soil were 1.03 and 0.96 for the desert and grassland, respectively (Fig. 10). The value of 1.03 in the desert was slightly larger than that in Zhao et al. (2016), who reported 98.3% of water consumption allocated to ET determined by sap flow measurement of a Calligonum mongolicum community in the desert of this study from 2008 to 2010. Except for the difference in ET determination, distinct meteorological conditions may be responsible for the disparity. Annual mean precipitation in Zhao et al. (2016) was 113.3 mm, being larger than that (104.7 mm) in this study. The maximum rainfall of 50.6 mm occurred in September 2010 functioned like an irrigation, resulting in the reduced soil evaporation and infiltration deep enough beyond the root zone of shallow-rooted 10

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Fig. 8. Comparison in (a) soil water content (θ), (b) clay content, (c) silt content, and (d) sand content in the 0–10, 10–20, and 20–30 cm soil layers among the Halaxylon ammodendron communities with different planting ages.

this study are more representative at a regional scale. Groundwater fluctuations affect the dynamics of soil moisture and ET processes in the grassland. Soil water contents in the 2.8–3.0 m layer of desert and in the 0.8–1.0 m layer of grassland increased with the shallowing groundwater levels (Fig. 11). However, the correlation between groundwater level and soil water content was not clear enough during the study period. The ratios of cumulative net recharge to the summation of cumulative precipitation and cumulative net recharge into the 0–1 m soil varied from 0.55 to 0.98 for the desert and from 0.66 to 0.94 for the grassland over the 25 occasions. The temporal average

in the grassland (Scott et al., 2014). However, the heterogeneous aeration, moisture and salinization of upper soil and groundwater level lead to the differences in species, coverage and aboveground biomass of the grassland over space. Difference in the NPP estimation methods may be another reason for the discrepancy. Liu et al. (2017) estimated the NPP of different vegetation types based on regression analysis using 250-m resolution NDVI as independent variable and experimental NPP data from several plots as dependent variable. The NPP data in this study has a 30-m spatial resolution and was verified by the aboveground biomass at all surveyed locations. The WUE values obtained in

Fig. 9. The relationship between (a) monthly ET and monthly NDVI, and (b) cumulative ET and cumulative NPP of the desert and grassland. 11

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Fig. 10. Temporal series in the ratios of cumulative ET to the summation of cumulative net recharge amount and cumulative precipitation of the desert and grassland from June 2011 to October 2013.

season and 28% in wet season. Higher contributions of groundwater in this study indicate the importance of groundwater in maintaining the natural ecosystems in the arid northwestern China. This percentage is larger than the findings in the similar or same regions. It has been reported that groundwater accounted for 60.6% and 50% of water consumption in a Calligonum mongolocum community (Zhao et al., 2016) and a Nitraria sphaerocarpa community (Zhou et al., 2017) in the desert of this study, respectively. Liu et al. (2018) showed that > 85% of xylem water of shrub layer was derived from groundwater, which contributed > 35% of total ET of the Haloxylon ammodendron community in the Gurbantonggut Desert during 2014. Differences in the

values were 0.75 and 0.79, respectively, indicating that groundwater supplied > 75% of water requirement for natural ecosystems. This result agreed with previous reports that groundwater is an important source of soil water to support the photosynthesis and transpiration of plants and soil evaporation (Lowry and Loheide, 2010; Soylu et al., 2014). For example, Miller et al. (2010) found that approximately 80% of total ET of oak trees during June and August came from groundwater in a California oak savanna. Groundwater fluctuation occupied up to 50% of the variance in the daily ET in the Spreewald wetland of northeastern Germany (Fahle and Dietrich, 2014). Barbeta and Penuelas (2017) found that plant use of groundwater was 49% in dry

Fig. 11. The temporal series of (a) the groundwater level at the weather station and soil water content in the 2.8–3.0 m soil layer at the location (#13) close to the well and (b) the groundwater level on the Heihe River bank and soil water content in the 0.8–1.0 m soil layer at the location (#30) close to the well. 12

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annual amount and seasonal partitioning of precipitation, the depth to groundwater, and species composition may explain the different groundwater contributions. Greater groundwater supply in the grassland indicates that the importance of groundwater increased with the decrease in groundwater level. This concurs with the outcome in Liu et al. (2017) that groundwater use efficiency decreased along a gradient of increasing groundwater level. The groundwater use efficiencies were 0.12 and 0.33 g C m−2 mm−1 in the desert and grassland, respectively. Precipitation supplied not more than 25% water to ET and its use efficiencies were 0.04 and 0.11 g C m−2 mm−1 in the desert and grassland, respectively. The rainfall use efficiency of the desert was remarkably lower than that (0.26 g C m−2 mm−1) of the sandy desert in Liu et al. (2017), who reported no groundwater supply due to deep groundwater levels below 6.3 m.

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5. Conclusions This study investigated the temporal dynamics of soil water balance components and their relations with plant growth of natural ecosystems, and estimated the use efficiency of precipitation and groundwater in an arid region of northwestern China. Annual mean ET of the desert and grassland were 402.0 and 532.4 mm, respectively, and annual mean net primary productivity of the grassland (233.4 g C m−2) was 3.8 times that of the desert. Water exchange in the desert soils was dominated by recharge into the 0–1 m soil from layers deeper than 3 m, and annual mean value of which was 303.4 mm. Recharge alternated with discharge events in soils of the grassland, with annual mean recharge and discharge amounts of 494.2 and 50.1 mm for the 0–1 m layer and 641.1 and 228.9 mm for the 0–3 m soil profile, respectively. Groundwater supplied about 75% of water requirement for natural vegetation, and its use efficiencies were 0.12 and 0.33 g C m−2 mm−1 in the desert and grassland, respectively. The use efficiency of precipitation were 0.04 and 0.11 g C m−2 mm−1, respectively. Ecological measures guaranteeing great utilization of precipitation pulses are vital in the desert. Seedling of herbaceous species with sparse or dense trichomes at the interspace and under the canopy of shrubs may be promising to capture the small rainfall and dewdrop and to reduce soil evaporation after rainfall events. Transplanting of shrubs with large amounts of shallow lateral roots can effectively utilize the shallow soil water. In view of the importance of groundwater to natural vegetation, reducing the irrigation consumption of groundwater and controlling the expansion of oasis cropland are foremost to guarantee the ecological water requirement and to maintain the sustainability of natural ecosystems. Keep or increase current area and layout of grassland may benefit the water cycle and carbon sequestration in arid inland river basins of northwestern China. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (No: 41701251), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA19040500) and the Key Frontier Program of Chinese Academy of Sciences (QYZDJSSW-DQC043). The authors thank Mr. Yunzhi Tao and Miss Fei Han for their help in vegetation survey. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop Evapotranspiration – Guidelines for Computing Crop Water Requirements. In: FAO Irrigation and Drainage Paper

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