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Capacity and Distribution of Water Stored in the Vadose Zone of The Original Research Core Ideas Capacity and Distribution of Water • Vadose zone water is up to Stored in the Vadose Zone 3.1 ´ 1012 m3 in the Chinese Loess Plateau. of the Chinese Loess Plateau • The deep vadose zone (below 5 m) stores 92.4% of the total vadose Yuanjun Zhu, Xiaoxu Jia,* Jiangbo Qiao, Andrew Binley, zone water. Robert Horton, Wei Hu, Yunqiang Wang, and Ming’an Shao • Vadose zone water distribution is Water stored in the vadose (unsaturated) zone provides the majority of water uneven, depending on loess thick- ness and precipitation. required by plants and buffers water resources; thus, it is central to understand- • Vadose zone water accounts for ing ecological and hydrological processes in the Chinese Loess Plateau (CLP) with 42.1% of water resources in the its thick loess deposits. We used multisource data on soil water content (SWC) and Chinese Loess Plateau. vadose zone thickness, combined with a spatial interpolation method, to quan- tify the vadose zone water and further deduce the water resource composition in the CLP. Vadose zone water is approximately 3.1 ´ 1012 m3 (±27.5%) in the CLP, 92.4% of which is stored in the deep vadose zone (>5 m and above the ground- water table). The water resources composition of the CLP comprises precipitation, river water, vadose zone water, and the saturated zone water (shallow groundwa- ter), accounting for 2.1, 0.1, 42.1, and 55.7%, respectively. Although a large amount of water exists in the vadose zone, the SWCs in the upper (<5 m) and deep vadose zones are 47.4 and 65.3%, respectively, of the mean field capacity, both being at a low level. Our findings bridge the knowledge gaps on the deep vadose zone water and water resources composition in the CLP, providing the basis for deci- sion-making on balancing revegetation and water resources conservation. Abbreviations: BD, bulk density; CLP, Chinese Loess Plateau; GWT, groundwater table; RBFI, radial basis function interpolation; SWC, soil water content; WS, water storage; WSC, water storage capacity. Y. Zhu, J. Qiao, and M. Shao, State Key Lab. of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F Univ., Yan- Vadose (unsaturated) zone water accounts for only a small proportion (3.8%) gling Shaanxi, 712100, China; X. Jia, Key Lab. of Ecosystem Network Observation and of the Earth’s land surface freshwater (Gleick, 1993), but it plays a vital role in terres- Modeling, Institute of Geographic Sciences trial ecosystems by supplying the majority of water required by plants (Federer, 1979; and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, China; Schlesinger and Jasechko, 2014), regulating water flows in the soil–plant–atmosphere A. Binley, Lancaster Environment Centre, continuum (Gardner and Ehlig, 1963; Passioura, 1982), and linking the components of Lancaster Univ., Bailrigg Lancaster, LA1 4YQ, UK; R. Horton, Dep. of Agronomy, Iowa State the water cycle (precipitation, runoff, evapotranspiration, and groundwater) (Federer, Univ., Ames, Iowa, 50011-1010; W. Hu, New 1979; Yang et al., 2000; Chen et al., 2008). Quantifying this water is, thus, the basis of Zealand Institute for Plant & Food Research understanding soil water–plant relationships, water-based processes in the soil–plant– Limited, Private Bag 4704, Christchurch 8140, New Zealand; Y. Wang, Institute of Earth atmosphere continuum, and the water cycle. For such a quantification, both soil water Environment, Chinese Academy of Sciences, content (SWC) and the depth of the vadose zone are required, and various methods Xi’an Shaanxi, 710061, China. *Corresponding author ([email protected]). and techniques are applied to obtain SWC in a profile (Dalton et al., 1984; Zhou, 2001; Huisman et al., 2003; Wang and Qu, 2009). However, the quantification is still typically limited to shallow soil, ranging from a very thin depth (5 cm) to a normal rooting depth Received 20 Nov. 2018. Accepted 9 Aug. 2019. (typically up to 4 m) and the depth of deep-rooted plants (21 m), due to the limitations of sampling depth and SWC measurement techniques (Wang et al., 2011, 2015; McColl Citation: Zhu, Y., X. Jia, J. Qiao, A. Binley, R. Hor- et al., 2017). ton, W. Hu, Y. Wang, and M. Shao. 2019. Capacity Loess is a clastic, predominantly silt-sized sediment that is formed by the accumula- and distribution of water stored in the vadose zone of the Chinese Loess Plateau. Vadose tion of wind-blown dust, and it or similar sediments cover 10% of the Earth’s land surface Zone J. 18:180203. doi:10.2136/vzj2018.11.0203 (Liu, 1985; Smalley et al., 2011). The Chinese Loess Plateau (CLP) has the largest and thickest loess deposits (92.2 m on average) around the world and is implementing reveg- etation to remediate its fragile ecosystem under the conditions of limited water resources, soil erosion, and highly frequent human activities (Liu, 1985; Fu et al., 2017; Zhu et al., © 2019 The Author(s). This is an open access article distributed under the CC BY-NC-ND license 2018, 2019). The revegetation, i.e., the “Grain for Green” initiative, has greatly changed (http://creativecommons.org/licenses/by-nc- nd/4.0/). the ecological and hydrological systems in the CLP, typically represented by a significant Vadose Zone Journal | Advancing Critical Zone Science increase in vegetation coverage, widely distributed dried soil layers, the vadose zone of the CLP. The objectives of this study were to: and sharp decreases in runoff and sediment discharging into the (i) calculate the capacity of water in the vadose zone, (ii) evaluate Yellow River (Wang et al., 2009, 2016; Feng et al., 2016; Wei et the spatial distribution of the vadose zone water, and (iii) deduce al., 2016). Therefore, there is growing concern about the rational the water resources composition in the CLP. use of water resources to achieve sustainable development of the ecosystem (Li, 1982; Yang, 2001; Feng et al., 2016; Fu et al., 2017; Jia et al., 2019). Materials and Methods Vadose zone water is central to this issue. First of all, it is Study Area almost the only water source available to crops and other vegeta- The CLP is located in the middle reaches of the Yellow River tion in the CLP (Huang et al., 2003; Li and Huang, 2008), thus and covers an area of 6.4 ´ 105 km2 (Fig. 1). It is not completely directly and intensively affecting crop production and revegeta- covered by loess because of the effects of soil genesis and landforms. tion performance (Hou et al., 1991; Li, 2001; Chen et al., 2015). The northwest part of the CLP is covered by aeolian sandy soil due The revegetation could, in turn, consume an immense amount to its short distance to the source of loess and its low precipitation. of soil water to meet increased transpiration needs (Feng et al., The westernmost part is mainly desert soil, and the southernmost 2016; Li et al., 2016; Jia et al., 2017; Ren et al., 2018), leading to and easternmost parts are mainly stony soils. The region exclusive a rapid reduction of soil water in the root zone, further affecting of the areas with aeolian sand, desert, and stony soils is the typi- vegetation succession and percolation (Hou et al., 1991; Huang cal loess region of the CLP, with an area of 3.6 ´105 km2 (Fig. 1). and Gallichand, 2006; Jia et al., 2017). This kind of water is mostly In addition, there are mountains inside the typical loess region. stored in the root zone, usually in the upper loess layers. The water The loess is does not easily accumulate in these mountainous areas stored in the deep vadose zone (beyond root depth) also has impor- because of steep slopes and forest vegetation. These areas, with an tant ecological and hydrological functions. It is a potential and area of 3.5 ´104 km2, were excluded in our analysis. Hence, the precious water source for some deeply rooted plants (e.g., forest determination of vadose zone water was conducted throughout the and shrub vegetation), especially as the upper soil water is largely typical loess region with the thick and continuous loess deposits, consumed (Li et al., 2008; Liu et al., 2010; Cheng et al., 2014; with an area of 3.2 ´105 km2 (Fig. 1). Wang et al., 2015). Also, it can buffer water resources (storing pre- cipitation) and thus become a primary source of recharge to the Water Storage Estimation Methods deep soil water and groundwater (Huang and Gallichand, 2006; The loess vadose zone is very deep. Previous studies have Gao et al., 2015; Li et al., 2017), impacting base flow to rivers and shown that the depths of most plant roots and precipitation infil- discharge of the rivers into the Yellow River, which has reportedly tration are within 5 m (Shan and Liang, 2006; Cheng et al., 2008; decreased in recent decades (Li et al., 2016; Wang et al., 2016). Zhao et al., 2012; Zhang et al., 2014; Bai et al., 2016; Fang et al., A few efforts have been made to quantify vadose zone water at 2016; Jia et al., 2019). With this consideration, the loess vadose various scales in the CLP (Li and Huang, 2008; Chen et al., 2010; zone was divided into two layers: the upper vadose zone (the upper Wang et al., 2011, 2015; Jian et al., 2015; Jia et al., 2017). Observations from the north–south sampling sites in the CLP have shown that 0- to 5-m soil water storage varies from 61 to 277 mm m−1 soil, depending on veg- etation type and climate (Jia et al., 2017). However, the maximum depth of SWC observation reported is only 21 m (far less than the average loess thickness) and the observed sites are few, coupled with a of technology for deep SWC observation, resulting in little informa- tion on the water stored in the deep loess vadose zone.
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