Spatial distribution of water-active soil layer along the south-north transect in the Loess Plateau of China

Abstract

Soil water is an important composition of water recycle in the soil-plant-atmosphere continuum. However, intense water exchange between soil-plant and soil-atmosphere interfaces only occurs in a certain layer of the soil profile. For deep insight into water active layer (WAL, defined as the soil layer with a coefficient of variation in soil water content >10% in a given time domain) in the Loess Plateau of China, we measured soil water content (SWC) in the 0.0-5.0 m soil profile from 86 sampling sites along an approximately 860-km long south-north transect during the period 2013-2016. Moreover, a dataset contained four climatic factors (mean annual precipitation, mean annual evaporation, annual mean temperature and mean annual dryness index) and five local factors (altitude, slope gradient, land use, clay content and soil organic carbon) of each sampling site was obtained. In this study, three WAL indices (WAL-T (the thickness of WAL), WAL-CV (the mean coefficient of variation in SWC within WAL) and WAL-SWC (the mean SWC within WAL)) were used to evaluate the characteristics of WAL. The results showed that with increasing latitude, WAL-T and WAL-CV increased firstly and then decreased. WAL-SWC showed an opposite distribution pattern along the south-north transect compared with WAL-T and WAL-CV. Average WAL-T of the transect was 2.0 m, suggesting intense soil water exchange in the 0.0-2.0 m soil layer in the study area. Soil water exchange was deeper and more intense in the middle region than in the southern and northern regions, with the values of WAL-CV and WAL-T being 27.3% and 4.3 m in the middle region, respectively. Both climatic (10.1%) and local (4.9%) factors influenced the indices of WAL, with climatic factors having a more dominant effect. Compared with multiple linear regressions, pedotransfer functions (PTFs) from artificial neural network can better estimate the WAL indices. PTFs developed by artificial neural network respectively explained 86%, 81% and 64% of the total variations in WAL-T, WAL-SWC and WAL-CV. Knowledge of WAL is crucial for understanding the regional water budget and evaluating the stable soil water reserve, regional water characteristics and eco-hydrological processes in the Loess Plateau of China.

Deep soil water <sup>18</sup>O and <sup>2</sup>H measurements preserve long term evaporation rates on China's Loess Plateau

“…Therefore, the invariable isotope compositions of deep soil 215 water indicate that deep soil water is free of seasonal variation in evaporation and precipitation infiltration (Thomas et al, 2013;Sprenger et al, 2016;Cheng et al, 2014). This is consistent with previous isotope studies (Wan and Liu, 2016;Yang and Fu, 2017) and is supported by the observation of soil water dynamics in this region (Zhao et al, 2019). Moreover, the reduced variations of stable isotopes in deep soils can be attributed to the mixing processes in the deep unsaturated zone since it has the long residence time (DePaolo et al, 2004;Thomas et al, 2013).…”

“…Precipitation and soil evaporation are two main factors to the variation of soil water isotopes; however, soil evaporation is limited to a few to dozens of centimeters only (Cheng et al, 2014) and precipitation infiltration depth rarely exceeds two meters at our study area (Jin et al, 2018;Zhao et al, 2019). Therefore, the invariable isotope compositions of deep soil 215 water indicate that deep soil water is free of seasonal variation in evaporation and precipitation infiltration (Thomas et al, 2013;Sprenger et al, 2016;Cheng et al, 2014).…”

Factors determining soil water heterogeneity on the Chinese Loess Plateau as based on an empirical mode decomposition method

“…Many field experiments have been conducted on the Chinese Loess Plateau to reveal spatial and temporal distributions of soil water (He et al, 2019;Liang et al, 2019;Zhao et al, 2019;Yu et al, 2020), while influences of soil physical properties on SWC were seldomly been studied. Geostatistics have also been used to assess heterogeneity of surface soil water (Zou et al, 2019); however, the analysis of SWC using this method was greatly affected by spatial scale (Lian et al, 2019).…”

Effects of Climatic Change on Soil Hydraulic Properties during the Last Interglacial Period: Two Case Studies of the Southern Chinese Loess Plateau

“…Therefore, the impact of the precipitation on soil hydraulic properties might be reduced. For preferential flow, studies showed that the modern water active layer was no more than 2 m on the Loess Plateau [63], whereas the modern climate of the CLP is wetter than that of the last glacial period, and the impact of water flow on soil pedogenesis is a time consuming process [52]. For the intact and typical paleosol layer, the influence of precipitation on soil hydraulic properties may not have been so severe, and the hydraulic properties of the paleosol were mainly determined by the paleoclimate when it developed.…”