Mean Soil Water Storage Research Articles

Response of soil water to long-term revegetation, topography, and precipitation on the Chinese Loess Plateau

The Chinese Loess Plateau (CLP) rose to prominence for its fragmented terrain and fragile ecosystems. Large-scale revegetation was applied to control soil and water loss there. However, revegetation resulted in unexpected water shortages. In this study, we investigated the temporal-spatial distribution of soil water in the northern CLP based on 13-year soil water content (SWC) data of four different revegetation types on a loessial slope and 4-year data of a gully system to study the response of soil water to long-term revegetation, topography, and precipitation. Results showed that in the 0–400 cm soil depth under shrub, grass, natural fallow, and millet, soil water storage (SWS) was –102.8, –129.3, 54.9, and 39.3 mm higher after 13-year of restoration, respectively. Although soil water under shrubs- and grasslands became more replenished under high precipitation, planting these water-intensive vegetation types aggravated soil desiccation. The gully intensified soil water spatial heterogeneity, with mean SWS within 0–480 cm soil depth of 902, 712, and 746 mm at the gully bottom, edge, and bank, respectively. The deviation index among the four revegetation types and among gully bottom, edge, and bank were 0.43 and 0.26, respectively, which indicated that revegetation types had a more obvious effect on SWS. Given that more concentrated rainfall under global climate change, both revegetation type and topography should be considered during ecological restoration in semi-arid areas. These findings can deepen our comprehension of the hydrological processes on the CLP and provide a reference for the development of ecological restoration policies under future climate change.

Improving prediction accuracy of soil water storage through reducing sampling frequency

Knowledge of spatiotemporal dynamics of soil water storage (SWS) is essential for hydrological modeling and vegetation restoration in semi-arid areas. However, characterizing the temporal stability of SWS at a regional-scale requires time-consuming and labor-intensive manual sampling. Moreover, the influence of soil depth on temporal stability of SWS is not systematic. This study aimed to investigate the influences of sampling frequency and soil depth on SWS and SWS temporal stability. We measured soil moisture at 20-cm intervals in the soil profiles to a depth of 3 m using a neutron probe at 135 locations along a 1340-m long transect on 14 dates from 2012 to 2013. Results showed that sampling frequency did not influence the mean SWS (P < 0.05), while sampling frequency significantly affected temporal stability characteristics including Spearman’s rank correlation coefficient (rs), standard deviation of mean relative difference (SDRD), the number of locations with SDRD < 5%, and the representative locations. Temporal stability of SWS increased with the increasing soil thickness and depth, which increases the possibility of the number of representative locations in deep soil. Although the mean SWSs of all soil depths can be predicted accurately at each sampling frequency, the prediction accuracy improved when sampling frequency was reducing. The values of R2 ranged from 0.769 to 0.978 at 15-day sampling frequency, and from 0.987 to 0.998 at 45-day sampling frequency. Soil moisture stability may be more important than the soil water regime during prediction of soil moisture. These findings can provide guidelines for optimizing soil moisture sampling strategies and benefit management of water resources in semi-arid watershed.

Influence of tied-ridge with biochar amendment on runoff, sediment losses, and alfalfa yield in northwestern China

BackgroundLoss of organic matter and mineral nutrients to soil erosion in rain-fed agriculture is a serious problem globally, especially in China’s Loess Plateau. As a result, increasing rainwater usage efficiency by tied-ridge-furrow rainwater harvesting with biochar is expected to improve agricultural productivity. Nonetheless, with limited knowledge on tied-ridge-furrow rainwater harvesting with biochar, small-scale farmers face the challenge of adoption, thus, the rationale for this study.Materials and methodsA field experiment was conducted to determine the influence of open-ridging (OR) and tied-ridging (TR) with bio-degradable film on ridges and biochar in furrows on runoff, sediment losses, soil moisture, fodder yield, and water use efficiency (WUE) on sloped land, using flat planting (FP) without ridges and furrows as control, during alfalfa-growing year (2020).ResultsRunoff in flat planting (30%), open ridging (45%), and tied ridging (52%) were decreased with biochar to the extent where sediment was decreased in flat planting (33%), open ridging (43%), and tied ridging (44%) as well. The mean runoff efficiency was lower in flat planting (31%), open ridging (45%), and tied ridging (50%) in biochar plots compared to no-biochar plots. In biochar and no-biochar plots, soil temperature on ridges of TR was higher than that on OR, which was higher than FP during alfalfa growing season. Soil temperature in furrows during alfalfa growing season in biochar and no-biochar plots were in the order FP > OR > TR. Mean soil water storage for FP, OR, and TR, in biochar plots was higher than in no-biochar plots. This indicates biochar has a beneficial impact on open riding. Total annual net fodder yield (NFY) was significantly (p = 0.00) higher in treatments in the order TR > OR > FP. Tied ridging had a significant effect on actual fodder yield (AFY) in biochar plots, while open ridging significantly affected AFY in no-biochar plots. Annual total mean NFY and AFY increased by 8% and 11% in biochar plots compared to no-biochar plots. In biochar and no-biochar plots, water use efficiency was in the order TR > OR > FP. Conclusively, water use efficiency was significantly higher (p = 0.01) in biochar plots compared to no-biochar plots.ConclusionWhen crop production is threatened by soil erosion and drought, mulched tied-ridge with biochar is beneficial to crop growth in rain-fed agriculture, according to this research. Smallholder farmers should be trained on applying this technique for water-saving to mitigate runoff, soil erosion, sediment losses, and improve food security in semiarid areas.

Soil Water Storage Temporal Stability With and Without Outliers

The knowledge on the temporal stability of spatial variability of soil water storage in the crops&amp;rsquo; root zone is of fundamental importance for soil and water management. The objective of this work was to characterize the temporal distribution of water storage in a Latossolo vermelho amarelo and identify field locations with spatial patterns of high, intermediate and low soil water storage, in 13 samplings every 14 days. The assessed period included periods of drying and water recharge of the soil, along which soil water content was determined at 60 sampling points arranged in a 5 &amp;times; 5 m grid covering an area of 1250 m2 (25 &amp;times; 50 m). Soil water content was determined by means of a neutron probe, at soil depths of 0.2, 0.4, 0.6, 0.8 and 1.0 m. Soil water storage was calculated by Simpson&amp;rsquo;s rule and data were analyzed by the temporal persistence of the spatial pattern. Maximum values of soil water storage were obtained at the portion of the area with water flux concentration (sampling points 4, 28 and 57), with and without outliers, and low values of soil water storage were obtained at the highers levels of the site (sampling points 12, 18 and 19), with and without outliers. The sites representing the mean soil water storage were 32, 51 and 11, considering outliers, and 8, 11 and 53, without considering outliers.

Soil Water Dynamics and Winter Wheat Performance under Precipitation Change in the Loess Tableland Region, China

The responses of soil water and winter wheat (Triticum aestivum L.) to changes in precipitation are crucial for stability of wheat yield and soil water conservation in the Chinese Loess Tableland Region (LTR). A 2‐yr (2014‐2016) experiment was conducted in the LTR to investigate soil water dynamics and winter wheat performance under three precipitation levels: Rc (normal), R+ (increased by one‐third of normal), and R− (decreased by one‐third of normal) by a real‐time precipitation allocation system. Soil water storage (SWS) at a depth of 0–3.8 m showed positive (R+) or negative (R−) responses to precipitation. The mean SWS in different soil layers followed an order of R+ &gt; Rc &gt; R−. During the experiment, the 0–3.8 m SWS decreased by 34.5 mm (R+), 183.8 mm (Rc), and 218.9 mm (R−). The shoot biomass and grain yield varied little among three precipitation levels in 2014–2015, but was reduced by 50% in R− compared with Rc in 2015–2016. The R− and Rc treatments had higher water use efficiency (WUE) than R+ in 2014–2015, and Rc was the highest in 2015–2016. The proportions of soil water to water consumption in R+ and Rc increased from 22.5% in 2014–2015 to 42.5% in 2015–2016 on average, and for R− was stable over 2‐yr (average 44.5%). Soil water plays a critical role in winter wheat performance under changes in precipitation, which should guide winter wheat production and soil water management in the future.Core Ideas Precipitation changes soil water storage but does not change the regime of soil water changing with time. Response of winter wheat yield to precipitation change has hysteresis effect. Supplementary effect of soil water compensates for the negative effect of decreased precipitation. Performance of supplementary effect is determined by precipitation pattern.

Impacts of Climate Change on the Hydro-Climate of Peninsular Malaysia

In this study, a regional climate model was used to dynamically downscale 15 future climate projections from three GCMs covering four emission scenarios (SRES B1, A1FI, A1B, A2) based on Coupled Model Intercomparison Project phase 3 (CMIP3) datasets to 6-km horizontal resolution over the whole Peninsular Malaysia. Impacts of climate change in the 21st century on the precipitation, air temperature, and soil water storage were assessed covering ten watersheds and twelve coastal regions. Then, by coupling a physical hydrology model with the regional climate model, the impacts of the climate change on river flows were assessed at the outlets of ten watersheds in Peninsular Malaysia. It was found that the increase in the 30-year mean annual precipitation from 1970–2000 to 2070–2100 will vary from 17.1 to 36.3 percent among the ten watersheds, and from 22.9 to 45.4 percent among twelve coastal regions. The ensemble average of the basin-average annual mean air temperature will increase about 2.52 °C to 2.95 °C from 2010 to 2100. In comparison to the historical period, the change in the 30-year mean basin-average annual mean soil water storage over the ten watersheds will vary from 0.7 to 10.9 percent at the end of 21st century, and that over the twelve coastal regions will vary from −1.7 to 15.8 percent. Ensemble averages of the annual mean flows of the 15 projections show increasing trends for the 10 watersheds, especially in the second half of the 21st century. In comparison to the historical period, the change in the 30-year average annual mean flows will vary from −2.1 to 14.3 percent in the early 21st century, 4.4 to 23.8 percent in the middle 21st century, and 19.1 to 45.8 percent in the end of 21st century.

Interactive effects of autumn tillage with mulching on soil temperature, productivity and water use efficiency of rainfed potato in loess plateau of China

Low precipitation and temperature fluctuations are key limiting factors that influence the potato tuber yield in rainfed areas of the Loess Plateau, China. The implementation of tillage with mulching during fallow after the autumn harvest may be an effective management strategy to help reduce soil evaporation and conserve more soil water for spring sown potato. We conducted a three-year field experiment in a semiarid area of the Loess Plateau in 2014–2015, 2015–2016, and 2016–2017 to study the interactive effects of autumn tillage and mulching practices during fallow on the soil temperature, moisture, tuber yield, and water-use efficiency of potato. The treatments comprised three tillage systems in the main plots, i.e., no-tillage (NT), subsoiling (ST), and plowing tillage (PT), and three mulching methods in the subplots, i.e., straw mulch (SM), plastic-film mulch (PM), and no mulch (NM), in a replicated split plot design. Autumn tillage with mulching during fallow significantly influenced the soil water storage, especially in the early growing season of potato. Among the mulching methods, SM and PM increased the mean soil water storage by 6.3% and 5.9% (P < 0.05), respectively, compared with the NM plots. Conservation tillage combined with PM (NT×PM and ST×PM) significantly increased the topsoil temperature, whereas conservation tillage combined with SM (NT×SM and ST×SM) significantly decreased the temperature, which could have improved the topsoil conditions for potato tuber yield formation in the three experimental years. Among the tillage systems, the PM plots increased the mean topsoil temperature by 2.4°C (P < 0.05), whereas the SM plots reduced the temperature by 3.0°C (P < 0.05) during the whole period. The beneficial effects of the changes in moisture and temperature in the PM and SM plots improved the tuber yield of potato, in comparison with NM plots. Tillage and mulching had significant interaction effects on the potato tuber yield and water-use efficiency. The highest mean potato tuber yields were obtained in the ST×SM and NT×SM treatments, respectively, which were 41.0% and 35.3% higher (P < 0.05) than those with the PT×NM treatment. The three-year mean water-use efficiency increased significantly by 50.5% and 39.9% (P < 0.05) under the ST×SM and NT×SM treatments, respectively, and the three-year mean rainwater-use efficiency increased by 30% and 29% (P < 0.05) when compared with CT×NM treatment. Overall, our findings demonstrate that conservation tillage with SM and PM significantly increased the potato tuber yield and water-use efficiency. Considering the years with warm and dry climate conditions, we recommend conservation tillage with SM may be a more efficient method for improving the potato productivity, WUE and RUE in the semiarid region of the Loess Plateau.

Intensive land restoration profoundly alters the spatial and seasonal patterns of deep soil water storage at watershed scales

Soil-water storage (SWS) highly correlates with changes to land use and land restoration, and is an important variable for managing water resources and environmental restoration in water-limited terrestrial ecosystems. The Gully Land Consolidation project (GLC) was implemented in 2011 on the Chinese Loess Plateau and it may influence the spatial distribution and temporal variability of SWS at the watershed scale. However, there have been limited studies assessing the magnitude and pattern of such changes. We measured SWS to a depth of 5 m nine times over four seasons across two watersheds－one treated by the GLC project (GT-T, n = 89) and the other untreated (GT-U, n = 72). We then used correlation analysis, principle component analysis, multiple linear regression analysis, generalized linear model, and regression kriging to evaluate the seasonal variability, as well as spatial patterns, of SWS. Mean SWS in the spring, summer, autumn, and winter ranged from 830 to 934 mm in the GT-T watershed and from 681 to 771 mm in the GT-U watershed. Land use was the most important factor for predicting SWS in the two watersheds. After land management as a result of the GLC project, SWS exhibited different spatial patterns in the GT-T watershed compared with the GT-U watershed. Dynamics of SWS between adjacent seasons in the GT-U watershed displayed more variability than that in the GT-T watershed. In both watersheds, soil water volume (SWV, i.e., SWS per unit area × watershed area) declined from spring to summer (-3.9% in the GT-T and -13.4% in the GT-U), and increased from summer to autumn (11.7% in the GT-T and 20.7% in the GT-U). After the whole year, SWV displayed different degrees of increase in the two watersheds (10.6% in the GT-T, 2.7% in the GT-U) due to the effect of land restoration. The ratios of SWV in the gully land filled in the GT-T watershed to the SWV of the whole GT-T watershed for the four seasons ranged from 21.4–23.3%, with a mean value of 22%. Our results provide new insights regarding how human engineering of landscapes (as exemplified by GLC project) affect the spatial distribution of SWS and the dynamics of SWV storing capacity at the watershed scale. Understanding the dynamics of SWS/SWV between seasons is essential to evaluate, model and manage the soil water resources in human-affected watersheds.

Temporal stability of soil water storage in multiple soil layers in high-elevation forests

Understanding soil water storage (SWS) dynamics in soil profiles is important for hydrological modeling and restoration of vegetation in semi-arid areas. Using the temporal stability method, we aimed to investigate the temporal stability of SWS at 0–40, 40–80, and 80–120 cm depth, and to identify representative sites for reliable estimates of the mean SWS in the permanent forest plot in the Qilian Mountains of China. Further, we wanted to identify correlations between temporal stability of SWS and soil, topography, and properties of the vegetation. Soil water content at soil depths 0–120 (for locations 1–40) and 0–70 cm (for locations 41–60) were measured using time-domain reflectometry (TDR) on 52 dates from 2016 to 2017.Results revealed that time-averaged mean SWS for the three layers differed significantly (P < 0.05), and the temporal changes and spatial variations of the mean SWS all decreased with increasing soil depth. Based on either the Spearman correlation coefficient or the standard deviation of relative difference (SDRD) index, the temporal stability of SWSs within the soil profiles under different soil layers were strong, and the number of time-stable locations increased with increasing soil depth, indicating that the SWS intended to be more temporally stable in deeper soil. One time-stable site can be representative of the mean SWS for multiple soil layers for the whole plot, and can accurately estimate the mean SWS for the three soil layers (R2 > 0.86, P < 0.001). Moreover, these time-stable locations should be those having average LAI, SBD or relatively low canopy interception loss compared to the corresponding field means. Soil bulk density, canopy interception rate, and aboveground biomass of mosses can significantly (P < 0.05) affect the stability of SWSs in this high-elevation forests. Such effects, however, differed among the different soil layers due to different conditions of soil characteristics, distribution of root biomass, and freeze–thawing processes. These results suggest that the influence of vegetation properties of the temporal stability of SWS should attract more attention and that both the relative importance of and interactions among different determining factors is helpful for better understanding the mechanistic determinants of SWS temporal stability in these areas.

The effects of rotating conservation tillage with conventional tillage on soil properties and grain yields in winter wheat-spring maize rotations

Intensive tillage in conventional tillage systems reinforces water stress effects on crop growth, limiting yields from dryland agriculture. Conservation tillage can reduce soil evaporation and conserve more soil water in fields, but long-term, mono-conservation tillage may lead to low crop yields. The rotation of conventional tillage with conservation tillage may offset some of the defects generated by the mono-tillage practices of either conventional or conservation tillage, improve crop yields and provide better soil conditions. A long-term tillage rotation experiment (2007–2016) was established to assess the effects of rotating conservation tillage with conventional tillage on soil water and crop productivity in a winter wheat-spring maize rotation field in Heyang County, Shaanxi Province, a typical semi-arid region of the Loess Plateau, China. Four tillage treatments were applied over ten years, as follows: ST/CT (subsoiling was performed during the first year then rotated with conventional tillage in the second year), CT/NT (conventional tillage during the first year and then rotated with no-tillage in the second year), NT (no tillage applied in any year) and CT (conventional tillage applied annually). Compared with CT and NT, the CT/NT rotation significantly decreased the soil bulk density and increased the soil porosity in the 0–60 cm soil layer after ten years (P < 0.05). The CT/NT and ST/CT rotations increased the minimum soil temperature during the wheat growth season and decreased the maximum soil temperature during the maize growth season. In fallow periods, the mean soil water storage and soil water content values for the CT/NT rotation were 8.7% and 4.8–10.1% higher than those of the CT treatment, respectively. The CT/NT rotation consumed more soil water during the winter wheat growth season and less soil water during the spring maize growth season compared to the NT and CT treatments. Over ten years, the ST/CT rotation produced higher crop yields (winter wheat: 5231 kg ha−1, spring maize: 8388 kg ha−1), while the CT/NT rotation had a higher WUE (winter wheat: 15.5 kg ha−1 mm−1, ST/CT: 20.6 kg ha−1 mm−1) than either NT or CT alone. The CT/NT rotation also had an increased straw yield (mean value: 9952 kg ha−1) and economic profit (6776 yuan ha−1). Moreover, the CT/NT and ST/CT rotations provided an optimal SOC distribution in the 0–60 cm soil layer. With respect to comprehensive productivity, the CT/NT rotation provided relatively better soil conditions and crop yields during the winter wheat-spring maize rotations. We recommend using the CT/NT rotation as the optimal tillage system for the sustainable production of crops under conditions of semi-arid agricultural production in the Loess Plateau of China.

Profile distribution of soil moisture in the gully on the northern Loess Plateau, China

Gully is a typical geographical unit and a key component of the slope–gully system on the northern Loess Plateau. Understanding the distribution of soil moisture in gullies aids water resource regulation and ecological restoration. To investigate temporal–spatial variability of soil moisture, we measured values of soil water content (SWC) within 0–500 cm by using a neutron probe on 19 occasions from October 2014 to July 2017 in the gully and bank of a developed gully in Liudaogou catchment of Shenmu County. Results showed that the values of space–averaged soil water storage (SWS) in the gully were significantly (P < 0.01) higher than those in the bank. Soil clay content (SCC) and topography (including site elevation and distance from the gully head) were the most important factors affecting SWS of 0–500 cm in the gully. Topography can indirectly control SWS by redistribution of rainfall and soil properties. SWS and SCC rose with increasing distance from the gully head to the outlet. SWS in the gully exhibited strong temporal stability, and two time–stable sites around the middle section can be used to represent mean SWS within 500 cm depth in the gully. The gully sidewall significantly (P < 0.05) aggravated drought of soils in the bank between 150 and 250 cm from the gully edge. Gullies make the temporal–spatial patterns of soil moisture complex in slope–gully system. These results were expected to improve understanding of soil moisture distribution in the gully region and aid vegetative restoration of the northern Loess Plateau.

Spatial and temporal variability of 0‐ to 5‐m soil–water storage at the watershed scale

AbstractDynamic relationships among rainfall patterns, soil water distribution, and plant growth are crucial for sustainable conservation of soil and water resources in water‐limited ecosystems. Spatial and temporal variation in deep soil water content at a watershed scale have not yet been characterized adequately due to the lack of deep soil water data. Deep soil–water storage (SWS) up to a depth of 5 m (n = 73) was measured at 19 sampling occasions at the LaoYeManQu watershed on the Chinese Loess Plateau (CLP). At a depth of 0–1.5 m, the annual mean SWS was highly correlated with rain intensity, and the correlation decreased with depth, but within the layers at 1.5–5.0 m, the changes in SWS indicated a lag between precipitation and the replenishment of soil water. Geostatistical parameters of SWS were also highly dependent on depth, and the mean SWS presented similar spatial structures in two adjacent layers. Temporal stability of SWS as indicated by mean relative difference, standard deviation of the relative difference (SDRD), and mean absolute bias error (MABE) was significantly weaker at the shallow than at deeper layers. Soil separates and organic carbon content controlled the spatial pattern of SWS at the watershed scale. One representative location (Site 57) was identified to estimate the mean SWS in the 1‐ to 5‐m layer of the watershed. Semivariograms of the SDRD and MABE were best fitted by an isotropic spherical model, and their spatial distributions were depth‐dependent. Both temporal stability and spatial variability of SWS increased over depth. This study is helpful for deep SWS estimation and sustainable management of soil and water on the CLP, and for other similar regions around the world.

Soil Water Dynamics in Apple Orchards of Different Ages on the Loess Plateau of China

Core Ideas Soil water dynamics in apple orchards (initially 7 and 17 yr old) were studied. Mean soil water content was consistently higher in the 7‐yr‐old orchard. Mean soil water storage was consistently lower in the 17‐yr‐old orchard. A dried soil layer had formed in the 17‐yr‐old orchard. Tree age should be considered when formulating soil water management regimes. Knowledge of soil water in apple (Malus domestica Borkh.) orchards is necessary for formulating robust management strategies and sound planning of apple plantations with limited water resources, such as on the Chinese Loess Plateau. In this work, we assessed the relationship between the age of trees and soil water dynamics in apple orchards that were initially 7 and 17 yr old in the Wangdonggou watershed on the Loess Plateau. For this, we monitored soil water content (SWC, measured using a neutron probe) and calculated soil water storage (SWS), available SWS (ASWS), and soil desiccation along 0‐ to 600‐cm soil profiles in both orchards from May to September in 2012, 2013, 2014, and 2015. For both orchards, annual mean SWCs along the 0‐ to 600‐cm profile differed during the past 4 yr. Mean SWC along the profile was consistently higher in the 7‐yr‐old orchard at both monthly and annual timescales. Accordingly, monthly mean SWS and ASWS along the 0‐ to 600‐cm soil profiles were consistently lower in the 17‐yr‐old orchard. Moreover, a dried soil layer (DSL) had formed in the soil of the 17‐yr‐old orchard and there was slightly desiccation in the 320‐ to 600‐cm layers, but no DSL was detected in the 7‐yr‐old orchard during the four study years. Tree age was mostly responsible for the differences in soil water dynamics in apple orchards. Therefore, it is important to take tree age into account when considering soil water management regimes in orchards in both the focal region of the Loess Plateau and other water‐limited regions around the world.

Temporal stability of soil water storage and its influencing factors on a forestland hillslope during the rainy season in China’s Loess Plateau

Large-scale vegetation restoration in China’s Loess Plateau has been initiated by the central government to control soil and water losses since 1999. Knowledge of the spatio-temporal distribution of soil water storage (SWS) is critical to fully understand hydrological and ecological processes. This study analysed the temporal stability of the SWS pattern during the rainy season on a hillslope covered with Chinese pine (Pinus tabulaeformis Carr.). The soil water content in eight soil layers was obtained at 21 locations during the rainy season in 2014 and 2015. The results showed that the SWS at the 21 locations followed a normal distribution, which indicated moderate variability with the coefficients of variation ranging from 14 to 33%. The mean SWS was lowest in the middle slope. The spatial pattern of SWS displayed strong temporal stability, and the Spearman correlation coefficient ranged from 0.42 to 0.99 (p < 0.05). There were significant differences in the temporal stability of SWS among different soil layers (p < 0.01). The spatial patterns of SWS distribution showed small differences in different periods. The best representative locations of SWS were found at different soil depths. The maximum RMSE and MAE at 0–1.6 m soil depth for the rainy season were 4.27 and 3.54 mm, respectively. The best representative locations determined during a short period (13 days) can be used to estimate the mean SWS well for the same rainy season, but not for the next rainy season. Samples of SWS collected over a fortnight during the rainy season were able to capture the spatial patterns of soil moisture. Roots were the main factor affecting the temporal stability of SWS. Rainfall increased the temporal stability of the soil water distribution pattern. In conclusion, the SWS during the rainy season had a strong temporal stability on the forestland hillslope.

Estimation of spatial mean soil water storage using temporal stability at the hillslope scale in black locust (Robinia pseudoacacia) stands

Black locust (Robinia pseudoacacia) has been extensively planted on the Loess Plateau for soil and water conservation, but afforestation with this non-native tree has produced some negative effects such as soil desiccation. Knowledge of the soil water storage (SWS) within soil profiles in black locust stands is thus critical for optimization of forest management for sustainable production. The objectives of this study were to evaluate the temporal stability of SWS to identify the most time-stable locations (MTSLs) for reliably estimating the mean SWS and to further investigate the factors influencing its temporal stability. The SWS values of various soil layers (0–1, 1–2, and 2–3m) were measured from May 2014 to October 2015 at 70 locations along two 187-m long transects on a hillslope covered with black locust in the Loess Plateau, China. A total of 18 SWS datasets were collected over the period of measurement. Results indicated that the temporal variation of spatial mean SWS decreased and the spatial variation of SWS increased with increasing soil depth. A strong temporal persistence in all soil layers was demonstrated by high Spearman's rank correlation coefficients (P<0.01), with the temporal stability in deep soil layers relatively more stable than in shallow layer. Among four methods (MRD, mean relative difference; SDRD, standard deviation of relative difference; ITS, index of temporal stability; and MABE, mean absolute bias error) used to identify the MTSLs for the estimation of the mean SWS, the ITS provided the best results. The MTSLs identified in diverse layers of the first transect were then used to successfully predict the mean SWS along the second transect (R2>0.91). Elevation, soil saturated hydraulic conductivity, leaf area index, and fine root area density were the major factors influencing the temporal stability of SWS. The results of this study were useful for estimating mean SWS and could improve soil water management and hydrological applications in sloping black locust forested areas of the Loess Plateau.

Spatiotemporal Characteristics of Soilwater Storage Along Regional Transect on the Loess Plateau, China

Knowledge of the spatiotemporal variability of soil water storage (SWS) is important for vegetation restoration and water resources optimization in semi‐arid areas. This study investigated the spatiotemporal characteristics of SWS in the 0–5 m profile and identified representative sites for reliably estimating the mean SWS along regional transect of the Loess Plateau. Mean relative difference and the associated standard deviation (SDRD) were used to assess temporal stability (TS) of profile SWS. The time‐averaged mean SWS in the 5‐m profile generally decreased from south to north following the decreasing rainfall gradient, with a mean value of 704 mm. Spatiotemporal analysis showed that temporal variations in SWS decreased with increasing soil depth, while the spatial variations increased. Comparisons of SDRD and the number of time‐stable locations with SDRD &lt;5% among various depths indicated that TS increased with increasing soil depth. The representative sites identified for the transect well reflected mean SWS of the five soil layers, and the accuracy of estimation increased with increasing soil depth. The TS of SWS patterns was controlled by precipitation, temperature, clay content, field capacity, soil organic carbon, elevation, slope gradient, and normalized difference vegetation index in the study area. This study demonstrates the feasibility of directly estimating the patterns of SWS using TS analysis on a large scale, which might be comprehensively controlled by the combined effects of climate, soil, topography, and vegetation.

Soil Water Storage Changes within Deep Profiles under Introduced Shrubs during the Growing Season: Evidence from Semiarid Loess Plateau, China

Water stored deep in the soil profile is the primary bio-available reservoir for regional vegetation in the semiarid Loess Plateau of China. However, the planting of introduced shrubs over many years as part of the “Grain to Green Program (GGP)” has consistently lead to dried soil in areas with severe water scarcity. Knowledge of soil water storage (SWS) changes within deep profiles in water-deficient regions is critical for the sustainable development of vegetation restoration. Caragana korshinskii K. (CK) and Hippophae rhamnoides L. (HR) are widely planted in the Loess Plateau to control soil erosion. We selected these two shrubs for a study on variations in deep soil water (100–500 cm) and identified the main factors affecting deep soil water storage replenishment (SWSR) during their growing seasons. The results indicated that the mean SWS at 100–500 cm depth under HR was significantly higher than that under CK at both the beginning (352.74 mm for CK and 644.79 mm for HR) and end of the growing season (311.95 mm for CK and 529.05 mm for HR) (p &lt; 0.01). In these ecosystems, SWS was only recharged below 340 cm under CK, which was due to vegetation characteristics. Under HR, however, soil water consumption exceeded recharge throughout the whole 100–500 cm profile. The SWSR at the 100–340 cm depth was mainly affected by sand content, which explained 28% of the variability of SWSR. At the 340–500 cm depth, the variability in SWSR was due to vegetation type. Therefore, expansion of the GGP should pay more attention to both soil water conditions and influencing factors, including appropriate vegetation selection and the altering of the microtopography.

Temporal persistence and stability of soil water storage after rainfall on terrace land

The “Grain for Green” Project on the Loess Plateau of China has converted a large area of cultivated slope farmland to terraces to reduce soil erosion and improve the ecological environment. The area of terraced land on the Loess Plateau was 32,600 km2 in 2010. Knowledge of the behavior of soil water storage (SWS) and its spatiotemporal distribution provides essential information concerning hydrological and ecological processes. This study aimed to analyze the temporal stability of SWS after rainfall for different time periods on terraces planted with jujube trees (Ziziphus jujuba Mill.). The SWS of 21 locations was obtained from July 19 to September 3 in 2014 and from August 01 to August 31 in 2015. Relative difference analysis and the nonparametric Spearman rank correlation test were used to check the temporal stability of SWS. The SWS on terraces was normally distributed and demonstrated moderate spatial variability. The SWS at three soil depths all showed strong temporal stability with stability in the order of 0–0.6 < 0–1.0 < 0–1.6 m. Rainfall significantly affected the temporal stability of SWS in the depth of 0–0.6 m. The number of representative locations of SWS was not constant. The best representative locations were not identical for different soil depths; however, a common representative location did exist for the different depths and periods. Moreover, the mean SWS generally increased from the upper to lower slope. The temporal stability of SWS was not significantly affected by rainfall but was significantly negatively correlated with sand content. The total root length and soil organic carbon content did not show any obvious correlations with mean SWS. It was feasible to use the representative locations of SWS to represent the mean SWS over a period of time. However, the cumulative absolute error increased with cumulative days. In conclusion, SWS in the deeper soil depths indicated greater temporal stability than that in shallow soil depth during wet period. The best representative locations for SWS were depth and time dependent.

Temporal stability of soil water storage analysis by the improved mean relative difference in two deposited-soil farmlands

Temporal stability of soil water storage (TSSWS) is of considerable interest in estimating the mean soil water storage (SWS) across the study area for validating the remote sensing products and reducing the observation efforts. However, the general mean relative difference index usually balances the positive and negative relative difference values. Thus, it may find the weakly representative site for mean SWS estimation. Revised mean relative difference, i.e., the absolute value of general relative difference, was used in two deposited soil farmlands (DFs) to analyze the TSSWS. The soil water contents were measured at 18 sites of each DF in 1.5 years. The results showed that revised mean relative difference index was more accurate than normal one in finding the best location to represent mean SWS in the DFs. This can be applicable to sites where SWS is not consistently higher or less than the average value. The SWS in the location which was near to the middle DF showed the strongest temporal stability and behaved the best location to estimate the mean SWS. Revised mean relative difference may be a better criterion for the application of temporal stability tool in soil water field observation.

Can shallow-layer measurements at a single location be used to predict deep soil water storage at the slope scale?

Summary Knowing the amount of soil water storage (SWS) in agricultural soil profiles is important for understanding physical, chemical, and biological soil processes. However, measuring the SWS in deep soil layers is more expensive and time consuming than in shallower layers. Whether deep SWS can be predicted from shallow-layer measurements through temporal stability analysis (TSA) remains unclear. To address this issue, the soil water content was measured at depths of 0–1.6 m (0.2-m depth intervals) at 79 locations along an agricultural slope on 28 occasions between July 2013 and October 2014. SWSs values were then calculated for the 0–0.4, 0.4–0.8, 0.8–1.2, 1.2–1.6, and 0–1.6 m soil layers. The SWS exhibited strong temporal stability, with mean Spearman’s ranking coefficients ( r s ) of 0.83, 0.92, 0.83, and 0.79 in the 0–0.4, 0.4–0.8, 0.8–1.2, and 1.2–1.6 m soil layers, respectively. As expected, the most temporally stable location (MTSL1) accurately predicted the average SWS of the corresponding soil layer, and the values of absolute bias relative to mean (ARB) were lower than 3% for all of the investigated soil layers. Using TSA, deep-layer SWS information could be predicted using a single-location measurement in the 0–0.4 m soil layer. The mean ARB values between the observed and predicted mean SWS values were 2.9%, 4.3%, 3.9%, and 2.7% in the 0.4–0.8, 0.8–1.2, 1.2–1.6, and 0–1.6 m soil layers, respectively. The prediction accuracy of the spatial distribution generally decreased with increasing depth, with linear determination coefficients ( R 2 ) of 0.93, 0.79, 0.72, and 0.84 for the four soil layers, respectively. The proposed method could further expand the application of the temporal stability technique in the estimation of SWS.