Shallow Soil Water Content Research Articles

Effects of coal mining disturbance on spatial and temporal distribution of soil water content in Northwest China‐based on 3D EBK model

AbstractNorthwest China is both an important coal storage base and an ecologically fragile area, and soil water content (SWC) is a key factor limiting the ecological development of Northwest China. Revealing the characteristics of spatial and temporal distribution changes of soil water content in mining areas under coal mining disturbance and the influencing mechanism is crucial for the protection of water resources in mining areas. In this study, the soil water content (depth 0–1000 cm) of a typical coal mine subsidence area in the western part of Shendong Coal Group was monitored in situ for 1 year after mining, and the absolute value, variability, and spatial distribution of soil water content were temporally analysed by combining classical statistics, 2D Ordinary Kriging and 3D Empirical Bayesian kriging spatial interpolation. sequence analysis. The results showed that the shallow SWC (0 ~ 60 cm) was distributed horizontally in bands, and gradually increased along the direction from northwest to southeast; with the increase of coal mining time, the absolute value of SWC decreased by 0.81% ~ 33.58%, and the coefficient of variation decreased and then increased, with the range of variation from 2.19% ~ 34.49%. The deep SWC (100 ~ 1000 cm) was stratified vertically and increased with soil depth, and the shallow and deeper soil moisture would gradually migrate to the middle layer under the influence of coal mining. In addition, this paper accurately portrays the three‐dimensional spatial and temporal distribution of soil water content by the 3D EBK model, which further reveals the mechanism of coal mining's influence on soil water content. This study can provide technical and data support for predicting and evaluating the potential impacts of mining activities on the water cycle, and help mining areas to formulate policies for managing and protecting water resources.

Effect of Nostoc commune cover on shallow soil moisture, runoff and erosion in the subtropics

Nostoc commune (Nostoc commune Vaucher ex Born & Flahault) is widely distributed in grasslands within the subtropical region. Because of its morphological changes are more sensitive to moisture, it may affect the soil’s response to infiltration-runoff processes, thus significantly influencing soil moisture dynamics, runoff, and soil loss. However, the exact effects of this phenomenon are not yet fully understood. This study established a one-year monitoring experiment of soil moisture, runoff and soil erosion in the karst area of Southwest China from September 2021 to November 2022. Using bare soil (CK) as the control, two treatments of Nostoc commune cover at 100 % coverage (C1) and 50 % coverage (C2) were set up in 3 repetitions to investigate the shallow soil’s response to Nostoc commune cover and rainfall and to quantify the effects of land cover and rainfall type on runoff and soil loss. The results showed that Nostoc commune cover significantly increased the shallow-soil water content, and the effect of increasing soil water content was more significant with increasing coverage. The cumulative soil water increment (CSWI) varied greatly under different types of rainfall due to the high water-absorbing capacity of Nostoc commune. The CSWI showed C1 < C2 < CK under light rain and moderate rain events, while the CSWI showed C1 > C2 > CK under heavy rain and rainstorm events. The temporal pattern of soil moisture changed greatly under different cover treatments and rainfall events. In particular, the response time under different rainfall types varied from 10 min to 250 min. The runoff depth and soil loss in CK were 1.19–2.15 times and 1.92–14.43 times that of C1 and C2, respectively. This indicates that Nostoc commune cover has a significant effect on controlling runoff and soil erosion. In addition, the sensitivity of runoff and soil loss to the maximum 10-min rainfall intensity was higher than that of the maximum 30-min rainfall intensity, which was attributed to the rapid swelling of Nostoc commune in the hydration process, which greatly reduced runoff and soil erosion. This study can contribute to further understanding the hydrological process and soil erosion characteristics under different surface covers in humid karst areas.

How residual plastic film affects the soil water–salt and cotton growth at the seeding stage

AbstractCotton holds a significant position among cash crops in China, with Xinjiang serving as the primary cotton‐growing region within the country. Agricultural practices involving plastic mulching have gained importance. However, the presence of residual plastic film affects the growth of cotton seedlings as well as the soil environment. Our study aimed to explore the impact of residual plastic film accumulation on the growth of cotton seedlings and the associated soil water‐salt environment. This was done through a barrel planting experiment, utilizing soil from cotton fields with varying durations of mulching. The study showed that when the mulching period exceeded 12 years (residual plastic film content of 260.77 kg hm−2), residual plastic film had a significant impact on cotton seedling growth and the soil environment. Residual plastic film reduces the seedling germination rate of cotton by up to 11.11% (7 days after irrigation, 15 years of film mulching, residual plastic film content of 309.88 kg hm−2). It also impedes the growth of cotton seedlings; plant height, stem thickness, and leaf area were reduced by 34.52%, 10.73%, and 37.18%, respectively (19 days after irrigation, 15 years of film mulching, residual plastic film content of 309.88 kg·hm−2). Compared with the treatment without residual plastic film, the shallow soil water content in the T5 treatment decreased by 1.19% (10 days after irrigation, 15 years of film mulching, residual plastic film content of 309.88 kg·hm−2). Residual plastic film in cotton fields subjected to long‐term mulching obstructs the normal growth and development of cotton seedlings while also impacting the transport of soil water and salt. Therefore, we recommend adopting biodegradable mulch or regularly recycling residual plastic film within mulched areas to prevent its accumulation and subsequent negative impacts.

Potential of ground-penetrating radar to calibrate electromagnetic induction for shallow soil water content estimation

Ground-penetrating radar (GPR) and electromagnetic induction (EMI) are used to determine and map soil water content (SWC) in the agricultural landscape. While GPR provides a straightforward estimation of SWC, EMI requires site-specific calibrations mainly based on point-scale measurements such as time domain reflectometry (TDR). However, there is a significant difference in the measurement volumes between EMI and point-scale TDR measurements. This study aimed to enhance the calibration of EMI for estimating SWC in the agricultural landscape by leveraging the larger sampling volumes provided by GPR. Apparent electrical conductivity (ECa) from a multi-coil EMI sensor and dielectric constant from GPR with two center frequencies (500 MHz and 250 MHz) were collected as soil proxies along with TDR measurements. Calibration models were developed under irrigated conditions and the model evaluation was conducted under natural moisture conditions. Correlations were assessed between the proxies from GPR and EMI with TDR-derived SWCs. Simple linear regression (SLR) models were developed between EMI and GPR data to predict SWC. Strong positive correlations (r ≥ 0.80) were observed between the proxies (GPR and the shorter inter-coil spacing (0.32 m) of EMI) and TDR-measured SWC. ECa data from vertical and horizontal coil orientations of the shorter inter-coil spacings were selected to develop SLR models with GPR. The SLR models with the GPR 500 MHz frequency showed a higher coefficient of determination (R2 > 0.70) for both coil orientations of EMI. Results showed the potential of using GPR to calibrate EMI for shallow SWC estimation (below 0.5 m). Nevertheless, the EMI estimated SWCs were not effectively verified with GPR-estimated SWCs, which could be attributed to specific site conditions in the study area. Further research must focus on improving the calibration and model evaluation by incorporating the variability of other soil parameters (soil porosity, pore-water conductivity, and soil salinity) that may affect the SWC–ECa relationship.

Selective removal of non-woody species released water limitation on vegetation community stagnated at early successional stages in a humid karst region

The use of plantations and natural restoration enhance vegetation coverage and contribute significantly to the hydrologic functions of degraded terrestrial ecosystems. Researchers have investigated the development and use of plantations, however, few studies looked at naturally-restored vegetation, especially for early successional areas. We applied a series of treatments and their impacts on water status and the dominant woody species were monitored. Our goal is to enhance the hydrologic functions of naturally-restored karst vegetation that stagnated as early successional shrubby-grassland. Our results show that vegetation on both the upper and lower slopes intercepted approximately 50 % of the rainfall because of the dense community structure. With more than 60 % of the community-level root biomass concentrated in the upper 20 cm soil depth, shallow soil water content (SSWC) decreased continuously even when there was scattered light rain events. On the lower slopes, competition-reducing oriented removal of non-woody species significantly reduced canopy interception of water, which contributed to the increase of SSWC and water availability for the remaining dominant woody species. Moreover, dominant woody species in the treated plot exhibited more negative leaf carbon isotope values than the same species in the control plot, indicating released water limitation on plant growth because of the treatment. On the contrary, this treatment exhibited negligible effects on SSWC on the upper slope, environmental water status could only be improved by improving soil water storage capacity. Our results highlight strong competition on SSWC for naturally-restored vegetation on karst hillslopes that stagnated during the early successional stages. In lower slope position where soil is relatively deep, competition-reducing oriented treatment alone can release water limitations on plant growth and provide opportunities for the settlement of species with different hydrologic niches, thus providing a path for enhancing hydrologic functions of naturally-restored karst vegetation.

Energy Distribution Characteristics and Environmental Impact Factors in Different Regions of the Summer Monsoon Transition Zone

The summer monsoon transitional zone is a typical transitional ecosystem that is, also ecologically fragile. It is critical to explore the land surface energy balance and the water and heat transfer processes in this region for water resource management. However, the processes of surface water and heat transfer throughout the transition zone and the effects of environmental factors on energy distribution are understudied. Therefore, we analyzed the surface energy distribution and the environmental factors influencing it in the eastern and western transition zone from July to September, and compared the differing effects of meteorological factors and vegetation on the energy distribution between the two regions. The results show that the main consumption item of the available energy in the west of the transition zone is the latent heat flux. The same is true in the eastern region from July to August, but it changes to sensible heat flux in September. The factors influencing the Bowen ratio in the two regions were ground temperature difference, vapor pressure deficit, shallow soil water content, and drought stress index. Nevertheless, the Bowen ratio in the western region was more sensitive to water factors, whereas that in the eastern region was more sensitive to energy factors. The relationship between the Bowen ratio and the normalized difference vegetation index in the eastern region was more regular. This study provides a scientific basis for reasonably guiding agricultural production in the transition area.

Effect of soil water content changes caused by ridge-furrow plastic film mulching on the root distribution and water use pattern of spring maize in the Loess Plateau

Ridge-furrow plastic film mulching (RFPM) is an efficient planting technique for rainwater harvesting in rainfed regions of the Loess Plateau, China. However, relevant data on root water use patterns under RFPM remain sparse. To clarify the water use pattern of roots under this planting technique and determine the root characteristic parameters related to water absorption, this study conducted a two-year field experiment using spring maize with three treatments: flat planting (FP), and RFPM with two different ridge:furrow ratios (40 cm:70 cm and 70 cm:40 cm). Hydrogen and oxygen stable isotopes (δD and δ18O) of different water sources were measured, and the MixSIAR model was used to quantify the contributions of different water sources to spring maize root water uptake. The results showed that the root characteristic parameters of spring maize under the three treatments decreased with increasing soil depth and were mainly distributed in the shallow (0–20 cm) and middle (20–60 cm) soil layers. Compared to FP, under RFPM the distribution proportions of the root characteristic parameters in shallow soil were higher, which may be related to the increase in shallow soil water content. The main water-absorbing soil layers of spring maize under the three treatments were the same: 0–20 cm, 20–40 cm, 40–60 cm, 40–60 cm, and 0–20 cm in the V6, V12, R1, R3, and R6 growth stages, respectively. However, compared to FP, under RFPM the water use proportion of spring maize of shallow soil increased, especially under the larger ridge to furrow ratio, and correlation analysis showed that this phenomenon might be related to the RFPM increasing the proportion of the spring maize root characteristic parameters in shallow soil. Correlation analysis further showed that the root length density and root surface area density might be the key characteristic parameters for determining root water uptake.

Effect of Shallow-Buried High-Intensity Mining on Soil Water Content in Ningtiaota Minefield

Shallow-buried high-intensity mining (SHM) activities commonly in China’s western mining area will lead to the decrease of groundwater level and soil water content (SWC), which will aggravate the further deterioration of the local fragile ecological environment. In this study, the applicability and limitations of six typical soil dielectric models were comprehensively evaluated based on ground penetrating radar (GPR) technology and shallow drilling methods. Moreover, experiments were performed to test the variation of SWC in Ningtiaota minefield affected by the SHM. The results show that the fitting effect of the four empirical models and two semi-empirical models on the clay is better than that of the medium sand. Among the six models, the Ledieu model has the best performance for medium sand, and the Topp model for clay. After SHM, the shallow SWC decreases as a whole. The decreasing range is 4.37–15.84%, showing a gradual downward trend compared with the one before mining. The shorter the lagging working face distance, the greater the drop of SWC will be. The longer the lagging working face distance, the smaller the drop of SWC will be showing a gradual and stable trend.

Responses of shallow soil water content in Artemisia ordosica community to different rainfall patterns

The shrub species, Artemisia ordosica, commonly occurs in the eastern Hobq desert. Here, we used a micrometeorological observation system to continuously monitor the rainfall and soil water content in 0-10, 10-30, and 30-50 cm soil layers during the growing season from 2016 to 2018. The dynamic spatial and temporal changes in soil water content under different rainfall patterns were examined, and the replenishing effects of rainfall events on soil water content and water infiltration characteristics were analyzed. The results showed that soil water content of the surface layer in the A. ordosica community had significant seasonal and vertical variation under rainfall fluctuation. Rainfall amount and soil water content before rain were the main factors controlling soil water replenishment and infiltration. The soil surface layer (0-10 cm) was sensitive to rainfall, and the rainfall of 3.8 mm began to replenish this layer. The responses of 10-30 cm soil layer to rainfall was slower, more than 8.6 mm rainfall being needed for effective replenishment. The response of the 30-50 cm soil layer to rainfall was even more delayed, and replenishment at this depth could not be achieved until the rainfall exceeded 11.8 mm. The water infiltration rate increased with rainfall amount and decreased with soil depth, while water infiltration depth was positively correlated with the rainfall amount and soil water content before rainfall. During the study period, rainfall of <10 mm occurred predominantly, accounting for 78.4% of the total rainfall events. The rainfall mainly replenished soil layer above 30 cm, and the replenishment of deep soil was very limited, which was not conducive to the growth of deep-rooted species. Therefore, rainfall patterns directly affected the composition, distribution, and succession of plant communities in this area.

Relationships between soil water content, evapotranspiration, and irrigation measurements in a California drip-irrigated Pinot noir vineyard

The Central Valley of California relies on irrigation for crop production, but water resources, particularly groundwater, have reached a critical state due to extended drought periods and overuse by irrigated agriculture. The purpose of the Grape Remote sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX) project is to improve efficiency in vineyard irrigation through field measurements and modeling efforts using remote sensing. In this study, we analyze multi-year timeseries of ground-based soil moisture and evapotranspiration (ET) measurements collected during GRAPEX to identify patterns that may inform future irrigation recommendations based on remotely sensed ET. The study focuses on field data collected in two adjacent Pinot noir vineyards near Lodi, CA from 2013 to 2017. The data used for this analysis are reference evapotranspiration (ETref), actual evapotranspiration (ETa), mean daytime soil water content (SWC) measured to a depth of 90 cm, precipitation, and irrigation. The relationship between SWC and the ratio of ETa/ETref (used as a soil moisture proxy indicator) changes throughout the spring and summer months due to advancing phenological stages and management practices. In early spring, when the interrow cover crop is the primary source of ET, ETa is strongly correlated with SWC. As the vine canopy grows in, the strong correlation breaks down as the vines begin to access to water beyond the depth of the soil moisture sensors. As the soil profile dries out during the summer, the correlation between ETa and SWC once again emerges as the vines become strongly dependent on irrigation. The dynamic interaction between the upper and lower root zone profile and evaporative demand means that the key to understanding vineyard water status using relatively shallow SWC observations is to use them in conjunction with ETa data, so that when both SWC and ETa timeseries are highly correlated vine water status is well defined. This suggests, delaying the initiation of irrigation during the period of rapid vine growth until a clear relationship between SWC and ETa emerges would both reduce the total amount of water used and give the grower more control over vine growth and grape quality affected by water status.

Soil Water Content Estimation Using High-Frequency Ground Penetrating Radar

The rapid high-precision and nondestructive determination of shallow soil water content (SWC) is of vital importance to precision agriculture and water resource management. However, the low-frequency ground penetrating radar (GPR) technology currently in use is insufficient for precisely determining the shallow SWC. Therefore, it is essential to develop and use a high-precision detection technology to determine SWC. In this paper, a laboratory study was conducted to evaluate the use of a high-frequency GPR antenna to determine the SWC of loamy sand, clay, and silty loam. We collected soil samples (0–20 cm) of six soil types of loamy sand, clay, and silty loam and used a high-frequency (2-GHz) GPR antenna to determine the SWC. In addition, we obtained GPR data and images as well as characteristic parameters of the electromagnetic spectrum and analyzed the quantitative relationship with SWC. The GPR reflection two-way travel times and the known depths of reflectors were used to calculate the average soil dielectric permittivities above the reflectors and establish a spatial relationship between the soil dielectric permittivity ( ε ) and SWC ( θ ), which was used to estimate the depth-averaged SWC. The results show that the SWC, which affects the attenuation of wave energy and the wave velocity of the GPR signal, is a dominant factor affecting the soil dielectric permittivity. In addition, the conductivity, magnetic soil, soil texture, soil organic matter, and soil temperature have substantial effects on the soil dielectric permittivity, which consequentially affects the prediction of SWC. The correlation coefficients R2 of the “ θ ~ ε ” cubic curve models, which were used to fit the relationships between the soil dielectric permittivity ( ε ) and SWC ( θ ), were greater than 0.89, and the root-mean-square errors were less than 2.9%, which demonstrate that high-frequency GPR technology can be applied to determine shallow SWC under variable hydrological conditions.

白刺沙包浅层土壤水分动态及其对不同降雨量的响应

白刺沙包浅层土壤水分动态及其对不同降雨量的响应

Contributions of throughfall, forest and soil characteristics to near-surface soil water-content variability at the plot scale in a mountainous Mediterranean area

Soil water-content (SWC) variability in forest ecosystems is affected by complex interactions between climate, topography, forest structure and soil factors. However, detailed studies taking into account the combined effects of these factors are scarce. This study's main aims were to examine the control that throughfall exerts on local spatial variation of near-surface soil water-content and to combine this information with forest structure and soil characteristics, in order to analyze all their effects together. Two stands located in the Vallcebre Research Catchments (NE Spain) were studied: one dominated by Quercus pubescens and the other by Pinus sylvestris. Throughfall and the related shallow SWC were monitored in each plot in 20 selected locations. The main characteristics of the nearest tree and soil parameters were also measured. The results indicated that mean SWC increment at the rainfall event scale showed a strong linear relationship with mean throughfall amount in both forest plots. The % of locations with SWC increments increased in a similar way to throughfall amount in both forest plots. The analyses considering all the effects together indicated again that throughfall had a significant positive effect in both forest plots, while soil litter depth showed a significant negative effect for the oak plot but lower statistical significance for the pine plot, showing a comparable –although more erratic– influence of the organic forest floor for this plot. These results, together with lower responses of SWC to throughfall than expected in rainfall events characterized by low preceding soil water-condition and high rainfall intensity, suggest that litter layer is playing an important role in controlling the soil water-content dynamics. The biometric characteristics of the nearest trees showed significant but very weak relationships with soil water-content increment, suggesting that stemflow and throughfall may act at lower distances from tree trunk than those presented in our study.

Seasonal and inter-annual variability of the Bowen smith ratio over a semi-arid grassland in the Chinese Loess Plateau

Energy partitioning between grassland sensible (H) and latent (LE) heat fluxes is critical in determining boundary layer development, the hydrological cycle, weather and climate. In this report, we describe the seasonal and inter-annual variability of the Bowen ratio (β) over semi-arid grassland of the Loess Plateau in the north boundary belt of the summer monsoon of China. β was directly measured by an eddy covariance system over a 6-year period (2007–2012), during which the studied grassland experienced a large range in precipitation, extending from 555.5 mm (2007) to 262.8 mm (2011). The annual Bowen ratio (βa) based on annual energy budgets ranged from 0.90 (2007) to 1.93 (2011) with an annual mean value of 1.32, while the annual midday (10:00–16:00 GMT + 8) β ranged from 2.07 (2007) to 4.19 (2011) with an annual mean value of 3.02. The mean midday β during the dry years (3.66, 2009, 2010 and 2011) was higher than that observed during the wet years (2.39, 2007, 2008 and 2012). The 6-year annual mean values of the bulk surface resistance (Rs), aerodynamic resistance (Ra) and climatological resistance (Ri) varied from 272.27 to 410.10 s m−1, 31.62–34.89 s m−1 and from 64.5 to 87.13 s m−1, respectively, with annual mean values of 337.08, 32.83 and 73.05 s m−1, respectively. The normalized difference vegetable index (NDVI) had a direct influence on β as mediated by Rs, and there was a logarithmic function between ET/ETeq and Rs in the semi-arid grassland. A positive trend between β and the surface resistance (Rs*) normalized by the climatological and aerodynamic resistance was observed at this site. Moreover, the annual effective precipitation frequency was significantly related to the midday β during the growing and non-growing seasons, which indicated that the annual effective precipitation, not the annual precipitation, indirectly affected the annual β through its effect on the shallow soil water content (SWC). The results suggested that inter-annual variations in β was primarily controlled by an annual effective precipitation frequency and seasonal variations in β was directly affected by NDVI as mediated by Rs over the semi-arid grassland in the Chinese Loess Plateau.

A Comparison of Ground‐Penetrating Radar Early‐Time Signal Approaches for Mapping Changes in Shallow Soil Water Content

Core Ideas Early‐time amplitude analysis requires understanding of site, system, and methodology. The AEA and CFA both have benefits and disadvantages. Time window selection is important when shallow reflectors may be present. Early time GPR results are highly frequency dependent. Improving irrigation efficiency requires accurate assessment of the soil moisture distribution in time and space, but obtaining accurate observational data is challenging. Early‐time signal (ETS) amplitude analysis of ground‐penetrating radar (GPR) data may permit such rapid noninvasive characterization. In this study we performed controlled irrigation experiments using a multifrequency GPR system to compare two statistics used to quantify the ETS: average envelope amplitude (AEA) and carrier frequency amplitude (CFA). Supporting data were provided by direct measurements, electrical resistivity imaging (ERI), and synthetic modeling. In our first experiment, both statistics successfully related the ETS for 250 and 400 MHz GPR data to increasing water content. However, the 400 MHz AEA lost sensitivity at later stages of the irrigation process, whereas the 400 MHz CFA remained sensitive to changes in water content. The 1000 MHz data did not show the expected relationships, possibly due to shallow reflectors, such as the wetting front, which the higher frequency antennae would have a greater chance of detecting, as supported by synthetic modeling. In our second experiment, we focused on the effect of the time window on calculating ETS statistics. We demonstrate that, when there is interference in the ETS, using a shorter time window instead of the more common first positive half cycle improves correlation with soil moisture content. Our work shows that the GPR ETS data respond to changes in soil water content in similar fashion to ERI data.

A linear mixed effect (LME) model for soil water content estimation based on geophysical sensing: a comparison of an LME model and kriging with external drift

Soil water content (SWC) is a critical attribute in precision irrigation. Direct measurements are costly and relative sparse, so there is interest in methods to predict SWC at unsampled sites from sampled data. The precision of such predictions can be improved, if covariates are incorporated into the predictor. To do that, two efficient ways are: (1) linear mixed effect model (LME), in which the spatial process is obtained by splitting the total variability in a systematic term or mean effect, a spatially correlated component and a random noise, and (2) kriging with external drift (KED), a nonstationary geostatistical technique, assuming covariates to have a linear effect on target variable. Geoelectric sensors provide noninvasive information on soil. Their outputs are quite sensitive to SWC; therefore, they can be used as covariates in SWC predictor. The objective of this work was to compare LME and KED, based on geophysical sensing, to estimate shallow SWC. The surveys were conducted in a south-eastern Italy field, using GPR at two frequencies, 600 and 1,600 MHz, and EMI. Volumetric soil water contents were measured with a Time Domain Reflectometer at 96 locations. Three LMEs, using three correlation functions (spherical, exponential, and Gaussian), were used and compared with KED using a set of cross-validation criteria. The mixed models showed a quite similar behavior even if exponential model outperformed the other two. The covariates used in the mixed models and in KED predictor were different and the results of cross-validation showed a slight out-performance of KED.

Interannual variation of the Bowen ratio in a subtropical coniferous plantation in southeast China, 2003-2012.

The interannual variation of the Bowen ratio, through its effect on the warming extent of available energy to the ecosystem land surface air, heavily influences the ecosystem microclimate and affects the hydrological cycle at both regional and global scales. Although the precipitation amount in southeast China is not expected to change greatly as a result of climate change, the precipitation frequency may be altered in the future. We explored the interannual variation of the Bowen ratio and its affecting mechanisms based on eddy covariance measurements in a subtropical plantation in southeast China during 2003–2012. The results indicated that the annual mean Bowen ratio was 0.35±0.06, with a range of 0.29–0.45. The Bowen ratio during the dry season (July-October) positively correlated with the annual Bowen ratio (R2 = 0.85, p<0.001). The effective precipitation frequency during the dry season, through its positive effect on shallow soil water content, indirectly and negatively affected the annual Bowen ratio. Between 2003 and 2012, the annual Bowen ratio exhibited a marginally significant decreasing trend (p = 0.061), meanwhile the effective precipitation frequency and shallow soil water content during the dry season increased significantly (p<0.001). The annual Bowen ratio may decrease further if the effective precipitation frequency and shallow soil water content during the dry season follow similar trends in the future. The warming effect of available energy to the surface air of our studied plantation may decline with the decreasing annual Bowen ratio.

Internal hydraulic redistribution prevents the loss of root conductivity during drought

Shrubs of the Great Basin desert in Utah are subjected to a prolonged summer drought with the potential consequence of reduced water transport capability of the xylem due to drought-induced cavitation. Hydraulic redistribution (HR) is the passive movement of water from deep to shallow soil through plant roots. Hydraulic redistribution can increase water availability in shallow soil and ameliorate drought stress, providing better soil and root water status, which could affect shallow root conductivity (Ks) and native root embolism. We tested this hypothesis in an Artemisia tridentata Nutt. mono-specific stand grown in a common garden in Utah. We enhanced HR artificially by applying a once a week deep-irrigation treatment increasing the water potential gradient between deep and shallow soil layers. Plants that were deep-watered had less negative water potentials and greater stomatal conductance and transpiration rates than non-watered control plants. After irrigation with labeled water (δD), xylem water in stems and shallow roots of watered shrubs was enriched with respect to control shrubs, a clear indication of deep water uptake and HR. Shallow root conductivity was threefold greater and shrubs experienced lower native embolism when deep-watered. We found clear evidence of water transfer between deep and shallow roots through internal HR that delayed depletion of shallow soil water content, maintained Ks and prevented root embolism. Overall, our results show a positive effect of HR on root water transport capacity in otherwise dry soil, with important implications for plant water status.

Monitoring Shallow Soil Water Content Under Natural Field Conditions Using the Early‐Time GPR Signal Technique

It has been recently demonstrated that the early‐time portion of the ground‐penetrating radar (GPR) signal, consisting of the direct air and ground wave events, is dependent on the shallow subsurface bulk electromagnetic properties of the material; these properties are strongly controlled by the water content in this material. While several controlled experiments have been conducted to study the effects of water content variations on the antenna–material coupling, they considered a limited range of moisture variations and soil textures. Furthermore, those previous experiments did not consider highly dynamic shallow moisture responses that would be encountered under natural field conditions. For these reasons, general acceptance of this method requires that it be tested in real‐life applications. Our paper evaluates the early‐time GPR technique under natural field conditions where surface roughness, lithology, lateral heterogeneities, vegetation and water content dynamics are not controlled. We assess the capacity of the early‐time amplitude technique over the complete annual cycle of soil moisture conditions at three textural sites. To evaluate the sensitivity of the early‐time amplitudes to subsurface water content variations, we compare the early‐time results acquired using the enveloped amplitude of the first part of GPR signals with the bulk dielectric permittivity obtained from concurrently collected common‐midpoint direct ground wave velocity and gravimetric water content measurements. Our results demonstrate that the early‐time method can yield near‐surface permittivity information that is consistent with that obtained from direct ground wave velocity measurements, and accurate predictions of shallow soil moisture conditions.

Early-Time GPR Signal Attributes to Estimate Soil Dielectric Permittivity: A Theoretical Study

High-frequency electromagnetic (EM) surveys have shown to be valuable techniques in the study of soil water content due to the strong dependence of soil dielectric permittivity with moisture content. This quantity can be determined by analyzing the average value of the early-time instantaneous amplitude of ground-penetrating radar (GPR) traces. We demonstrate the reliability of this approach to evaluate the shallow soil water content variations from standard fixed-offset GPR data by simulating the data over different likely EM soil conditions. A linear dipole model that uses a thin-wire approximation is assumed for the transmitting and receiving antennas. The homogenous half-space model is used to calculate the waveform instantaneous amplitude values averaged over different time windows. We analyzed their correlation with the soil surface dielectric parameters, and we found a clear inverse linear dependence on the permittivity values. Moreover, we evaluated how different kinds of noise affect this correlation, and we determined the influence of the electrical conductivity on the trace attributes. Finally, through a two-layered medium, we estimated the effect on the GPR signal of a shallow reflector, we analyzed how its presence can carry out inaccuracies in the soil surface dielectric permittivity estimation, and we determined the best time window to minimize these errors.