Surface Soil Water Content Research Articles

Responses of wetland soil bacterial community and edaphic factors to two-year experimental warming and Spartina alterniflora invasion in Chongming Island

Climate warming and plant invasions are among the most serious threads to biodiversity and stability in terrestrial ecosystems. A particular challenge is to predict how soil microbial communities and edaphic factors will change under future conditions. Despite evidence that soil microbial community structure can be shifted by experimental warming and plant invasion, few studies were conducted to quantify the interactive effect of both factors. Here, we compared soil microbial diversity, bacterial community, and soil physiochemical properties in both surface (0–5 cm) and subsurface (5–20 cm) soils of two-year open-top chambers (OTCs) warming and Spartina alterniflora (S. alterniflora) invasion treatments with no warming and Phragmites australis (P. australis) treatments. Results showed that surface soil water content, total organic carbon, total nitrogen (TN), and the ratio of TN to total phosphorous (N:P) were significantly decreased by experimental warming, while pH was significantly enhanced. Soil TN was significantly enhanced in the mixed community of P. australis and S. alterniflora compared with single species community. Soil TP was gradually decreased with the proportion of S. alterniflora increasing by experimental warming. Soil bacterial α diversity was not changed by either experimental warming or S. alterniflora invasion. Experimental warming significantly shifted soil bacterial community, whereas S. alterniflora invasion and their interaction showed no impact. Redundancy analysis showed soil bacterial community structure was highly correlated with soil edaphic factors (i.e. pH, TN, and NH4+-N). Prediction of microbial function profiles revealed that several bacterial functions, especially carbon and nitrogen metabolism pathways, were adjusted to tolerate warming. Overall, our results demonstrated that experimental warming significantly altered soil bacterial community and physiochemical properties of S. alterniflora and P. australis communities, and should be considered in ecologically sustainable development of Chongming Island and wetland ecosystem.

Estimation of soil surface water contents for intertidal mudflats using a near-infrared long-range terrestrial laser scanner

Estimations of the soil surface water contents and distributions play a key role in the ecological, environmental, and topographical investigations for intertidal mudflats. However, existing techniques have limitations. Long-range terrestrial laser scanners (TLSs) can record the co-located intensity value which refers to a measure of the backscattered laser from each scanned point. Most long-range TLSs emit near-infrared lasers that can be strongly absorbed by water. Thus, the intensity values can be used as proxies for water contents. In this study, the intensity data of long-range TLSs are corrected for the incidence angle and distance effects to quantitatively estimate the soil surface water contents of intertidal mudflats. A case study for a mudflat in Chongming Island, Shanghai, China, is conducted. Results indicate that compared with traditional techniques, the corrected intensity data of long-range TLSs are extremely effective data sources for a quick, accurate, and detailed estimation of water contents for large-area mudflats. The estimation root mean square error is approximately 3%. Furthermore, the 3D distributions of the water contents can be accurately mapped by combining the point cloud of the mudflats to potentially analyze the intrinsic association among water contents and topography, vegetation coverage, and habitation of creatures in mudflats.

Effects of planting patterns on rainwater use efficiency and potato tuber yield in seasonally arid areas of southwestern China

China is the top potato producing country in the world, and Yunnan Province (in Southwest China) makes a significant contribution (10.1 %) to the total production. The uneven rainfall distribution causes seasonal spring drought in the region. To resolve this issue, we evaluated the effects of cultivation modes on the soil temperature, soil water content, and potato yield. Four cultivation modes were evaluated: A1, flat-planting at sowing, double ridges after emergence; A2, double ridges in plastic film covered with soil, potato planted along the larger ridge side; A3, double ridges in plastic film mulching covered in soil; and A4-control (CK), double ridges, consisting of two different plastic film covering modes (A2, A3) and two covering modes without a plastic film (A1, A4). The field experiments were conducted for three years (2014, 2015, and 2016) in three regions (S1, S2, and S3) of Yunnan in southwestern China. The results showed that the effects of cultivation patterns on the soil temperature and humidity were not the same in the three ecological regions. Before mound closure, compared to the CK treatment, higher soil temperature was observed at the S1 and S2 regions using the A3 mode, which was 5.15 °C and 3.4 °C, respectively. The soil moisture was 4 % (S1), 50.8 % (S2), and 10.0 % (S3) higher with the A2 mode than under the CK mode in the three ecological zones, but the soil moisture was 27.1 % lower with the A3 mode than that with the CK mode at S2. Multiple regression analysis revealed the optimum surface soil temperature (18.9–20.8 °C) and water content (58.6 %–70.4 %) for seedling emergence and potato tuber yield. The emergence, tuber growth, and yield were mainly related to the soil temperature and soil water content at S1 and S2, respectively, while at S3, both factors contributed significantly. Overall yield differed significantly at the cultivation sites, but the average yield was similar in each year at each site. The highest stable potato yield occurred with the A3 mode at S1, and with the A2 mode at S2 and S3. Our research provides suitable methods for potato production in different rain-fed and seasonally arid areas.

Surface soil water content dominates the difference between ecosystem and canopy water use efficiency in a sparse vineyard

Water use efficiency (WUE) is an important eco-physiological index that reflects the relationship between ecosystem water and carbon cycles. Investigating the ecosystem WUE contributes to a better understanding of the water-carbon coupling mechanism in ecosystems. We explored the responses of the daily ecosystem WUE (WUEe) and canopy WUE (WUEc) to several influential factors, the dominant biophysical factor and the dominant water/carbon flux component of the difference between WUEe and WUEc based on continuous measurements collected via an eddy covariance system, sap flow system, and microlysimeters over two growing seasons from 2017 to 2018 in a sparse vineyard in northwestern China. The results showed that the seasonal averages for WUEe and WUEc were 0.52 g C kg H2O−1 and 1.96 g C kg H2O−1, respectively. WUEe and WUEc responded weakly to the air temperature (Ta) and showed a negative linear correlation to the photosynthetic active radiation (PAR) and vapor pressure deficit (VPD). WUEe decreased significantly with the rising soil water content (SWC) while WUEc showed a weak response. Both WUEe and WUEc exhibited positive linear relationships with the leaf area index (LAI). The results from a structural equation model (SEM) indicated that among the biophysical controllers, SWC and LAI were the top two controlling factors on the difference between WUEe and WUEc, and SWC played a dominant role. The difference between WUEe and WUEc increased with the rising SWC and decreased with the rising LAI. Among the flux components, including soil evaporation (E), transpiration (T), net ecosystem productivity (NEP), and ecosystem respiration (Reco), E dominated the discrepancy between WUEe and WUEc due to the strong control of SWC on E. This study highlights the strong control by the SWC in regulating the ecosystem WUE under a sparse canopy in an arid area.

Intra-storm time stability analysis of surface soil water content

Time stability analysis is an important and economical tool to upscale point soil water content (SWC) to larger scales. However, little is known whether and how precipitation affects characteristics of time stability of SWC during rainfall. Based on a high-resolution dataset, we found that medium- and heavy- rainfall events substantially reorganized spatial patterns of SWC, and that the time-stable locations identified for inter-rainfall period produced three to six times larger prediction errors during medium- and heavy- rainfall periods compared with before rainfall events. Accordingly, hydrological application needs to be applied with caution when based on predicated SWC from presumed time-stable locations.

Soil Evaporation in a Shaded Coffee Plantation Derived From Eddy Covariance Measurements

AbstractLittle is known about soil evaporation (Esoil) in shaded coffee plantations. In this study, Esoil and its proportion to total evapotranspiration and rainfall (P) were quantified for a densely shaded Arabica coffee plantation (~80% tree cover) in central Veracruz, Mexico, using eddy covariance (EC) measurements made above the tree canopy and at the boundary between the stem space and canopy of the coffee crop. While measurements above the canopy were made continuously during the 1‐year study period (October 2016 to September 2017), this was not possible for the above‐soil level due to logistical considerations, and hence, annual Esoil was calculated from a relationship between the ratio of the actual to the equilibrium Esoil and surface soil water content. This empirical model was calibrated using 31 days of EC measurements, representing the range in surface soil water content observed during the study period. Examining the above‐canopy and above‐soil EC measurements for quality yielded similar results with regard to stationarity (99% and 85% of the time, respectively) and degree of energy balance closure (81% and 96%, respectively). The permanent and relatively dense vegetation cover provided by the shade trees and coffee plants reduced net radiation at the soil surface to only about 12% of the above‐canopy value. Consequently, Esoil was a minor fraction of evapotranspiration (10–13%) and P (8–9%), with the spread reflecting uncertainties related to the failure to close the energy balance. In addition to the low‐energy availability, Esoil may have been reduced by the presence of a litter layer.

Unraveling location-specific and time-dependent interactions between soil water content and environmental factors in cropped sandy soils using Sentinel-1 and moisture probes

Advances in sensor technologies provide the potential for real-time monitoring of soil moisture status. Little work has been conducted to study the spatial and temporal variations of soil water content at the field scale and the interactions between soil water content and environmental controlling factors in cropped fields during the growing season. In this study, we assessed the spatial and temporal variations of surface soil water content (0–0.1 m) using seven in-situ soil moisture probes and Sentinel-1 satellite data over the 2016 and 2017 cropping seasons in irrigated fields of the Central Sands Plains, Wisconsin, USA. An empirical multiple linear regression model was developed between the soil water content with Sentinel-1 backscatter data. The model was evaluated with leave-one-group-out cross-validation, which showed an R2 of 0.60, mean error of –0.00 m3 m−3, root mean square error of 0.02 m3 m−3, and Lin’s concordance correlation coefficient of 0.75. Soil volumetric water content (VWC) and soil water deficit (SWD) were mapped at 30-m resolution every 6–12 days across one field (DU field at the Wallendal farm). Using two statistical metrics (“first metric” and “last metric”) and a bootstrapping approach, the location-specific and time-dependent interactions between estimated soil water content and environmental controlling factors across the DU field during the cropping season were evaluated, and it was found that such interactions were affected by initial soil water content and plant stress status. The robustness of the model needs to be improved in the future by collecting more VWC measurements in different soil and crop conditions. The method has the potential to provide a baseline soil moisture map and can be coupled with mechanistic models and subsoil VWC measurements for improved irrigation management and water use efficiency under a changing climate.

Identifying agronomic options for better potato production and conserving water resources in the agro-pastoral ecotone in North China

Potato is a main food crop in the agro-pastoral ecotone (APE) of North China, but its yield is low and highly variable. Agronomic managements, such as irrigation, fertilization and intensive cropping systems, has been used for increasing potato yield but resulted in negative environmental sequences (e.g., soil dryness and groundwater table decline) in such arid and semi-arid regions. This study aims to quantify the agronomic options for better potato production and groundwater conservations based on field experiments together with APSIM-Potato model. The long-term simulated crop yield, economic income and environmental impact (surface soil water content and groundwater table) were analyzed for the different cropping systems of potato (continuous planting, CP; alternating planting and fallowing, APF; long-term continuous fallowing, LF) and irrigation scheduling (rainfed and one-four irrigations). The calibrated APSIM-Potato performed well in simulating the responses of soil water dynamics, leaf area index (LAI), biomass, evapotranspiration (ET) and yield of potato to different irrigation treatments. Long-term (1981–2010) simulated potato yield could be increased by 43.4–90.2% from rainfed yield if irrigation was applied from 60 to 240 mm and the yield gap between the potential and rainfed yields could be narrowed by 41.5–85.9%. Across the APE, CP with four irrigations (240 mm) produced the maximum income of 18,445 Yuan·ha−1 while WUE was the highest for both CP and APF with two irrigations (120 mm). However, such irrigation amounts would decrease the groundwater table by 4.1–42 m across the APE during this period. Rainfed CP could enhance the groundwater table by 0–3 m but decrease available soil water content in 1 m depth by 15.5 mm per decade from 1981 to 2010. Rainfed APF and LF increase available soil water content in 1 m depth by 2–79 mm and 100–143 mm respectively. The study suggested that CP would decrease the soil moisture significantly and increase the risk of soil desertification in the APE. APF with one irrigation by harvesting rainwater could be a feasible way of maintaining soil moisture and groundwater conservation if government subsidy could guarantee the incomes of local farmers in the APE in North China.

Soil moisture and nitrate-nitrogen dynamics and economic yield in the greenhouse cultivation of tomato and cucumber under negative pressure irrigation in the North China Plain

A field experiment was carried out for two years to investigate the benefits of negative pressure water supply on surface soil water content, nitrate-nitrogen (NO3−-N) distribution in the soil profile, economic yield and water and fertilizer use efficiency of tomato and cucumber under greenhouse cultivation in the North China Plain. The experiment included two irrigation treatments: drip irrigation with nutrient solution (DIN) and negative pressure irrigation with nutrient solution (NIN). The results showed that the NIN treatment had a relatively stable soil moisture (about 87% of field capacity), and the fluctuation of soil water content in the 0–20 cm soil layer was 20.6%–25.0% during the experiment period in 2014–2015, which was less than the range of 19.2%–28.1% in the DIN treatment. In both the DIN and NIN treatments, the NO3−-N at the end of the four growing seasons was mainly distributed in the 0–40 cm soil layer and showed a gradually increasing trend as the number of cultivation years increased. Compared with the DIN treatment, the NO3−-N content in the 0–60 cm layer of the NIN treatment was significantly decreased by 19.7%–28.0% after the fourth growing season. The NIN treatment produced the highest economic yield with lower water and nutrient input than the DIN treatment, however, no significant difference was observed in tomato and cucumber yield in the two years. Average irrigation water use efficiency (WUEi) and partial factor productivity of fertilizer (PFPf) over the study period were all significantly improved under the NIN treatment relative to the DIN treatment, with increases of 26.2% and 25.7% (P < 0.05), respectively. Negative pressure water supply not only maintained a high fruit yield, but significantly increased WUEi and PFPf, indicating a great advantage in water and fertilizer saving compared with drip irrigation.

Grazing Crop Residues Has Less Impact in the Short‐Term on Soil Properties than Baling in the Central Great Plains

Core Ideas Corn residue grazing had little to no impact on soil health indicators. Residue baling increased wind erosion risks by 22 to 56%. Baling reduced near‐surface soil water content by 8 to 32%. Soil type or precipitation amount did not influence residue removal impacts on soil properties. Residue grazing can be a more sustainable practice than baling Cattle grazing and baling of corn (Zea mays L.) residues are common practices in integrated crop–livestock systems, but their impacts on soil properties are not well understood on a regional scale. We conducted a 3‐yr study across a precipitation gradient (ranging from 400 to 804 mm) in the central Great Plains to evaluate soil compaction, structural quality, water content, fertility, sorptivity, and microbial biomass in response to corn residue grazing and baling. Six on‐farm sites varying in soil texture, organic matter, crop rotation, and irrigation practices were established with three residue treatments: control, grazing, and baling. Sites were predominantly under conservation tillage management. Residue baling reduced residue cover by 57% and residue grazing by 17% relative to the control. At most sites, residue baling reduced near‐surface soil water content by 8 to 32% and increased wind erodible fraction by 22 to 56% compared with the control but grazing generally had no effect. Residue grazing and baling had small or no negative effects on other measured soil properties regardless of soil type or precipitation amount. When changes in soil properties did occur, they were due to differences in grazing duration or agronomic management. For example, grazing during spring at one of the sites increased soil bulk density by 7% (1.17 vs. 1.26 Mg m−3) relative to the control. This short‐term study indicates that residue grazing, in general, does not impact soil properties, but residue baling at high rates (&gt;50%) could increase wind erosion risks and reduce surface soil water content in this region.

Moisture pulse-reserve in the soil-plant continuum observed across biomes.

The degree to which individual pulses of available water drive plant activity across diverse biomes and climates is not well understood. It has previously only been investigated in a few dryland locations. Here, plant water uptake following pulses of surface soil moisture, an indicator for the pulse-reserve hypothesis, is investigated across South America, Africa and Australia with satellite-based estimates of surface soil and canopy water content. Our findings show that this behaviour is widespread: occurring over half of the vegetated landscapes. We estimate spatially varying soil moisture thresholds at which plant water uptake ceases, noting dependence on soil texture and proximity to the wilting point. The soil type and biome-dependent soil moisture threshold and the plant soil water uptake patterns at the scale of Earth system models allow a unique opportunity to test and improve model parameterization of vegetation function under water limitation.

Water storage effect of soil freeze-thaw process and its impacts on soil hydro-thermal regime variations

In the soil freeze/thaw phenomenon, the soil water phase and energy budget change, which can affect the soil-vegetation-atmosphere system. Herein, the basic role of freeze–thaw–process (FTP) in soil water and heat transport and its impact on soil moisture and temperature variation were investigated using the observations and numerical experiments with and without FTP in Community Land Model version 4.5 (CLM4.5) on the Tibetan Plateau (TP). The results averaged over the whole TP revealed that no soil FTP (i.e., no water phase change occurred) resulted in lower surface soil temperature by about -1.02 ℃ in freeze-thaw (FT) period, while surface soil temperature is higher by about 0.91 ℃ and surface soil moisture is drier by about -0.02 mm3/mm3 in after-thaw (AT) period. Soil FTP has a water storage effect, the storage index (SI) can reach 0.95 at surface layer. Without soil FTP, the surface soil water content at the AT period can be reduced by ∼10% (SI is decreased by about -0.05) as a result of enhanced evaporation; the soil moisture memory was shortened by about -20 days in March. Accordingly, surface latent heat decreased by -1.07 W/m2, while the surface sensible heat increased by 4.72 W/m2. The effects of FTP on soil water and heat transport at deep layer are distinguished from that at the surface layer. These imply that the biases of soil temperature and moisture simulation in models during FT period could lead to large uncertainties in estimating climate effects of TP. Under climate warming background in recent decades, with the delaying of soil freeze-beginning date and advancing of soil thaw-end date, changes of soil FTP regime would lead to loss of spring soil moisture.

Soil Texture Effects on Surface Resistance to Bare‐Soil Evaporation

AbstractModeling of surface energy balance and the separation of evapotranspiration to its component fluxes require quantification of evaporation from land surfaces. The nonlinear relationship between surface heat fluxes and the hydration state of soil surfaces present a challenge to remote estimation of surface evaporation rates. We study the often‐overlooked role of soil properties in determining surface evaporation resistance. We present a framework for quantifying how the ratio of actual to potential evaporation rates varies with changes in surface water content for different soil textures. The model uses the evaporative characteristic length (a soil‐dependent active depth of evaporation) and soil resistance to capillary flow across that region. Predictions were in good agreement with flux tower measurements of bare‐soil evaporation from 10 soil textural classes. The study offers a simple and physically based method for incorporating surface evaporation resistance into land‐surface models considering soil type and surface water content.

Effect of Vegetation on the Energy Balance and Evapotranspiration in Tallgrass Prairie: A Paired Study Using the Eddy-Covariance Method

We carried out a paired study in tallgrass prairie to evaluate the influence of vegetation on the energy exchange and evapotranspiration. Two eddy-covariance systems were installed over two adjoining sites, one of which was denuded of vegetation, with the adjacent, control site kept undisturbed. Our year-long investigation shows that, for quantifying the ground surface heat flux, the soil heat storage above the soil plates is more important than the sub-surface soil heat flux, both temporally and in magnitude. The incorporation of the soil heat storage, therefore, is indispensable for energy balance closure in areas with short vegetation. At our control site, we observed a critical threshold of 0.17 m3 m−3 in the surface (top 0.3 m) soil water content, whereby the energy partitioning is significantly affected by the presence of the photosynthetically active vegetation when the surface soil water content is higher than this critical threshold. The pattern of energy partitioning approaches that of the treated site when the surface soil water content is lower than this threshold (during drought), because of the suppression of plant physiological activities. This threshold also applies to the surface conductance for water vapour at the control site, where yearly evapotranspiration is 728 ± 3 mm (versus 547 ± 2 mm for the treated site). Thus, the soil water content and presence of active vegetation are the key determinants of energy partitioning and evapotranspiration. Any land-cover changes or vegetation-management practices that alter these two factors may change the energy and water budgets in tallgrass prairie.

Temporal dynamics of subsurface soil water content estimated from surface measurement using wavelet transform

Temporal variation of water in the vadose zone is important to understand processes such as solute transport and nutrient cycling. Measurements of soil water content (SWC) in the subsurface are less common than those near the surface and predictions using numerical models are limited by data availability. Wavelet decomposition of surface measurements of SWC could improve modeling of subsurface SWC by segregating features at different temporal scales and projecting them to the subsurface. The objectives of this work were to: 1) predict subsurface SWC using surface SWC and a combination of wavelet analysis and linear regression, and 2) investigate relationships between soil properties and the movement of soil water at various temporal scales, s. Climate data and SWC at various depths were collected from eight sites in the Atlantic Coastal Plain of the USA. Soil water retention and hydraulic conductivity (k) functions of each horizon were optimized by comparing measured and predicted (using HYDRUS-1D) soil water contents. Each time series of SWC was decomposed into 50 scale components using the Mexican Hat wavelet and later reduced to 5 group components with minimal impact on the characteristics of the signal. Changes in the values of each group component with depth were represented with transfer coefficients that could be estimated with predictors derived from particle size distributions and optimized soil hydraulic functions. Prediction depth and saturated k were the two most important predictors for s < 256 h, while k at −10 kPa was the best predictor for 256 h < s < 724 h, and the median value of particle size diameters for s > 724 h. Subsurface soil water content can be reasonably predicted with the proposed approach, particularly when vertical movement of soil water is unrestricted.

Diversity of Drought Tolerance in the Genus Vigna.

Wild relatives of crop plants are thought as reservoir of prominent genetic resources for abiotic stress tolerance. However, insufficient information on genetic variation and phenotypic traits restricts their use for crop breeding. This study focused on wild species of genus Vigna (family Fabaceae) originated from highly humid to arid regions. To clarify the diversity of drought tolerance during the vegetative stage, 69 accessions, including 15 domesticated, and 54 wild accessions, were evaluated under two drought conditions of non-terminal and terminal stresses. In the non-terminal drought condition, the plants were grown in pipes of different heights where surface soil water content decreased faster in pipes with greater height. Relative shoot biomass was used for tolerance evaluation and we identified 19 drought tolerant accessions. Almost of them were wild accessions showing higher relative shoot biomass than that in the domesticated accessions. Domesticated species were mostly classified as drought susceptible but could be improved using tolerant conspecific wild ancestors with cross-compatibility. The tolerance was related with higher plant water status presumably due to small water consumption. However, the variation of drought tolerance could not be explained by simple tolerance factor alone, and other tolerance mechanisms such as deep rooting and increasing in root biomass were found in the tolerant accessions. In the terminal drought condition, the plants were grown in small pots, and the watering was stopped to expose them extreme and rapid soil water scarcity. The tolerance was evaluated as the number of days until wilting. However, the accessions found to be tolerant in the pot experiment were not the same as those in the pipe experiment. In this condition, plant water status was not related with the length of days to wilting. This indicates that different mechanisms are necessary for adaptation to each of the non-terminal and terminal drought conditions. Many accessions were tolerant to one of the conditions, although we identified that some accessions showed tolerance in both experiments. The great diversity in drought tolerance in the genus Vigna might serve to both improve crop drought tolerance and understand the mechanisms of adaptation in drought-prone environments.

Responses of soil and aggregate-associated organic carbon mineralization to throughfall reduction in a Pinus massoniana plantation.

To examine the effects of altered precipitation pattern on the mechanism of forest soil carbon (C) emission under climate change scenarios, we established plots with 50% reduction in throughfall in a subtropical Pinus massoniana plantation. The effects of throughfall reduction on the mineralization of both soil and aggregate-associated organic C were quantified by laboratory incubation at constant temperature. The results showed that the cumulative mineralization of organic C in the 1-2 mm aggregates was higher than that in other sizes of aggregates. In the dry and wet seasons, the surface soil water content in the throughfall reduction plots was 82.1% and 82.7% of that in the control, but the mass fractions of 0.106-0.25 mm soil aggregates in the throughfall reduction plots increased by 1.8% and 4.2% than the control, respectively. The throughfall reduction treatment significantly reduced the mineralization rate of labile C pools (k1) of soil and micro-aggregates in the dry season, increased the mineralization rate of recalcitrant C pool (k2) of soil and <1 mm aggregates, and had no significant effects on the cumulative mineralization of soil organic C. Results from the correlation analyses revealed significant and positive correlations between k1 of soil and that of the micro-aggregates and between k2 of soil and that of the 0.25-1 mm aggregates. Under the influence of aggregate structure, water condition and soil organic carbon content, the throughfall reduction suppressed the mineralization of labile organic C while stimulated that of recalcitrant organic C.

Identifying representative sites to simultaneously predict hillslope surface and subsurface mean soil water contents

Many approaches have been proposed to identify the representative sampling sites for estimating the spatial mean soil water contents. However, comparisons on these approaches have seldom been conducted to simultaneously predict the surface and subsurface mean soil water contents. In this study, five approaches were evaluated in identifying representative sites to estimate the surface and subsurface mean soil water contents on a typical hillslope in Taihu Lake Basin, China. They were temporal stability analysis (TSA), k-means clustering with environmental factors as inputs (EFs), combinations of TSA and EFs (EFs + TSA), k-means clustering with surface soil water contents as inputs (Theta), and combinations of TSA and Theta (Theta+TSA). The correlation coefficient (r) and root mean squared error (RMSE) between estimated and measured mean soil water contents were used to evaluate the accuracies during the calibration period (the first 25 dates) and validation period (the last 18 dates). Results showed the optimal number of representative sites on this hillslope was six. When >6 representative sites were selected, the TSA had the lowest accuracies for estimating both surface and subsurface mean soil water contents during validation period (mean RMSE ≥ 0.011 m3 m−3). The Theta and Theta + TSA had better accuracies in estimating surface mean soil water contents during both calibration and validation periods (mean RMSE < 0.007 m3 m−3). However, to estimate surface and subsurface mean soil water contents simultaneously, the EFs and EFs + TSA were more promising (mean RMSE < 0.011 m3 m−3 during validation period), especially the EFs which only required one-time collection of environmental factors. These findings will be beneficial for choosing proper approach to calibrate and validate the remote sensed soil water contents.

Accounting for soil porosity improves a thermal inertia model for estimating surface soil water content

Soil thermal inertia (P), a property that controls the temporal variation of near-surface temperature, has been used to estimate surface water content (θ) in remote sensing studies. The accuracy of θ estimates, however, is affected by surface soil porosity (n). We hypothesize that n can be derived using a simple linear n-P relationship of a dry surface soil layer, and that accounting for n improves the accuracy of θ estimation using a P(θ) model. The P of a surface layer was measured by using the heat pulse method during a drying period, and the feasibility of estimating θ with a P(θ) model that included n was explored. The approach was also tested with published P values derived from meteorological data and MODIS data against in situ θ measurements at two field sites in Arizona, USA. The results on a partially vegetated shrubland indicated that by using the P-derived n, the P(θ) model provided more accurate θ estimates than by using the literature n values. Discrepancies between modeled θ and in situ θ measurements were observed at small θ values, which were caused in part by the fact that the modeled θ represented soil layers a few millimeters thick, while the in situ measurements represented θ at the 5-cm depth. The new n-P function has potential for estimating surface θ accurately using multi-scale P data on bare soils or on sparsely vegetated lands.

Canopy cover effects on local soil water dynamics in a tropical agroforestry system: Evaporation drives soil water isotopic enrichment

AbstractDespite the widely held assumption that trees negatively affect the local water budget in densely planted tree plantations, we still lack a clear understanding of the underlying processes by which canopy cover influences local soil water dynamics in more open, humid tropical ecosystems. In this study, we propose a new conceptual model that uses a combination of stable isotope and soil moisture measurements throughout the soil profile to assess potential mechanisms by which evaporation (of surface soil water and of canopy‐intercepted rainfall) affects the relationship between surface soil water isotopic enrichment (lc‐excess) and soil water content. Our conceptual model was derived from soil water data collected under deciduous and evergreen plants in a shade grown coffee agroforestry system in Costa Rica. Reduced soil moisture under shade trees during the “drier” season, coinciding when these trees were defoliated, was largely the result of increase soil water evaporation as indicated by the positive relationship between soil water content and lc‐excess of surface soil water. In contrast, the evergreen coffee shrubs had a higher leaf area index during the “drier” season, leading to enhanced rainfall interception and a negative relationship between lc‐excess and soil water content. During the wet season, there was no clear relationship between soil water content and between lc‐excess of surface soil water. Greater surface soil water under coffee during the dry season may, in part, explain greater preferential flow under coffee compared with under trees in conditions of low rainfall intensities. However, with increasing rainfall intensities during the wet season, there was no obvious difference in preferential flow between the two canopy covers. Results from this study indicate that our new conceptual model can be used to help disentangling the relative influence of canopy cover on local soil water isotopic composition and dynamics, yet also stresses the need for additional measurements to better resolve the underlying processes by which canopy structure influences local water dynamics.