Critical Soil Water Content Research Articles

The dependence of evaporative efficiency of vegetated surfaces on ground cover mass fractions in vegetated soils in mesic ecosystems

AbstractBare soil evaporation has been studied extensively, but less is certain regarding how site‐specific features, especially the overstory tree canopy and ground covers, mediate evaporation processes. Inspired by recent advances on modelling bare soil evaporative efficiency (SEE), this study explored SEE over a range of soil substrates and ground cover types, with and without the presence of an overstory canopy in three mesic ecosystems in Canadian Rocky Mountains. A significant relationship was found between the critical soil water content and ground cover mass fractions across various ground cover types, both with and without the presence of an overstory canopy. This relationship is expected to be prevalent across various ecosystems. Moreover, a simple approach for modelling SEE of vegetated surfaces and a correction method to account for below‐canopy SEE is also proposed. The model yields satisfactory simulations, and the approach is expected to be widely applicable, given the strength that its parameters are easily acquired, and its formulations are simple and straightforward. While the model may be particularly suited to mesic ecosystems, the underlying mechanism of SEE suggests that this model can also be applied in dryer conditions. This approach will greatly improve ET parameterization in land‐surface models (LSMs) and increase our knowledge of the global water cycle and ecosystem responses under climate change impacts.

Drought and recovery in barley: key gene networks and retrotransposon response.

During drought, plants close their stomata at a critical soil water content (SWC), together with making diverse physiological, developmental, and biochemical responses. Using precision-phenotyping lysimeters, we imposed pre-flowering drought on four barley varieties (Arvo, Golden Promise, Hankkija 673, and Morex) and followed their physiological responses. For Golden Promise, we carried out RNA-seq on leaf transcripts before and during drought and during recovery, also examining retrotransposon BARE1expression. Transcriptional data were subjected to network analysis. The varieties differed by their critical SWC (ϴcrit), Hankkija 673 responding at the highest and Golden Promise at the lowest. Pathways connected to drought and salinity response were strongly upregulated during drought; pathways connected to growth and development were strongly downregulated. During recovery, growth and development pathways were upregulated; altogether, 117 networked genes involved in ubiquitin-mediated autophagy were downregulated. The differential response to SWC suggests adaptation to distinct rainfall patterns. We identified several strongly differentially expressed genes not earlier associated with drought response in barley. BARE1 transcription is strongly transcriptionally upregulated by drought and downregulated during recovery unequally between the investigated cultivars. The downregulation of networked autophagy genes suggests a role for autophagy in drought response; its importance to resilience should be further investigated.

Functional physiological phenotyping and transcriptome analysis provide new insight into strawberry growth and water consumption.

Global warming is expected to increase agricultural water scarcity; thus, optimized irrigation schedules are important and timely for sustainable crop production. Deficit irrigation, which balances crop growth and water consumption, has been proposed, but the critical threshold is not easily quantified. Here, we conducted experiments on strawberry plants subjecting progressive drought following various water recovery treatments on the high-throughput physiological phenotyping system "Plantarray". The critical soil water contents (θcri), below which the plant transpiration significantly decreased, were calculated from the inflection point of the transpiration rate (Tr) - volumetric soil water content (VWC) curve fitted by a piecewise function. The physiological traits of water relations were compared between the well-watered plants (CK), plants subjecting the treatment of rewatering at the point of θcri following progressive drought (WR_θcri), and the plants subjecting the treatment of rewatering at severe drought following progressive drought (WR_SD). The results showed that midday Tr, daily transpiration (E), and biomass gain of the plants under WR_θcri treatment were equivalent to CK during the whole course of the experiment, but those under WR_SD treatment were significantly lower than CK during the water stress phase that could not recover even after rehydration. To explore the gene regulatory mechanisms, transcriptome analysis of the samples collected 12h before, 12h post and 36h post water recovery in the three treatments was conducted. GO and KEGG enrichment analyses for the differentially expressed genes indicated that genes involved in mineral absorption and flavonoid biosynthesis were among the most striking transcriptionally reversible genes under the WR_θcri treatment. Functional physiological phenotyping and transcriptome data provide new insight into a potential, quantitative, and balanceable water-saving strategy for strawberry irrigation and other agricultural crops.

Load-bearing capacity and critical water content of the coffee plantation soil with management in full sun and shaded

New management practices applied to coffee crops may influence the soil’s capacity to tolerate vertical stresses. This paper aimed to evaluate the influence of two coffee crop management systems on the soil load-bearing capacity and critical soil water content to agricultural machinery traffic. This study was performed in the experimental area of the Federal Institute of the Southeast of Minas Gerais – Rio Pomba college, in Rio Pomba city, Minas Gerais, Brazil. Dystrophic Red-Yellow Oxisol (Latossolo Vermelho-Amarelo distrófico) (LVA7) [...]

Going underground: soil hydraulic properties impacting maize responsiveness to water deficit

PurposeAlthough the coordination between stomatal closure and aboveground hydraulics has extensively been studied, our understanding of the impact of belowground hydraulics on stomatal regulation remains incomplete. Here, we investigated whether and how the water use of maize (Zea mays L.) varied under hydraulically contrasting soil textures. Our hypothesis is that a textural-specific drop in the hydraulic conductivity is associated with a distinct decrease in transpiration during soil drying.MethodsMaize plants were grown in contrasting soil textures (sand, sandy loam, loam) and exposed to soil drying. We measured the relationships between transpiration rate, soil water content as well as soil and leaf water potential. We used a soil-plant hydraulic model to reproduce the experimental observations and infer the hydraulic conductance of the soil-plant system during soil drying.ResultsWe observed the impact of soil texture on plant response to soil drying in various relationships. The soil water potentials at which transpiration decreased were more than one order of magnitude more negative in loam than in sand. The soil-plant conductance decreased not only abruptly but also at less negative soil water potentials in sand than in sandy loam or loam. Stomata closed at less negative leaf water potentials in sand than in loam. The model predictions matched well the experimental observations.ConclusionThe results elucidated that the critical soil water content and potential at which plants close stomata depends on the soil texture. These findings support our plea to include soil properties for understanding and predicting stomatal regulation during soil drying.

Water repellency prediction in high‐organic Greenlandic soils: Comparing vis–NIRS to pedotransfer functions

AbstractSoil water repellency (SWR) is a common phenomenon across agricultural soils of South Greenland that can negatively affect soil functions. Existing methods to measure SWR as a function of water content (w) are laborious. This study was conducted to compare the potential of visible–near‐infrared spectroscopy (vis–NIRS) as an alternative method to pedotransfer functions (PTFs) for predicting four SWR indices in 143 agricultural soils from South Greenland (clay, 0.016–0.172 kg kg–1; organic carbon (OC), 0.009–0.241 kg kg–1). Pedotransfer functions were established by multiple linear regression based on OC, clay, and pH. Partial least squares regression (PLS‐R) and interval PLS‐R were applied to build vis–NIRS prediction models (vis–NIR range 400–2,500 nm). The area under the SWR–w curve (SWRarea) and the critical soil water content (wnon) were accurately predicted by PTFs (R2 = .90; R2adj = .91) while the SWR after 60 °C pretreatment (SWR60), and the integrative repellency dynamic index (IRDI) were predicted less accurately (R2adj = .36; R2adj = .27). Vis‐NIRS models with variable selection performed at a better or close to the same level of accuracy as PTFs (SWRarea, R2 = .88; wnon, R2 = .90; SWR60, R2 = .63; IRDI, R2 = .54). This study demonstrated vis–NIRS as a valuable alternative to PTFs for rapid assessment of SWR and as a tool for SWR mitigation for farmers in South Greenland. The results may well apply to other regions with similar texture and OC ranges, but further testing is required.

Unraveling the Genetic Architecture of Two Complex, Stomata-Related Drought-Responsive Traits by High-Throughput Physiological Phenotyping and GWAS in Cowpea (Vigna. Unguiculata L. Walp).

Drought is one of the most devasting and frequent abiotic stresses in agriculture. While many morphological, biochemical and physiological indicators are being used to quantify plant drought responses, stomatal control, and hence the transpiration and photosynthesis regulation through it, is of particular importance in marking the plant capacity of balancing stress response and yield. Due to the difficulties in simultaneous, large-scale measurement of stomatal traits such as sensitivity and speed of stomatal closure under progressive soil drought, forward genetic mapping of these important behaviors has long been unavailable. The recent emerging phenomic technologies offer solutions to identify the water relations of whole plant and assay the stomatal regulation in a dynamic process at the population level. Here, we report high-throughput physiological phenotyping of water relations of 106 cowpea accessions under progressive drought stress, which, in combination of genome-wide association study (GWAS), enables genetic mapping of the complex, stomata-related drought responsive traits “critical soil water content” (θcri) and “slope of transpiration rate declining” (KTr). The 106 accessions showed large variations in θcri and KTr, indicating that they had broad spectrum of stomatal control in response to soil water deficit, which may confer them different levels of drought tolerance. Univariate GWAS identified six and fourteen significant SNPs associated with θcri and KTr, respectively. The detected SNPs distributed in nine chromosomes and accounted for 8.7–21% of the phenotypic variation, suggesting that both stomatal sensitivity to soil drought and the speed of stomatal closure to completion were controlled by multiple genes with moderate effects. Multivariate GWAS detected ten more significant SNPs in addition to confirming eight of the twenty SNPs as detected by univariate GWAS. Integrated, a final set of 30 significant SNPs associated with stomatal closure were reported. Taken together, our work, by combining phenomics and genetics, enables forward genetic mapping of the genetic architecture of stomatal traits related to drought tolerance, which not only provides a basis for molecular breeding of drought resistant cultivars of cowpea, but offers a new methodology to explore the genetic determinants of water budgeting in crops under stressful conditions in the phenomics era.

A model for estimating Ag-MAR flooding duration based on crop tolerance, root depth, and soil texture data

Agricultural Managed Aquifer Recharge (Ag-MAR) is an emerging MAR technique that uses agricultural fields as percolation basins to recharge the underlying aquifers. Ag-MAR can be a beneficial solution for storing excess surface water, however, if not managed properly it can potentially harm the soil and crops planted on the field at the time of recharge, ultimately leading to yield loss. Root zone residence time (RZRT), defined as the duration that the root-zone can remain saturated (or nearly saturated) during Ag-MAR without crop damage, is a key factor in Ag-MAR since extended periods of saturation in the root-zone can damage crops. Here we propose a simple RZRT model for estimating a safe Ag-MAR flooding duration based on hydraulic parameters deduced from soil texture, crop tolerance to saturation, effective root depth, and critical soil water content, which is the point where soil re-aeration occurs during drainage. We tested the model with different hydraulic parameter sets and compared the results to observed data and HYDRUS simulations. Using fitted and unfitted hydraulic parameters the average error of the predicted Ag-MAR flooding duration was less than 5 h, and up to a few days, respectively. Consequently, for crops with low flooding-tolerance, the model should be used with caution, but for more tolerant crops, the model provides reasonable predictions. The model also provides a first approximation of the possible amount of water that can be applied during an Ag-MAR event. Based on the RZRT model, we evaluated the Ag-MAR potential of various crops and effective root depths for each of the USDA soil texture classes. A spreadsheet containing the RZRT model including hydraulic parameters, and crop properties is publicly available and can be used as a learning tool or to estimate Ag-MAR flooding duration for different soils. The proposed model can be easily integrated into Ag-MAR assessment tools.

Potential of ecological modelling and smart-drainage development for mitigating adverse effects of future global change-type droughts for the Estonian forest sector

Abstract Global change-type droughts will become more frequent in the future and threaten forest ecosystems around the globe. A large proportion of the Estonian forest sector is currently subject to artificial drainage, which could probably lead to negative feedbacks when water supply falls short because of high temperatures and low precipitation during future drought periods. In this short article, we propose a novel research perspective that could make use of already gathered data resources, such as remote sensing, climate data, tree-ring research, soil information and hydrological modelling. We conclude that, when applied in concert, such an assembled dataset has the potential to contribute to mitigation of negative climate change consequences for the Estonian forest sector. In particular, smart-drainage systems are currently a rare phenomenon in forestry, although their implementation into existing drainage systems could help maintain the critical soil water content during periods of drought, while properly fulfilling their main task of removing excess water during wet phases. We discuss this new research perspective in light of the current frame conditions of the Estonian forest sector and resolve some current lacks in knowledge and data resources which could help improve the concept in the future.

Mandarin irrigation scheduling by means of frequency domain reflectometry soil moisture monitoring

The accurate estimation of plant water needs is the first step for achieving high crop water productivity. The main objective of the work was to develop an irrigation scheduling procedure for mandarin orchards under Mediterranean conditions based on replacing the amount of consumed water using reference values of soil moisture according to different phenological periods. The proposed methodology includes a definition part where the threshold values were established relating the trees’ stem water potential and the volumetric soil water content measured with Frequency Domain Reflectometry probes. A second part includes the steps for standardizing measurements from capacitance probes by using the LEACHM hydrological simulation model to take into account the sensor-to-sensor variations. Finally, an extrapolation procedure based on soil water retention curves was used for adapting critical soil water content thresholds to different soil conditions. Field evaluations were made in a citrus orchard located in eastern Spain during two seasons. Standardize critical soil water contents were: i) 24 % vol. for post-harvest, bloom - fruit set and phase III of fruit growth; ii) 27 % vol. for phase I of fruit growth, and iii) 29 % vol. for phase II of fruit growth with average daily air vapour pressure deficit values ranging between 0.2 - 0.4; 0.9–1.1 and 1.1–1.3 kPa, respectively. When implemented in the orchard, the sensor-based strategy resulted in water saving of 26 % respect to a control treatment, irrigated using the standard FAO-56 approach, without significant differences in yield and increasing the crop water productivity by 33 %. In conclusion, we suggest that the determination and use of the critical soil water content is a useful tool for scheduling irrigation. The proposed standardization and extrapolation methodology allows the irrigation strategy to be applied to other mandarin orchards under similar climatic conditions.

Modelling root water uptake under deficit irrigation and rewetting in Northwest China

AbstractThe spatial and temporal distribution of root water uptake (RWU) under deficit irrigation are critical factors for crop growth. The SWAP (soil–water–atmosphere–plant) model was applied to analyze the pattern of RWU for winter wheat (Triticum aestivum L.) under three irrigation levels: no water deficit (100% evapotranspiration [ET]), moderate water deficit (80% ET) and severe water deficit (60% ET). The 2–yr experiments indicated that SWAP was highly accurate (mean relative error [MRE] <21.7%, root mean square error [RMSE] <0.07 cm3 cm−3) in simulating the soil water content (SWC). Root water uptake was significantly (P < 0.01) different in the 0‐ to 60‐cm soil layer. The 0‐ to 60‐cm soil layer was the main source of RWU, and the average value accounted for 89.4% of the total root zone. Water stress had the greatest adverse effect on heading to grain filling, reducing RWU by 0.0026 cm3 cm−3 d−1. The critical SWC was 67.9% of the field capacity, when the RWU dropped to 95% of the control treatment. After rewetting, compensation and hysteresis effects on RWU were observed. The ranking of RWU recovery ability after rewetting was: emergence to jointing > jointing to heading > grain filling to maturity > heading to grain filling. Recovery time of RWU was 2 to 11 d and gradually increased with growth stage. The simplified RWU model established using path analysis and regression performed well (R2 = 0.836; P < 0.01) for RWU. This provided a more convenient way to accurately estimate RWU with fewer variables.

Stochastic Analysis of the Soil Water Content Standard Deviation‐Mean Value Relationships: On the Physical Significance of the Critical Mean Soil Water Content

AbstractThe spatial heterogeneity in the soil hydraulic properties was considered here as the main cause of the spatial variability in soil water content, θ. The aim of the study was (i) to identify the critical mean soil water content, Θcr, below which the standard deviation of the soil water content, σθ, can only diminish, and (ii) to provide a mathematical tool capable of linking Θcr to inherent soil hydraulic properties. The relationships between σθ and the mean value of θ, Θ, were analyzed by a first‐order analysis, based on a stochastic continuum presentation of steady‐state flow in 3‐D, spatially hetrogeneous, unsaturated soils, using the Gardner‐Russo model to describe the constitutive relations locally. Novel results of the study suggested that the mean pressure head, H, associated with Θcr, Hcr = H (Θcr), is an inflection point of the mean soil water capacity function, C(H) = dΘ/dH. Results of numerical simulations of flow in 3‐D, spatially heterogeneous, unsaturated soils, using the van Genuchten (1980, https://doi.org/10.2136/sssaj1980.03615995004400050002x) model to describe the constitutive relations locally, suggested that the results of the first‐order analysis might be applicable to more realistic situations, involving transient, nonmonotonous flows and more complicated descriptions of the local constitutive relations.

Lessons learnt from semi-arid wetland degradation. Las Tablas de Daimiel National Park

The Tablas de Daimiel National Park wetland developed in an area characterized by the cyclic nature of droughts, excessive aquifer exploitation and an evident aridification trend driven by climate change. The park and its surroundings have been exploited since prehistoric times and they are currently deeply anthropized to the point that both the physical survival of the wetland as well as its ecological function entirely depend on human action. A severe drought between 2006 and 2009 together with the drawdown of the water table caused the dry out of the system, reed overgrowth, disappearance of cut-sedge and sub-aquatic Chara spp. meadows and a smouldering peat fire. During this same period surface water, soil and groundwater were sampled for physical, hydrological and hydrochemical characterization. Upon drainage, the system functions as an artificial recharge system and becomes eutrophic showing large nutrient and salt content in soils and water. High water transmissivity capacities in the unsaturated zone and anthropic park management (soil compacted by heavy machinery, recirculation of low quality groundwater, reed reaping, water transfers, etc.) condition groundwater pollution. Increased knowledge of the physical-environment has allowed us to build a conceptual model of the surface water – groundwater interactions and to develop management tools to support the park management during a system dry out. Amongst these actions, we propose an unsaturated flow model for soil moisture simulation than can be used to predict critical soil water content for reed overgrowth or peat combustion risk.

Relative soil water content as a factor for wind erodibility in soils with different texture and aggregation

Abstract The soil water content is one of the main factors influencing the beginning of soil particle movement by the wind. Although, some authors studied the effect of soil water content on threshold wind velocity and have defined a value to prevent wind erosion, the critical relative soil water content (HRc), most of them used disturbed and/or sieved soil samples. Under these conditions, the effect of aggregation, i.e. the natural state of the soil, cannot be evaluated. So, our objective was to determine HRc in soils with different textures in Semiarid Argentinian Pampas Region (SAPR) using uncrushed soil samples under variable wind velocities. Wind tunnel simulations were performed at three wind speeds (2.1, 8.0 and 10.5 m s−1) during 3 min in each case. Results indicated that the quantity of collected soil (Q) decreased with increasing relative soil water contents (HR) up to a HRc value at which it became negligible. HRc values were lower in fine textured soils than in coarse soils. All soils showed increasing HRc values as wind velocity increased from 2.1 to 10.5 m s−1, while the wind velocity effect is more important in coarse textured soils. HRc was negatively related to micro-aggregation in a linear way. For that reason, soils with high fine silt and clay content need lower HRc values to inhibit wind erosion. In conclusion, not only texture is important factor in defining the HRc value at a specific wind velocity but also the micro and macro-aggregation of the soils.

Carbon and water flux patterns of a drought-prone mid-succession ecosystem developed on abandoned karst grassland

Successional ecosystems developed spontaneously on agricultural lands cover a substantial area worldwide, particularly in Europe and North America. The functioning of these ecosystems in terms of carbon and water fluxes remains fairly under-investigated. Here, carbon and water exchanges of a spatially heterogeneous tree-shrub-grassland mosaic developed on former semi-dry calcareous grassland in the sub-Mediterranean region of Slovenia were studied. Using eddy covariance, the annual, seasonal and daily dynamics of net ecosystem exchange (NEE) and water vapor exchange were examined, together with their environmental controls during the period August 2008–December 2012. Over this period, the ecosystem appeared to be a carbon sink but the annual carbon fixation had a span of more than four-fold, from 82 to 351gCm−2yr−1. This variability was largely explained by different durations of drought events that occurred during the summer despite the high and relatively evenly distributed rainfall. This is related to the poor ecosystem rain use efficiency, only 0.10–0.35gCm−2yr−1 was taken up per mm of rain and the estimated deep water drainage ranged between 44 and 72% of the incoming rain. Drought periods were identified by deriving the critical soil water content (SWC) (here 0.145m3m−3) at which evapotranspiration and carbon fluxes started to be limited by water availability. The estimated annual gross primary productivity was shown to be linearly dependent on the duration of the longest annual drought period and not on the drought intensity. Despite a severe drought in the summer of 2012, the ecosystem proved quite resilient to drought stress and restored its carbon uptake after several rain events. As shown by the light response curves, the light dependency of carbon exchange was affected by seasonality and environmental factors, particularly SWC, vapor pressure deficit (VPD) and air temperature (Ta). Interaction between the factors was also observed; negative effect of low SWC was significantly enhanced in the conditions of higher Ta or higher VPD.

Effects of amendments and hydrogel application on water retention, water release pattern, growth and yield performance of soybean-wheat cropping system

In a field experiment, water retention and release pattern of a sandy loam (Typic Haplustept) soil was studied along with growth and yield performance during 2009–10 in soybean-wheat cropping system. Main treatment included three amendment types namely FYM, tank soil applied @ 5 t ha−1 and no amendments and sub-treatments included three rates of super hydrogel i.e. 5, 2.5 and 0 kg ha−1, applied at 5-7cm depth along with seed in rows using a manual seed drill. Soil water content at various suction and soil water content changes during drying cycle after watering were monitored. Results revealed that changes in soil water content per unit change in suction became negligible after 100 kPa, therefore soil water content at 100 kPa suction was labeled as critical soil water content (CSWC). Soil water characteristic showed that at all suctions, water retained in gel treatments was always higher. In soybean, when pan evaporation ranged between 4–7 mm day,−1 CSWC in FYM with gel treated plots reached in 14 days, whereas, in other treatments CSWC arrived early. In wheat, during early growth when pan evaporation ranged between 1–1.8 mm day−1, CSWC arrived in 28 days in FYM with gel @ 5 kg ha−1 treatment. Application of hydrogel @ 2.5 kg ha−1 significantly increased the dry biomass, root growth, grain yield and water use efficiency over no hydrogel, while further increased in hydrogel application @ 5 kg ha−1 growth parameters and water use efficiency increased marginally. Results further indicated that a mixture of both FYM and hydrogel was more efficient for improving hydraulic properties of the soil and crop growth parameters than applying each of them alone.

Bacterial impact on the wetting properties of soil minerals

Soil–water repellency (SWR) is a widely observed phenomenon with severe impacts, but a physicochemical framework to explain the process of SWR development is still a major field of research. Recent studies have shown that microbial biomass residues, in particular cell fragments, contribute significantly to the formation of soil organic matter (SOM) and can decrease wettability. It was also shown that osmotic stress increases the hydrophobicity of bacterial cell surfaces. If microorganisms are an important source of SOM, the attachment of cells and their residues on mineral grains should decrease wettability of minerals, and the effect should be more pronounced in case of osmotic stress. Cultures of Pseudomonas putida, either unstressed or exposed to osmotic stress, and cell fragments were mixed with minerals and the impact on surface wetting properties was investigated by determining the solid-water contact angle (CA). Attachment of bacteria to quartz surfaces resulted in a significant increase in hydrophobicity of the surfaces (CA increase by up to 90°), in particular for stressed cells. Cell fragments and cytosol were also found to decrease wettability significantly (CAs of up to 100°). These findings may explain various phenomena related to SWR, like critical soil–water content, and may be one important explanation for the formation of SWR after irrigation with treated sewage effluents. The results also support the hypothesis of a microbial origin of SWR, in which macromolecular biological structures may have a greater impact than specific classes of organic compounds.

Relationship between the severity, persistence of soil water repellency and the critical soil water content in water repellent soils

Soil water repellency (SWR) causes reduced soil water storage and enhanced runoff and reduced ecosystem productivity. As such, characterization of SWR is a prerequisite for effective environmental management. The objectives of this study were to determine the relationship between the severity of SWR and its persistence and to determine the soil critical water content (CWC). Soils were collected from thirteen soil sites; five from natural jack pine (Pinus banksiana) ecosites (AE1, AE2, SV 10, 26 and 27), six from reclaimed/disturbed sites (ALFH, CPA, SS trial, ATS, SCB, and SW30) located in the Athabasca Oil Sands region and two from agricultural sites (Goodale and Melfort) in Central Saskatchewan, Canada. The severity of SWR as a function of persistence was assessed by measuring the change of water drop contact angles (modified sessile drop method) with time (WDPT, water drop penetration time). The CWC was determined for all the soils by measuring water drop contact angles on soils with predetermined water contents from oven dried to 20% (kgkg−1). In natural, reclaimed and agricultural soils, a high severity (contact angle) of repellency does not necessarily denote long persistence (WDPT) or high CWC. Measurement of severity and persistence is related to the differences and changes in surface energy between water and the soil surface respectively. Although the CWC gives us the water content at which above it SWR is negligible, the trend between contact angle and increasing water content proved to be more informative. Characterizing and understanding the severity, persistence and CWC together are valuable when determining the effects of SWR on hydrological processes as they have different mechanisms compared to one another.

Water availability in different soils in relation to hydrogel application

A study was conducted on PVC columns in the laboratory to investigate the effect of the application of hydrogel in modifying the hydraulic properties of different soil types. The main treatments consisted of four different soil types, namely sand, alluvial sandy loam, red sandy loam and black clay. The sub-treatments consisted of three levels of gel application: 0.7%, 0.5% and 0% (by weight). A given level of gel was mixed in the top 10cm-layer of soil. In all soil columns, 5cm of water was applied, and soil samples were taken with a core auger on the fourth, seventh, 14th and 22nd days after watering (DAW). The undisturbed soil of the rings of the core assembly was used for preparing the soil water retention curve, and the soil of the core was used for determining saturated hydraulic conductivity (Ks), bulk density and gravimetric soil water content.The results of measuring soil water content (SWC) during drying revealed that in sand treated with a 0.7% gel application, the water release rates were relatively uniform throughout the entire drying period, whereas in black clay, water release rates were very high initially (4–7DAW) but fell appreciably afterwards (less than 0.5% per day).The result of the soil water characteristic curve revealed that water release per unit suction change in the 0–10kPa range (unavailable to plants) in soil samples not treated with gel was higher compared to soil samples treated with gel, except in alluvial soil, for which rates were similar. However, the water release per unit suction change in the 10–100kPa range (available to plants) in soil samples not treated with gel was significantly lower compared to that in soil samples treated with gel for all soil types.The above results suggest that gel significantly improved the readily available water capacity (RAWC) of the soils. Furthermore, the exceptionally low value of the water released per unit suction change pattern in all soil types for soil water suction of >100kPa indicated that 100kPa can be considered the critical limit of soil water potential from the water availability standpoint. The efficacy of the gel in improving RAWC was confirmed by the increased midpoint moisture (MPM), reduced median pore diameter (dm) and moisture release index (Im).The time at which a critical SWC (SWC corresponding to 100kPa) was reached was studied in order to further examine the suitability of gel in improving soil water retention in different soil types. The drying pattern of soils highlighted that under low evaporative condition, the critical SWC with the 0.7% gel treatment reached approximately 7, 14, 22 and 4DAW in red sandy loam, alluvial sandy loam, sand and black clay soils, respectively. The above-mentioned results thus reaffirmed the suitability of gel for sandy soil because it improved the water availability of the sandy soil for a longer period (nearly 22days, which corresponds to the irrigation interval of most agricultural crops), while gel was found unsuitable for black soil, in which the critical soil water content was reached early (4–7days).

Corn and Velvetleaf (Abutilon theophrasti) Transpiration in Response to Drying Soil

Soil water availability is the most important factor limiting crop yield worldwide. Understanding crop and weed transpiration in response to water supply may provide valuable insight into the mechanisms of crop yield loss in water-limited environments. A greenhouse experiment was conducted to quantify corn and velvetleaf transpiration in response to drying soil. Five plants of each species were well watered by adding back the equivalent water loss each day to reach field capacity, and five plants were subjected to drought stress (dry-down) by not replacing lost water. Normalized daily transpiration of dry-down plants was regressed on soil water content expressed as the fraction of transpirable soil water (FTSW). The critical soil water content below which plants begin to close their stomates occurred at FTSWcr= 0.36 ± 0.015 for corn and 0.41 ± 0.018 for velvetleaf. Total water transpired did not differ among species. Velvetleaf also responded to drought by senescing its oldest leaves, whereas corn mainly maintained its leaf area but with rolled leaves during peak drought stress. During a short-term drought, corn is expected to perform better than velvetleaf because it maintains full transpiration to a lower FTSW and does not senesce its leaves. Under severe long-term drought, the species that closes its stomates at greater FTSWcrwill conserve water and increase its chances of survival. Moreover, senescing all but the youngest leaves may ensure at least some seed production. Research is needed to evaluate the effects of soil water supply on corn–velvetleaf interference in the field.