Critical Soil Moisture Research Articles

Global influence of soil texture on ecosystem water limitation.

Low soil moisture and high vapour pressure deficit (VPD) cause plant water stress and lead to a variety of drought responses, including a reduction in transpiration and photosynthesis1,2. When soils dry below critical soil moisture thresholds, ecosystems transition from energy to water limitation as stomata close to alleviate water stress3,4. However, the mechanisms behind these thresholds remain poorly defined at the ecosystem scale. Here, by analysing observations of critical soil moisture thresholds globally, we show the prominent role of soil texture in modulating the onset of ecosystem water limitation through the soil hydraulic conductivity curve, whose steepness increases with sand fraction. This clarifies how ecosystem sensitivity to VPD versus soil moisture is shaped by soil texture, with ecosystems in sandy soils being relatively more sensitive to soil drying, whereas ecosystems in clayey soils are relatively more sensitive to VPD. For the same reason, plants in sandy soils have limited potential to adjust to water limitations, which has an impact on how climate change affects terrestrial ecosystems. In summary, although vegetation-atmosphere exchanges are driven by atmospheric conditions and mediated by plant adjustments, their fate is ultimately dependent on the soil.

Responses of Khao Dawk Mali 105 and Pathum Thani 1 Rice to Different Soil Moisture Regimes on Physiological Traits and Productivity

Khao Da wk Mali 105 (KDML105) and Pathum Thani 1 (PTT1) are two popular rice cultivars in Thailand; KDML105 is known for its unique texture and fragrance and PTT1 for its high yield and ability to be grown year-round in various conditions. This research aimed to (1) study the morphological and physiological processes of two rice cultivars KDML105 and PTT1 under six levels of soil moisture including 20, 30, 40, 50, 60, and 70% available water holding capacity (AWHC) in clay soil and sandy clay loam soil (SCL); (2) identify the critical soil moisture content (CMC) using the physiological response to drought. All physiological traits decreased with reduced soil moisture levels across all growth stages and soil types. The 20% AWHC maximum reduced plant height (1.61-folds), predawn leaf water potential (LWPpd, 3.07-folds), net photosynthesis rate (3.26-folds), stomatal conductance (17.00-folds), chlorophyll fluorescence (1.28-folds), dry matter (2.70-folds), grain yield (3.91-folds) in flowering stage. All physiological traits positively correlated with dry matter and grain yield in both cultivars and both soils, but the highest correlation was found between LWPpd with dry matter (0.884–0.961**) and grain yield (0.790–0.936**) in PTT1, or dry matter (0.883-0.973**) and grain yield (0.818–0.945**) in KDML105. Therefore, LWPpd was used to evaluate the CMC from the linear regression between LWPpd and soil moisture contents. At the seedling stage, the CMCs of PTT1 were at 60% AWHC in both soils, while KDML105 was at 60 and 70% AWHC in clay and SCL soils, respectively. At the tillering and flowering stages, the CMCs of both cultivars were at 70% AWHC in both soils. The results indicate that CMC values vary with soil texture, rice cultivar, and growth stage. This information is crucial for optimizing irrigation practices, such as implementing alternate wetting and drying systems tailored to specific soil textures and rice cultivars.

Mapping critical soil moisture thresholds of water stress for global grasslands

The critical soil moisture threshold (SMth) of plant water stress distinguishes whether vegetation is in an energy-limited or water-limited regime. Accurate quantification of SMth for grasslands holds significant implications for drought monitoring and efficient grassland management. However, our understanding of SMth within global grasslands remains limited. In this study, we investigated the relationship between SMth and environmental factors and mapped the global SMth for grasslands based on the estimated SMth across FLUXNET sites. Firstly, SMth was identified and estimated for 18 distinct grassland sites. Then, the factors significantly affecting SMth were investigated and the global SMth for grasslands was predicted using an empirical linear model. Our results showed that SMth was significantly positively correlated with mean annual soil moisture (SM), mean annual precipitation (PRE), and maximum annual LAI (LAImax), while negatively correlated with vapor pressure deficit (VPD) and shortwave radiation (RAD). These factors collectively constrain SMth well, with the coefficient of determination of 0.79. The estimated mean SMth for global grasslands from 2000 to 2021 is 27.8%, with a range from 1.3% to 55.2%. The SMth values are lower in the northern hemisphere than that in the southern hemisphere. The highest values of SMth, exceeding 40.0%, are mainly distributed in Brazil, Peru, Southern Europe, New Zealand and the Tibetan Plateau. In contrast, the lowest values, failing below 20.0%, are mainly distributed in Northern Canada, Greenland, Central African Savannah and Kazakhstan. The SMth is markedly lower in the arid region than that in the tropical and temperate regions. The quantified SMth in this study could provide a direct way to determine whether and to what extent grasslands are subjected to water stress, which would greatly facilitate the study of drought or water stress effects on vegetation growth for grassland ecosystems.

Precipitation-induced landslide risk escalation in China’s urbanization with high-resolution soil moisture and multi-source precipitation product

Landslides pose a formidable natural hazard. Accurate risk assessment of landslides triggered by precipitation heavily relies on hydrometeorological factors, specifically precipitation and soil moisture. However, the insufficient ground-based observations and the coarse spatio-temporal resolution hinder the performance of landslide prediction. It is not clear what hydrometeorological thresholds and triggering mechanisms are more likely to trigger landslides in China, particularly in the context of rapid urbanization. To address these questions, this study investigated 1504 shallow landslide events in Chinese urban and non-urban areas from 2007 to 2019. It utilized daily 1 km soil moisture at various depths (20–100 cm) and multi-source precipitation datasets, including gauge-based gridded dataset, in conjunction with three multi-source fusion precipitation products (Multi-Source Weighted-Ensemble Precipitation − MSWEP, the Climate Hazards Group InfraRed Precipitation with Station dataset − CHIRPS, and the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement − GPM-IMERG), along with dynamic urban impervious area datasets. It aims to determine the optimal multi-source precipitation predictor, the critical soil moisture depth that triggers landslides, and to establish the hydrometeorological thresholds for landslides. Additionally, the impact of urbanization on landslide occurrences was estimated by comparing antecedent precipitation accumulation, soil moisture, and impervious surface ratio dynamics between urban and non-urban areas. The results indicated that a combination of 2-day cumulative CHIRPS precipitation and 100 cm soil moisture provided the most accurate predictions for landslides in urban regions of China (accuracy = 88.5 %), outperforming interpolated ground-based observations and other fusion products. Specifically, landslides become more prone when antecedent cumulative rainfall surpasses 97.42 mm in 2 days and soil moisture exceeds 39.6 % m3/m3 saturation in China. Urban areas experienced high antecedent precipitation levels, lower precipitation (64.40 mm) threshold and soil moisture threshold (38.9 %), and shorter durations at landslide sites compared to non-urban areas (71.90 mm, 41.4 %, and 7 days, respectively). The process of urbanization is observed to decrease soil moisture levels while concurrently elevating rainfall amounts. This phenomenon, combined with anthropogenic activities, including distance from roads and urban impervious surface expansion, contributes to 44.6 % of the causes of landslides by reducing slope stability and increasing the presence of loose material. These findings have implications for landslide warnings in urban areas with limited measurements and contribute to understanding urbanization’s impact on landslide risks in developing nations.

Global critical soil moisture thresholds of plant water stress

During extensive periods without rain, known as dry-downs, decreasing soil moisture (SM) induces plant water stress at the point when it limits evapotranspiration, defining a critical SM threshold (θcrit). Better quantification of θcrit is needed for improving future projections of climate and water resources, food production, and ecosystem vulnerability. Here, we combine systematic satellite observations of the diurnal amplitude of land surface temperature (dLST) and SM during dry-downs, corroborated by in-situ data from flux towers, to generate the observation-based global map of θcrit. We find an average global θcrit of 0.19 m3/m3, varying from 0.12 m3/m3 in arid ecosystems to 0.26 m3/m3 in humid ecosystems. θcrit simulated by Earth System Models is overestimated in dry areas and underestimated in wet areas. The global observed pattern of θcrit reflects plant adaptation to soil available water and atmospheric demand. Using explainable machine learning, we show that aridity index, leaf area and soil texture are the most influential drivers. Moreover, we show that the annual fraction of days with water stress, when SM stays below θcrit, has increased in the past four decades. Our results have important implications for understanding the inception of water stress in models and identifying SM tipping points.

Straw Use to Reducing Soil Compaction and Its Effect on the Biometric and Physiological Characteristics of Soybean and Maize Plants

ABSTRACT The objective of this study was to evaluate the straw to decrease the bulk density of sandy clay loam and clay soils and to determine the effect of this on the development of soybean and maize plants. The soils were compacted using Proctor test with different amounts of crushed straw on the soil surface corresponding to 0, 3, 6, 9 and 12 t ha−1. After this, the soils in pots were compacted with same levels of straw on its surface using the maximum bulk density each soil. Then, soybean and maize were sown and after 42 days, biometric characteristics and physiological parameters were evaluated. The results showed that the maximum bulk density and critical soil moisture for the sandy clay loam soil were 1.51 t m−3 and 0.15 kg kg−1, respectively, while those for the clay soil were 1.54 t m−3 and 0.19 kg kg−1. The straw reduced soil compaction for the two soils, resulting in lower soil bulk density with 9 and 8 t ha−1 of straw on the surface of sandy clay loam and clay soils, respectively. When evaluated the plant, the amounts 6 to 9 t ha−1 of straw promoted better initial development of soybean and maize plants. Thus, this study suggests that a minimum of 6 and 8 t ha−1 of straw be used on the surface of sandy clay loam and clay soils, respectively were efficient in promoting physical soil improvements and better plant development.

Preferential Hydrologic States and Tipping Characteristics of Global Surface Soil Moisture

AbstractA dynamic transition in soil hydrologic states through meteorological variability and terrestrial feedback governs soil‐vegetation‐climate (SVC) interactions, constrained by critical soil moisture (SM) thresholds. However, observational and scaling constraints limit critical SM threshold estimation at the remote‐sensing (RS) footprint scale. Using global surface SM (θRS) from NASA’s Soil Moisture Active Passive (SMAP) satellite, we characterize the seasonal preferential hydrologic states of θRS and derive three tipping characteristics to estimate the intensity (Mean Tipping Depth, ), frequency (Tipping Count, η), and duration (Mean Tipped Time, ) of the excursion of θRS from wet‐ to dry‐average conditions. The preferential state provides the seasonally dominant hydrological states of θRS, while tipping characteristics capture the ecosystem linkages of the dynamic transition in θRS hydrologic states. Globally, θRS predominantly exhibits a (unimodal) dry‐preferential state, especially over arid/semi‐arid drylands and a unimodal wet‐preferential θRS state in high‐latitude boreal forests and tundra biomes. Prevalence of (bimodal) bistable θRS state overlaps with regions of strong positive SM‐precipitation coupling and monsoonal climate in semi‐arid/subhumid climates. Seasonal preferential hydrologic states co‐vary with the regional variability in plant water stress threshold and land‐atmospheric coupling strength. Tipping characteristics of θRS show sensitivity to intra‐biome variability in SVC coexistence patterns and display high skill in partitioning global ecoregions. While and η are climate‐controlled, is moderated by soil and vegetation through their influence over θRS drydown during water‐limited evapotranspiration. Preferential states and tipping characteristics find applications in quantifying SVC coexistence patterns, climate model diagnosis, and assessing ecosystem sensitivity to climate change.

Sensitivity of the Land–Atmosphere Coupling to Soil Moisture Anomalies during the Warm Season in China and its Surrounding Areas

Significant temporal and spatial variability in soil moisture (SM) is observed during the warm season in China and its surrounding regions. Because of the existence of two different evapotranspiration regimes, i.e., soil moisture-limited and energy-limited, averaging the land–atmosphere (L–A) coupling strength for all soil wetness scenarios may result in the loss of coupling signals. This study examines the daytime-only L–A interactions under different soil moisture conditions, by using two-legged metrics in the warm season from May to September 1981–2020, partitioning the interactions between SM and latent heat flux (SM–LH, the land leg) from the interactions between latent heat flux and the lifting condensation level (LH–LCL, the atmospheric leg). The statistical results reveal large regional differences in warm-season daytime L–A feedback in China and its surrounding areas. As the soil becomes wetter, the positive SM–LH coupling strength increases in arid regions (e.g., northwest China, Hetao, and the Great Indian Desert) and the positive feedback shifts to the negative one in semi-arid/semi-humid regions (northeast and northern China). The negative LH–LCL coupling is most pronounced in wet soil months in arid regions, while the opposite is true for the Tibetan Plateau. In terms of intraseasonal variation, the large variability of SM in north China, the Tibetan Plateau, and India due to the influence of the summer monsoon leads to the sign change in the land segment coupling index, comparing pre- and post-monsoon periods. To further examine the impact of SM anomalies on L–A coupling and to explore evapotranspiration regimes in the North China Plain, four sets of sensitivity experiments with different soil moisture levels over a period of 10 years were conducted. Under relatively dry soil conditions, evapotranspiration is dominated by the soil moisture-limited regime with positive L–A coupling, regardless of external moisture inflow. The critical soil moisture value separating a soil moisture-limited and an energy-limited regime lies between 0.24 m3/m3 and 0.29 m3/m3. Stronger positive feedback under negative soil moisture anomalies may increase the risk of drought in the North China Plain.

Smart greenhouse construction and irrigation control system for optimal Brassica Juncea development.

This paper contributes to smart greenhouses and IoT (Internet of Things) research. Our pioneering achievement centers on successfully designing, constructing, and testing a 30m2 smart greenhouse, explicitly focusing on the cultivation and development of Brassica Juncea, a mustard variety commonly grown in Vietnam. The construction phase entailed the meticulous integration of diverse IoT technologies and systems, culminating in the creation of a finely tuned environment to meet the unique needs of Brassica Juncea cultivation. Notably, our research team has realized the physical infrastructure and developed and implemented a robust web interface. This interface empowers users to monitor and remotely control the smart greenhouse conveniently. It provides real-time visualization of critical parameters, including temperature, humidity, soil moisture, and light intensity, enabling precise monitoring and supporting informed decision-making in crop management. In addition to the web interface, we have meticulously designed and completed an Android mobile application, further enhancing accessibility and convenience. This mobile app allows users to monitor and control the smart greenhouse while on the move. It is imperative to underscore that this work marks a significant milestone as the first complete smart greenhouse IoT solution dedicated to developing Brassica Juncea. Our pioneering accomplishments not only advance the frontiers of innovative greenhouse and IoT research but also contribute substantially to the progress of intelligent agriculture.

A Soil Moisture Profile Conceptual Framework to Identify Water Availability and Recovery in Green Stormwater Infrastructure

The recovery of soil void space through infiltration and evapotranspiration processes within green stormwater infrastructure (GSI) is key to continued hydrologic function. As such, soil void space recovery must be well understood to improve the design and modeling and to provide realistic expectations of GSI performance. A novel conceptual framework of soil moisture behavior was developed to define the soil moisture availability at pre-, during, and post-storm conditions. It uses soil moisture measurements and provides seven critical soil moisture points (A, B, C, D, E, F, F″) that describe the soil–water void space recovery after a storm passes through a GSI. The framework outputs a quantification of a GSI subsurface hydrology, including average soil moisture, the duration of saturation, soil moisture recession, desaturation time, infiltration rates, and evapotranspiration (ET) rates. The outputs the framework provide were compared to the values that were obtained through more traditional measurements of infiltration (through spot field infiltration testing), ET (through a variety of methods to quantify GSI ET), soil moisture measurements (through the soil water characteristics curve), and the duration of saturation/desaturation time (through a simulated runoff test), all which provided a strong justification to the framework. This conceptual framework has several applications, including providing an understanding of a system’s ability to hold water, the post-storm recovery process, GSI unit processes (ET and infiltration), important water contents that define the soil–water relationship (such as field capacity and saturation), and a way to quantify long-term changes in performance all through minimal monitoring with one or more soil moisture sensors. The application of this framework to GSI design promotes a deeper understanding of the subsurface hydrology and site-specific soil conditions, which is a key advancement in the understanding of long-term performance and informing GSI design and maintenance.

Assessment of Crop-Water Requirement of Alfalfa Using FAO-CROPWAT Model-8.0

The current study was conducted at the Farming System Research Station, situated within the Department of Agronomy at the College of Agriculture, Junagadh Agricultural University in Junagadh, Gujarat, India. This research took place during the winter cycle (Rabi season) 2021 to 2022 and 2022 to 2023. In this study, the CROPWAT crop irrigation schedule provides an overall Evapotranspiration (ETc) value of 549.6 millimeters as the optimal water requirement for achieving maximum crop production with sufficient water supply. The actual water use by the crop during its growth period amounted to 544.2 millimeters and total net irrigation applied was 507.9 millimeters, while the gross irrigation amounted to 725.6 millimeters. The irrigation efficiency was 100%. Comparing this approach to irrigation scheduling based on the Irrigation Water to Crop Potential Evapotranspiration (IW/CPE) ratio, using CROPWAT enabled the precise management of irrigation water. It ensured that the alfalfa crop received the appropriate amount of water according to its specific growth stage requirements. The CROPWAT model data indicates that there was no yield reduction, even with a 100% critical depletion of available soil moisture. Additionally, the seasonal yield response factor was calculated to be 0.80%. These findings emphasize the effectiveness of the irrigation strategy employed, resulting in optimal crop yields without yield losses due to critical soil moisture depletion.

Uncertainty in Projected Critical Soil Moisture Values in CMIP6 Affects the Interpretation of a More Moisture‐Limited World

AbstractEvaporation is controlled by soil moisture (SM) availability when conditions are not extremely wet. In such a moisture‐limited regime, land‐atmosphere coupling is active, and a chain of linked processes allow land surface anomalies to affect weather and climate. How frequently any location is in a moisture‐limited regime largely determines the intensity of land feedbacks on climate. Conventionally this has been quantified by shifting probability distributions of SM, but the boundary between moisture‐limited and energy‐limited regimes, called the critical soil moisture (CSM) value, can also change. CSM is an emergent property of the land‐atmosphere system, determined by the balance of radiative, thermal and kinetic energy factors. We propose a novel framework to separate the contributions of these separate effects on the likelihood that SM lies in the moisture‐limited regime. We confirm that global warming leads to a more moisture‐limited world. This is attributed to reduced SM in most regions: the moisture effect. CSM changes mainly due to shifts in the surface energy budget, significantly affecting 27.7% of the globe in analyzed climate change simulations. However, consistency among Earth system models regarding CSM change is low. The poor agreement hints that variability of CSM in models and the factors that determine CSM are not well represented. The fidelity of CSM in Earth system models has been overlooked as a factor in water cycle projections. Careful assessment of CSM in nature and for model development should be a priority, with potential benefits for multiple research fields including meteorology, hydrology, and ecology.

Critical soil moisture thresholds of plant water stress in terrestrial ecosystems.

Plant water stress occurs at the point when soil moisture (SM) limits transpiration, defining a critical SM threshold (θcrit). Knowledge of the spatial distribution of θcrit is crucial for future projections of climate and water resources. Here, we use global eddy covariance observations to quantify θcrit and evaporative fraction (EF) regimes. Three canonical variables describe how EF is controlled by SM: the maximum EF (EFmax), θcrit, and slope (S) between EF and SM. We find systematic differences of these three variables across biomes. Variation in θcrit, S, and EFmax is mostly explained by soil texture, vapor pressure deficit, and precipitation, respectively, as well as vegetation structure. Dryland ecosystems tend to operate at low θcrit and show adaptation to water deficits. The negative relationship between θcrit and S indicates that dryland ecosystems minimize θcrit through mechanisms of sustained SM extraction and transport by xylem. Our results further suggest an optimal adaptation of local EF-SM response that maximizes growing-season evapotranspiration and photosynthesis.

A model framework to investigate the role of anomalous land surface processes in the amplification of summer drought across Ireland during 2018

AbstractDue to its latitude and ample year‐round rainfall, Ireland is typically an energy‐limited regime in the context of soil moisture availability and evapotranspiration. However, during the summer of 2018, regions within the country displayed significant soil moisture deficits, associated with anomalous atmospheric forcing conditions, with consequent impacts on the surface energy balance. Here, we explore the utility of a physically based land surface scheme coupled with observational, global gridded reanalysis and satellite‐derived data products to analyse the spatial and temporal evolution of the 2018 summer drought event in Ireland over grassland, which represents the dominant agricultural land‐cover. While the surface–air energy exchanges were initially dominated by atmospheric anomalies, soil moisture constraints became increasingly important in regulating these exchanges, as the accumulated rainfall deficit increased throughout the summer months. This was particularly evident over the freer draining soils in the east and southeast of the country. From late June 2018, we identify a strong linear coupling between soil moisture and both evapotranspiration and vegetation response, suggesting a shift from an energy‐limited evapotranspiration regime into a dry or soil water‐limited regime. Applying segmented regression models, the study quantifies a critical soil moisture threshold as a key determinant of the transition from wet to dry evaporative regimes. These findings are important to understand the soil moisture context under which land–atmosphere couplings are strongest in water‐limited regimes across the country and should help improve the treatment of soil parameters in weather prediction models, required for subseasonal and seasonal forecasts, consequently enhancing early warning systems of summer climate extremes in the future.

Camelina germination under osmotic stress − Trend lines, time-courses and critical points

Camelina [Camelina sativa (L.) Crantz] has increased cold, heat, and drought tolerance and decreased susceptibility to diseases and pests than oilseed rape (Brassica napus L.). Because water deficit at sowing leads to unsatisfactory stand establishment due to irregular seed germination and emergence, the aim of this study was to understand the response of camelina germination under osmotic stress and identify critical soil moisture levels for successful establishment. Two spring cultivars, NS Slatka and NS Zlatka, developed at the Institute of Field and Vegetable Crops Novi Sad, Serbia, were compared under 9 levels of osmotic stress, ranging from 0 MPa to −1.6 MPa. Polyethylene glycol was used to obtain the osmotic potential of the solutions. Results showed that the tested cultivars did not decrease germination under mild and medium osmotic stress levels (down to −0.8 MPa). However, germination significantly decreased in both cultivars under higher levels of osmotic stress, and NS Zlatka was more sensitive. Germination speed significantly increased at −0.4 MPa. The estimated osmotic potentials to stop germination were −1.45 MPa for NS Slatka and - 1.46 MPa for NS Zlatka. Time to 50% germination also showed a significant bi-linear trend in response to osmotic potential, but in the opposite direction than the one observed in germination. Inflection points were recorded at −0.77 MPa for NS Slatka and −0.78 MPa for NS Zlatka, thereafter time to 50% of germination rapidly increased. This study confirmed that camelina can withstand increased levels of drought stress at germination, so it could be considered a more suitable option than oilseed rape on marginal land, or environments with irregular precipitation.

Experimental study and performance evaluation of a ground heat exchanger with different intermittent ratios and soil thermal properties

The temperature behavior as well as the heat transfer evaluation of a ground heat exchanger (GHE) are investigated under different intermittent operation strategies, different amount of water added in soil at the initial moment of heating and recovery phase. For this purpose, small-scale experiments are respectively carried out in a soil-box system and a soil-column system. The results from the soil-box experiment showed that the dimensionless temperature rise rate and recovery rate of the soli represent a downtrend with an increasing on-off ratio. The effect of heat source on its nearby soil temperature rise would fade away after the pump operation time exceeds a certain value. In addition, the larger heat transfer mainly occurs in the initial 4 h during the heating and recovery phase. Thus, the further experiment performed in the 4h-4 h mode indicates that the better temperature recovery and heat transfer performance compared with the 12h-12 h mode. Compared with soil humidification at the recovery phase, soil humidification at the heating phase has the better heat transfer of soil, and the values of the temperature rise and recovery rate at 13 cm are respectively increased by 141.9% and 63.6% when 150 ml water added in soil. Moreover, its advantage becomes more noticeable as the increasing distance of the testing position from the GHE. Additionally, it is found that there is a critical soil moisture content for the temperature recovery improvement. This research on the GHE give the idea to elevate its energy performance and may be helpful to validate the future theoretical study.

Uncovering the critical soil moisture thresholds of plant water stress for European ecosystems.

Understanding the critical soil moisture (SM) threshold (θcrit ) of plant water stress and land surface energy partitioning is a basis to evaluate drought impacts and improve models for predicting future ecosystem condition and climate. Quantifying the θcrit across biomes and climates is challenging because observations of surface energy fluxes and SM remain sparse. Here, we used the latest database of eddy covariance measurements to estimate θcrit across Europe by evaluating evaporative fraction (EF)-SM relationships and investigating the covariance between vapor pressure deficit (VPD) and gross primary production (GPP) during SM dry-down periods. We found that the θcrit and soil matric potential threshold in Europe are 16.5% and -0.7MPa, respectively. Surface energy partitioning characteristics varied among different vegetation types; EF in savannas had the highest sensitivities to SM in water-limited stage, and the lowest in forests. The sign of the covariance between daily VPD and GPP consistently changed from positive to negative during dry-down across all sites when EF shifted from relatively high to low values. This sign of the covariance changed after longer period of SM decline in forests than in grasslands and savannas. Estimated θcrit from the VPD-GPP covariance method match well with the EF-SM method, showing this covariance method can be used to detect the θcrit . We further found that soil texture dominates the spatial variability of θcrit while shortwave radiation and VPD are the major drivers in determining the spatial pattern of EF sensitivities. Our results highlight for the first time that the sign change of the covariance between daily VPD and GPP can be used as an indicator of how ecosystems transition from energy to SM limitation. We also characterized the corresponding θcrit and its drivers across diverse ecosystems in Europe, an essential variable to improve the representation of water stress in land surface models.

Anatomy of the 2018 agricultural drought in the Netherlands using in situ soil moisture and satellite vegetation indices

Abstract. The soil moisture status near the land surface is a key determinant of vegetation productivity. The critical soil moisture content determines the transition from an energy-limited to a water-limited evapotranspiration regime. This study quantifies the critical soil moisture content by comparison of in situ soil moisture profile measurements of the Raam and Twente networks in the Netherlands, with two satellite-derived vegetation indices (near-infrared reflectance of terrestrial vegetation, NIRv, and vegetation optical depth, VOD) during the 2018 summer drought. The critical soil moisture content is obtained through a piece-wise linear correlation of the NIRv and VOD anomalies with soil moisture on different depths of the profile. This non-linear relation reflects the observation that negative soil moisture anomalies develop weeks before the first reduction in vegetation indices: 2–3 weeks in this case. Furthermore, the inferred critical soil moisture content was found to increase with observation depth, and this relationship is shown to be linear and distinctive per area, reflecting the tendency of roots to take up water from deeper layers when drought progresses. The relations of non-stressed towards water-stressed vegetation conditions on distinct depths are derived using remote sensing, enabling the parameterization of reduced evapotranspiration and its effect on gross primary productivity in models to study the impact of a drought on the carbon cycle.

Impact of Soil Characteristics and Moisture Content on the Corrosion of Underground Steel Pipelines

External corrosion of underground steel structures is a major environmental and economic burden. The severity of such detriment depends on many factors that encompass: soil properties, moisture content, total soluble salts, the presence of microorganism, and method of metal protection. The investigative endeavor put forth bestows prime attention to the effect of soil attributes and moisture loading on the corrosion of submerged steel structures. The exposure to four discrete soil types (saline, clay, calcareous and sand) was explored in this study. Electrochemical linear polarization measurements were deployed as a means of assessing corrosion performance. The results revealed corrosion event severities endured by steel to be strongly underpinned by soil overall characteristics and moisture content in the vicinity. Corrosion potentials were found to possess more negative values with rising soil moisture content. The corrosion rate was rendered consistently peaking in coincidence with 20% (by wt.) moisture contents pertinent to sand, and with a corresponding 25% content in clay, saline and calcareous soils. The corrosivity of soil at the critical soil moisture content increases in the order: calcareous < clay < saline < sand. Scanning electron microscope observations confirmed the existence of corrosion product (loose and porous) with evidence of pitting attack. EDX analysis proved that the corrosion product was mainly composed of iron oxide. Plate counts revealed the absence of bacteria which warrants the exclusion of any microbial detriment as a contributing factor that might underpin corrosion.

The Relationship between Soil Moisture and Soil Water Repellency Persistence in Hydrophobic Soils

In this work, we modelled the response of soil water repellency (SWR) persistence to the decrease in moisture in drying soils, and we explored the implication of soil particle size distribution and specific surface area on the SWR severity and persistence. A new equation for the relationship between SWR persistence and soil moisture (θ) is described in this paper. The persistence of SWR was measured on ten different hydrophobic soils using water drop penetration time (WDPT) at decreasing levels of gravimetric water content. The actual repellency persistence showed a sigmoidal response to soil moisture decrease, where Ra(θ)=Rp/1+eδ(θ−θc). The suggested equation enables one to model the actual SWR persistence (Ra) using θ, the potential repellency (Rp) and two characteristic parameters related to the shape of the response curve. The two parameters are the critical soil moisture θc, where the Ra increase rate reaches its maximum, and the parameter δ affecting the steepness of the curve at the inflexion point of the sigmoidal curve. Data shows that both soil carbon and texture are controlling the potential SWR in New Zealand pastures.