Variability Of Soil Water Content Research Articles

The Composition of Nitrogen-Fixing Microorganisms Correlates With Soil Nitrogen Content During Reforestation: A Comparison Between Legume and Non-legume Plantations.

Numerous reforestation projects have been conducted to improve soil fertility in degraded forests, often causing alterations to the soil microbial communities. However, it remains unclear whether microbial functional groups are affected and how these groups correlate with an increase in the nutrient contents during reforestation. We investigated the abundance and composition of free-living nitrogen-fixing microorganisms (diazotrophs) by quantifying and sequencing the marker gene nifH in bulk soils from five reforestation approaches, including legumes and non-legumes, in subtropical China. The relationships between diazotrophic community attributes and soil nitrogen (N) content [NO3−, NH4+, and microbial biomass N (MBN)] were examined under various approaches. Abundance of diazotrophs was highest in the native tree plantation (Schima spp. and Michelia macclurei) and Acacia mangium monoculture (AM), and lowest in the Pinus massoniana monoculture. The diazotrophic abundance correlated positively with soil organic matter and water content while there was a negative correlation to pH. The composition of diazotrophic community differed significantly among the five reforestation approaches examined and was closely correlated with variations in soil pH, NH4+ and water content. Diazotrophic community composition was closely related to soil NH4+ content, whereas abundance was not. The AM contained higher NH4+, NO3− and MBN contents than the other reforestation approaches, which may be associated with the indicator species of diazotrophs (Actinobacteria, Proteobacteria, and Firmicutes). However, there were more indicator species of Proteobacteria in the mixed Acacia plantation (Acacia mangium and Acacia crassicarpa) than in AM, which might have contributed to the remarkedly lower N content compared to AM. Overall, the soil N content under reforestation appeared to be more related to the composition of diazotroph community than to the abundance of diazotrophs.

Two-dimensional monitoring of soil water content in fields with plastic mulching using electrical resistivity tomography

Plastic mulching (PM) has become an important agricultural practice to improve crop yields worldwide, while there is still a lack of methods to quantify the complex spatial variations of soil water content (SWC) in the PM field. In this study, a methodology for using Electrical Resistivity Tomography (ERT) to get SWC information in the PM field was presented. Its performance in monitoring SWC was validated, and the spatial variation of SWC was analyzed using the ERT results. A simplified Waxman and Smits model was selected to calibrate the pedo-physical relationship, and it showed good performance (coefficient of determination > 0.92). With the calibrated model, the SWC obtained using ERT showed good agreement with soil moisture sensors in different soil layers (RMSE < 0.027 cm3 cm−3). The ERT results showed that rainfall and drying events have different effects on SWC at different growing stages. At the early stage, rainfall and drying events mainly influenced SWC on the bare strip, while at the later stage, rainfall and drying events had more obvious effects on the zone near the planting hole. On a seasonal scale, a higher SWC was not only found in the middle part of the mulched strip, but also in the bare strip, while a lower SWC was found in positions near the planting hole. At the same time, a high-resolution ERT measurement revealed that the SWC was also largely influenced by the soil heterogeneity. As such, SWC in the mulched strip was not necessarily higher than in the bare strip as we had supposed, but showed a high degree of irregularity in two dimensions. Considering the irregularity of SWC in two dimensions, our study calls for replacing point-scale measurement with two-dimensional monitoring methods when acquiring SWC information in a field with PM.

Response of plants water uptake patterns to tunnels excavation based on stable isotopes in a karst trough valley

Karstic aquifers are very sensitive and fragile to environmental changes. With more tunnels excavated in karst areas, groundwater levels are increasingly lowered and resulted in changed natural hydrogeological flow system and groundwater-dependent vegetation, soil and hydrology of surface water systems. Water, especially groundwater, is a limited and key factor and significant driving force for karst ecosystem processes. However, the effects of tunnels excavation on the ecosystems in karst areas remain largely unknown. This study aimed at investigating whether there were variations in soil water contents and shifts of plants water uptake patterns between rainy and dry seasons in a karst trough valley affected by tunnels excavation, and comparing their differences with those in a tunnel-free karst trough valley in southwest China. Monthly soil water contents at two soil layers of the upper 20 cm and lower 20–40 cm were measured, and soil water at two soil layers, subcutaneous water and plant (arbor and shrub) xylem water were sampled in February (mid dry season) and September (late rainy season) 2017, respectively, and the δ2H and δ18O of those waters were analyzed. The IsoSource model based on dual stable isotopes of δ2H and δ18O was used to estimate the contributions of different sources to the plant xylem water. Although the soil water contents at both valleys showed similar vertical and temporal variations resulted primarily from the evaporation and precipitation, the soil water contents of two soil layers in the tunneling affected valley were significantly lower than that of the tunnel-free valley at both seasons. Plants uptake water pattern changed from a dominant soil water source during the rainy season to a dominant subcutaneous water source during the dry season at both valleys. However, plants extracted more water from the subcutaneous zone in the tunneling affected karst trough valley than that of the tunnel-free karst trough valley at both seasons, of which the proportion of plants water uptake from the subcutaneous zone was 33% in rainy season and 76% in dry season, respectively, in the tunneling affected karst trough valley, while the proportion of plants water uptake from the subcutaneous zone was 24% in rainy season and 59% in dry season, respectively, in the tunnel-free karst trough valley. The above results indicated that tunnels excavation decreased the soil water contents and then changed plants water uptake patterns in karst areas.

Design of water reuse storage facilities in Sustainable Urban Drainage Systems from a volumetric water balance perspective

This paper presents a methodology for designing water reuse storage facilities as part of Sustainable Urban Drainage Systems (SUDS) in urban catchments. The method analyzes the whole water balance of the catchment. The contributions to the balance are irrigation and precipitation; the outlets are evapotranspiration, seepage and discharge to the conventional sewage system. The internal system variations are the volume of water to be locally reutilized and the soil water content variation. A cost function that includes the costs of irrigation, discharge to the conventional sewer system and reuse of water locally is proposed to estimate the optimum volume of water to be reused. This approach for SUDS design goes beyond traditional events-based perspectives oriented to damage prevention. This method conceives stormwater as a resource and seeks its optimal use through the design of SUDS. Several types of urban catchments were studied, and the results show that the proposed methodology can be applied either for simulating SUDS behavior in urban catchments or for estimating the optimum volume of water to be locally reused.

Variations in Soil Water Content and Evapotranspiration in Relation to Precipitation Pulses within Desert Steppe in Inner Mongolia, China

Neither single nor discontinuous hydrological observation data can truly reflect periodic changes in soil moisture under natural conditions or interrelationships between various water sources. Therefore, in this study, precipitation pulse characteristics and variations in the soil water content (SWC) and actual evapotranspiration (ETa) in relation to pulses are explored through a field multi-water continuous observation system set in desert steppe in Inner Mongolia, China. A comparison between precipitation events in the growing seasons of 2016 and 2017 shows that precipitation events that are greater than 10 mm are the main cause of dramatic interannual precipitation variations in this region. A single small precipitation event has a limited impact on SWC and provides no obvious increase in the SWC within the top 10 cm soil layer. The precipitation interval ratio (P/I) is suitable for comparing water stresses of different drying-wetting cycles, and correlations between soil layers are found to be closer in humid years than in dry years. In this study, three modes of interpulse ETa in the desert steppe are discussed: a stable ETa mode under a water-sufficient condition, an attenuation ETa mode, and a stable ETa mode under extreme drought conditions.

Effects of subsoiling and straw returning on soil physical properties and maize production in Yellow River irrigation area of Gansu, China

A field experiment was conducted to examine the effects of subsoiling 35 cm with maize straw returning, subsoiling 35 cm without maize straw returning, and rotary tillage without maize straw returning on soil compaction, soil bulk density, soil infiltration, soil water content in 0-100 cm depth, nutrients uptake and production of maize on sierozem in the Gansu Yellow River irrigated area in 2015-2017. Compared with subsoiling 35 cm without maize straw returning and rotary tillage without maize straw returning, subsoiling 35 cm with maize straw returning significantly decreased the soil compaction and soil density in 0-40 cm depth. Compared with that in 2015 (before experiment), soil compaction and soil bulk density in subsoiling 35 cm with straw returning was decreased by 42.6% and 7.0%, respectively, after harvest in 2017. Compared with other treatments, subsoiling 35 cm with straw returning had the lowest variation of soil compaction (6.1%) and soil bulk density (3.2%) in 0-40 cm depth before sowing and after harvest in 2016 and 2017. The soil infiltration rate in subsoiling 35 cm with straw returning was significantly improved by 33.6% compared with rotary tillage without maize straw returning. Subsoiling 35 cm with straw retention could significantly increase soil water content and decrease water variation in 0-100 cm soil depth in spring (before maize sowing) and autumn (after maize harvest). Compared with rotary tillage without maize straw returning, water storage in subsoiling 35 cm with straw retention was increased by 15.5% and 5.6% in spring and autumn, respectively. The water use efficiency was enhanced by 32.4%. Furthermore, subsoiling 35 cm with straw retention could increase maize economic yield and biomass yield by 25.6% and 33.3%, compared with rotary tillage without straw retention. Subsoilng and straw retention could promote nutrient absorption, with N, P2O5 and K2O uptake increased by 49.6%, 51.5% and 37.6%, compared with rotary tillage. Overall, our results suggested that subsoiling 35 cm straw retention could improve soil characteristics, stabilize the phy-sical properties of the plough layer, increase soil water content in the 0-100 cm soil layer, and reduce water variation in spring and autumn. Consequently, it was the best management to promote the water and nutrient utilization of maize and achieve high yield. Our findings could provide theoretical basis for further research on the construction technology of the plough layer in Gansu irrigation area.

Field Water Balance Closure with Actively Heated Fiber-Optics and Point-Based Soil Water Sensors

While traditional soil water sensors measure soil water content (SWC) at point scale, the actively heated fiber-optics (AHFO) sensor measures the SWC at field scale. This study compared the performance of a distributed (e.g., AHFO) and a point-based sensor on closing the field water balance and estimating the evapotranspiration (ET). Both sensors failed to close the water balance and produced larger errors in estimated ET (ETε), particularly for longer time periods with &gt;60 mm change in soil water storage (ΔSWS), and this was attributed to a lack of SWC measurements from deeper layers (&gt;0.24 m). Performance of the two sensors was different when only the periods of ˂60 mm ΔSWS were considered; significantly lower residual of the water balance (Re) and ETε of the distributed sensor showed that it could capture the small-scale spatial variability of SWC that the point-based sensor missed during wet (70–104 mm SWS) periods of ˂60 mm ΔSWS. Overall, this study showed the potential of the distributed sensor to provide a more accurate value of SWS at field scale and to reduce the errors in water balance for shorter wet periods. It is suggested to include SWC measurements from deeper layers to better evaluate the performance of the distributed sensor, especially for longer time periods of &gt;60 mm ΔSWS, in future studies.

Water percolation in a thick unsaturated loess layer considering the ground‐atmosphere interaction

AbstractThe key objective of this paper is to advance our present understanding of how surface water infiltrates in thick unsaturated loess, which is found in arid and semiarid regions of the world, considering the ground‐atmosphere interaction. In situ data for a period of 1 year in thick loess layer at a site in the Loess Plateau of China that has groundwater table at 97.5 m depth were collected for achieving this objective. Climate factors, mainly rainfall and actual evaporation, were measured. In addition, variations of soil temperature and water content at different depths in the unsaturated zone were also measured. The data were used to interpret the water percolation characteristics by dividing the thick unsaturated zone into three zones; namely, (i) surface zone, which constitutes the top 1.0 m, (ii) unsteady zone, which is from 1.0 to 7.0 m, and (iii) steady zone, which is below 7.0 m. In the surface zone, soil temperature and water content are sensitive to climate factors. There is a variation of water content associated with the cumulative influence of infiltration and evaporation in the precipitation and nonprecipitation periods, respectively. In the unsteady zone, the water content is relatively constant; however, temperature varies in different seasons. Water percolation in this zone is both in liquid and vapour phases. In the steady zone, both soil temperature and water content are constant during the entire investigation period. The percolation velocity in this zone is approximately 1.23 × 10−8 m/s or 0.39 m/year, which suggests that it will take approximately 230.8 years for surface water to pass through the thick unsaturated zone and recharge the groundwater.

Temporal variability of soil water content and penetration resistance under different soil management practices

This study analyzed the temporal induced variability of soil penetration resistance (PR) and soil water content (SWC) of eight soil treatments, which ranged four tillage systems (conventional tillage [CT], reduced tillage [RT], no-tillage [NT], and fallow no-tillage [FNT]), to four soil management treatments (abandoned soil [that is, control], compacted soil, CT on compacted soil [tilled], and application of super absorbent polymers [SAP] on abandoned soil). The treatments were arranged in 20 × 4 m (65.6 × 13.1 ft) plots with four replicates. The temporal variability of soil PR and SWC was determined using four measurement repetitions over time, from 2014 to 2015 (December, February, April, and June). A total of 64 soil samples (four replications × eight treatments × two soil depth layers) were collected for the analysis of the temporal variations of SWC at four different times. According to the study, soil PR and SWC were influenced by soil management and tillage systems. Also, the study indicated that PR and SWC varied over time. For different treatments, a second degree polynomial correlation between PR and soil depth was observed during all measurement times. Tillage practice on compacted soils resulted in a decrease in PR during all measurement times and in both depths. In general, for most of the investigated soil treatments, a low variability of SWC between treatments was obtained at a given measurement time. It was discovered that the effect of soil management and soil depth was significant for SWC, but the interaction between depth and management was not statistically significant for any measurement time. The effect of soil treatment and soil depth, and the interaction between them, was significant for PR at p

Sugarcane straw removal effects on soil water storage and drainage in southeastern Brazil

To enhance global bioenergy production, the Brazilian industry9s interest in using sugarcane (<i>Saccharum officinarum</i>) straw as a feedstock for electricity and cellulosic ethanol production has increased recently. However, indiscriminate straw removal may negatively affect key soil functions related to water infiltration, retention, and availability to plants. Therefore, a microlysimeter system with a 0.2 m soil column was built to evaluate the effects of straw removal rates on water drainage and water storage in a sandy clay loam soil from a commercial sugarcane field in southeastern Brazil. The experiment was conducted in an open area where the lysimeter system was subjected to rainfall. After assembly, the treatments consisting of four straw amounts of 0, 3, 6, and 12 Mg ha⁻¹ were applied, representing straw removal rates of 100% (bare soil), 75%, 50%, and 0%, respectively. The water drainage was quantified and collected for 17 months, and the soil water storage was measured over a two-month period. The soil water holding capacity was calibrated by an inverse modeling approach minimizing the differences between soil water storage measured in the field and simulated by a water balance model. Our results reveal that the amount of water drained from bare soil was 30% lower than no removal treatment. Bare soil had the lowest water storage for most of the days, indicating a greater susceptibility to losing water through evaporation and drying. The long-term simulation for early, mid, and late sugarcane harvest seasons suggests longer periods of water restriction to crop growth under higher straw removal rates (i.e., 100% and 75%), potentially impairing sugarcane growth and thus yield. Also, the findings suggest that partial residue maintenance, up to 6 Mg ha⁻¹ of straw (50% removal) on the soil surface, seems to be sufficient to prevent excessive water losses and daily variations of soil water content, sustaining soil water storage rates above the critical point for sugarcane growth.

The Texas Soil Observation Network:A Comprehensive Soil Moisture Dataset for Remote Sensing and Land Surface Model Validation

Core Ideas TxSON is a Core Cal/Val Site for the NASA SMAP, SMOS, and Sentinel programs. TxSON consists of 40 in situ locations nested within a 36‐km EASE2 grid cell. Locations monitor soil moisture, temperature, and precipitation at 5, 10, 20, and 50 cm. Hourly, quality‐controlled data from 2015 to 2018 are available. The spatiotemporal variability of soil water content (SWC) at the remote sensing scale requires dense monitoring for calibration and validation. Here, we present an overview of the Texas Soil Observation Network (TxSON), an intensively monitored area in the semiarid rangelands of the central Texas Hill Country. TxSON is a dense network consisting of 40 in situ locations nested at 36, 9, and 3 km within the Equal‐Area Scalable Earth Grid and serves as a Core Calibration and Validation Site for NASA's Soil Moisture Active Passive mission. The 4‐yr dataset consists of hourly SWC measured at 5, 10, 20, and 50 cm. The SWC data are upscaled using arithmetic, Voronoi, and inverse distance weighting for 36‐, (2) 9‐, and (5) 3‐km grid cells. We present the site selection, environmental characteristics, network design, quality assurance (QA), upscaling algorithms, and data structure. Ancillary data include bulk density, particle size, and carbon content for each site and depth. We also provide automated QA for each location and scripts to use our binary flagging in upscaling methods. To summarize, the 36‐km grid cell has a mean bulk density of 1.34 ± 0.18 g cm−3 and a loam textural class. The in situ SWC has a root mean square error of 0.029 m3 m−3 against gravimetric data from 14 field campaigns. TxSON continues to add new locations with additional dense networks planned in other ecotones of the Edwards Plateau. The time series data along with scripts to import, plot, and upscaled SWC are available at https://doi.org/10.18738/T8/JJ16CF.

Variations in Soil Water Content, Infiltration and Potential Recharge at Three Sites in a Mediterranean Mountainous Region of Baja California, Mexico

In this research, we examined temporal variations in soil water content (θ), infiltration patterns, and potential recharge at three sites with different mountain block positions in a semiarid Mediterranean climate in Baja California, Mexico: two located on opposing aspects (south- (SFS) and north-facing slopes (NFS)) and one located in a flat valley. At each site, we measured daily θ between 0.1 and 1 m depths from May 2014 to September 2016 in four hydrological seasons: wet season (winter), dry season (summer) and two transition seasons. The temporal evolution of θ and soil water storage (SWS) shows a strong variability that is associated mainly with high precipitation (P) pulses and soil profile depth at hillslope sites. Results shows that during high-intensity P events sites with opposing aspects reveal an increase of θ at the soil–bedrock interface suggesting lateral subsurface fluxes, while vertical soil infiltration decreases noticeably, signifying the production of surface runoff. We found that the dry soil conditions are reset annually at hillslope sites, and water is not available until the next wet season. Potential recharge occurred only in the winter season with P events greater than 50 mm/month at the SFS site and greater than 120 mm/month at the NFS site, indicating that soil depth and lack of vegetation cover play a critical role in the transport water towards the soil–bedrock interface. We also calculate that, on average, around 9.5% (~34.5 mm) of the accumulated precipitation may contribute to the recharge of the aquifer at the hillslope sites. Information about θ in a mountain block is essential for describing the dynamics and movement of water into the thin soil profile and its relation to potential groundwater recharge.

Investigating plant root effects on soil electrical conductivity: An integrated field monitoring and statistical modelling approach

AbstractPlants have been shown to affect soil water content and temperature. Previous studies were conducted mainly in forestry and agricultural soils, where conditions of soil and vegetation are different from those in an urban landscape. In an urban landscape, the influence of plant roots on electrical conductivity, soil water content and temperature is still not clear. This study aims to investigate the effects of soil water content and temperature on electrical conductivity in vegetated soils through an integrated field monitoring and computational modelling approach. A new relationship between soil electrical conductivity and water content as well as temperature is proposed. Field monitoring was conducted in both vegetated (tree species) and bare soils. The monitoring included measurements of soil water content, soil temperature and soil electrical conductivity. This was followed by response surface regression modelling. Measured results show that soil temperature at shallow depths was lower in vegetated soil than that in the bare soil. This observation was also consistent with the higher soil water content and hence, higher electrical conductivity under tree canopy. The model developed could predict nonlinear relationships between electrical conductivity and soil temperature and water content. Uncertainty analysis indicated normal distribution for electrical conductivity under variation of soil temperature and water content. © 2018 John Wiley &amp; Sons, Ltd.

Spatial heterogeneity of soil water content under introduced shrub (Caragana korshinskii) in desert grassland of the eastern Ningxia, China

Combined with geographic information system (GIS) data, the spatial heterogeneity of soil water in the 0-200 cm soil layer of the desert steppe with the introduction of the shrub Caragana korshinskii in eastern Ningxia was studied by classical statistical and geostatistical methods. It was found that the soil water content of each soil layer in the 0-200 cm profile was relatively low, ranging from 0.6% to 19.0% (averaged 4.4%). The coefficient of soil water content variation ranged from 49.5% to 86.3%, which indicated moderate spatial heterogeneity. The variation of soil water content at the 0-60 cm and 80-120 cm soil layers fitted the spherical model, while that at the 60-80 cm soil layer fitted the exponential model and that at the 120-200 cm soil layer fitted the Gaussian model. Soil water content at different soil layers showed different degrees of spatial autocorrelation. The nugget/sill [C0/(C0+C)] of soil water content in the 0-40, 60-80, and 120-200 cm soil layers ranged from 26.1% to 49.9%, with moderate spatial correlations. The nugget/sill of soil water content in the 40-60 cm, 80-100 cm, and 100-120 cm soil layers ranged from 15.5% to 22.1%, with strong spatial correlations. The scale of spatial heterogeneity in soil water content (A0) varied among soil layers. The A0 was relatively large in the 0-20 and 20-40 cm soil layers (37.10-45.18 km) and small in the 40-200 cm soil layers (3.58-8.66 km). The results indicated that the introduction of C. korshinskii shrubs into desert grassland for vegetation recovery in recent decades have depleted soil water and accelerated the depletion of soil water in the deep layer (40-200 cm), leading to spatial heterogeneity and fragmentation of soil water (0-200 cm) with stronger effects at deep soil layer.

Spatial variability of soil water content and related factors across the Hexi Corridor of China

Soil water content (SWC) is a key factor limiting ecosystem sustainability in arid and semi-arid areas of the Hexi Corridor of China, which is characterized by an ecological environment that is vulnerable to climate change. However, there is a knowledge gap regarding the large-scale spatial distribution of SWC in this region. The specific objectives of this study were to determine the spatial distribution patterns of SWC across the Hexi Corridor and identify the factors responsible for spatial variation of SWC at a regional scale. This study collected and analyzed SWC in the 0–100 cm soil profile from 109 field sampling sites (farmland, grassland and forestland) across the Hexi Corridor in 2017. We selected 17 factors, including land use, topography (latitude, longitude, elevation, slope gradient, and slope aspect), soil properties (soil clay content, soil silt content, soil bulk density, saturated hydraulic conductivity, field capacity, and soil organic carbon content), climate factors (mean annual precipitation, potential evaporation, and aridity index), plant characteristic (vegetation coverage) and planting pattern (irrigation or rain-fed), as possible environmental variables to analyze their effects on SWC. The results showed that SWC was 0.083 (±0.067) g/g in the 0–100 cm soil profile and decreased in the order of farmland, grassland and forestland. The SWC in the upper soil layers (0–20, 20–40 and 40–60 cm) had obvious difference when the mean annual precipitation differed by 200 mm. The SWC decreased from southeast to northwest following the same pattern as precipitation, and had a moderate to strong spatial dependence in a large effective range (75–378 km). The SWC showed a similar distribution and had no significant difference between soil layers in the 0–100 cm soil profile. The principal component analysis showed that the mean annual precipitation, geographical position (longitude and latitude) and soil properties (soil bulk density and soil clay content) were the main factors dominating the variance of environmental variables. A stepwise linear regression equation showed that plant characteristic (vegetation coverage) and soil properties (soil organic carbon content, field capacity and soil clay content) were the optimal factors to predict the variation of SWC. Soil clay content could be better to explain the SWC variation in the deeper soil layers compared with the other factors.

Effect of elevated carbon dioxide on soil hydrothermal regimes and growth of maize crop (Zea mays L.) in semi-arid tropics of Indo-Gangetic Plains.

To see the effect of climate change on the variation of soil hydrothermal regimes and growth of maize crop, an experiment was conducted in free-air carbon dioxide enrichment (FACE)facility during the kharif season of 2015 at Climate Change Facility of Indian Agricultural Research Institute, New Delhi, India. Under elevated CO2 and ambient condition, surface bulk density (BD) were 1.38Mgm-3 and 1.44Mgm-3, respectively but BD were not significantly different. During different days after sowing (DAS), in 0 to 10-cm soil depth, soil water content (SWC) in FACE varied between 14.58-20.70%, whereas in ambient condition, SWC variations were in between 19.33-22.94%. In 10 to 20-cm soil depth, SWC ranged in between 20.47-27.14% in FACE and 23.57-25.42% in ambient condition for different DAS. It is also observed that the arrival of peak surface ST was 1h early in elevated CO2 condition. Photosynthetic rate increased by 5.7% on 44 DAS and 18.1% on 70 DAS under elevated carbon dioxide condition. Elevated carbon dioxide had reduced the stomatal conductance but the reduction was not significant. Like variation in air temperature for climate change, more intensive study is required to see the effect of climate change on soil temperature and its effect on crop growth.

Soil water content variability drives productivity responses of a model grassland system to extreme rainfall events under elevated CO2

Climate change is significantly altering rainfall patterns globally and will likely cause increases in extreme rainfall events. Grassland systems are particularly vulnerable to these changes as their productivity is strongly mediated by soil water content (SWC). SWC mean and variability are driven by the amount of rainfall received as well as the distribution through time of that rainfall. In this study, we used a model grassland system in a controlled glasshouse experiment to identify whether SWC mean or variability is a stronger driver of productivity. We then examined how extreme rainfall events alter this driver and the resulting effect this has on productivity and biomass allocation under ambient and elevated carbon dioxide (CO2). Rainfall amount was held constant, but distribution through time varied (control, one in 20 years event, one in 100 years event). SWC variability was a stronger driver of productivity (mesocosm biomass) than SWC mean, with increasing extreme rainfall event magnitude resulting in greater SWC variability. Surprisingly, elevated CO2 only had a small effect on these productivity and biomass allocation responses which may be due to the relatively small CO2 difference tested. Our results suggest that distribution of rainfall in time is an important driver of grassland productivity and that increases in extreme rainfall events, for a given total rainfall, will result in reduced grassland productivity.

A comparative assessment of water use by Acacia longifolia invasions occurring on hillslopes and riparian zones in the Cape Agulhas region of South Africa

The detrimental impacts of invasive alien plants on ecosystems and water resources have raised concerns in arid and semi-arid countries like South Africa where the average precipitation is approximately 500 mm/yr, which is below the world average of around 860 mm/yr. Several studies have examined the effects of invasive alien plants such as the Australian Acacias on the water resources. However, few studies have quantified the differences in water use between hillslope and riparian Acacia longifolia invasions. A. longifolia is one of the aggressive invader species in South Africa even on hillslopes that contribute substantially to runoff generation. Therefore, the encroachment of invasive alien plants has the potential to reduce runoff, thereby adversely affecting the available water downstream. This paper aims to; 1) compare transpiration rates of A. longifolia growing on hillslopes and along riparian areas, 2) establish the key drivers for water use by this species, and 3) estimate the hydrological impacts of the invasions at the catchment scale in the Heuningnes catchment, in the Western Cape Province of South Africa. Transpiration by the trees was measured using the heat pulse velocity sap flow method. Automatic weather stations and soil moisture sensors were used to monitor weather and soil water content variations at each site. The results showed that, at the stand level the riparian A. longifolia transpired two times more water (∼596 mm/yr) than on the hillslope (∼242 mm/yr). During years with above average rainfall, the water use rates by the invasions was estimated to be ∼579 mm/yr on the hillslope and could be as much as ∼1348 mm/yr at the riparian site. Thus, the hypothesis that riparian trees use more water than invasions on non-riparian areas was accepted in this study. At the catchment scale (740 km2), the estimated water use by the invasions was 20.5 Mm3. Clearing of all the invasions in the study catchment is likely to make 17 Mm3/yr of water available. Hence the clearing of A. longifolia along the riparian corridors should be prioritised as this will lead to water savings at the catchment scale.

Variation of soil water content of fixed sand dune at 2014 in the Horqin sandy land

The fixed-point observation data of soil water content (SWC) in the growing season (from May to September) of 2014 in the semi-arid region of Horqin Sandy Land was employed to study the characteristics of SWC changes in fixed sand dune. The results showed that: 1) The SWC was relatively high in May. The SWCs in May and June were significantly higher than those in July, August and September, and were significantly different from those in July, August and September. From June to August, the SWC generally showed a decreasing trend. In September, the SWC did not recover obviously and remained at a low level. 2) The SWC was the lowest in the surface layer (0-10 cm), followed by the deep layer (90-150 cm), and highest in the shallow layer (10-90 cm), and there was a large difference among the three layers. The change of SWC with the soil depth showed a reversed S-shaped curve, i.e. “first increase, then decrease, and finally increase again”. The SWC was the highest (3.96%) in the soil layer of 30-50 cm, and the lowest (1.46%) in the soil layer of 0-10 cm. In the soil layer below 110 cm, the SWC showed a little change with the soil depth and tended to be stable.

Nested Scales of Spatial and Temporal Variability of Soil Water Content Across a Semiarid Montane Catchment

AbstractTopographic redistribution of water has been represented by various terrain metrics (e.g., topographic wetness index, slope, and upslope accumulated area). This type of landscape characterization has promoted the use of terrain metrics to inform how spatial patterns of soil volumetric water content (VWC) influence streamflow, ecological processes, and associated nutrient fluxes. However, evaluation of what these static terrain metrics reflect has only been accomplished in a few catchments. Additionally, previous research suggests that relationships between topographic metrics and VWC could be different across catchments through time. Here we measured VWC from snowmelt through summer drydown across a semiarid montane catchment. Using a spatially nested sampling design, we assessed the spatiotemporal variability of VWC from plot (tens of meters) to landscape scales (hundreds of meters). Variance of riparian area VWC increased as the catchment dried, while upland variance decreased, highlighting the utility of delineating distinct landscape units when considering spatial variability of moisture, rather than calculating statistics across the landscape as a whole. In contrast to previous research, we found that the relationship between VWC and topographic metrics persisted through the dry catchment state, suggesting that patterns of topographic redistribution of water during snowmelt continued to influence dry season VWC despite variability in plot scale vertical processes (e.g., evapotranspiration). Future research should focus on resolving the relationship between catchment moisture state and VWC variability as a function of wetness state, seasonality, and magnitude of precipitation, topography, and soil depth.