Variations In Soil Moisture Content Research Articles

Spatial explicit soil moisture analysis: pattern and its stability at small catchment scale in the loess hilly region of China

AbstractSoil moisture is essential for plant growth and terrestrial ecosystems, especially in arid and semi‐arid regions. This study aims to quantify the variation of soil moisture content and its spatial pattern as well as the influencing factors. The experiment is conducted in a small catchment named Yangjuangou in the loess hilly region of China. Soil moisture to a depth of 1 m has been obtained by in situ sampling at 149 sites with different vegetation types before and after the rainy season. Elevation, slope position, slope aspect, slope gradient and vegetation properties are investigated synchronously. With the rainy season coming, soil moisture content increases and then reaches the highest value after the rainy season. Fluctuation range and standard deviation of soil moisture decrease after a 4‐month rainy season. Standard deviation of soil moisture increases with depth before the rainy season; after the rainy season, it decreases within the 0‐ to 40‐cm soil depth but then increases with depths below 40 cm. The stability of the soil moisture pattern at the small catchment scale increases with depth. The geographical position determines the framework of soil moisture pattern. Soil moisture content with different land‐use types is significantly increased after the rainy season, but the variances of land‐use types are significantly different. Landform and land‐use types can explain most of the soil moisture spatial variations. Soil moisture at all sample sites increases after the rainy season, but the spatial patterns of soil moisture are not significantly changed and display temporal stability despite the influence of the rainy season. Copyright © 2013 John Wiley & Sons, Ltd.

2D and 3D soil moisture imaging using a sensor-based platform moving inside a subsurface network of pipes

Summary In this study a prototype sensor-based platform moving inside a subsurface network of pipes with the task of monitoring the soil moisture content is presented. It comprises of a mobile platform, a modified commercial soil moisture sensor (Diviner 2000), a network of subsurface polyvinylchloride (PVC) access pipes, driving hardware and image processing software. The software allows the composition of two-dimensional (2D) or three-dimensional (3D) images with high accuracy and at a large scale. The 3D soil moisture images are created by using 2D slices for better illustration of the soil moisture variability. Three case studies of varying soil moisture content using an experimental soil tank were examined. In the first case study, the irrigation water was applied uniformly on the entire tank surface. In second and third case studies, the irrigation water was applied uniformly only on the surface of the intermediate and last part of the soil tank respectively. The processed images give a detailed description of the soil moisture distribution of a layer at 15 cm depth under the soil surface in the tank. In all case studies that have been investigated, the distribution of soil moisture was characterized by a significant variability (difference between poorly and well-drained regions) of the soil tank. A very poorly-drained region was located in the middle of the soil tank, while well-drained soil areas were located southwest and northeast. The knowledge of the spatial and temporal distribution of soil moisture is a valuable tool for proper management of crop irrigation.

COMPARISON OF LONG·TERM. TRANSPLRAnON OF DIFFERENT TREE SPECIES IN SEPARATE CANOPY LAYERS IN A KANDVAN FOREST GARDEN

Kandyan Forest Gardens (KFG s) are a specific vegetation type covering a considerableland area of the Central Province of Sri Lanka. Therefore, water use by KFG s form animportant component of the catchment water balance of this area. KFG s consist of amixture of tree species whose canopies are arranged in different vertical layers.The study was conducted in a typical Kandyan Forest Garden located at Pilapitiya,Pilimathalawa within the agroecological region ~IJ.·hA vegetation survey was carried outin this garden and it showed 201 trees belonging to 24 different species. Out of these, twotree species were selected to represent upper and middle canopy layers. They were jak(Artocarpus heterophyllus) and toona (Cedrella laona). Transpiration of these trees wasmeasured as the sapflow in their trunk using thermal dissipation probes. Measurements ofthe weather parameters were also taken. The incident solar radiation was measured usingthe tube solari meters. Relative humidity in the open and inside the KFG was measured bytwo solid state sensors. The soil moisture content al five soil depths (i.e. 20, 40, 60, 80 and100 em) was measured by gravimetric sampling. All the data except soil moisture weretaken at 30 second time intervals integrated over five minutes and stored in the data logger.Tile measurements that were taken during the period from 25.12.2001 to 20.02.2002 wereanalyzed.Both species showed a similar pattern of daily variation of sapflow. However, the sapflowwas substantially greater in jak than in toona. The total water use of jak for theexperimental period was 3881.25 kg/tree. The corresponding value of toona was 462.83kg/tree. The upper canopy jak received a greater irradiance than middle canopy toona.There was a linear relationship between the radiation incident on jak and toona canopies.Daily water use of the upper canopy jak showed a clear linear relationship with dailyirradiance on its canopy. However, the water use of toona did not show such a relationshipIn both species, there were significant negative linear relationships between daily sapflowand daily mean relative humidity. However, air temperature did not have a significanteffect on sapflow of both jak and toona trees. The canopy leaf area of jak remained more orless constant during the experimental period. But in toona, the canopy leaf area increasedduring the latter parr of the experimental period. Water use of both jak and toona did notshow a significant relationship with the variation of soil moisture content of the top l m ofthe soil profile. This indicated that the trees were extracting water from the deeper layers ofthe soil profile. It can be concluded from results of the present study that water use of aKFG is dominated by large upper canopy trees and that it is largely determined by theincident solar radiation. This study also showed that large trees in this ecosystem maintaintheir high levels of water use even during dry periods of the year by extracting water fromdeeper layers of the soil profile.

Evaluation of soil thermal diffusivity algorithms at two equatorial sites in West Africa

<p>This study presents comparisons between six algorithms used in the calculation of apparent thermal diffusivity (K<sub>h</sub>) of the topsoil during measurement campaigns conducted at two equatorial sites. It further investigates the effects of transient and seasonal variations in soil moisture content (theta) on the estimation of K<sub>h</sub>. The data used comprise soil temperatures (T) measured at depths of 0.05 m and 0.10 m, and theta within the period of transition from the dry season to the wet season at Ile Ife (7.55˚ N, 4.55˚ E), and for the peak of the wet season at Ibadan (7.44˚ N, 3.90˚ E). The thermal diffusivity, K<sub>h</sub>, was calculated from six algorithms, of: harmonic, arctangent, logarithmic, amplitude, phase, and conduction-convection. The reliability of these algorithms was tested using their values to model T at a depth of 0.10 m, where direct measurements were available. The algorithms were further evaluated with statistical indices, including the empirical probability distribution function of the differences between the measured and modeled temperatures ([delta capitalized]T). The maximum absolute values of [delta capitalized]T for the six algorithms investigated were: 0.5˚C, 0.5˚C, 0.5˚C, 1˚C, 1˚C and 1˚C, respectively. K<sub>h</sub> showed an increasing trend as theta increased from the dry season to the peak of the wet season, with R<sup>2</sup> = 0.70 for the harmonic algorithm. The accuracy of all of the algorithms in modeling T reduced with transient variations of theta. The harmonic, arctangent and logarithmic algorithms were the most appropriate for calculating K<sub>h</sub> for the region of study. The empirical relation between theta and K<sub>h</sub> and the values of K<sub>h</sub> obtained in this study can be used to improve the accuracy of meteorological and hydrological models.</p>

Spatio-temporal variability of soil moisture and its effect on vegetation in a desertified aeolian riparian ecotone on the Tibetan Plateau, China

► Soil moisture content is one of the limiting factors for natural vegetation succession. ► Soil moisture in the riparian ecotone showed strong spatial dependence. ► Precipitation and water level were the main determining environmental factors. ► The correlation between vegetation coverage and soil moisture increased with depth. ► The results have important implications for vegetation restoration on the Tibetan plateau. Soil moisture content is one of the limiting factors for natural vegetation succession in alpine river valleys on the Tibetan Plateau. However, its spatio-temporal variability and effect on artificial vegetation restoration has rarely been reported. In this paper, we conducted a case study to examine the spatio-temporal variability and vertical characteristics of soil moisture by setting up a 100 × 120 m experimental plot on aeolian sandy land in a riparian ecotone and analyzing the result data using a geo-statistical approach. The soil moisture contents at different depths all showed strong temporal variability, with the mean soil moisture ranging from 4% to 6% in spring, from 6% to 14% in summer and from 9% to 12% in autumn. The Gaussian semi-variogram model was the best fitted theoretical model for the spatial structure of soil moisture contents in different seasons, and soil moisture at the field scale showed strong spatial dependence. The spatial patterns of soil moisture all demonstrated strong similarity between depths with the strongest between 0–20 cm and 20–40 cm depths in autumn. The soil moisture content over the upper 60 cm was the key factor that restricted the seed germination and seedling growth, and it affected vegetation coverage and density. The precipitation, variations of river water level, elevation as well as landform types were the main factors determining the variations of soil moisture content in different seasons. The results have important implications for the ongoing vegetation restoration in the study area.

Water Use and Water Productivity of Sugarbeet, Malt Barley, and Potato as Affected by Irrigation Frequency

Successful irrigation management is one of the most important agronomic practices for achieving profitable yield and maximizing crop water productivity (CWP) while maintaining environmental quality by minimizing water losses to runoff and deep drainage. This study was conducted to compare the influence of two irrigation frequencies on crop water use (CWU) and CWP of sugarbeet (Beta vulgaris L.), malt barley (Hordeum vulgare L.), and potato (Solanum tuberosum L.) on a sandy loam soil in the semiarid northern Great Plains. The irrigation frequencies compared were: high frequency (HF) irrigation with biweekly application of small irrigation quantities, and the conventional low frequency (LF) with weekly application of large irrigation quantities. Irrigation frequency was varied based on either 15 mm (HF) or 30 mm (LF) cumulative crop evapotranspiration replacements. Seasonal CWU amounts were determined using the water balance equation of sugarbeet, malt barley, and potato under HF and LF irrigations for 2007, 2008, 2009, 2010, and 2011. No significant differences due to irrigation frequency were found for yield, CWU, and CWP of sugarbeet (root and sucrose), malt barley, or potato. Small differences in CWU values between HF and LF irrigations were due to variations in soil moisture content in the soil profile and drainage losses below the 0.91‐m soil depth. Conventional LF irrigation thus can sustain yield, improve water use, and reduce net economic input as feasibly as HF irrigation practices when a self‐propelled automated sprinkler system is used on a sandy loam soil.

Retrieval of daily evolution of soil moisture from satellite-derived land surface temperature and net surface shortwave radiation

Soil moisture is a key parameter in water balance, and it serves as the core and link in atmosphere–vegetation–soil–groundwater systems. Soil moisture directly affects the accuracy of the simulation and prediction conducted by hydrological and atmospheric models. This article aims to develop a new model to retrieve the daily evolution of soil moisture with time series of land surface temperature (LST) and net surface shortwave radiation (NSSR). First, for the time series of soil moisture, LST and NSSR daytime data were simulated by the common land model (CoLM) with different soil types in bare soil areas. Based on these data, the variations between soil moisture and LST-NSSR during the daytime with different soil types were analysed, and a plane function was used to fit the daily evolution of soil moisture and the time series of LST and NSSR data. Further study proved that the coefficients of the soil moisture retrieval model are not sensitive to soil type. Then, a relationship model between the daily evolution of soil moisture and the time series of LST-NSSR was developed and validated using the data simulated by CoLM with different soil types and different atmospheric conditions. To demonstrate the feasibility of the soil moisture retrieval method proposed in this study, it was applied to the African continent with data from the METEOSAT Second Generation Spinning Enhanced Visible and Infrared Imager (MSG–SEVIRI) geostationary satellite. The results show that the variation of soil moisture content can be quantitatively estimated directly by the method at the regional scale with some reasonable assumptions. This study can provide a new method for monitoring the variation of soil moisture, and it also indicates a new direction for deriving the daily variation of soil moisture using the information from the time series of the land surface variables.

Tree induced soil suction and slope stability

This paper provides an investigation of moisture migration patterns in the vicinity of mature trees and soil slope stability. A numerical model is presented for unsaturated moisture flow incorporating a sink term to represent water uptake by a tree. The resulting variation in soil moisture content (or suction) is then included within an extended slope stability analysis that employs a method of estimating shear strength as a function of suction in addition to net mean stress. The moisture transfer equation is solved via application of a finite element method. The resulting water uptake predictions have been validated previously and were shown to be in reasonable agreement with field measured data. The current work provides an assessment of the importance of this aspect of the problem on slope stability. Results show that seasonal soil suctions generated by a tree can influence the facture of safety against soil slope failure in the order of 8%. The research provides a further contribution to the overall assessment of slope stability. It may be included in routine calculations where maintenance of rail/road embankments may involve alteration (or removal) of vegetation. It may also be applied to vegetated natural slopes subject to deforestation or development.

Estimation of Root Zone Soil Moisture Using Apparent Thermal Inertia With MODIS Imagery Over a Tropical Catchment in Northern Thailand

Understanding the variability of soil moisture content (SMC) is important for studying ecohydrological processes because it provides insights into surface water and energy balances. To comprehend the dynamics of SMC under different land use/cover types in tropical environments, this study proposes an apparent thermal inertia (ATI) approach with the Moderate Resolution Imaging Spectroradiometer (MODIS) imagery for estimating root zone SMC. Root zone SMC at depths of 10 cm, 100 cm, and 200 cm were estimated for seven sites over a northern Thailand catchment from 2005 to 2008, and compared with in situ observations. Pearson correlation coefficient and the Nash-Sutcliffe efficiency coefficient between the ATI-MODIS retrieved SMC and the measurements were respectively 0.80, 0.84, and 0.84, and 0.57, 0.537, and 0.492 for the 10 cm, 100 cm, and 200 cm depths. Root Mean Square Errors were 0.055, 0.025 and 0.029 (m3 m-3) for the three respective depths. Potential issues of the mixed land cover types within the 1 km MODIS pixel were examined; the mixed agricultural land cover types at two of the seven sites with agricultural activities such as irrigation water use might have affected the SMC estimation. Overall, the ATI-MODIS approach performed well, particularly for the 100 cm depth.

Anatomical Basis for Optimal Use of Water for Maintenance of Three Xerophytic Plants

Three xerophytic plant species namely Agave americana Linn., Aloe vera Tourn. and Linn. and Euphorbia milii Des Moul. were propagated in a greenhouse each with 5 varying soil moisture contents i.e. 1.25%, 2.5%, 5%, 10%, and 20% and subjected to 4 watering frequencies i.e. daily, weekly, biweekly and monthly. Euphorbia milii was the most xerophytic species having relatively lower rate of transpiration than Aloe vera and Agave americana. It was suggested that the high rate of transpiration in Aloe vera and Agave americana may be due to the large tetracytic stomata as compared to the small paracytic stomata of Euphorbia milii. It was also observed that Aloe vera was least tolerant of high soil moisture in daily watering as well as low soil moisture in monthly regime. Agave americana and Euphorbia milii were species that were more robust with capacity to cope well with low and high watering regimes than Aloe vera.

Physically-based modeling of topographic effects on spatial evapotranspiration and soil moisture patterns through radiation and wind

Abstract. In this paper, simulations with the Soil Water Atmosphere Plant (SWAP) model are performed to quantify the spatial variability of both potential and actual evapotranspiration (ET), and soil moisture content (SMC) caused by topography-induced spatial wind and radiation differences. To obtain the spatially distributed ET/SMC patterns, the field scale SWAP model is applied in a distributed way for both pointwise and catchment wide simulations. An adapted radiation model from r.sun and the physically-based meso-scale wind model METRAS PC are applied to obtain the spatial radiation and wind patterns respectively, which show significant spatial variation and correlation with aspect and elevation respectively. Such topographic dependences and spatial variations further propagate to ET/SMC. A strong spatial, seasonal-dependent, scale-relevant intra-catchment variability in daily/annual ET and less variability in SMC can be observed from the numerical experiments. The study concludes that topography has a significant effect on ET/SMC in the humid region where ET is a energy limited rather than water availability limited process. It affects the spatial runoff generation through spatial radiation and wind, therefore should be applied to inform hydrological model development. In addition, the methodology used in the study can serve as a general method for physically-based ET estimation for data sparse regions.

Comparison of the Temporal Heterogeneity of Soil Moisture of Different Ecosystems at Red Soil Hillside Fields

This paper investigates the differences in the temporal heterogeneity of soil moisture between natural restoration ecosystem without disturbance (named as eco-1), restoration ecosystem with disturbance (named as eco-2) and shrubs ecosystem (named as eco-3) in red soil hillside fields. The probe soil moisture measured at 20 and 40 cm below the soil surface Hydrosense Portable Time Domain Reflectometry (TDR) system. The results showed that: (1) Daily measurements during a growing season showed significantly temporal interactions between vegetation and water. Soils under eco-2 (restoration ecosystem with disturbance) and eco-3 (shrubs ecosystem) were wettest at the start of the growing season but dries at the end; (2) The coefficient of variation of soil moisture content over time during the growing season was significantly higher in eco-2 than in eco-1, and at two depths beneath the soil surface (20 and 40 cm).

Comparison of the Temporal Heterogeneity of Soil Moisture of Different Ecosystems at Red Soil Hillside Fields

This paper investigates the differences in the temporal heterogeneity of soil moisture between natural restoration ecosystem without disturbance (named as eco-1), restoration ecosystem with disturbance (named as eco-2) and shrubs ecosystem (named as eco-3) in red soil hillside fields. The probe soil moisture measured at 20 and 40 cm below the soil surface Hydrosense Portable Time Domain Reflectometry (TDR) system. The results showed that: (1) Daily measurements during a growing season showed significantly temporal interactions between vegetation and water. Soils under eco-2 (restoration ecosystem with disturbance) and eco-3 (shrubs ecosystem) were wettest at the start of the growing season but dries at the end; (2) The coefficient of variation of soil moisture content over time during the growing season was significantly higher in eco-2 than in eco-1, and at two depths beneath the soil surface (20 and 40 cm).

Soil moisture content retrieval based on apparent thermal inertia for Xinjiang province in China

In the arid to semi-arid regions of north-western China, soil moisture is the main hydrological driver for vegetation growth. With the launch of the Moderate Resolution Imaging Spectroradiometer (MODIS), the local MODIS reception capacity and the strong pressure on water resources in the province, the detection and mapping of soil moisture content (SMC) has become a major issue for the regional water management authorities of the province.In this article, we apply the apparent thermal inertia approach to quantify SMC in the soils of the province of Xinjiang using locally received MODIS data. We report on SMC mapping for the entire province for the year 2005.For the estimation, diurnal land surface temperature (LST (Tσ)), LST difference (ΔTσ) and broadband albedo (α0) were applied to determine the space–time variability of SMC. The retrieval of SMC was based on the rationale that high apparent thermal inertia (Iτ) values correspond to high SMCs and low Iτ values correspond to the minimal ones. To enable the application of the technique, a new classification of soil texture was established based on an existing Chinese soil type classification.Typically only topsoil surface moisture content is retrieved with thermal remote sensing (RS). However, SMC retrieval for a 1 m soil profile was performed by applying a semi-empirical modelling approach. The model uses a two-layer water balance equation, and its SMC (θg) input is based on its linear relationship with the soil moisture saturation index (θsi) at time t.For validation purposes, the automatic weather station and time domain reflectometry (TDR) monitoring network included eight sites in the province, including the Mosuowan and Tianshan snow sites and the Turpan, Bayangburk, Kuerle, Yinsu, Alagan and Yiganbujima TDR sites for which data for the year of 2005 were acquired by the Xinjiang Meteorological Bureau (XMB), the Tarim Management Bureau (TMB) and the Xinjiang Institute for Ecology and Geography (XIEG).When time series of SMC determined by using Iτ are compared with the measurements at the different validation sites, regression curve slopes of the validation relationships vary between 0.499 and 0.922. The R2 values vary between 0.25 and 0.83. The minimum and maximum root mean square errors (RMSEs) are 0.001 and 0.028, respectively. Results suggest that apparent thermal inertia application is quite suitable for θg retrieval of a 1 m soil moisture profile in an arid to semi-arid region. The Aqua MODIS 10-day mean soil moisture product is proven to deliver quantitatively correct SMC imagery representing seasonal changes quite realistically.

Sensitivity in the Overland Reintensification of Tropical Cyclone Erin (2007) to Near-Surface Soil Moisture Characteristics

Abstract This study investigates the impact of abnormally moist soil conditions across the southern Great Plains upon the overland reintensification of North Atlantic Tropical Cyclone Erin (2007). This is tested by analyzing the contributions of three soil moisture–related signals—a seasonal signal, an along-track rainfall signal, and an early postlandfall rainfall signal—to the intensity of the vortex. In so doing, a suite of nine convection-permitting numerical simulations using the Advanced Research Weather Research and Forecasting model (WRF-ARW) is used. Of the signals tested, soil moisture contributions from the anomalously wet months preceding Erin are found to have the greatest positive impact upon the intensity of the vortex, though this impact is on the order of that from climatological soil moisture conditions. The greatest impact of the early rainfall signal contributions is found when it is added to the seasonal signal. Along-track rainfall during the simulation period has a minimal impact. Variations in soil moisture content result in impacts upon the boundary layer thermodynamic environment via boundary layer mixing. Greater soil moisture content results in weaker mixing, a shallower boundary layer, and greater moisture and instability. Differences in the intensity of convection that develops and its accompanying latent heat release aloft result in greater warm-core development and surface vortex intensification within the simulations featuring greater soil moisture content. Implications of these findings to the tropical cyclone development process are discussed. Given that the reintensification is shown to occur in, apart from land, an otherwise favorable environment for tropical cyclone development and results in a vortex with a structure similar to developing tropical cyclones, these findings provide new insight into the conditions under which tropical cyclones develop.

Removing the effect of soil moisture from NIR diffuse reflectance spectra for the prediction of soil organic carbon

Field measurement using NIR spectroscopy is becoming a popular method to provide in situ, rapid, and inexpensive estimation of soil organic carbon (SOC) content. However NIR reflectance is quite sensitive to external environmental conditions, such as temperature and soil moisture. In the field, the soil moisture content can be highly variable. It is a challenge to find a chemometric method that allows for prediction of soil organic carbon from spectra obtained under field conditions that is insensitive to variable moisture content. This paper utilises an external parameter orthogonalisation (EPO) algorithm to remove the effect of soil moisture from NIR spectra for the calibration of SOC content. The algorithm projects all the soil spectra orthogonal to the space of unwanted variation, and thus the variations of soil moisture can be effectively removed. We designed a protocol with 3 independent datasets to be used for calibration of NIR spectra: (1) the calibration dataset, which contains soil samples with measured spectra and SOC content under standard (or laboratory) condition (air-dried), (2) the EPO development dataset contains spectra under laboratory condition (air-dried samples) and spectra collected under field conditions (varying soil moisture content), and (3) the validation dataset contains spectra collected under field condition and measured SOC content. We conducted experiments using soils at different moisture contents in laboratory conditions. Using the EPO algorithm, we are able to remove the effect of soil moisture from the spectra, which resulted in improved calibration and prediction of SOC content.

The Surface Urban Energy and Water Balance Scheme (SUEWS): Evaluation in Los Angeles and Vancouver

Summary An urban energy and water balance model is presented which uses a small number of commonly measured meteorological variables and information about the surface cover. Rates of evaporation-interception for a single layer with multiple surface types (paved, buildings, coniferous trees and/or shrubs, deciduous trees and/or shrubs, irrigated grass, non-irrigated grass and water) are calculated. Below each surface type, except water, there is a single soil layer. At each time step the moisture state of each surface is calculated. Horizontal water movements at the surface and in the soil are incorporated. Particular attention is given to the surface conductance used to model evaporation and its parameters. The model is tested against direct flux measurements carried out over a number of years in Vancouver, Canada and Los Angeles, USA. At all measurement sites the model is able to simulate the net all-wave radiation and turbulent sensible and latent heat well (RMSE = 25–47 W m−2, 30–64 and 20–56 W m−2, respectively). The model reproduces the diurnal cycle of the turbulent fluxes but typically underestimates latent heat flux and overestimates sensible heat flux in the day time. The model tracks measured surface wetness and simulates the variations in soil moisture content. It is able to respond correctly to short-term events as well as annual changes. The largest uncertainty relates to the determination of surface conductance. The model has the potential be used for multiple applications; for example, to predict effects of regulation on urban water use, landscaping and planning scenarios, or to assess climate mitigation strategies.

Hydrologic feasibility of artificial forestation in the semi-arid Loess Plateau of China

Abstract. Hydrologic viability, in terms of moisture availability, is fundamental to ecosystem sustainability in arid and semi-arid regions. In this study, we examine the spatial distribution and after-planting variations of soil moisture content (SMC) in black locust tree (Robinia pseudoacacia L.) plantings in the Loess Plateau of China at a regional scale. Thirty sites (5 to 45 yr old) were selected, spanning an area of 300 km by 190 km in the northern region of the Shaanxi Province. The SMC was measured to a depth of 100 cm at intervals of 10 cm. Geographical, topographic and vegetation information was recorded, and soil organic matter was evaluated. The results show that, at the regional scale, SMC spatial variability was most highly correlated with rainfall. The negative relationship between the SMC at a depth of 20–50 cm and the stand age was stronger than at other depths, although this relationship was not significant at a 5 % level. Watershed analysis shows that the after-planting SMC variation differed depending upon precipitation. The SMC of plantings in areas receiving sufficient precipitation (e.g., mean annual precipitation (MAP) of 617 mm) may increase with stand age due to improvements in soil water-holding capacity and water-retention abilities after planting. For areas experiencing water shortages (e.g., MAP = 509 mm), evapotranspiration may cause planting soils to dry within the first 20 yr of growth. It is expected that, as arid and semi-arid plantings age, evapotranspiration will decrease, and the soil profile may gradually recover. In extremely dry areas (e.g., MAP = 352 mm), the variation in after-planting SMC with stand age was found to be negligible. The MAP can be used as an index to divide the study area into different ecological regions. Afforestation may sequentially exert positive, negative and negligible effects on SMCs with a decrease in the MAP. Therefore, future restoration measures should correspond to the local climate conditions, and the MAP should be a major consideration for the Loess Plateau. Large-scale and long-term research on the effects of restoration projects on SMCs is needed to support more effective restoration policies. The interaction between afforestation and local environmental conditions, particularly water availability to plants, should be taken into account in afforestation campaigns in arid and semi-arid areas.

Polynomial response of 2,4-D mineralization to temperature in soils at varying soil moisture contents, slope positions and depths

The herbicide 2,4-D [2,4-(dichlorophenoxy) acetic acid] is a widely used broadleaf control agent in cereal production systems. Although 2,4-D soil-residual activity (half-lives) are typicaly less than 10 days, this herbicide also has as a short-term leaching potential due to its relatively weak retention by soil constituents. Herbicide residual effects and leaching are influenced by environmental variables such as soil moisture and temperature. The objective of this study was to determine impacts of these environmental variables on the magnitude and extent of 2,4-D mineralization in a cultivated undulating Manitoba prairie landscape. Microcosm incubation experiments were utilized to assess 2,4-D half-lives and total mineralization using a 4 × 4 × 3 × 2 factorial design (with soil temperature at 4 levels: 5, 10, 20 and 40°C; soil moisture at 4 levels: 60, 85, 110, 135 % of field capacity; slope position at 3 levels: upper-, mid- and lower-slopes; and soil depth at 2 levels: 0–5 cm and 5–15 cm). Half-lives (t1/2) varied from 3 days to 51 days with the total 2,4-D mineralization (M T ) ranging from 5.8 to 50.9 %. The four-way interaction (temperature × moisture × slope × depth) significantly (p< 0.001) influenced both t1/2 and M T. Second-order polynomial equations best described the relations of temperature with t1/2 and MT as was expected from a biological system. However, the interaction and variability of t1/2 and MT among different temperatures, soil moistures, slope positions, and soil depth combinations indicates that the complex nature of these interacting factors should be considered when applying 2,4-D in agricultural fields and in utilizing these parameters in pesticide fate models.

Isotopic fractionation during soil uptake of atmospheric hydrogen

Abstract. Soil uptake of atmospheric hydrogen (H2) and the associated hydrogen isotope effect were studied using soil chambers in a Western Washington second-growth coniferous forest. Chamber studies were conducted during both winter and summer seasons to account for large natural variability in soil moisture content (4–50%) and temperature (6–22 °C). H2 deposition velocities were found to range from 0.01–0.06 cm s−1 with an average of 0.033 ± 0.008 cm s−1 (95% confidence interval). Consistent with prior studies, deposition velocities were correlated with soil moisture below 20% soil moisture content during the summer season. During winter, there was considerable variability observed in deposition velocity that was not closely related to soil moisture. The hydrogen kinetic isotope effect with H2 uptake was found to range from −24‰ to −109‰. Aggregate analysis of experimental data results in an average KIE of −57 ± 5‰ (95% CI). Some of the variability in KIE can be explained by larger isotope effects at lower (&lt;10%) and higher (&gt;30%) soil moisture contents. The measured KIE was also found to be correlated with deposition velocity, with smaller isotope effects occurring at higher deposition velocities. If correct, these findings will have an impact on the interpretation of atmospheric measurements and modeling of δD of H2.