•X-ray CT was effectively used to quantify soil seal/crust thickness.•Different micro-morphological zones within seal layers were revealed.•Rainfall had a strong and rapid impact on water transport ...and retention in soil.•The existence of a soil-dependent raindrop impact threshold was hypothesized.
This study delivers new insights into rainfall-induced seal formation through a novel approach in the use of X-ray Computed Tomography (CT). Up to now seal and crust thickness have been directly quantified mainly through visual examination of sealed/crusted surfaces, and there has been no quantitative method to estimate this important property. X-ray CT images were quantitatively analysed to derive formal measures of seal and crust thickness. A factorial experiment was established in the laboratory using open-topped microcosms packed with soil. The factors investigated were soil type (three soils: silty clay loam – ZCL, sandy silt loam – SZL, sandy loam – SL) and rainfall duration (2–14 min). Surface seal formation was induced by applying artificial rainfall events, characterised by variable duration, but constant kinetic energy, intensity, and raindrop size distribution. Soil porosities derived from CT scans were used to quantify the thickness of the rainfall-induced surface seals and reveal temporal seal micro-morphological variations with increasing rainfall duration. In addition, the water repellency and infiltration dynamics of the developing seals were investigated by measuring water drop penetration time (WDPT) and unsaturated hydraulic conductivity (Kun). The range of seal thicknesses detected varied from 0.6 to 5.4 mm. Soil textural characteristics and OM content played a central role in the development of rainfall-induced seals, with coarser soil particles and lower OM content resulting in thicker seals. Two different trends in soil porosity vs. depth were identified: i) for SL soil porosity was lowest at the immediate soil surface, it then increased constantly with depth till the median porosity of undisturbed soil was equalled; ii) for ZCL and SL the highest reduction in porosity, as compared to the median porosity of undisturbed soil, was observed in a well-defined zone of maximum porosity reduction c. 0.24–0.48 mm below the soil surface. This contrasting behaviour was related to different dynamics and processes of seal formation which depended on the soil properties. The impact of rainfall-induced surface sealing on the hydrological behaviour of soil (as represented by WDTP and Kun) was rapid and substantial: an average 60% reduction in Kun occurred for all soils between 2 and 9 min rainfall, and water repellent surfaces were identified for SZL and ZCL. This highlights that the condition of the immediate surface of agricultural soils involving rainfall-induced structural seals has a strong impact in the overall ability of soil to function as water reservoir.
Wildfires raise risks of floods, debris flows, major geomorphologic and sedimentologic change, and water quality and quantity shifts. A principal control on the magnitude of these changes is ...field‐saturated hydraulic conductivity (Kfs), which dictates surface runoff generation and is a key input into numerical models. This work synthesizes 73 Kfs datasets from the literature in the first year following fire at the plot scale (≤ 10 m2). A meta‐analysis using a random effects analysis showed significant differences between burned and unburned Kfs. The reductions in Kfs after fire, expressed by the ratio of Kfs Burned/Kfs Unburned, were 0.46 (95% confidence interval of 0.31‐0.70) combining wildfire and prescribed fire and 0.3 (95% confidence interval of 0.13‐0.71) for wildfire. No significant differences for Kfs were observed between wildfire and prescribed fire or moderate and high fire severity. Both Kfs magnitude and variability depended more on measurement method than measurement support area at the plot scale, with methods applying head ≥0.5 cm producing larger estimates of Kfs. It is recommended that post‐fire efforts to characterize Kfs for modeling or process‐based interpretations use methods that reflect the dominant infiltration processes: tension infiltrometers and simulated rainfall methods when soil matrix flow dominates and ponded head methods when macropore flow is critical. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.
Measured soil field‐saturated soil hydraulic conductivity in the first year after wildfire and prescribed fire in forests from 73 sites across the globe is synthesized and analyzed using meta‐analysis.
Improving soil physical properties by means of biochar application has been proposed in recent publications. The objective of this study was to investigate to what extent the addition of corn stover ...(CS) and biochars produced from the pyrolysis of corn stover feedstock (CS) at 350 and 550°C temperatures (CS-350, CS-550) affected aggregate stability, volumetric water content (θV), bulk density, saturated hydraulic conductivity (Ks) and soil water repellency of specific soils. Organic amendments (CS, CS-350, CS-550) were incorporated into a Typic Fragiaqualf (TK) and a Typic Hapludand (EG) soils at the rate of 7.18tCha−1, which corresponded to 17.3, 11.3 and 10.0tbiocharha−1 for the CS, CS-350 and CS-550 treatments, respectively. After 295d of incubation (T295), soils were sampled as (i) undisturbed samples for bulk density and Ks; and (ii) mildly disturbed samples for θV (at −15, −1, −0.3, −0.1, −0.08, −0.06, −0.04, and −0.02bar), aggregate stability and soil water repellency. The θV at time 0 (T0) was also determined at −15, −1 and −0.3 matric potentials for the different treatments. Biochar application significantly increased (P<0.05) aggregate stability of both soils, the effect of CS-550 biochar being more prominent in the TK soil than that in the EG soil, and the reverse pattern being observed for the CS-350 biochar. Biochar application increased the θV at each matric potential although the effect was not always significant (P<0.05) and was generally more evident in the TK soil than that in the EG soil, at both T0 and T295. Biochar addition significantly (P<0.05) increased the macroporosity (e.g., increase in θV at −0.08 to 0bar) in the TK soil and also the mesoporosity in the EG soil (e.g., increase in θV from −1 to −0.1bar). Both biochars significantly increased (P<0.05) the Ks of the TK soil, but only CS-350 biochar significantly increased (P<0.05) the Ks in the EG soil. Biochar was not found to increase the water repellency of these soils. Overall results suggest that these biochars may facilitate drainage in the poorly drained TK soil. However, the present results are biochar-, dose- and soil-specific. More research is needed to determine changes produced in other biochar, dose and soil combination, especially under field conditions.
•The effect of biochar on soil aggregation depended on soil and biochar types.•The other physical properties were in turn affected by the biochar amendment.•These changes were more evident in the Alfisol than in the Andisol.
Whether the hydraulic conductivity representative elementary volume (KREV) exists is a fundamental question for understanding the hydraulic behavior of fractured rock masses. The fracture density and ...size of rocks vary greatly, and are the two most important parameters affecting the existence of a KREV. The International Society for Rock Mechanics (ISRM) presented quantitative descriptions of fracture persistence and density, dividing the persistence into 5 rates and the density into 7 rates, defining 35 basic types of isotropic fractured rock masses with different persistence and densities. Based on ISRM classification suggestions, we construct an additional 40 anisotropic fractured rocks and conduct a systematic investigation of the KREVs of all 75 rock types. The 3D DFNs of the 75 rocks are established with the Monte Carlo method; the water flow in the DFNs is simulated with the finite difference method; the equivalent conductivities of each rock are calculated in 10 domain sizes and 21 different flow directions; and the optimum conductivity ellipsoid and the fitting errors of the equivalent directional conductivity vectors are analyzed to determine the existence and size of the KREVs. A KREV is more likely to exist in rock masses with a high fracture density and persistence. The effect of fracture density and persistence can be comprehensively represented by blockiness, defined as the ratio of the volume of isolated blocks formed by fractures to the total rock volume. A strong correlation is found between the blockiness and the existence of a KREV. A KREV always exists when the blockiness is higher than 0.5%, and its size is between 2 and 18 times the fracture spacing. A KREV may or may not exist in the range of 0.1% < B < 0.5%. When B < 0.1%, a KREV does not exist.
Saturated hydraulic conductivity (Ks) is a fundamental soil property that regulates the fate of water in soils. Its measurement, however, is cumbersome and instead pedotransfer functions (PTFs) are ...routinely used to estimate it. Despite much progress over the years, the performance of current generic PTFs estimating Ks remains poor. Using machine learning, high‐performance computing, and a large database of over 18,000 soils, we developed new PTFs to predict Ks. We compared the performances of four machine learning algorithms and different predictor sets. We evaluated the relative importance of soil properties in explaining Ks. PTF models based on boosted regression tree algorithm produced the best models with root‐mean‐squared log‐transformed error in ranges of 0.4 to 0.3 (log10(cm/day)). The 10th percentile particle diameter (d10) was found to be the most important predictor followed by clay content, bulk density (ρb), and organic carbon content (C). The sensitivity of Ks to soil structure was investigated using ρb and C as proxies for soil structure. An inverse relationship was observed between ρb and Ks, with the highest sensitivity at around 1.8 g/cm3 for most textural classes. Soil C showed a complex relationship with Ks with an overall positive relation for fine‐textured and midtextured soils but an inverse relation for coarse‐textured soils. This study sought to maximize the extraction of information from a large database to develop generic machine learning‐based PTFs for estimating Ks. Models developed here have been made publicly available and can be readily used to predict Ks.
Key Points
High accuracy machine learning‐based pedotransfer function models are developed to predict saturated hydraulic conductivity
Variable importance measures are used to identify and rank soil properties that are most important for predictions
Bulk density and organic carbon content are used as proxies to describe the effect of soil structural alterations in predictions
Several million hectares of salt-affected land in the western Songnen Plain, in northeast China, is characterized as hard saline-sodic soils because of their poor physical and chemical properties. ...Desulfurized gypsum, a by-product of thermal power plants for removal of sulfur from coal, may be used as an external source of Ca2+ for reclamation of saline-sodic soil in this plain because it is comparatively cheap, generally available, and has a high gypsum content (>900gkg−1). In the present study, the effects of desulfurized gypsum on physical and chemical properties of saline-sodic soil and growth and yield of rice (Oriza sativa L.) were assessed in a soil column experiment. Desulfurized gypsum was added to the saline-sodic soil (ECe=26.00dSm−1, SARe=397.22 (mmolcL−1)1/2, pH=10.48) at rates of 0.00gkg−1 (control), 15.00gkg−1 (100% gypsum requirement (GR) treatment) and 30.00gkg−1 (200% GR treatment). The results demonstrated that desulfurized gypsum significantly increased the wet stability of macro-aggregates (>0.25mm), the IR and the Ks of the saline-sodic soil. Soil salinity (ECe), sodicity (SARe) and pH decreased to 2.72dsm−1, 8.65 (mmolcL−1)1/2, and 8.05 in the 200% GR treatment, and 4.93dsm−1, 24.74 (mmolcL−1)1/2 and 8.75 in the 100% GR treatment from 24.65dsm−1, 390.60 (mmolcL−1)1/2 and 10.28 in the control, respectively. The yield of rice (O. sativa L.) increased from 0.0gplant−1 in control to 1.83gplant−1 and 2.45gplant−1 in 100% GR treatment and 200% GR treatment, respectively. Desulfurized gypsum is an effective amendment for saline-sodic soil in the west of Songnen Plain, northeast China.
► Desulfurized gypsum (DG) is a by-product of thermal power plants. ► DG has a high content of pure gypsum (>900gkg−1). ► DG was used as an external source of Ca2+ for reclamation of saline-sodic soil. ► DG improved physical and chemical properties of saline-sodic soil. ► DG also increased growth and yield of rice (Oriza sativa L.).
Mountainous areas are referred to as “water towers” since they are the source of water for many low‐lying communities. The hydrologic budgets of these areas, which are particularly susceptible to ...climate change, are typically poorly constrained. To address this, we analyzed the partitioning between baseflow and mountain block recharge (MBR) using a regional groundwater model of the northern Qinghai‐Tibet Plateau run with multiple scenarios. We found that ~19% of precipitation is recharged, approximately 35% of which becomes MBR, while 65% discharges as baseflow. This partitioning is relatively independent of the recharge rate but is sensitive to exponential depth decrease of hydraulic conductivity (K). The MBR is more sensitive to this exponential decrease in K than baseflow. The proportion of MBR varies from twice to half of baseflow as the decay exponent increases by more than fivefold. Thus, the depth dependence of K is critical for quantifying hydrologic partitioning in these sensitive areas.
Key Points
Regional groundwater flow is modeled and analyzed for the mountainous region of the Qinghai‐Tibet Plateau
Eighteen to 20% of precipitation recharges the groundwater system and 65% of the groundwater discharges as stream baseflow, while 35% becomes mountain block recharge
Depth dependency of hydraulic conductivity controls the partitioning of groundwater recharge between baseflow and mountain block recharge
•The reduction of Ksat was related to the decreased EC and increased sodicity.•The dispersed clay in leachate was not necessarily corresponding to Ksat reduction.•Low density soil columns maintained ...higher Ksat compared to the compacted soils.•High porosity and macropores enabled dispersed clay to move through soil column.
Using saline and sodic waters for irrigating agricultural lands has increased worldwide, which may cause adverse effects on soil porosity, structural stability and permeability. However, few studies have investigated the effect of clay swelling, dispersion and migration on the reduction of saturated hydraulic conductivity (Ksat) by analysing the total porosity and pore size change through the profile. In this study, two soils with contrasting mineralogy was used and packed at two different bulk densities (1.0 and 1.2 g cm−3). Soil columns were leached with three types of sodium adsorption ratio (SAR) solutions, i.e. good quality water (GQW), SAR 10 and SAR 50 solutions, before distilled water (low ionic strength) was leached through. The results indicated that the application of distilled water decreased Ksat for all soils, particularly in the initially sodic water treated columns. The Ksat of leaching with distilled water in low density columns were generally higher, but showed more degree of reduction than more compacted columns, mainly related to the high initial porosity and macropores that enabled dispersed clay to move through the soil column and out in the leachate. The application of distilled water significantly reduced the proportion of macro and mesopores, whereas increased the proportion of micropores in both soils, particularly for soil columns initially treated high SAR solutions. When the dispersed clay was trapped within the column, the depth of clay accumulation was affected by a combined effect of packing density, soil type and water quality.
•The mean of conditional K realizations differs from the conditional effective K.•Conditional effective K is harmonic and geometric mean in 1&2-D HT, respectively.•Conditional Monte Carlo Simulation ...and SLE yield similar results.
This study first discusses the conditional mean, realizations, and effective hydraulic conductivity in a theoretical framework. It then introduces Monte Carlo simulation (MCS) algorithms for constraining the outcome by either hydraulic conductivity (K) samples or hydraulic head (h) measurements from the hydraulic tomographic survey (HT). It demonstrates that kriging using K measurements leads to a conditional mean K field, while inverse modeling using successive linear estimator (SLE) with head measurements of HT yields the conditional effective K field. The effects of conditioning using K measurements are different from those using heads. Besides, the conditional effective K leads to the unbiased prediction of the head that honors the observed head at measurement locations. More importantly, the study reveals that the harmonic and geometric means of conditional realizations of K fields of MCS, given head measurements, are equivalent to the conditional effective K in one- and two-dimensional flows, respectively. The first-order approximation in the SLE results in a conditional covariance similar to that from MCS with smaller magnitudes. Despite the difference, all approaches predict unbiased conditional mean head behaviors.
•Soil hydrological properties are improved after 20 years of vegetation restoration.•Shrubland and forest have higher KS than grassland and cropland in the Loess Plateau.•Soil porosity and bulk ...density explain 93.8% of soil hydrological properties variation.•Locally-developed PTF of KS using bulk density, Clay and Silt shows good performance.
Soil hydrological properties play a key role in soil hydrological processes. However, the effect of long-term vegetation restoration on soil hydrological properties and the corresponding influencing mechanisms remains poorly understood. Here, three soil hydrological properties including saturated water-holding capacity (SWHC), field capacity (FC) and saturated hydraulic conductivity (Ks), as well as several basic soil properties in the Zhifanggou watershed of the Loess plateau were investigated. The variations in SWHC, FC and Ks under different vegetation restoration types and their dominant influencing factors were analyzed. Moreover, we collected available Ks data from peer-reviewed publications to determine the land use with the largest Ks across the entire Loess Plateau. The results showed that SWHC FC and Ks were increased after 20 years of vegetation restoration. The higher Ks was found in shrubland and forest in the whole Loess Plateau. Compared with cropland, Ks in shrubland was increased by 87.10% at 0–20 cm, 48.89% at 20–40 cm, and 18.37% at 40–100 cm, respectively, indicating that the impact of revegetation on Ks were most obvious in the upper soil layer. Bulk density (BD), total porosity (TP), capillary porosity (CP), noncapillary porosity (NCP) and soil organic matter (SOM) had a significant effect on SWHC, FC and Ks for different land-use types (P < 0.01). Soil porosity (i.e., TP, CP and NCP) and BD, soil chemical properties (i.e., SOM and pH), and soil particle composition explained 93.8%, 59.2%, and 13.4% of the total variance in soil hydrological properties (i.e., SWHC, FC and Ks), respectively. This indicates that soil porosity and BD are the dominant factors affecting soil hydrological properties. Moreover, soil particle composition played an important role in regulating Ks, with the contribution of 38.6%. The established pedotransfer function (PTF) of Ks using BD, clay and silt content had a better performance than two existing PTFs. This research provides a more systematic and comprehensive understanding of the soil hydrological effect of vegetation restoration in the Loess Plateau.
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