Drainable Porosity Research Articles

Exploring the Soil Structure Analysis of a Subtropical Watershed by Introducing a Blanket Physico‐Hydraulic Health Index

ABSTRACTMultiple disturbed and undisturbed soil samples were collected to assess the soil structure health in support of water use and management in a subtropical watershed, which is composed of shallow Entisols, near to Canguçu, RS state—Brazil. Various physico‐hydraulic properties related to soil structure, such as bulk density, total, micro and macroporosity, saturated hydraulic conductivity, organic carbon, soil water retention curve (SWRC), and the ranges of plant available water (PAWC) and drainable porosity (DP) were measured to be analyzed. The hydraulic‐energy indices of soil aeration and water retention were determined by integrating the areas under SWRC. Combining these properties with the contents of sand and clay, a unique blanket soil physico‐hydraulic health index (BSPHI) was formulated and applied to investigate the impact of land use and management on the soils of the Ellert Creek Watershed (ECW), thereby supporting soil physical health analysis and, consequently, the analysis of water and human health. The favorable outcomes found in the ECW soils under annual crops indicated good root aeration in the surface layer due to the balance between DP and PAWC. The BSPHI results showed that only about 19% of the areas exhibited unhealthy physical health. The soil structure in pasture areas of ECW was not significantly impacted by the extensive cattle grazing, whereas lands under native forest served as a control for the natural soil quality status. The BSPHI successfully captured small changes in soil structure, regardless of land use and management, highlighting its potential as a promising tool for evaluating soil health by assessing the soil physical quality.

Developing a new unsteady‐state equation for the design of subsurface drainage systems suitable for the Delta region, Egypt

AbstractThe main goal of this research is to obtain a new unsteady‐state equation for the design of subsurface drainage systems that provides a more combatable solution under conditions in the Delta region, Egypt, while simultaneously avoiding all obstacles, difficulties and points of argument in other unsteady‐state equations. The point of argument in unsteady‐state assumptions is the initial water table shape. Then, the study reached a constant called Csh, which represents the initial water table shape and was proven to depend on drain depth (d.d.). Moreover, the new equation avoids using the drainable porosity of soil (f), which is the greatest restriction in using unsteady‐state equations. The new equation replaces f with another constant called Cs, which can be easily determined by using tables or graphs depending on drain depth (d.d.) and hydraulic conductivity (k). Another new important variable is also used in the new equation: the hydraulic head after 6 days of irrigation (h). Using this parameter with the initial water table (h0) and the water table after t days (ht) ensures more accurate results. Then, the new equation, which can be called the Safaa equation, can result in an optimum solution for the Delta region, Egypt, because it provides spacing between laterals (L), which results in the desired performance with the least cost.

Can hydraulic-energy-indices be effectively used to describe the saturated hydraulic conductivity?

The saturated hydraulic conductivity (Ks) and water retention curve (SWRC) parameters are important properties for simulating soil hydrological processes and characterizing soil conservation around the world. Therefore, Ks and SWRC are related with the soil physical quality (SPQ) and several SPQ indices can be derived from SWRC, such as the pore size distribution, relative field capacity, plant available water, drainable porosity, and soil hydraulic-energy indices (SHEI). It is well known that the soil structure can be assessed by using SHEI, but a possible physical relationship between Ks and SHEI was not examined yet. Therefore, the objective of this study was to investigate the behavior of Ks as function of SHEI for several soil textural classes. If this relationship be proved, then SHEI might be applied to improve the Ks prediction by PTF models. In this work, a data set of 395 measured SWRC's were fitted to the vG equation to obtain the SHEI to verify whether they are statistically correlated and physically dependent on Ks. The resulting parametric and non-parametric correlation results were split up according to six textural classes. The significant influence of Ks on at least one of the absolute SHEI (Aa or WRa) was verified on the numerical scale when all textures were grouped and on numerical and pF scales for clayey and silty textures. Ks showed significant impact on Aa and WRa indices in four textural classes. Furthermore, Ks had influence on the sum Aa + WRa denoted in pF scale for five of the six textural classes, with a significant linear correlation in the clayey texture when log (Aa + WRa) was applied. The significant and high correlation of Ks on the ratios WRa/AWC and Aa/φD was also observed in four of the six classes, and therefore the use of these indices is recommended for the development of PTFs for Ks prediction.

A Nonlinear Recession Model for Horizontal Aquifers

AbstractRecession analysis is a powerful tool for determining the hydraulic characteristics of a riparian aquifer. The basis of the method relies on two analytical solutions of the nonlinear Boussinesq equation: one applies to a uniform water table profile in a semi‐infinite aquifer while the other pertains to a hypothetical initial profile in a finite aquifer. Both solutions assume that the water depth in the adjoining stream is negligible. In the present work, a nonlinear solution of the one‐dimensional Boussinesq equation is derived using the traveling wave approximation. The solution better represents the field conditions as it pertains to a uniform water table profile in a finite aquifer that is adjoining a stream with a non‐zero water level. It allows the derivation of simple algebraic expressions for the recession of the water table, the flow rate, and the associated drainage volume. Approximations relating the discharge rate to the outflow volume were also obtained for practical implementation in the estimation of the hydraulic parameters of the aquifer. A comparison of the proposed methodology of parameter estimation with the standard method of recession analysis showed that the present approach is superior as it avoids the pitfalls associated with the classical method, especially when daily discharge values with inherent observational errors are used. Application of the recession model to real‐world cases demonstrates its effectiveness in the estimation of the catchment‐scale hydraulic conductivity and drainable porosity.

Physical and Chemical Properties of Hidimundige (Crown Choking) Affected and Healthy Arecanut Gardens of Chitradurga District, Karnataka, India

The present investigation was undertaken at College of Agriculture, Shivamogga during 2018-2019 to delineate the causes for hidimundige syndrome in arecanut. Soil samples from two depths 0 to 30 cm and 30 to 60 cm were collected at 60 cm away from the trunk of arecanut from both healthy and affected gardens of Holalkere, Hiriyur, Chitradurga and Hosadurga taluks. The collected soil samples were analyzed for various physical and chemical properties and also for macro and micro nutrients. The results of the investigation indicated that soils coming under affected gardens showed the high mean clay content than healthy gardens. There was a significant increase in bulk density of affected gardens compared to healthy gardens in Holalkere (from 1.63-1.84 Mg m-3) and Hosadurga (from 1.84-1.96 Mg m-3) taluk at lower depth. Whereas, the drainable porosity decreased significantly in affected gardens compared to healthy gardens of Holalkere (from 33.54-25.69 %) and Hosadurga (from 25.79-21.23 %) taluk at lower depth. In all taluks, majority of the soils from both healthy and affected arecanut gardens were neutral to alkaline in reaction. The study showed that availability of N, P, K, S, Fe, Mn, Cu and B and also the content of exchangeable Ca and Mg was quite heterogeneous in both healthy and affected gardens. Whereas, available zinc status in the soil showed significant decrease in affected gardens compared to healthy gardens of Hiriyur, Chitradurga and Holalkere taluks.

A comparison of undisturbed soil cores with restructured columns using a new one‐step outflow model

AbstractUndisturbed soil cores may be best for studying field processes in the laboratory, but core variability can make them difficult to use. The goal of this study was to develop a method to make packed columns that have one‐step outflow curves similar to those of undisturbed cores. Undisturbed 15.2‐cm‐long cores and bulk soil were obtained from a sandy loam permanent pasture (Pasture) and a sandy clay loam uncultivated field (Springhill). Ordinary‐packed columns were made by compressing bulk soil to the average core bulk density, and layered columns were made by duplicating the average bulk densities of three core layers. Outflow was recorded from pressure head of −7.6 to −133 cm from newly packed columns and after drying to produce shrinkage cracks. A new outflow model that included initial water content (θi), final water content (θf), and time to reach 63.2% outflow (Τ) was fit to the data by adjusting a time exponent β (average r2 = 0.995). Column parameters were compared with cores by equivalence testing. Drying reduced θi and θf, possibly due to shrinkage cracks that increased drainable porosity. Columns of the Pasture soil had significantly greater Τ than natural cores, possibly due to lack of root‐formed pores in the columns. Overall, layered and dried columns were most similar to natural cores and less variable, and cumulative one‐step outflow curves from undisturbed cores and packed columns can be quantified with a simplified four‐parameter model. These methods and results may help make packed soil columns more representative of undisturbed soil.

Denitrifying Bioreactor In Situ Woodchip Bulk Density

Highlights The bulk density of woodchips in denitrifying bioreactors in the field is unknown. In situ bulk density estimation methods were developed for use during construction or excavation. Dry bulk densities of aged woodchips at bioreactor bottoms were lower than previous literature values. Moisture and particle size and density explained some, but not all, of the variation in in situ bulk densities. Abstract. Woodchip bulk density in a denitrifying bioreactor governs system hydraulics, but this prime physical attribute has never been estimated in situ. The objectives were twofold: (1) to establish estimates of in situ woodchip bulk density at bioreactors in the field, and (2) evaluate causal factors for and resulting impacts of these estimates. Proof-of-concept bulk density methods were developed at a pilot-scale bioreactor using three ways to estimate volume: surveying the excavated area, pumping the excavation full through a flow meter, and using iPhone Light Detection and Ranging (LiDAR). These methods were then further tested at two new and three old full-size bioreactors. Additional ex situ (off-site) testing with the associated woodchips included analysis of bulk density along a moisture gradient and particle size, particle density, wood composition, and hydraulic property testing. In situ dry bulk densities based on the entire volume of the new bioreactors (206-224 kg/m3) were similar to values from previous lab-scale studies. In situ estimates for woodchips at the bottom of aged bioreactors (22-mo. to 6-y) were unexpectedly low (120-166 kg/m3), given that these woodchips would presumably be the most compacted. These low moisture-content corrected dry bulk densities were influenced by high moisture contents in situ (>70% wet basis). The impacts of particle size and particle density on bulk density were somewhat mixed across the dataset, but in general, smaller woodchips had higher dry bulk densities than larger, and several woodchips sourced from the bottom of bioreactors had low particle densities. Although dry bulk densities in the zone of flow in bioreactors in the field were shown to be relatively low, the resulting permeability coefficients under those packing conditions did not differ from those of the original woodchips. The LiDAR-based volume estimation method was the most practical for large-scale, full-size evaluations and allowed high precision with small features (e.g., vertical reactor edges, drainage fittings). Keywords: Compaction, Cone penetrometer, Drainable porosity, LiDAR, Moisture content, Survey.

Denitrifying bioreactor woodchip sourcing guidance based on physical and hydraulic properties

Denitrifying bioreactors using woodchips reduce nitrate loads in agricultural outflows, but there is a lack of practical guidance for woodchip selection from a hydraulic design perspective. The objectives of this study were to develop relationships between particle size, porosity, bulk density, and saturated hydraulic conductivity (Ksat) for a range of commonly available woodchip types to improve woodchip selection guidance and default design model values. Twenty woodchip types from across the US Midwest had their sizes quantified using both sieve analysis and manual axis measurements. Bulk and particle densities and porosity of the twenty types were measured using relatively easy “jar tests” and a selected eight types had bulk density determined across four compaction levels in larger permeameters. Saturated hydraulic conductivity was determined for six types each at three compaction levels. Mean conductivities generally ranged from 3.1 to 7.4 to 0.51–1.8 cm/s for uncompacted and highly compacted woodchips, respectively, and this work newly documented maximum limits of woodchip compaction occurred generally at bulk densities of 190 to 260 kg/m3. Woodchip types with D50 in the 13.5–16.8 mm range had predictable and relatively consistent drainable porosity and Ksat values across bulk densities, facilitating the development of predictive models (R2 ≥ 0.51; p < 0.0001). Even so, Ksat values were highly variable both within and between woodchip types, especially at the lowest compaction level, which creates challenges for precisely estimating this term for design models. Woodchip types with D50 > 12 mm and D90 > 26 mm with uniformity coefficients of 2–3.1 that were chip-shaped (length: width ratio < ∼3.5) were recommended for standardized denitrifying bioreactor designs.

Analytical Expressions of Drainable and Fillable Porosity for Layered Soils Under Shallow Groundwater Environments

AbstractThis study presents the analytical expressions of drainable and fillable porosity for layered soils under shallow groundwater environments. The expressions of drainable porosity λd and fillable porosity λf for two‐layered soils are first derived with water table depth change ∆d→0 under dynamic soil moisture conditions. The expressions of λd and λf consider the dynamic soil moisture conditions through the assumption of vertical fluxes in the unsaturated zone, varying as functions of water table elevation h. The analytical expressions of λd and λf have the same formula form but different values under evapotranspiration and recharge conditions, which is also verified using the numerical calculation. The case comparisons indicate that the vertical distribution of soil layers and the fluxes have significant effects on the performance of λd and λf. Our proposed expressions could effectively characterize the effects of soil layering and vertical fluxes on λd and λf. They could improve the accuracy of the previous expressions for single‐layer soils, and complement the lack of description for layered soils under dynamic soil moisture conditions in previous expressions. Furthermore, under the hydrostatic assumption (static equilibrium of soil water pressure distribution) in the unsaturated zone, the expressions of drainable porosity λd* and fillable porosity λf* for two‐layered soils with large water table fluctuations are derived based on λd and λf (asterisks in λd* and λf* is to distinguish them from λd and λf derived based on ∆d→0). Verification with drainage experimental data indicates that the proposed expressions can be extended to well capture the λd* and λf* changes for two‐layered soils. Application scenarios suggest that our analytical expressions of drainable and fillable porosity are useful to improve the application performance of related subsurface modeling and groundwater estimation for layered soils under shallow water table environments.

Effect of stockpiling time on donor-peat hydrophysical properties: Implications for peatland restoration

Northern peatlands are an important global climate regulator storing approximately one-third of the global carbon pool, however the degradation of these ecosystems from land-use change can switch peatlands to persistent and long-term sources of atmospheric carbon dioxide. Active restoration is often required to return degraded peatlands to a net carbon sink. The peat-block restoration technique, where intact peat blocks are extracted from a donor peatland and transferred to restore peatlands where the remnant peat is non-existent, contaminated, and/or undergoes seasonal flooding is increasingly being adopted as a peatland restoration technique given the carbon sequestration that can occur immediately post-restoration. However, donor peat blocks often need to be temporarily stockpiled during the restoration process due to logistical constraints. The dewatering of the peat blocks during this stockpiling period may alter hydrophysical peat properties that sustain critical peatland ecohydrological functionality and ultimately affect peatland restoration success. Yet, the hydrophysical evolution of stockpiled peat blocks remains unknown. Here, we examine how peat block stockpiling time (3, 7, 11, and 14 months and a reference site) impacts peat hydrophysical properties and sphagnum moss photosynthesis, both of which are critical for peatland restoration success. Stockpiling peat differentially impacted the hydrophysical properties between the shallower and deeper peats, where little to no impact from stockpiling was observed in the shallower peats, regardless of stockpiling time. Rather, as stockpiling time increased, there was a marked decrease in macroporosity (pores >75 μm) and mobile porosity (drainable porosity at approximately −100 hpa) at depths below 20 cm but the water conducting matrix porosity (defined as mobile porosity minus macroporosity) was not significantly different than the reference samples. However, stockpiling created inhospitable conditions for sphagnum mosses., as chlorophyll fluorescence ratio was below 0.3, indicating little to no photosynthesis of the stockpiled peat during summertime drought conditions. Taken together, we suggest limiting stockpiling time as much as possible would be advantageous for using the stockpiled peat blocks for the peat-block restoration technique or other restoration efforts, such as floating mat creation.

An inverse model-based Bees algorithm for estimating ratio of hydraulic conductivity to drainable porosity

A major step in designing a subsurface drainage system is calculation of drainpipe spacing. Nearly, all unsteady-state drainpipe-spacing equations use ratio of hydraulic conductivity (Ks) to drainable porosity (f) in the form of κ=Ksf. This work developed a novel inverse model based on Bees Algorithm to estimate the κ. We examined the efficiency of the developed inverse model to appraise the κ using the existing data sets at laboratory and field scales. For each data set, we compared the performance of the developed inverse model with those of a dimensionless curve generated by the authors (generalized dimensionless curve), Skaggs’s (1976) dimensionless curve being proportional to the data set (Skaggs’s dimensionless curve), a drainpipe discharge method, and a direct method. The results showed that the best statistical indicator values were achieved for each data set when the drainage equations utilized the κ deduced from the developed inverse model. All the studied techniques performed the worst for Skaggs et al.’s (1973) data at 57.5 hrs. after drainage start. For this data set, the studied techniques were, respectively, ranked as the developed inverse model, Skaggs’s dimensionless curve, the generalized dimensionless curve, and the direct method. Even in this worst case, the global performance index was improved almost 93% when the drainage equation utilized the κ appraised by the developed inverse model, instead of that appraised by Skaggs’s dimensionless curve. The overall performance comparison of the three inverse techniques evaluated in this study revealed that the root mean square error and coefficient of determination related to the developed inverse model are the best ones, followed by those related to Skaggs’s dimensionless curve, and the developed inverse model, respectively. In summary, the results revealed that, compared to the other studied techniques, the developed inverse model is the most efficient technique to estimate the κ, which is promising for practical purposes at the laboratory and field scales.

Non‐Darcy Flow Through a Natural Streambed in a Disconnected Stream

AbstractSeepage flux from disconnected streams can be a main source of groundwater recharge in arid and semiarid regions. The seepage into the streambed will respond to a change in stream stage; however, the relationship between seepage flux and stream stage is uncertain due to a lack of in situ observations. This study investigated the change in seepage flow regime with stream stage in a natural streambed through field measurements along a 50.1 km long disconnected stream. Seepage fluxes were quantified using a seepage meter method involving the measurement and simulation of the variable stream stages in a seasonal stream. Flow regimes in the streambed were then evaluated using a developed model. Two seepage flow regimes were observed, the Darcy flow regime and the non‐Darcy flow regime, with the change of the stream stage in the streambed. Our analysis of the mechanisms of seepage flow indicates that drainable porosity and particle size of streambed media are the key factors controlling non‐Darcy seepage flow behavior. We demonstrated that the Reynolds number based on the particle diameter d30 instead of d50 can provide a more accurate criterion for non‐Darcy flow in natural heterogeneous streambeds. Model evaluation showed that predicted streambed seepage regimes and fluxes were consistent with field observations. Together, these results provide new insights into the seepage mechanisms, which have significant implications for the establishment of a better seepage model and the joint development of regional surface water and groundwater.

Increased rainfall may place saline/sodic soils on the tipping point of sustainability

Increased rainfall is increasing the risk of the capillary movement of sodium (Na⁺) and other salts from buried marine sediments to the soil surface in the North American northern Great Plains. These salts reduce productivity and resilience while increasing their effect on the environment. Understanding the interactions among management, climate, cropping system, and soil is the first step toward implementing effective management plans. Unfortunately, while much work has been conducted, little is known about the effective management of semiarid dryland saline/sodic soils. This study determined the influence of soil depth on hydraulic conductivity and changes in soil Na⁺ (mg Na⁺ kg^–1 soil) to electrical conductivity (EC_1:1) (dS m^–1) ratio following high spring rainfall in 2019 in three soils. The soil parent materials were Glaciolacrustrine underlaid at a depth of approximately 15 m by marine sediments that contained Na⁺ and other salts. The landscape positions included in the study were a well-drained shoulder, moderately well-drained backslope, and a poorly drained toeslope soil. Based on the soil classification, shoulder and backslope subsoils were not predicted to be salt affected, while the toeslope soil was predicted to contain a natric soil horizon. Rainfall in 2018, 2019, and 2020 was 46, 76, and 37 cm, respectively, and soil cores were collected prior to and following the 2019 high rainfall. Soil cores were separated into the 0 to 7.5, 50 to 57.5, 82.5 to 90, 92.5 to 100, and 105 to 112.5 cm segments. Samples from 2018 were analyzed for soil EC_1:1, pH, ammonium acetate extractable cations, soil particle size, available water at field capacity, drainable porosity, soil bulk density, and saturated hydraulic conductivity. Samples from 2019 were analyzed for EC_1:1 and ammonium acetate extractable Na⁺. Across the sampling sites, shoulder and backslope soils had higher saturated hydraulic conductivities than the toeslope soils. Saturated hydraulic conductivities were negatively correlated to pH (<i>r</i> = –0.55, <i>p</i> &lt; 0.01), the Na⁺ to EC_1:1 ratio (<i>r</i> = —0.66, <i>p</i> &lt; 0.01), extractable Na⁺ (<i>r</i> = —0.56, <i>p</i> &lt; 0.01), and sand content (<i>r</i> = —0.66, <i>p</i> &lt; 0.01), and positively correlated to the silt content (<i>r</i> = 0.65, <i>p</i> &lt; 0.01). A comparison between the saturated hydraulic conductivity and the Na⁺ to EC_1:1 ratio suggests that saturated conductivities approached 0 cm h^–1 when the Na⁺ (mg Na⁺ kg^–1) to EC_1:1 (dS m^–1) ratio exceeded 600. The 2019 high rainfall increased the risk of soil dispersion in the lower soil depths (&gt;82.5 cm). For example, in the shoulder soil at the 105 to 112.5 cm depth, EC_1:1 decreased 0.936 ± 0.254 dS m^–1 from 2018 to 2019, whereas the exchangeable Na⁺ increased 688 ± 283 mg kg^–1 soil. Our findings suggest that a climate change-induced shift in rainfall patterns can increase salinity and sodicity risks in northern Great Plains subsurface soils. Salinity and sodicity risks are expanding into zones not previously identified as at risk, and improving or maintaining the productivity of these soils requires careful planning.

Self organizing hydrological processes in a runoff source area

Understanding the hydrology of runoff source areas is crucial for predicting floods and evaluating chemical transport. Numerically modeling water fluxes in the source areas is quite complex. However, the self-organization of complex hydrological systems makes it possible to simplify watershed models by considering the landscape functions. The limitation is that input values are not known a priori. This study seeks to find the soil physical parameters governing the hydrology of runoff source areas in humid climates and use them in a surrogate simulation model to predict the runoff and the perched water table height. The site chosen was a 5.4-ha, periodically saturated runoff source area with a shallow perched water table and a humid temperate climate. The only inflow was from precipitation. Perched water table depths at five locations and the outflow were measured continuously. Measurements showed that the outflow was negligible 24 h after a rain event. It indicated that a quasi-static equilibrium had been established, in which the capillary pressure decreased linearly with depth to zero at the shallow groundwater. The soil–water retention function determined the soil water distribution and the drainable porosity. Runoff was generated by saturation excess and equaled the rainfall minus the empty pore volume. Based on these observations, a simple spreadsheet-based surrogate model was developed to calculate the air-filled pore volume by accounting for daily precipitation and evaporation. The surrogate model accurately predicted daily discharge and water table height using climatic data and a literature-based water retention function. In our case, the quasi-static equilibrium was restored within a day. Thus, the small-scale nonlinear physics that determines the soil’s water flow rate before equilibrium is reached were irrelevant when the time step is one day. Studies in similar runoff source areas could utilize this approach and create their specific surrogate models.

Root Biomass Distribution and Soil Physical Properties of Short-Rotation Coppice American Sycamore (Platanus occidentalis L.) Grown at Different Planting Densities

Short rotation woody crops (SRWCs) provide sustainable, renewable biomass energy and offer potential ecosystem services, including increased carbon storage, reduced greenhouse gas emissions, and improved soil health. Establishing SRWCs on degraded lands has potential to enhance soil properties through root and organic matter turnover. A better understanding of SRWC planting density and its associated root turnover impacts on soil–air–water relations can improve management. In this study, we investigate the effects of planting density for a low-input American sycamore SRWC (no fertilization/irrigation) on soil physical properties for a degraded agricultural site in the North Carolina piedmont. The objectives were (1) to estimate the distributions of coarse and fine root biomass in three planting densities (10,000, 5000, and 2500 trees per hectare (tph)) and (2) to assess the effects of planting density on soil hydraulic properties and pore size distribution. Our results show that planting at 10,000 tph produced significantly higher amounts of fine root biomass than at lower planting densities (p &lt; 0.01). In the 25,000 tph plots, there was significantly higher amounts of coarse root biomass than for higher planting densities (p &lt; 0.05). The 10,000 tph plots had lower plant available water capacity but larger drainable porosity and saturated hydraulic conductivity compared with lower planting densities (&lt;0.05). The 10,000 tph plots total porosity was more dominated by larger pore size fractions compared with the 5000 and 2500 tph. Generally, our findings show similar patterns of soil hydraulic properties and pore size distributions for lower planting densities. The results from 10,000 tph indicate a higher air-filled pore space at field capacity and more rapid drainage compared with lower planting densities. Both characteristics observed in the 10,000 tph are favorable for aeration and oxygen uptake, which are especially important at wet sites. Overall, the results suggest that improved soil health can be achieved from the establishment of American sycamore SRCs on marginal lands, thereby providing a green pathway to achieving environmental sustainability with woody renewable energy.

Differentiated influence of the double porosity of the chalk on solute and heat transport

Abstract Chalk porosity plays a decisive role in the transport of solutes and heat in saturated chalk. From a geological point of view, there are at least two types of porosity: the porosity of pores corresponding to the micro-spaces between the fossil coccoliths that form the chalk matrix and the porosity owing to the micro- and macro-fractures (i.e. secondary porosity). For groundwater flow, the fracture porosity is a determining factor at the macroscopic scale. The multiscale heterogeneity of the porous/fractured chalk induces different effects on solute and heat transport. For solute transport considered at the macroscopic scale, tracer tests have shown that the ‘effective transport porosity’ is substantially lower than the ‘effective drainable porosity’. Moreover, breakthrough curves of tracer tests show an important influence of diffusion in a large portion of the ‘immobile water’ (‘matrix diffusion’) together with rapid preferential advection through the fractures. For heat transport, the matrix diffusion in the ‘immobile water’ of the chalk is hard to distinguish from conduction within the saturated chalk.

Testing numeric and logarithmic scales for hydraulic-energy indices and functions to assess physical earthworm effects on soil structure

Sufficient soil physical quality (SPQ) is needed to support the provision of multiple ecosystem services including food and fibre production, cycling of nutrients, cycling and storage of carbon, filtration of water and a habitat for biodiversity. Several SPQ indices are used to ascertain the SPQ of a soil and are often derived from the soil water retention curve (SWRC), built with multiple volumetric water content (θ) points across the pressure head (h) range, from saturation to oven-dried soil. The integral area under the SWRC has been used as an indicator for evaluating SPQ. The present study compared five hydraulic-energy SPQ indices and two cumulative hydraulic-energy functions based on the SWRC integral calculated on θ[log10(h)] basis and θ(h) in terms of their ability to track and refine soil structural information over time. For this, both approaches were applied to the SWRC data developed to examine the relationship between SPQ and earthworm abundance under dung pat and non-dung pat treatments from deposition to decay and beyond at a grassland field site in Ireland. Results showed that plant available water content (PAWC) and drainable porosity (DP) can be examined with either numeric (θ(h)) and logarithmic (θ[log10(h)]) scales and outcomes were not significantly different. Although the absolute hydraulic-energy SPQ indices of aeration and water retention presented distinct results, they both resulted in the same information with respect to soil structure status and this was consistent across numeric or logarithmic scales. However, the relative hydraulic-energy indices of aeration, water retention and air-water revealed more information with respect to water retention and drainage processes than when calculated on a logarithmic basis. Over the study duration, it was shown that more than 85% of the integral energy was used for storing water in the PAWC range and was dominant over the aeration process regardless of the treatment imposed. The results also indicate high and medium effect sizes for some parameters suggesting that a larger population or sample size could enhance the correlations and significance above those found in the present study.

Coupling of a subsurface drainage model with a soil reservoir model to simulate drainage discharge and drain flow start

The environmental impact of subsurface drainage and agricultural activities has been widely studied in the literature. Agricultural subsurface drainage modifies the hydrological behavior and accelerates the transfer of pollutants of agricultural origin into surface water during the drain flow period, which is often limited to winter. The main objective of this study is to present a modeling approach allowing an accurate modeling of subsurface drainage discharge and prediction of drain flow start times, by integrating a new conceptual soil reservoir, managing the water flow in the unsaturated zone, to the SIDRA model. A comparison of the model results with field measurements of drainage discharge at the “La Jaillière” site (France) shows that such a model can efficiently simulate drainage discharge (KGE values >0.75) and predict, with good accuracy, the drain flow start time (with a median value of 5 days and a standard deviation of 10 days). The split simple test conducted for the model calibration and validation shows that the model is temporally robust. A sensitivity analysis conducted using the Sobol method on the five model parameters reveals that the drainage discharge simulation is mainly sensitive to the hydraulic conductivity and drainable porosity parameters. On the other hand, the date delimiting drain flow start is sensitive to the soil reservoir parameters. The model's ability to accurately predict the start of drain flow serves to avoid the application of farm inputs (pesticides or fertilizers) during this critical period in order to limit their transfer to surface waters.

The three-dimensional groundwater salinity distribution and fresh groundwater volumes in the Mekong Delta, Vietnam, inferred from geostatistical analyses

Abstract. Over the last decades, economic developments in the Vietnamese Mekong Delta have led to a sharp increase in groundwater pumping for domestic, agricultural and industrial use. This has resulted in alarming rates of land subsidence and groundwater salinization. Effective groundwater management, including strategies to work towards sustainable groundwater use, requires knowledge about the current groundwater salinity distribution, in particular the available volumes of fresh groundwater. At the moment, no comprehensive dataset of the spatial distribution of fresh groundwater is available. To create a 3D model of total dissolved solids (TDS), an existing geological model of the spatial distribution and thickness of the aquifers and aquitards is updated. Next, maps of drainable porosity for each aquifer are interpolated based on the sedimentological description of the borehole data. Measured TDS in groundwater, inferred TDS from resistivity measurements in boreholes and soft incomplete data (derived from measurements in boreholes and data from domestic wells) are combined in an indicator kriging routine to obtain the full probability distribution of TDS for each (x,y,z) location. This statistical distribution of TDS combined with drainable porosity yields estimates of the volume of fresh groundwater (TDS &lt; 1 g L−1) in each aquifer. Uncertainty estimates of these volumes follow from a Monte Carlo analysis (sequential indicator simulation). Results yield an estimated fresh groundwater volume for the Mekong Delta of 867 billion cubic metres with an uncertainty range of 830–900 billion cubic metres, which is somewhat higher than previous assessments of fresh groundwater volumes. The resulting dataset can for instance be used in groundwater flow and salt transport modelling as well as aquifer storage and recovery projects to support informed groundwater management decisions, e.g. to prevent further salinization of the Mekong Delta groundwater system and land subsidence, and is available at https://doi.org/10.5281/zenodo.4441776 (Gunnink et al., 2021).

Land use, management and physical attributes of dense Ferralsols in tropical northeastern Brazil

In Brazil, Haplic Ferralsols (Densic) are commonly known to have physical limitations, including higher bulk density and less drainable porosity, less water infiltration and greater resistance to root growth. However, there are extensive areas with these soils in Brazil that are important for agricultural production. Because maintaining and/or recovering the physical quality of these soils is an important task to maintain agricultural sustainability, the objective of this study was to evaluate the impact of different use and management systems on the physical quality and total organic carbon content of Haplic Ferralsols (Densic) in northeastern Brazil. The evaluated systems were: conventional planting (CC), pasture (PP) and two agroforestry systems: one 7 years (AF7) and the other 12 years old (AF12). An area of secondary forest with remnants of the Atlantic Forest Biome (SF) was selected and sampled as a reference. The total organic carbon content (TOC), flocculation index (FI) and particle density (Dp) were determined. Saturated hydraulic conductivity (Ko), macroporosity (MaP), microporosity (MiP), total porosity (TP), soil bulk density (Ds) and aggregate stability were also determined in samples collected in the 0– to 0.10, 0.10 to 0.20 and 0.20– to 0.40 m soil layers. The variables that best distinguished the agroforestry systems from the other systems were GMD, MaA, WMD and TOC. Correlations of these variables, all related to the soil structure in the AF7 and AF12 systems, contribute to maintaining or regenerating soil aggregation. None of the soil management systems were grouped with the forest area, although areas in the agroforestry systems had lower impact on the physical soil quality parameters compared to the CC and PP systems.