Negative Pressure Head Research Articles

Comment on “Modified expression for unsaturated hydraulic conductivity according to Mualem–van Genuchten to allow proper computations at low pressure heads” by M. Heinen

AbstractThis comment concerns evaluation of the Mualem–van Genuchten (MvG) unsaturated soil hydraulic functions at very low (negative) pressure heads as described recently in an important study by Heinen. He showed that below some critical pressure head, the unsaturated hydraulic conductivity should be approximated by a power function, even when evaluated using double precision computations. We show that a more precise approximation is possible when the approximation is formulated in terms of effective fluid saturation (Se) rather than the pressure head, h. More general constraints are also provided when the approximated hydraulic conductivity equation should be used. The alternative formulation and constraints were implemented earlier in the SOHYP and RETC software packages.

Comparison of Soil Hydraulic Properties Estimated by Steady- and Unsteady-Flow Methods in the Laboratory

In this study, soil hydraulic conductivity (K) and soil sorptivity (S) values estimated by applying various steady- and unsteady-flow methods using cumulative infiltration data of three disturbed soils (sandy loam, loam, clay) obtained from a disc infiltrometer in the laboratory at various negative pressure heads were compared. The steady-flow methods used were those of Ankeny et al. and Reynolds and Elrick as well as Logsdon and Jaynes, while the unsteady-flow methods were those of Haverkamp et al. (two-term (2T) and three-term (3T) infiltration equations) and Zhang. The method of White et al., which is a steady-flow method but also uses unsteady-flow infiltration data, was also examined. The results showed that the three steady-flow methods, as well as the Zhang equation, for values of the van Genuchten coefficient n > 1.35, tend to give similar values of K. The 2T infiltration equation with β = 0.6 provided hydraulic conductivity values greater than those estimated by the steady-state methods but gave negative K values in some cases. The values of the coefficients C1 and C2 of the 2T equation were affected by the infiltration time. The coefficient C1 increased while C2 decreased with increasing time when the cumulative linearization method (CL) was applied, but the change in C1 tended to be smaller than that in C2. The inverse solution of the 3T equation using the Excel Solver application for β = 0.75 and β = 1.6, when positive values of K were obtained, approached better the K values estimated by the steady-flow methods compared with those estimated using β = 0.6. Regarding the estimation of S from the unsteady-flow equations (2T, 3T, Zhang), comparable S values were obtained by all equations. The differences between the S values of the various methods are smaller compared to those of K, and S is less affected than K in terms of time. The problem of negative estimates of K might be attributed to the fact that the soils used in this study are classified as soils situated in the domain of lateral capillarity or are not completely homogeneous or soil compaction is observed at some depth. In the case where the soils are not completely homogeneous, the Sequential Infiltration Analysis (SIA) method with β = 0.75 corresponding to the soil types studied was proved to be effective in estimating K values.

Hydro-mechanical modelling of swell behaviour of bentonite buffer

Hydro-mechanical modelling of swell behaviour of bentonite buffer

Divergent effects of food waste derived hydrochar on hydraulic properties and infiltration in a sandy soil

Hydrothermal carbonization of food waste (FW) and application of the solid product (hydrochar, HC) in agriculture, is expected to be an effective way to promote the circulation in food-energy-water nexus. However, little is known on the effects of applying FW-derived HC as a soil amendment on soil properties and functions. The current study investigated the effects of FW-derived HC on soil hydraulic properties and infiltration process. Two types of FW-derived HC were mixed into the top 0–20 cm soil layer at a rate of 1 % and 2 % (w/w) in a soil column experiment with grass growth. In-situ infiltration experiment was conducted within the soil column using a disc infiltrometer at different negative pressure (−20 cm, −6 cm, and −2 cm water head). Unsaturated hydraulic conductivity (K0) and sorptivity (S0) at each negative pressure head was estimated by fitting experimental cumulative infiltration data to a 4-terms infiltration model. Saturated hydraulic conductivity (Ks) and soil water retention curve (SWRC) were measured using undisturbed soil cores taken from the soil column. Soil pore size distribution and soil water constants were estimated from the SWRCs. The results indicate that application of FW-derived HC significantly (p < 0.05) affected soil structure by increasing meso (0.03–0.0002 mm) and macro (> 0.03 mm) pores, which can be attributed to the promoted soil aggregation. Saturated hydraulic conductivity was significantly increased and water retention characteristics were changed with significantly (p < 0.05) increased soil water holding capacity and plant available water content. Besides, FW-derived HCs were found hydrophobic and can render wettable sandy soil water repellent. The K0 and S0 were significantly (p < 0.05) decreased and cumulative infiltration was reduced when soil was wetted from an initial dry condition. Negative effect was found on grass growth with significantly (p < 0.05) decreased aboveground biomass after HCs application. Our study indicates that agriculture application of FW-derived HC as a soil amendment could effectively improve soil fertility by recycling nutrient elements and improving soil structure. However, attention should be paid to the hydrophobicity and phototoxicity of FW-derived HC to avoid the related detrimental influences on plant growth and environment.

Evaluation of Soil Hydraulic Parameters Calculation Methods Using a Tension Infiltrometer

In the present work, a review for the methodologies that have been proposed to calculate the main soil hydraulic properties, hydraulic conductivity (K) and sorptivity (S), at negative pressure heads near to saturation of the soil using a tension infiltrometer is presented. These hydraulic properties can be calculated either from the analysis of steady flow or from early time observations. In particular, the main steady state methods described here are those of Ankeny et al., Reynolds and Elrick, and White and Sully, which are all based on Wooding’s equation. As for the transient flow, the approaches of Haverkamp et al. (complete, two-, three-, four-, five-terms expansions), Zhang and two different linearization methods are examined for the estimation of S and K. Generally, in steady state methods studied, a sequence of pressure heads is applied on the same disc (Ankeny et al., Reynolds and Elrick) or a unique pressure head is applied on a single disc radius (White and Sully), while in transient methods, a unique pressure head is applied on a single disc radius (Zhang and Haverkamp et al.). The conditions of their application and the way of calculating the soil parameters included into each method are critically commented. This gives to the researchers the opportunity to choose the appropriate method and a way to analyze the experimental data.

Numerical Analysis of an Unsaturated Capillary Barrier Cover System

Inclined multi-layered barriers can be used to protect underlying waste storage facilities. The intended barriers can be used to confine the infiltration through implementation of the capillary barrier effect. In this study, the effect of rainfall, evaporation, and transpiration on the hydraulic properties of inclined covers was assessed by performing a series of simulations using HYDRUS-2D numerical models. The material of the intended layers included clay loam soil as a seepage control layer, sandy soil as a moisture retention layer, and gravel as a capillary break layer. Based on the key results of numerical analyses, Lateral diversion in the interface between the seepage control layer and moisture retention layer occurred as a result of the significant slope of said layers and the low permeability of the moisture retention layer. At the reduced degree of saturation, water did not move easily from the seepage control layer to the moisture retention layer as well as from the moisture retention layer to the capillary break layer due to the low hydraulic conductivity. The negative pressure head in the seepage control layer had minimal effect on the water content in the moisture retention layer. Hence, the performance of this protective earthen cover can, then, be guaranteed due to the current climatological conditions.

Exploring the effect of varying soil organic matter contents on current and future moisture supply capacities of six Italian soils

The Available Soil Water Capacity (AWC) is standard data in most soil databases and expresses soil water contents in the rootzone between field capacity (FC; –33 kPa) and permanent wilting point (WP; −1500 kPa). Literature suggests that increasing the content of soil organic matter (SOM) of a given soil does not significantly increase AWC and this has important implications when estimating soil moisture supply to crops and evaluating the potential for climate mitigation. For most crops, the real FC values vary between −10 and −50 kPa in different soils and WP values between −800 and −1500 kPa. Thus standard values for AWC of FC and WP do not represent field conditions in many soils. When exploring AWC for six Italian soil series, ranging from clay to sandy, AWC values at increasing %SOM were lower in clay soils and higher in sand as compared with actual conditions, which could be explained by considering the shape of the corresponding moisture retention curves. Rather than focus on static AWC values to define moisture supply to plants, real or actual soil moisture supply capacities (MSC) can be obtained by dynamic modeling of the soil-water-atmosphere-plant system, including a “sink-term” indicating a continuous relation between water uptake and negative pressure head of soil water and evaporative demand. Also, only models allow exploration of the effects of future severe IPCC climate scenario RCP 8.5. Thus, studying MSC for the six Italian soil series showed that MSC values were: (i) on average 30% higher than the corresponding AWC; (ii) distinctly different for the six soils; (iii) affected by declines of 1–9% as a result of the effects of future climate scenarios.; (iv) not significantly affected by increases of %SOM when considering climate change, except for the sand. Generalizations as to the effect of future climate scenarios and %SOM on MSC can only be realistic when modeling is performed for soil series in different climate zones.

Soil water characteristic curve test and saturated-unsaturated seepage analysis in Jiangcungou municipal solid waste landfill, China

In this study, an analysis of municipal solid waste (MSW) is undertaken at the Jiangcungou landfill in Xi’an, China. Using waste specimens gathered from the landfill, the soil water characteristic curve (SWCC) is determined at the laboratory, and the effects of landfill age and dry density on the SWCC parameters are analyzed. Leachate level trends during the landfill lifecycle are examined by performing saturated-unsaturated seepage analysis with the consideration of precipitation, as well as the inner source of water. These results are then summarized into three primary points. First, the shallow waste in the Jiangcungou landfill lost its water relatively quickly at small negative pressure heads, while the deep waste lost its water much more gradually. In addition, the deeper waste exhibited a lower volumetric water content when the matrix suction was less than 1.4kPa, but the result was opposite when the matrix suction was more than 2.0kPa. Second, the saturated volumetric water content and steepness of the SWCC decreased as the dry density increased, and the residual volumetric water contents increased and the inverse of the air-entry pressure decreased with increasing landfill age. Third, layers three, five and six demonstrated the highest leachate levels in the eighth landfill stage especially layer five, and these merit the greatest attention in monitoring and construction. These findings may provide a basis for the management of the Jiangcungou landfill.

A Note on One- and Three-Dimensional Infiltration Analysis from a Mini Disc Infiltrometer

Disc infiltrometers are used to characterize soil hydraulic properties. The purpose of this study was to determine the difference between three- and one-dimensional infiltration and to calculate the infiltration shape parameter γ from a proposed analytical infiltration equation. One- and three-dimensional infiltration tests were done on three repacked soils (loam, sandy loam, and silty clay loam) for two negative pressure heads. A mini disc infiltrometer of a radius of 22.5 mm with suction that ranged from −5 mm to −70 mm was used. The difference between experimental three- and one-dimensional cumulative infiltration was linear with time, which confirmed the proposed equation. In this study, the shape parameter γ seems not to be seriously affected by the soil type and acquires values from 0.561 to 0.615, i.e., smaller than the value γ = 0.75, which is widely used. With these values, the criteria proposed for calculating hydraulic conductivity using three-dimensional infiltration data may be fulfilled in most soils.

Comparison of different assimilation methodologies of groundwater levels to improve predictions of root zone soil moisture with an integrated terrestrial system model

Abstract The linkage between root zone soil moisture and groundwater is either neglected or simplified in most land surface models. The fully-coupled subsurface-land surface model TerrSysMP including variably saturated groundwater dynamics is used in this work. We test and compare five data assimilation methodologies for assimilating groundwater level data via the ensemble Kalman filter (EnKF) to improve root zone soil moisture estimation with TerrSysMP. Groundwater level data are assimilated in the form of pressure head or soil moisture (set equal to porosity in the saturated zone) to update state vectors. In the five assimilation methodologies, the state vector contains either (i) pressure head, or (ii) log-transformed pressure head, or (iii) soil moisture, or (iv) pressure head for the saturated zone only, or (v) a combination of pressure head and soil moisture, pressure head for the saturated zone and soil moisture for the unsaturated zone. These methodologies are evaluated in synthetic experiments which are performed for different climate conditions, soil types and plant functional types to simulate various root zone soil moisture distributions and groundwater levels. The results demonstrate that EnKF cannot properly handle strongly skewed pressure distributions which are caused by extreme negative pressure heads in the unsaturated zone during dry periods. This problem can only be alleviated by methodology (iii), (iv) and (v). The last approach gives the best results and avoids unphysical updates related to strongly skewed pressure heads in the unsaturated zone. If groundwater level data are assimilated by methodology (iii), EnKF fails to update the state vector containing the soil moisture values if for (almost) all the realizations the observation does not bring significant new information. Synthetic experiments for the joint assimilation of groundwater levels and surface soil moisture support methodology (v) and show great potential for improving the representation of root zone soil moisture.

Effect of the soil water dynamics on nitrous oxide emissions

Microbial N2O production may occur via denitrification, a reaction which is influenced by the soil's water content. This study aimed to test the effect of soil water dynamics on N2O production and transport. The treatments consisted of two levels of soil bulk density (BD), negative pressure head applied on the soil cylinder (H), and saturated hydraulic conductivity of the ceramic plate placed at the bottom of the soil sample (K). We controlled the water status of repacked soil samples during two wetting-drying cycles, by using a multistep outflow system. The matric potential, outflow, N2O, and CO2 fluxes were recorded over time. A brief N2O peak occurred at the beginning of soil drying: N2O produced and entrapped in the soil during the wetting phase was released during soil drying with the increase in soil gas diffusion. Similar peaks dynamics were observed for CO2, implying that a physical process was involved. A relationship was observed to occur with maximum N2O fluxes increasing exponentially with cumulative drainage. Indeed, during drying, high N2O fluxes were measured when the air-entry potential was reached, i.e., when gas pathways were available for fast N2O transport in the gas phase. Then, maximum and cumulative N2O fluxes were highest for low BD and fast water flow during drying. Samples with the highest BD had smaller pore sizes, leading to low outflows at a given negative pressure head, and giving more time for further reduction of N2O to N2. We ranked the importance of the parameters controlling cumulative N2O fluxes: H>BD>K.

Diurnal Soil Water and Root Water Uptake/Nitrogen Dynamics in the Wastewater-Irrigated Pepper Field

ABSTRACTTo study the effect of wastewater irrigation in pepper fields, this research analyzed the principle of the water vapor–heat–nitrogen coupling transport and the diurnal rule of root water/nitrate uptake dynamics in wastewater-irrigated pepper fields. In an outdoor experiment, the soil water content, negative pressure head, and temperature at depths of 10, 25, 40, 55, and 70 cm were measured hourly for 480 h in July 2013. The modified HYDRUS-1D code was used to analyze soil water and root water/nitrogen uptake dynamics. The numerical analyses provided insight into the diurnal movement of the soil water and root water/nitrogen uptake dynamics driven by the gradients of the pressure heads and temperatures. The measured and simulated data agree well with each other. When the original nitrate-nitrogen (NO3-N) content in the soil is high, additional wastewater irrigation will not increase the root uptake of NO3-N. Thus, excessive wastewater irrigation may cause groundwater nitrogen contamination.

The relevance of in-situ and laboratory characterization of sandy soil hydraulic properties for soil water simulations

Field water flow processes can be precisely delineated with proper sets of soil hydraulic properties derived from in situ and/or laboratory experiments. In this study we analyzed and compared soil hydraulic properties obtained by traditional laboratory experiments and inverse optimization tension infiltrometer data along the vertical direction within two typical Podzol profiles with sand texture in a potato field. The main goal was to identify proper sets of hydraulic parameters and to evaluate their relevance on hydrological model performance for irrigation management purposes. Tension disc infiltration experiments were carried out at four and five different depths for both profiles at consecutive negative pressure heads of 12, 6, 3 and 0.1 cm. At the same locations and depths undisturbed samples were taken to determine Mualem–van Genuchten (MVG) hydraulic parameters (θr, residual water content, θs, saturated water content, α and n, shape parameters and Kls, lab saturated hydraulic conductivity) in the laboratory. Results demonstrated horizontal differences and vertical variability of hydraulic properties. The tension disc infiltration data fitted well in inverse modeling using Hydrus 2D/3D in combination with final water content at the end of the experiment, θf. Four MVG parameters (θs, α, n and field saturated hydraulic conductivity Kfs) were estimated (θr set to zero), with estimated Kls and α values being relatively similar to values from Wooding’s solution which used as initial value and estimated θs corresponded to (effective) field saturated water content, θf. The laboratory measurement of Kls yielded 2–30 times higher values than the field method Kfs from top to subsoil layers, while there was a significant correlation between both Ks values (r = 0.75). We found significant differences of MVG parameters θs, n and α values between laboratory and field measurements, but again a significant correlation was observed between laboratory and field MVG parameters namely Ks, n, θs (r ⩾ 0.59). Assessment of the parameter relevance in 1-D model simulations, illustrated that the model over predicted and under predicted top soil-water content using laboratory and field experiments data sets respectively. The field MVG parameter data set resulted in better agreement to observed soil-water content as compared to the laboratory data set at nodes 10 and 20 cm. However, better simulation results were achieved using the laboratory data set at 30–60 cm depths. Results of our study do not confirm whether laboratory or field experiments data sets are most appropriate to predict soil water fluctuations in a complete soil profile, while field experiments are preferred in many studies.

Soil Hydraulic Parameters of Bare Soil Plots with Different Soil Structure Inversely Derived from L‐Band Brightness Temperatures

The structure of the surface layer of the soil is strongly influenced by soil tillage practices, with important consequences for the hydraulic properties and soil moisture dynamics in the top soil layer. In this study, during four 28‐d periods, we monitored L‐band brightness temperatures and infrared (IR) temperatures over bare silt loam soil plots with different soil surface structure: tilled, seedbed, and compacted plots. Differences in absolute and normalized L‐band brightness temperatures between the plots indicated that plot specific roughness, soil moisture contents, and soil hydraulic properties might be inverted from L‐band brightness temperatures using a coupled radiative transfer, roughness correction, and soil hydrological model. The inversely estimated surface roughness parameters compared well with those derived from laser profiler measurements. The estimated saturated water contents of the tilled and seedbed plots were larger than the one of the compacted plot, and the unsaturated hydraulic conductivity was smaller in the former plots than in the compacted plot for more negative pressure heads. These differences in hydraulic properties translated into larger dynamics of the simulated soil moisture during a 28‐d measurement period in the tilled and seedbed plots than in the compacted plot. This difference could be confirmed qualitatively but not quantitatively by in situ soil moisture measurements. Furthermore, differences in simulated actual evaporation rates between the plots were confirmed by observed differences in measured IR temperatures. The results indicate that effects of soil management on soil surface roughness and soil hydraulic properties could be inferred from L‐band brightness temperatures.

On the applicability of unimodal and bimodal van Genuchten–Mualem based models to peat and other organic soils under evaporation conditions

Summary Soil moisture is one of the key parameters controlling biogeochemical processes in peat and other organic soils. To understand and accurately model soil moisture dynamics and peatland hydrological functioning in general, knowledge about soil hydraulic properties is crucial. As peat differs in several aspects from mineral soils, the applicability of standard hydraulic functions (e.g. van Genuchten–Mualem model) developed for mineral soils to peat soil moisture dynamics might be questionable. In this study, the hydraulic properties of five types of peat and other organic soils from different German peatlands have been investigated by laboratory evaporation experiments. Soil hydraulic parameters of the commonly-applied van Genuchten–Mualem model and the bimodal model by Durner (1994) were inversely estimated using HYDRUS-1D and global optimization. The objective function included measured pressure heads and cumulative evaporation. The performance of eight model set-ups differing in the degree of complexity and the choice of fitting parameters were evaluated. Depending on the model set-up, botanical origin and degree of peat decomposition, the quality of the model results differed strongly. We show that fitted ‘tortuosity’ parameters τ of the van Genuchten–Mualem model can deviate very much from the default value of 0.5 that is frequently applied to mineral soils. Results indicate a rather small decrease of the hydraulic conductivity with increasing suction compared to mineral soils. Optimizing τ did therefore strongly reduce the model error at dry conditions when high pressure head gradients occurred. As strongly negative pressure heads in the investigated peatlands rarely occur, we also reduced the range of pressure heads in the inversion to a ‘wet range’ from 0 to −200 cm. For the ‘wet range’ model performance was highly dependent on the inclusion of macropores. Here, fitting only the macropore fraction of the bimodal model as immediately drainable additional pore space seems to be a practical approach to account for the macropore effect, as the fitting of the full bimodal model led to only marginal further improvement of model performance. This keeps the number of parameters low and thus provides a model that is more easily managed in pedotransfer function development and practical applications like large scale simulations. Our findings point out first options to improve the performance of the frequently-used simple single-domain models when they are applied to organic soils. We suggest further performance evaluation of these models during wetting periods when they are known to fail to describe preferential and non-equilibrium flow phenomena.

Moisture Migration Mechanism in Granite Residual Soil Embankment under the Combined Action of Rainfall and Capillary

Taking the granite residual soil embankment of Fujian Province as an example, this paper investigated the moisture migration mechanism in granite residual soil embankment based on the finite difference technology. The results show that the moisture migration of the shallow layer of the embankment displays up first and down later in dry seasons. In rainy seasons, the negative pressure head of the embankment’s upper soil increases, and range of rainfall variation of the initial period is much sharper than the later period. Meanwhile, at the toe of slope, the rain firstly supplies the groundwater, making it a weak location. After the rain, the contour line of negative pressure head in the upper embankment gradually becomes sparse, and the gradient gradually decreases in the embankment. The fluctuation of negative pressure head on foundation surface decreases with the depth of groundwater. As to the low embankment, the change of the embankment moisture mainly depends on the external weather conditions when the groundwater level is deep and the moisture is mainly affected by groundwater when the groundwater level is shallow.

The auto-regulative capability of pitcher irrigation system

Among traditional irrigation systems, pitcher irrigation is one of the most efficient. Water seeps out of a buried pitcher due to the pressure head gradient across the wall of the pitcher directly into the root zone of the irrigated crop. The pressure gradient results from positive pressure head inside pitcher and negative pressure head at the outer surface of the pitcher which is in contact with soil. Laboratory experiments were conducted to quantify the influence of evaporation, which controls the negative pressure condition in the soil, on the seepage rate of pitchers and to explore the auto-regulative capability of the pitcher irrigation system. Two groups of pitchers, group A with high hydraulic conductivity and group B with low hydraulic conductivity, were filled with water and placed in a closed chamber at varying temperature and humidity. The pan evaporation rate (Ep) inside chambers ranged from 1 mm/d when the temperature and relative humidity were kept around 20 °C and 97% to 16 mm/d at 45 °C and relative humidity of 40%. A positive and significant correlation was found between seepage rate of pitchers and Ep with an average R 2 of 0.97. The average seepage rates of all pitchers at Ep values of 1 and 16 mm/d were 125 mL/d to as high as 1020 mL/d, respectively. Differences in the seepage rates in pitchers from group A and group B were high at low Ep and low at high Ep values. At low Ep, seepage rate was more affected by water pressure head inside pitchers and the hydraulic conductivity whereas at high Ep values the negative pressure head at the outer surface of pitchers were more important. The average seepage rate of type A pitchers was 200% higher than that of type B at Ep value of 1 mm/d but for the high Ep value of 16 mm/d the corresponding increase in the seepage rate of type A compared to type B was only 4%. Thus, evaporation rate controls seepage from the pitcher and the system is auto-regulative.

Coupling of runoff processes and sediment transport in mountainous watersheds underlain by different sedimentary rocks

AbstractRunoff and sediment transport were observed in two mountainous watersheds with different lithologies. One watershed is underlain by chert and the other by sandstone and shale. The chert watershed consists of bedrock cliffs and shallow regolith (0–2 m), whereas the sandstone–shale watershed is covered with a deeper regolith (1–5 m). In the sandstone–shale watershed, a large spring located in the channel head contributed to storm runoff. Tensiometric response in regolith above the spring showed negative pressure head throughout storm runoff, thus implying that the subsurface flow in bedrock predominated even during storm runoff. In the chert watershed, the runoff coefficient was larger than in the sandstone–shale watershed. Tensiometric response in regolith showed positive pressure head during storm runoff, indicating generation of rapid subsurface stormflow. Concentration of subsurface flow from side slopes increased peak streamflow in the downstream direction. In these two watersheds, different subsurface flowpaths, including bedrock fractures, facilitated different modes of sediment transport. Groundwater outflow at the spring caused seepage erosion in the sandstone–shale watershed, whereas peak streamflow transported bedload supplied by rockfall in the chert watershed. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Plant-cover effects on hydrology and pedogenesis in a sandy vadose zone

The hydrology of three field lysimeters was monitored after 15 years of plant growth to examine pedogenic impacts of plant-induced changes to water budgets, flow, and chemical denudation. Between 70% and 87% of annual water discharge from all lysimeters (“sandboxes”) was generated from short (usually ≤5 days) precipitation events and longer (usually ≤25 days) snowmelt events. These infiltration events occur only 10–23% of the time throughout the year. Average annual evapotranspiration (ET) from the red pine sandbox was 73% of precipitation and 1.4–2.6 times larger than ET from the grass and moss/lichen boxes, respectively. In situ unsaturated hydraulic conductivity estimates, based on Darcy's law, are on the order of 10 −8 m/s and did not appear to vary significantly with plant cover. The upper zone of each sandbox is characterized by rapid, large changes in the magnitude of pressure head (−700 to −6 cm), water content (1 to 9%), and magnitude and direction of the hydraulic gradient. The vertical extent of this upper zone varies with plant cover; it extends to a depth between 30 and 70 cm beneath moss/lichen and grass whereas the upper zone beneath red pine extends to at least 120 cm during the growing season. Synoptic field measurements of water content and pressure head were very different from laboratory measurements of hanging-column moisture-retention curves. The red pine sandbox consistently exhibited the smallest water contents and the most negative pressure heads. Upper-zone soil water samples, collected at small and large tension during events, had different silica concentrations, which supports the existence of a dynamic flow system consisting of water residing in large and small pore spaces, respectively. However, large-tension silica concentrations increased systematically with depth in all sandboxes, indicating chemical reaction progresses in bulk soil water along flow paths. A conservative estimate of the shortest mean soil water residence time in the sandboxes is 9 days, which is sufficient time for exchange and weathering reactions to occur. Silica concentrations at 95 cm beneath red pine were twice as large as those beneath the moss/lichen cover. However, larger water fluxes from the moss/lichen sandbox produced an annual silica denudation flux of 910 mol Si/ha/year, whereas silica denudation from the pine sandbox was only 470 mol Si/ha/year. These results suggest that water and solute uptake by rooting vegetation may actually decrease chemical denudation from young, soil-building ecosystems.

Soil surface characteristics influence on infiltration in black marls: application to the Super‐Sauze earthflow (southern Alps, France)

AbstractSoil surface characteristics (SSCs) have a strong influence on the infiltration properties at the soil surface. They can explain spatial variations in the infiltration processes observed under natural rainfall for relatively little‐differentiated material and can help us to understand the surface and subsurface hydrology of landslides, representative of some of black marls landscapes of the French southern Alps. This paper presents the results of investigations into the influence of SSCs using careful morphological description, in situ tension‐disc infiltrometry and statistical analysis (hierarchical ascending classification and stepwise discriminant analysis). The site studied is the Super‐Sauze earthflow. The selected infiltration and runoff variables were described for 233 microplots chosen all over the earthflow. The results show that an approach based on a description of relevant variables influencing water infiltration can validate a typology based on ‘expert’ knowledge. Hydraulic conductivity at saturation was measured on 36 undisturbed soil cores and at low negative pressure heads on 83 microplots: the dispersion exceeds three orders of magnitude at each pressure head. A statistical analysis of the data set obtained by describing selected variables showed the relevance of the black marl descriptors when defining types of SSC that enable us to distinguish significant hydrodynamical behaviour. This leads us to suggest a distribution of hydraulic conductivity curves near saturation for each SSC type, which can be used as a key to distribute local hydraulic conductivity values adapted to black marl hillslopes. As well as the types of SSC at a microplot scale leading to types of hydrodynamic properties, the typology of combinations of SSCs at a coarser scale should lead to types of hydrological response and to identification of hydrological and geomorphological response units. Copyright © 2003 John Wiley &amp; Sons, Ltd.