Soil Hydraulic Characterization Research Articles

A Holistic Irrigation Advisory Policy Scheme by the Hellenic Agricultural Organization: An Example of a Successful Implementation in Crete, Greece

The aim of this communication article is to present a successful irrigation advisory scheme on the island of Crete (Greece) provided by the Hellenic Agricultural Organization (ELGO DIMITRA), which is well adapted to the different needs of farmers and water management agencies. The motivation to create this advisory scheme stems from the need to save water resources while ensuring optimal production in a region like Crete where droughts seem to occur more and more frequently in recent years. This scheme/approach has three different levels of implementation (components) depending on the spatial level and end-users’ needs. The first level concerns the weekly irrigation bulletins in the main agricultural areas of the island with the aim of informing farmers and local water managers about crop irrigation needs. The second level concerns an innovative digital web-based platform for the precise determination of the irrigation needs of Crete’s crops at a parcel level as well as optimal adaptation strategies in the context of climate change. In this platform, important features such as real-time meteorological information, spatial data on the cultivation type of parcels, validated algorithms for calculating crop irrigation needs, an accurate soil texture map derived from satellite images, and appropriate agronomic practices to conserve water based on cultivation and the geomorphology of a farm are considered. The third level of the proposed management approach includes an open-source Internet of Things (IoT) intelligent irrigation system for optimal individual parcel irrigation scheduling. This IoT system includes soil moisture and atmospheric sensors installed on the field, as well as the corresponding laboratory soil hydraulic characterization service. This third-level advisory approach provides farmers with specialized information on the automated irrigation system and optimization of irrigation water use. All the above irrigation advisory approaches have been implemented and evaluated by end-users with a very high degree of satisfaction in terms of effectiveness and usability.

Comparing mini-disk infiltrometer, BEST method and soil core estimates of hydraulic conductivity of a sandy-loam soil

Saturated, Ks, and near-saturated, K, soil hydraulic conductivity control many hydrological processes but they are difficult to measure. Comparing methods to determine Ks and K is a means to establish how and why these soil hydrodynamic properties vary with the applied method. A comparison was established between the Ks and K values of a sandy-loam soil obtained, in the field, with the BEST (Beerkan Estimation of Soil Transfer parameters) method of soil hydraulic characterization and an unconfined MDI (mini-disk infiltrometer) experiment and, in the laboratory, with a confined MDI experiment and the CHP (constant-head permeameter) method. Using for the BEST calculations the soil porosity instead of the saturated soil water content yielded 1.4–1.1 times higher estimates of Ks and K, depending on the pressure head, and differences decreased in more unsaturated soil conditions. The confined MDI experiment yielded 22 % - 77 % higher K values than the unconfined MDI experiment, depending on the established pressure head, h0, and differences were not significant for h0 = −1 cm. In the close to saturation region, the soil hydraulic conductivity function predicted with BEST did not generally agree well with the Ks and K values obtained in the laboratory by a direct application of the Darcy’s law. In particular, BEST yielded a 5.6 times smaller Ks value than the CHP method and up to an 8.1 times higher K value than the MDI. Overall, i) the two application methods of the MDI yielded relatively similar results, especially close to saturation, and ii) there was not a satisfactory agreement between the field (BEST) and the laboratory (MDI plus CHP) determination of soil hydraulic conductivity close to saturation, unless a comparison was made with the same soil water content. The detected differences were probably attributable to soil spatial variability, overestimation of Ks in the laboratory due to preferential flow phenomena, underestimation of Ks in the field due to air entrapment in the soil and infiltration surface disturbance, inability of BEST to describe the actual soil hydraulic conductivity function at the sampled field site. Testing BEST predictions of Ks and K in other soils appears advisable and combining the MDI and CHP methods appears a rather simple means to make these checks. These additional investigations could improve interpretation of the differences between methods, which is an important step for properly selecting a method yielding Ks and K data appropriate for an intended use.

Testing the hydrodynamic behavior of a loam soil by beerkan infiltration runs with six heights of water pouring

Interpreting and simulating rainfall partition into infiltration and surface runoff has to consider that surface soil hydraulic properties, including saturated soil hydraulic conductivity (Ks) and sorptivity (S), could change even at very short temporal scales. Soil deterioration due to water impact can be tested in the field by the multi-height beerkan (MHB) method, that is, by pouring water on the infiltration surface from different heights. Soil hydraulic properties can be estimated coupling the MHB method with the steady version of the Beerkan Estimation of Soil Transfer parameters procedure (BEST-steady). The MHB method with six heights of water pouring (H) in the range 0.03–2 m was applied on a relatively dry loam soil to investigate height of water pouring effects on i) the established infiltration process, and ii) Ks, S, the scale parameter of the water retention curve (hg) and the characteristic microscopic pore radius (λm). Higher heights of water pouring generally induced a slowdown of the infiltration process, smaller S, Ks and λm values and higher |hg| values. The S, Ks, |hg| and λm vs. H relationships were statistically significant but the fitted relationships for S and Ks were stronger than those for |hg| and λm, indicating a different sensitivity of the considered parameters to the height of water pouring. Small or negligible soil deterioration was observed for both small (H ≤ 0.25 m) and large (H ≥ 1.5 m) heights. Between these two extremes, the soil deteriorated as H increased, suggesting that the external solicitations were high enough to overcome the resistance of the porous medium but not so high to determine a complete soil alteration. The tested methodology appears promising to determine the effects of water impact on the soil hydrodynamic behavior and it could be applied to perform a soil hydraulic characterization usable for modelling hydrological processes.

BEST-WR: An adapted algorithm for the hydraulic characterization of hydrophilic and water-repellent soils

Water-repellent soils usually experience water flow impedance during the early stage of a wetting process followed by progressive increase of infiltration rate. Current infiltration models are not formulated to describe this peculiar process. Similarly, simplified methods of soil hydraulic characterization (e.g., BEST) are not equipped to handle water-repellent soils. Here, we present an adaptation of the BEST method, named BEST-WR, for the hydraulic characterization of soils at any stage of water-repellency. We modified the Haverkamp explicit transient infiltration model, included in BEST for modeling infiltration data, by embedding a scaling factor describing the rate of attenuation of infiltration rate due to water repellency. The new model was validated using analytically generated data, involving soils with different texture and a dataset that included data from 60 single-ring infiltration tests. The scaling factor was used as a new index to assess soil water repellency in a Mediterranean wooded grassland, where the scattered evergreen oak trees induced more noticeable water repellency under the canopies as compared to the open spaces. The new index produced results in line with those obtained using the water drop penetration time test, which is one of the most widely test applied for quantifying soil water repellency persistence. Finally, we used BEST-WR to determine the hydraulic characteristic curves under both hydrophilic and hydrophobic conditions.

Hydraulic hysteresis of natural pyroclastic soils in partially saturated conditions: experimental investigation and modelling

In many geotechnical applications, especially in the study of weather-induced landslides, a reliable soil hydraulic characterization in unsaturated conditions is required. Currently, the experimental techniques that neglect the hydraulic hysteresis represent the greatest limitation to landslide forecasting. In this paper, a procedure to obtain an unsaturated soil hydraulic characterization on natural pyroclastic samples is proposed and verified. The approach enables the evaluation of the soil hydraulic properties along the main drying path and wetting/drying cycles to fully quantify the effects of the hydraulic hysteresis. Pyroclastic soil samples collected at a test site at Mount Faito in the Campania region (southern Italy) were tested. The experimental investigation consisted of a sequence of testing phases: a constant-head hydraulic conductivity test, a forced evaporation test followed by several wetting–drying cycles, and a drying test in a pressure plate apparatus. The hysteretic model proposed by Parker and Lenhard (1987) was adopted to fit the data, while inverse modelling of the forced evaporation tests allowed to derive the model parameters. Therefore, the main drying and wetting branches and the soil response to drying and wetting cycles from any reversal point were reproduced with the model, which suitably described the hysteretic behaviour of the pyroclastic soil under all conditions and along all paths.

Rapid and accurate measurement methods for determining soil hydraulic properties: A review

Abstract The determination of soil hydraulic properties is important in several environmental sciences but may be expensive and time consuming. Therefore, during the last decades, a great effort has been made in soil sciences to develop relatively easy, robust, and inexpensive methods for soil hydraulic characterization. In this manuscript, we reviewed and discussed different infiltrometer techniques in light of the available experimental applications. More specifically, we considered the simplified falling head (SFH) infiltrometer technique and the single-ring infiltration experiment of the Beerkan type. Concerning this latter method, we considered different algorithms for data analysis: two simplified methods based on the analysis of transient (TSBI) and steady (SSBI) Beerkan infiltration data, and the Beerkan Estimation of Soil pedoTransfer parameters algorithm (BEST), that allows to estimate the soil characteristics curves, i.e., the soil water retention curve and hydraulic conductivity functions. For a given method, after dealing briefly theory and practice, available literature references were reported to account for specific applications in order to provide findings on method validation and application. With the aim to provide practical information on available tools for a simpler application of the reviewed methods, several video tutorials were reported to show i) how to conduct correctly field experiments and ii) how to calculate saturated hydraulic conductivity or soil hydraulic functions using user-friendly tools for data analysis. Finally, details on a new automated single-ring infiltrometer for Beerkan infiltration experiments (i.e., construction, assembly and field use) were presented.

Assessing Water Infiltration and Soil Water Repellency in Brazilian Atlantic Forest Soils

This study presents the results of the soil hydraulic characterization performed under three land covers, namely pasture, 9-year-old restored forest, and remnant forest, in the Brazilian Atlantic Forest. Two types of infiltration tests were performed, namely tension (Mini-Disk Infiltrometer, MDI) and ponding (Beerkan) tests. MDI and Beerkan tests provided complementary information, highlighting a clear increase of the hydraulic conductivity, especially at the remnant forest plots, when moving from near-saturated to saturated conditions. In addition, measuring the unsaturated soil hydraulic conductivity with different water pressure heads allowed the estimation of the macroscopic capillary length in the field. This approach, in conjunction with Beerkan measurements, allowed the design better estimates of the saturated soil hydraulic conductivity under challenging field conditions, such as soil water repellency (SWR). This research also reports, for the first time, evidence of SWR in the Atlantic Forest, which affected the early stage of the infiltration process with more frequency in the remnant forest.

Testing an adapted beerkan infiltration run for a hydrologically relevant soil hydraulic characterization

Literature raises doubts about the usability of infiltrometer methods to characterize soils in a hydrological perspective since these methods often yield excessively high infiltration rates or saturated soil hydraulic conductivity, Ks, values. For a loam (AR) and a silty-clay (RO) soil, beerkan infiltration runs were adapted in the perspective to obtain usable soil data to predict rainfall partition into infiltration and rainfall excess. In particular, the initially nearly dry soil was sampled with different water volumes (15 or 30) and heights of water application (low, L, 0.03 m, and high, H, 1.5 m), and the BEST-steady algorithm was applied to determine sorptivity, S, and Ks. The H runs altered the surface soil layer more than the L runs but the response of the two soils to disturbance was different. For the AR soil, deterioration of the surface layer was almost complete close to the end of the run while, for the RO soil, it was concluded after applying one third of the overall used water volume. The least soil perturbing experiment (15L), that was carried out with the commonly recommended experimental protocol, yielded high S (91–118 mm/h0.5) and Ks (88–294 mm/h) values at the two sites, likely incompatible with formation of any rainfall excess. A soil perturbing experiment (30H) yielded significantly and substantially smaller S (26–40 mm/h0.5) and Ks (4–6 mm/h) values, that appeared potentially compatible with formation of rainfall excess. The gravitational potential energy, Ep, of the water used for the infiltration run explained most of the variance of both Ks and S. In conclusion, the infiltration run can be adapted in an attempt to induce a soil disturbance likely similar to that expected for a rainfall producing runoff. In a relatively dry initial status, complete deterioration can require less water for a silty-clay soil than a loam soil. The saturated soil hydraulic conductivity is more sensitive than soil sorptivity to the applied experimental methodology. Finally, the gravitational potential energy of the applied water could be used to establish a link between the hydrological process that has to be explained and a soil hydraulic characterization appropriate to reach the desired objective.

An open-source instrumentation package for intensive soil hydraulic characterization

We present a new open-source and modular instrumentation package composed of up to ten automatic infiltrometers connected to data acquisition systems for automatic recording of multiple infiltration experiments. The infiltrometers are equipped with differential transducers to monitor water level changes in a Mariotte reservoir, and, in turn, to quantify water infiltration rates. The data acquisition systems consist of low-cost components and operate on the open-source microcontroller platform Arduino. The devices were tested both in the laboratory and on different urban and agricultural soils in France and India. More specifically, we tested three procedures to treat the transducers readings, including a filtering algorithm that substantially improved the ability to determine cumulative infiltration from raw data. We combined these three procedures with four methods for estimating the soil parameters from infiltrometer data, showing pros and cons of each scenario. We also demonstrated advantages in using the automatic infiltrometers when infiltration measurements were hindered by: i) linearity in cumulative infiltration curves owing to gravity-driven flow, ii) an imprecise description of the transient state of infiltration, and iii) the occurrence of soil water repellency. The use of the automatic infiltrometers allows the user to obtain more accurate estimates of soil hydraulic parameters, while also reducing the amount of effort needed to run multiple experiments.

Modelling of an accidentally triggered shallow landslide in Northern Italy

The paper presents a case study of a landslide event, accidentally triggered by an unexpected and extraordinary infiltration in an otherwise stable slope. The work aims at modelling the slope failure mechanism with a simplified two-dimensional framework based on the formation of a perched water table. The landslide occurred in Val Venosta/Vinschgau Valley, in Northern Italy, in April 2010 and produced catastrophic consequences. It took place on a hillside with average slope angle 36° and affected an area of about 200 m2; the slip surface was located approximately 1.0 m below the slope profile, in the uppermost layers of a predominantly coarse, well-graded soil. A series of numerical simulations were performed to back-analyse the event, using a commercial computer program. The artificial water infiltration and water content evolution were modelled with a two-dimensional finite element model (FE) of the unsaturated/saturated domain with appropriate infiltration boundary conditions. The slope stability analyses were conducted with a classic limit equilibrium method (LE) and were performed at different time instants during the infiltration process. The soil-water retention curves and conductivity functions were defined according to the van Genuchten-Mualem model. The combined FE and LE simulations showed the gradual formation of a perched water table, whose associated localized pore pressure distribution resulted in the loss of the suction stabilizing effect and thus in the reduction of the safety factor. Although supported by basic soil mechanical and hydraulic characterization, the numerical simulations allowed to perform a back-analysis which effectively captured the timing of the event and the location and depth of the slip surface along the slope.

A general Beerkan Estimation of Soil Transfer parameters method predicting hydraulic parameters of any unimodal water retention and hydraulic conductivity curves: Application to the Kosugi soil hydraulic model without using particle size distribution data

Soil hydraulic characterization is crucial to describe the retention and transport of water in soil, but current methodologies limit its spatial applicability. This paper presents a cost-effective general Beerkan Estimation of Soil Transfer parameters (BEST) methodology using single ring infiltration experiments to derive soil hydraulic parameters for any unimodal water retention and hydraulic conductivity functions. The proposed method relies on the BEST approach. The novelty lies in the use of Kosugi hydraulic parameters without need for textural information. In addition, the method uses a quasi-exact formulation that is valid for all times, which avoids the use of approximate expansions and related inaccuracy. The new BEST methods were tested against numerically generated data for several contrasting synthetic soils, and the results show that these methods provide consistent hydraulic functions close to the target functions. The new BEST method is accurate and can use any water retention and hydraulic conductivity functions.

How does PTF Interpret Soil Heterogeneity? A Stochastic Approach Applied to a Case Study on Maize in Northern Italy

Soil water balance on a local scale is generally achieved by applying the classical nonlinear Richards equation that requires hydraulic properties, namely, water retention and hydraulic conductivity functions, to be known. Its application in agricultural systems on field or larger scales involves three major problems being solved, related to (i) the assessment of spatial variability of soil hydraulic properties, (ii) accounting for this spatial variability in modelling large-scale soil water flow, and (iii) measuring the effects of such variability on real field variables (e.g., soil water storage, biomass, etc.). To deal with the first issue, soil hydraulic characterization is frequently performed by using the so-called pedotransfer functions (PTFs), whose effectiveness in providing the actual information on spatial variability has been questioned. With regard to the second problem, the variability of hydraulic properties at the field scale has often been dealt with using a relatively simple approach of considering soils in the field as an ensemble of parallel and statistically independent tubes, assuming only vertical flow. This approach in dealing with spatial variability has been popular in the framework of a Monte Carlo technique. As for the last issue, remote sensing seems to be the only viable solution to verify the pattern of variability, going by several modelling outputs which have considered the soil spatial variability. Based on these premises, the goals of this work concerning the issues discussed above are the following: (1) analyzing the sensitivity of a Richards-based model to the measured variability of θ(h) and k(θ) parameters; (2) establishing the predictive capability of PTF in terms of a simple comparison with measured data; and (3) establishing the effectiveness of use of PTF by employing as data quality control an independent and spatially distributed estimation of the Above Ground Biomass (AGB). The study area of approximately 2000 hectares mainly devoted to maize forage cultivation is located in the Po plain (Lodi), in northern Italy. Sample sites throughout the study area were identified for hydropedological analysis (texture, bulk density, organic matter content, and other chemical properties on all the samples, and water retention curve and saturated hydraulic conductivity on a sub-set). Several pedotransfer functions were tested; the PTF‒Vereckeen proved to be the best one to derive hydraulic properties of the entire soil database. The Monte Carlo approach was used to analyze model sensitivity to two measured input parameters: the slope of water retention curve (n) and the saturated hydraulic conductivity (k0). The analysis showed sensitivity of the simulated process to the parameter n being significantly higher than to k0, although the former was much less variable. The PTFs showed a smoothing effect of the output variability, even though they were previously validated on a set of measured data. Interesting positive and significant correlations were found between the n parameter, from measured water retention curves, and the NDVI (Normalized Difference Vegetation Index), when using multi-temporal (2004–2018) high resolution remotely sensed data on maize cultivation. No correlation was detected when the n parameter derived from PTF was used. These results from our case study mainly suggest that: (i) despite the good performance of PTFs calculated via error indexes, their use in the simulation of hydrological processes should be carefully evaluated for real field-scale applications; and (ii) the NDVI index may be used successfully as a proxy to evaluate PTF reliability in the field.

Variability of near-surface saturated hydraulic conductivity for the clay soils of a small Sicilian basin

Proper characterization of saturated hydraulic conductivity, Ks, of clay soils in a hillslope or a basin is still a challenge for soil science. In this investigation, the clay soils of the Maganoce (Sicily, Italy) basin were sampled at 19 sampling sites by the BEST procedure of soil hydraulic characterization. More OM implied less compact conditions (decreasing dry soil bulk density; coefficient of determination, R2 = 0.67), more stability to water of the soil aggregates (increasing water stable aggregates; R2 = 0.83) and, consequently, higher Ks values (R2 = 0.54). Variability of Ks was lower in the steeper zones of the basin than in the flatter ones. A comparison with the Ks data collected in the past with the Simplified Falling Head (SFH) technique at the same sampling points revealed a similarity between the two datasets but also a tendency of this last technique to yield some small Ks values that were not obtained with BEST. Soil disturbance due to ring insertion that SFH requires and/or Ks anisotropy likely contributed to explain this result. In conclusion, organic matter content and topographic attributes can help to plan an effective clay soil sampling strategy. High Ks values are expected to be less uncertain than small values.

A test of water pouring height and run intermittence effects on single‐ring infiltration rates

AbstractAssessing how the infiltration process depends on the water impact energy improves interpretation of hydrological processes. Impact energies vary with the height of water pouring; that is, the distance between the water delivery point and the soil surface. The effects of the height of water pouring on infiltration in an initially near saturated soil can be tested in the field by two repeated Beerkan infiltration runs separated by a short pause (30 min) and using both low (non‐perturbing) and high (perturbing) heights of water application. The double two‐stage Beerkan run methodology was applied in two soils. The infiltration rate at the end of the perturbing stage of the experiment was 0.2–0.3 (sandy‐loam soil) and 0.15 (loam soil) times that obtained with the initial, nonperturbing stage whereas, without any perturbing effect, infiltration rates at the end of the second run were 0.5 times those at the end of the first run. Therefore, the methodology distinguished between a decrease in infiltration rate due to water redistribution during the pause and that attributable to soil surface perturbation. Maintaining a small depth of water (10 mm) on the infiltration surface was not an alternative to the classical Beerkan run for the non‐perturbing stage of the experiment since two times higher infiltration rates were measured with a greater hydrostatic pressure. In conclusion, the relationship between infiltration into an initially near saturated soil and the energy of the applied water can be determined directly in the field at any time with a simple and parsimonious experiment. This circumstance could improve soil hydraulic characterization for interpreting and simulating hydrological processes.

Soil Water Conservation: Dynamics and Impact

Human needs like food and clean water are directly related to good maintenance of healthy and productive soils. A good understanding of human impact on the natural environment is therefore necessary to preserve and manage soil and water resources. This knowledge is particularly important in semi-arid and arid regions, where the increasing demands on limited water supplies require urgent efforts to improve water quality and water use efficiency. It is important to keep in mind that both soil and water are limited resources. Thus, wise use of these natural resources is a fundamental prerequisite for the sustainability of human societies. This Special Issue collects 15 original contributions addressing the state of the art of soil and water conservation research. Contributions cover a wide range of topics, including (1) recovery of soil hydraulic properties; (2) erosion risk; (3) novel modeling, monitoring and experimental approaches for soil hydraulic characterization; (4) improvement of crop yields; (5) water availability; and (6) soil salinity. The collection of manuscripts presented in this Special Issue provides more insights into conservation strategies for effective and sustainable soil and water management.

Comparing Transient and Steady-State Analysis of Single-Ring Infiltrometer Data for an Abandoned Field Affected by Fire in Eastern Spain

This study aimed at determining the field-saturated soil hydraulic conductivity, Kfs, of an unmanaged field affected by fire by means of single-ring infiltrometer runs and the use of transient and steady-state data analysis procedures. Sampling and measurements were carried out in 2012 and 2017 in a fire-affected field (burnt site) and in a neighboring non-affected site (control site). The predictive potential of different data analysis procedures (i.e., transient and steady-state) to yield proper Kfs estimates was investigated. In particular, the transient WU1 method and the BB, WU2 and OPD methods were compared. The cumulative linearization (CL) method was used to apply the WU1 method. Values of Kfs ranging from 0.87 to 4.21 mm·h−1 were obtained, depending on the considered data analysis method. The WU1 method did not yield significantly different Kfs estimates between the sampled sites throughout the five-year period, due to the generally poor performance of the CL method, which spoiled the soil hydraulic characterization. In particular, good fits were only obtained in 23% of the cases. The BB, WU2 and the OPD methods, with a characterization based exclusively on a stabilized infiltration process, yielded an appreciably lower variability of the Kfs data as compared with the WU1 method. It was concluded that steady-state methods were more appropriate for detecting slight changes of Kfs in post-fire soil hydraulic characterizations. Our results showed a certain degree of soil degradation at the burnt site with an immediate reduction of the soil organic matter and a progressive increase of the soil bulk density during the five years following the fire. This general impoverishment resulted in a slight but significant decrease in the field-saturated soil hydraulic conductivity.

Previous Land Use Affects the Recovery of Soil Hydraulic Properties after Forest Restoration

Knowledge of soil hydraulic properties after forest restoration is essential for understanding the recovery of hydrological processes, such as water infiltration. An increase of forest cover may improve water infiltration and soil hydraulic properties, but little is known about the response and extent to which forest restoration can affect these properties. The purpose of this study was to investigate the effect of forest restoration on surface-saturated soil hydraulic conductivity (Ks), and to verify the Ks recovery to the pre-disturbance soil conditions. We sampled field Ks at the surface in Campinas municipality, São Paulo State, Brazil, at 18 plots under three land-cover types: (i) a pasture; (ii) a restored forest using a high-diversity mix of plantings (85 regional native species) of 9 years of age; and (iii) a remnant forest patch. We used the Beerkan method for soil hydraulic characterization. Bulk density (ρb), soil organic carbon content (OC), soil porosity and particle size data were also sampled. We found considerable differences in soil hydraulic properties between land-cover classes. The highest Ks were observed in remnant forest sites and the lowest Ks were associated with pasture sites. The Ks recovery differs markedly between restored forests. Our results strongly suggest that soil attributes and Ks recovery are influenced by the duration and intensity of land use prior to forest restoration. Attention needs to be given to management activities before, during and after forest restoration, especially where the soil is still compacted and Ks is low.

An assessment of the BEST procedure to estimate the soil water retention curve: A comparison with the evaporation method

The Beerkan Estimation of Soil Transfer parameters (BEST) procedure is an attractive, easy, robust, and inexpensive way for a complete soil hydraulic characterization but testing the ability of this procedure to estimate the water retention curve is necessary as relatively little information is available in the literature. In this investigation the soil water retention curve was predicted for four differently textured soils by applying three existing BEST algorithms (i.e., slope, intercept and steady) and the results compared with those measured by the standard Wind evaporation method. A sensitivity analysis of the infiltration constants, β and γ, was also carried out and their impact on the estimated retention curve scale parameter, hg, was evaluated. BEST-slope underestimated the soil water retention for three of the four soils under consideration, providing relatively low root mean squared differences between estimated and measured data (0.0261 cm3cm−3 ≤ RMSD ≤0.0483 cm3cm−3). For one site (PAL, sandy-loam soil), BEST-steady provided the lowest RMSD value (0.0893 cm3cm−3) among the considered algorithms, but the water retention was systematically overestimated as a consequence of a relatively higher difference between field and lab saturated soil water contents. A specific calibration performed for β and γ highlighted that: i) the water retention estimations by BEST-slope were more sensitive to β than those obtained by BEST-intercept and BEST-steady; ii) with the exception of PAL soil, the lowest RMSD values were obtained with BEST-slope. Estimation of the soil water retention curve was not significantly worse when reference values of infiltration constants (β = 0.6 and γ = 0.75) were used as detected by negligible differences in RMSDs as compared to calibrated β and γ. Therefore, it was concluded that the BEST-slope algorithm yielded predictions of water retention closer to the laboratory estimated ones than the alternative BEST algorithms (i.e. BEST-intercept and -steady). For these algorithms, the less accurate estimates of the water retention data were attributed to hg overestimations due to the independence of the retention curve scale parameter from γ.

Comparing Beerkan infiltration tests with rainfall simulation experiments for hydraulic characterization of a sandy‐loam soil

AbstractSaturated soil hydraulic conductivity, Ks, data collected by ponding infiltrometer methods and usual experimental procedures could be unusable for interpreting field hydrological processes and particularly rainfall infiltration. The Ks values determined by an infiltrometer experiment carried out by applying water at a relatively large distance from the soil surface could however be more appropriate to explain surface runoff generation phenomena during intense rainfall events. In this study, a link between rainfall simulation and ponding infiltrometer experiments was established for a sandy‐loam soil. The height of water pouring for the infiltrometer run was chosen, establishing a similarity between the gravitational potential energy of the applied water, Ep, and the rainfall kinetic energy, Ek. To test the soundness of this procedure, the soil was sampled with the Beerkan estimation of soil transfer parameters procedure of soil hydraulic characterization and two heights of water pouring (0.03 m, i.e., usual procedure, and 0.34 m, yielding Ep = Ek). Then, a comparison between experimental steady‐state infiltration rates, isR, measured with rainfall simulation experiments determining runoff production and Ks values for the two water pouring heights was carried out in order to discriminate between theoretically possible (isR ≥ Ks) and impossible (isR < Ks) situations. Physically possible Ks values were only obtained by applying water at a relatively large distance from the soil surface, because isR was equal to 20.0 mm h−1 and Ks values were 146.2–163.9 and 15.2–18.7 mm h−1 for a height of water pouring of 0.03 and 0.34 m, respectively. This result suggested the consistency between Beerkan runs with a high height of water pouring and rainfall simulator experiments. Soil compaction and mechanical aggregate breakdown were the most plausible physical mechanisms determining reduction of Ks with height. This study demonstrated that the height from which water is poured onto the soil surface is a key parameter in infiltrometer experiments and can be adapted to mimic the effect of high intensity rain on soil hydraulic properties.

Impacts of thinning of a Mediterranean oak forest on soil properties influencing water infiltration

AbstractIn Mediterranean ecosystems, special attention needs to be paid to forest–water relationships due to water scarcity. In this context, Adaptive Forest Management (AFM) has the objective to establish how forest resources have to be managed with regards to the efficient use of water, which needs maintaining healthy soil properties even after disturbance. The main objective of this investigation was to understand the effect of one of the AFM methods, namely forest thinning, on soil hydraulic properties. At this aim, soil hydraulic characterization was performed on two contiguous Mediterranean oak forest plots, one of them thinned to reduce the forest density from 861 to 414 tree per ha. Three years after the intervention, thinning had not affected soil water permeability of the studied plots. Both ponding and tension infiltration runs yielded not significantly different saturated,Ks, and unsaturated,K−20, hydraulic conductivity values at the thinned and control plots. Therefore, thinning had no an adverse effect on vertical water fluxes at the soil surface. MeanKsvalues estimated with the ponded ring infiltrometer were two orders of magnitude higher thanK−20values estimated with the minidisk infiltrometer, revealing probably soil structure with macropores and fractures. The input of hydrophobic organic matter, as a consequence of the addition of plant residues after the thinning treatment, resulted in slight differences in terms of both water drop penetration time, WDPT, and the index of water repellency,R, between thinned and control plots. Soil water repellency only affected unsaturated soil hydraulic conductivity measurements. Moreover,K−20values showed a negative correlation with both WDPT andR, whereasKsvalues did not, revealing that the soil hydrophobic behavior has no impact on saturated hydraulic conductivity.