Soil Unsaturated Hydraulic Conductivity Research Articles

Averaging or adding domain conductivities to calculate the unsaturated soil hydraulic conductivity

AbstractRecent models of the unsaturated hydraulic conductivity curve (UHCC) are the sum of separate UHCCs for domains of capillary water, film water, and water vapor. This requires parallel, noninteracting domains. A theoretical framework for aggregating domain conductivities to a bulk soil UHCC is presented to identify and possibly relax implicit assumptions about domain configuration. The paper develops arithmetic, harmonic, and geometric averages of the liquid‐water conductivities that can be arithmetically averaged with the vapor conductivity. However, current models for capillary and film conductivities are intrinsic, that is, valid within their respective domain. The vapor conductivity is a bulk conductivity, that is, it gives the conductivity of the gaseous domain as it manifests itself in the soil. Conversion relationships use the domain volume fractions as approximations of the as‐yet unknown weighting factors to convert between intrinsic and bulk conductivities. This facilitates consistent averaging of domain conductivities. Even with consistent averaging, a truly physically accurate model of the UHCC based on domain conductivities is fundamentally elusive. Nevertheless, models based on the three averages and the unweighted sum of the domain conductivities produce good fits to data for two soils but diverge in the dry range. The fitted curves for the capillary and film water depend on the averaging (or adding) method. Hence, they are not strictly characteristic of their respective domains. The true intrinsic domain conductivity functions may be impossible to determine.

Cover crop management systems improves soil quality and mitigate water erosion in tropical olive orchards

The production of olive trees in shallow soils in hillslopes requires adequate soil and water conservation systems. In this context, soil use and management besides vegetation cover contributes to better soil quality and the sustainability of the olive cultivation system. Thus, the objective was to determine the physical and chemical properties that are indicative of soil quality in areas of Dystrudepts, under different soil cover management systems in olive cultivation in tropical region. Olive tree culture field experiment was established in 2015, where soil was sampled over three years (2016, 2017 and 2021). The effect of five cover management systems were compared for each agricultural year, namely BS: bare soil in 2015/2016, 2016/2017 and 2017/2021; OBS: olive trees on bare soil in 2015/2016, 2016/2017 and 2017/2021; OSV: olive trees intercropped with mowed spontaneous vegetation in 2015/2016, 2016/2017 and 2017/2021; OJB: olive trees intercropped with jack beans (Canavalia ensiformis L.) in 2015/2016 and 2016/2017, and olive trees crowned intercropped with mowed spontaneous vegetation in 2017/2021; OMI: olive trees intercropped with millet (Pennisetum glaucum L.) in 2015/2016, olive trees intercropped with sunn hemp (Crotalaria juncea L.) in 2015/2017 and olive trees with spontaneous vegetations treated with herbicide (Roundup) in 2017/2021. The soil quality indices (SQI) proved to be sensitive to variations in different types of soil management from the second year of assessment, regarding water erosion. OSV and OJB showed the highest SQI, while OBS showed the lowest values. The SQI demonstrated an inverse correlation with soil and water losses starting from the second year of assessment. This highlights its efficacy in assessment the effects of adopted management on soil quality in the context of water erosion. Moreover, it reinforces the importance of vegetation cover in maintaining soil fertility, resulting in increase of soil organic matter and improving the structuring of Dystrudepts. The attributes that most correlated with the SQI were soil organic matter, effective cation exchange capacity, sum of bases, geometric mean diameter of aggregates, pH and unsaturated soil hydraulic conductivity. These attributes collectively provide valuable insights into soil health and water erosion susceptibility within the olive tree production system.

A Simple Model for Estimating the Hydraulic Conductivity of Unsaturated Soil

Describing the hydraulic conductivity of unsaturated soil is very important in predicting water transport. Most current models have complex forms and generally need to be calibrated by the measured unsaturated hydraulic conductivity curve. A simple model, by which it is possible to conveniently predict the unsaturated hydraulic conductivity, is proposed in this study. The soil–water characteristic curve and hydraulic conductivity curve are separated into three parts. The soil–water characteristic curve is represented by Fredlund and Xing’s equation. A simple model composed of three lines is proposed for estimating the hydraulic conductivity of unsaturated soil. The model parameters can be conveniently calibrated from the measured soil–water characteristic curve and saturated hydraulic conductivity. Finally, the proposed model is validated by the experimental data from different soils. The proposed model provides a simple approach to estimating the hydraulic conductivity of unsaturated soil, which is more convenient for practical application.

Soil water retention and hydraulic conductivity measured in a wide saturation range

Abstract. Soil hydraulic properties (SHPs), particularly soil water retention capacity and hydraulic conductivity of unsaturated soils, are among the key properties that determine the hydrological functioning of terrestrial systems. Some large collections of SHPs, such as the UNSODA and HYPRES databases, have already existed for more than 2 decades. They have provided an essential basis for many studies related to the critical zone. Today, sample-based SHPs can be determined in a wider saturation range and with higher resolution by combining some recently developed laboratory methods. We provide 572 high-quality SHP data sets from undisturbed, mostly central European samples covering a wide range of soil texture, bulk density and organic carbon content. A consistent and rigorous quality filtering ensures that only trustworthy data sets are included. The data collection contains (i) SHP data, which consist of soil water retention and hydraulic conductivity data, determined by the evaporation method and supplemented by retention data obtained by the dewpoint method and saturated conductivity measurements; (ii) basic soil data, which consist of particle size distribution determined by sedimentation analysis and wet sieving, bulk density and organic carbon content; and (iii) metadata, which include the coordinates of the sampling locations. In addition, for each data set, we provide soil hydraulic parameters for the widely used van Genuchten–Mualem model and for the more advanced Peters–Durner–Iden model. The data were originally collected to develop and test SHP models and associated pedotransfer functions. However, we expect that they will be very valuable for various other purposes such as simulation studies or correlation analyses of different soil properties to study their causal relationships. The data are available at https://doi.org/10.5880/fidgeo.2023.012 (Hohenbrink et al., 2023).

Evaluating mechanism and inconsistencies in hydraulic conductivity of unsaturated soil using newly proposed biochar conductivity factor

In the past few decades, numerous studies have been conducted to promote the use of biochar as a soil amendment and most recently, for compacted geo-engineered soils. In general, the definite trends of biochar effects on water retention and fertility of soils have been confirmed. However, the biochar effects on hydraulic conductivity, particularly unsaturated hydraulic conductivity of soil-biochar mix remain unclear, making it difficult to understand water seepage in both agricultural and geo-engineered infrastructures in semi-arid regions. This study examines the unsaturated hydraulic conductivity function derived based on the measurements of soil water characteristic curves of soil with biochar contents of 0%, 5% and 10%. A new parameter “biochar conductivity factor (BCF)” is proposed to evaluate the inconsistency in reported biochar effects on soil hydraulic conductivity and to interpret it from various mechanisms (inter- and intra- pore space filling, cracking, aggregation, bio-film formation and piping/internal erosion). The impact of biochar content on unsaturated hydraulic conductivity appears to reduce as the soil becomes drier with minimal effect in residual zone. Qualitative comparison of near-saturated hydraulic conductivity with test results in the literature showed that the BCF is generally higher for smaller ratio of sand to fine content (clay and silt). Moreover, the particle size of biochar may have significant influence on soil permeability. Future scope of research has been highlighted with respect to biochar production for its applications in agriculture and geo-environmental engineering. Long term effects such as root decay and growth, aggregation and nutrient supply need to be considered.Graphical

Estimating Unsaturated Hydraulic Conductivity in Unsaturated Soil Using Soil Moisture Sensors

Unsaturated hydraulic conductivity (K) is a measurement of soil water-retaining ability in unsaturated soil. The unsaturated hydraulic properties are necessary to predict the movement of water in unsaturated soil. The current study aims to create an alternative experimental device to estimate theKusing soil moisturesensors. The experimental set up was designed and fabricated in order to measure the volumetric water content using GS3 (Campbell) sensors at different heights. The data obtained from the sensors was validated using mass of moist sand column measured from electronic balance.TheKwas estimated with the help of retention and conductivity functions. The primary materials used in the study were solute-free water and sand. It was found that the reading from the sensors was in good match with the reading of the volumetric water content calculated from the mass of water in the sand column with a root mean square error of 0.0091. Soil water retention curve and hydraulic conductivity were plotted using Peters-Durner-Iden (PDI) model.The results were compared to the van Genuchten function. TheKprediction wasinitially equivalentfor both models, but the lateroverpredicts the falling rate of hydraulic conductivity with the increasing soil matric suction before underpredict after 200 cm matric suction on a sandy soil.PDI function was able to extrapolate the hydraulic conductivity film flow region of sandy soil.

Wolf prize awarded to Rien van Genuchten

Martinus Th. (“Rien”) van Genuchten has been awarded the 2023 Wolf Prize in Agriculture. The Wolf Prize is awarded to “outstanding scientists and artists from around the world for achievements in the interest of humanity and friendly relations among people.” The prize is awarded in the disciplines of medicine, agriculture, mathematics, chemistry, and physics. For agricultural sciences, the Wolf Prize is widely considered one of the world's most prestigious recognitions. Rien van Genuchten was born in 1945 in Vught, The Netherlands. He received his early education at the Agricultural University of Wageningen, The Netherlands, and his PhD at New Mexico State University in the United States. He then spent most of his career at the US Salinity Laboratory in Riverside, CA, before moving to the University of Rio de Janeiro, Brazil, and to Utrecht University, The Netherlands. One of his most influential publications describes a closed-form equation for predicting the hydraulic conductivity of unsaturated soils, known as the van Genuchten equation (van Genuchten, 1980). He was, together with Jirka Simunek, the developer of the widely used HYDRUS software package for simulating the movement of water, heat, and solutes in variably saturated media (Šimůnek et al., 2016). Early in his career, he also published a compendium on analytical solutions of the one-dimensional convection–dispersion equation (van Genuchten & Alves, 1982), a resource widely used to analyse solute transport at the column and field scale. Rien van Genuchten was the founding editor of Vadose Zone Journal, when he, 21 years ago, had the insight to initiate a new, multi-disciplinary journal to promote research related to the vadose zone, the zone between the soil surface and the groundwater table. This was at a time when environmental research was still in its infancy, and the new journal provided a common outlet for agronomy, soil science, hydrology, hydrogeophysics, biogeochemistry, civil engineering, and the emerging environmental sciences. Rien van Genuchten's foresight paved the way for innovative research and helped elevate vadose zone science to an established and respected field of research. The Wolf Prize was awarded to Rien van Genuchten “for his ground-breaking work in understanding water flow and predicting contaminant transport in soils.” On behalf of the editorial board of Vadose Zone Journal, we congratulate and celebrate Rien van Genuchten and his receipt of the Wolf Prize. Venkataraman Lakshmi: Conceptualization; writing—review and editing. Markus Flury: Conceptualization; writing—original draft; writing—review and editing.

Soil Hydraulic Conductivity and Soil Water Retention of Inland Peat on Various Land Covers (Case Study: Natural Peat and Burnt Peat)

This research aims to examine differences in the rate of saturated soil hydraulic conductivity (permeability) and unsaturated soil hydraulic conductivity (infiltration) and soil water retention with various pressures in peatland with different land covers. The study was conducted from June to October 2021. The research was conducted at the LAHG-Sebangau and at the KHDTK-Tumbang Nusa, central Kalimantan, Indonesia. Based on the results of this study, at the HA site has the highest saturated soil hydraulic conductivity (permeability) rate at a depth of 0-10 cm was classified as rather fast. The lowest saturated soil hydraulic conductivity (permeability) rate of the HS site at a depth of 20-30 cm was classified as a rather slow. The highest unsaturated soil hydraulic conductivity (infiltration) rate was at the HS site at a depth of 0-10 cm was classified as fast, while the lowest was at the natural forest (HA) at a depth of 10-20 cm classified as very slow. The soil physical characteristics: Soil water content ​​ranging from 474.769-631.364%, fiber content ranging from 8.00-22.67%, bulk density ranging from 0.12-0.19 g cm-3, porosity ranging from 71.61-85.63% and the color of the soil is dark brown to very blackish red dark. Meanwhile, the ability of the soil to accommodate maximum soil water (saturated conditions) with the highest pF 0 is at the HK site at depth of 10-20 cm was 377,92 cm3 cm-3, while the lowest is in the HR site at depth of 0-0 cm is 31.11 cm3 cm-3. The ability of the soil in maximum soil water holding (water control conditions) at pF 4.2 is highest in the HS site at depth of 20-30 cm was 61.10 cm3 cm-3, while the lowest is in the HR site at depth of 0-10 cm was 13.30 cm3 cm-3. The soil porosity value in the HS site was higher than that of the HR site, which is 85,830 and 82,130%. The weight value of particle at the HS site was higher compared to other land cover ranging from 0.74-0.86 g cm-3. The weight value of bulk density at the HS site ranging from 0.17-0.19 g cm-3 was higher than other land cover.

Estimativa da umidade e condutividade hidráulica do solo superficial usando termografia por infravermelhos no semiárido brasileiro

ABSTRACT This study introduces an exploratory technique for soil moisture and unsaturated hydraulic conductivity estimation, using infrared thermography. The technique consists in applying a certain volume of water to the soil surface and registering the temperature with an infrared camera. The applied water should have a lower or higher temperature than the soil surface in order to create a temperature gradient at the soil surface. Experiments were conducted in the Mimoso catchment, which is located in the semiarid region of the Brazilian state of Pernambuco, in two soils: Yellow Argisol and Fluvic Neossol. Experiments were conducted at two distinct periods of the day: in the morning with cooler soil surface and in the afternoon with hooter soil surface. Different volumes of water were applied to the soil surface originating different soil moisture. Water was applied at two distinct temperatures of 10 – 15 and 80 – 85°C, according to the initial soil surface temperature. The technique allowed us to map estimated soil moisture with values obtained with infrared thermography. When compared with soil sampling, a correlation coefficient higher than 0.70 was observed. Based on the data obtained, it was possible to estimate the topsoil unsaturated soil hydraulic conductivity, relevant to the analysis of hydrological processes, such as infiltration.

Unsaturated Hydraulic Conductivity Estimation—A Case Study Modelling the Soil-Atmospheric Boundary Interaction

Pore water pressure changes due to soil-atmospheric boundary interaction can significantly influence soil behaviour and can negatively affect the safety and stability of geotechnical structures. For example, prolonged rainfall events can lead to increased pore water pressure and lower strength; repeated cycles of pore water pressure changes can lead to degradation of strength. These effects are likely to become more severe in the future due to climate change in many parts of the world. To analyse the behaviour of soil subjected to atmospheric boundary interactions, several parameters are needed, and hydraulic conductivity is one of the more important and is difficult to determine. Hydraulic conductivity deduced from laboratory tests are often different from those from the field tests, sometimes by orders of magnitude. The problem becomes even more complicated when the soil state is unsaturated, where the hydraulic conductivity varies with the soil’s state of saturation. In this paper, a relatively simple alternative approach is presented for the estimation of the hydraulic conductivity of unsaturated soils. The method involved a systematic re-analysis of observed pore water pressure response in the field. Using a finite element software, the soil-atmospheric boundary interaction and related saturated/unsaturated seepage of an instrumented slope have been analysed, and results are compared with field measurements. The numerical model could capture the development of suction, positive pore water pressure and changes in water content with reasonable accuracy and demonstrated the usefulness of the hydraulic conductivity estimation method discussed in this paper.

Experimental Investigation of the Different Polyacrylamide Dosages on Soil Water Movement under Brackish Water Infiltration.

The use of soil conditioners in conjunction with brackish water irrigation is critical for the efficient development and use of brackish water as well as the enhancement of the structure of saline soil and stimulating crop growth. This study investigated the effects of different polyacrylamide (PAM) dosages (0, 0.02%, 0.04%, and 0.06%) on the water flow properties of sandy loam during brackish water infiltration using one-dimensional vertical and horizontal soil column infiltration experiments. The results showed that: (1) PAM could lower the soil infiltration rate and increase soil water retention performance under brackish water infiltration conditions. (2) PAM had a significant effect on the parameters of the Philip and Kostiakov infiltration models. The soil sorption rate S and the empirical coefficient λ were the smallest, and the empirical index β was the largest when the PAM dosage was 0.04%. (3) PAM dosage displayed a quadratic polynomial connection with the soil saturated water content and the saturated hydraulic conductivity. The soil saturated water content was highest when the PAM dosage was 0.04%, the intake suction hd of the Brooks-Corey model increased by 15.30%, and the soil water holding capacity was greatly improved. (4) Soil treated with PAM could absorb more water under the same soil water suction, whereas the soil unsaturated hydraulic conductivity and its growth rate decreased. The soil saturated diffusion rate Ds, as well as the soil water diffusion threshold, rose. Finally, the 0.04% PAM dosage could improve soil hydrodynamic characteristics under brackish water infiltration, which is beneficial for the efficient utilization of brackish water.

A novel frequency-domain integrated sensor for in-situ estimating unsaturated soil hydraulic conductivity

Unsaturated soil hydraulic conductivity (Ku) plays a significant role in agricultural, hydrological and ecological applications. However, the existing measurement technologies are failed to consecutively measure soil hydraulic conductivity in-situ, high cost or destructive to soil structure. Here we designed a novel frequency-domain integrated sensor that is combined with Richards’ equation to in-situ estimate Ku. The novel sensor has three perforated cylinder coaxial (PCC-1, PCC-2, PCC-3) probes and a stainless steel pin to measure soil water potentials at up (PCC-1), middle (PCC-2) and bottom (PCC-3) layers and soil water content at middle layer, respectively. After sensor calibration, drying experiments were conducted with sandy loam and clay loam. The Richards’ equation was used to in-situ estimate dynamics of Ku combining finite difference method with measurements of the novel sensor under certain assumptions, and thus neither additional parameters nor boundary condition was required. In order to assess the accuracy of the in-situ estimated Ku, the actual Ku were determined in laboratory using van Genuchten and Campbell hydraulic models as references. Then, a field experiment was conducted at an experimental farm to test the performance of the novel integrated sensor. The results showed that the in-situ estimated Ku in unsaturated range had a good agreement with the model calculated Ku in laboratory. The RMSEs (in log10) were 0.707 cm h−1 (van Genuchten) and 0.426 cm h−1 (Campbell) for sandy loam and 0.749 cm h−1 (van Genuchten) and 0.587 cm h−1 (Campbell) for clay loam. In near-saturated range, the in-situ estimated Ku were somewhat underestimated in comparison with the model calculated Ku. This may attribute to the limitation of accuracy of soil water potential sensor (TEROS 21) used for calibration, the insensitivity of drying method to the estimation of Ku or the effect of structural macropores or discontinuity of soil intrinsic hydraulic characteristics in near-saturated range. The overall trend of in-situ estimated Ku in the field is similar to that in laboratory. The consistent between the in-situ estimated and model calculated Ku in laboratory verified the availability and feasibility of the novel integrated sensor and the estimation of Ku in the field was achieved. In addition, the novel integrated sensor is low cost and reduces sensor-to-sensor uncertainties when multiple sensors were used, which has distinct advantages and potential application under field conditions.

Effect on Soil Hydrodynamic Parameters in Fields Mulched with Gravel for Different Periods of Time

Soil-water characteristic curve (SWCC) and soil hydraulic conductivity are important soil hydrodynamic parameters, which are of great significance in production practice. In 128 soil samples collected from 0-20, 20-40 cm layers at sampling scales of 32×32 m, the authors determined the effect of different periods of time on SWCC and soil hydraulic conductivity in fields mulched gravel. The water holding capacity of soil changes dramatically throughout time, with NEW having the maximum water holding capacity. As the planting time increases, the soil water holding capacity decreases. Both van Genuchten (VG) and Gardner models can fit SWCC in different time periods well, but the fitting accuracy of the VG model is higher. Geostatistics and Kriging interpolation are used to study the spatial variability of the VG model parameters of the SWCCs. The parameters θs, α, and n show a strong spatial correlation as a whole and are slightly affected by random factors. The changing trend of soil unsaturated hydraulic conductivity (K(θ)) in gravel-mulched fields is faster than the CHECK, and the NEW has the fastest change trend. The data suggest that fields mulched with gravel retain more water, with the NEW having the most noticeable water retention effect. The study’s findings can be used to investigate the regional variations in soil hydrodynamic characteristics in gravel mulched fields in arid locations.

Estimation of the hydraulic conductivity of unsaturated soil incorporating the film flow

In the conventional method for the estimation of the hydraulic conductivity of unsaturated soil, only the capillary water is considered in the estimation. However, it is known that a water film can be formed around soil particles due to the adsorptive force. The water may migrate in the soil through the water film. As a result, both water flow in the capillary water and water film should be considered in the estimation. In this paper, a new equation was proposed for the estimation of the hydraulic conductivity of unsaturated soil by considering the film flow. The proposed method was verified with the experimental data from the literature. It is recommended that both capillary water flow and film flow should be considered in the estimation of the hydraulic conductivity of unsaturated soil.

DIMENSIONAL ANALYSIS TO ESTIMATE THE UNSATURATED HYDRAULIC CONDUCTIVITY OF A SANDY LOAM SOIL IN THE AGRICULTURAL FIELD

Hydraulic conductivity in unsaturated soil controls water movement and measuring it in agricultural fields is a challenging task requiring time-consuming, costly, and skilled experimentation. This study was conducted to reduce the cost of experimentation through the development of an estimation model. The developed model is based on dimensional analysis to determine the value of hydraulic conductivity of unsaturated soil as it relates to soil moisture content, irrigation water electrical conductivity, and suction rate (pressure head). Data points were acquired from measurements of cumulative infiltration in the field, using a mini disc infiltrometer. The developed model gave a mean discrepancy ratio of 1.10 (the acceptable range is 0.5–2.0) and a mean percentage of relative errors of 9.96%. These values indicate that the dimensional analysis model is reliable for the prediction of sandy loam soil’s unsaturated hydraulic conductivity.

Theoretical method for the estimation of vapour conductivity for unsaturated soil

The liquid water movement is commonly considered in the seepage analysis related to the unsaturated soil (e.g, residual soil above the ground water table). The hydraulic conductivity of unsaturated soil is commonly considered to decrease with the increase in soil suction (or decrease in the degree of saturation). As a result, the estimated hydraulic conductivity of unsaturated soil from the conventional method is very low in the high suction range (i.e., beyond the residual suction). However, the experimental results from recent studies indicate that in the high suction zone, the hydraulic conductivity of unsaturated soil may increase with an increase in soil suction. In addition, recent studies also indicate that in the high suction zone (i.e., greater than 3100 kPa), water in soil moves mainly in the vapour form. As a result, the estimation of the hydraulic conductivity of unsaturated soil due to vapour flow in soil is crucial for engineers in the estimation of water transmission through a relative dry soil. In this note, the theoretical method is proposed for the estimation of vapour conductivity from the soil-water characteristic curve. The proposed method is rational and it has the theoretical basis without any empirical parameter. It is observed that the estimated results from the proposed method agree well with the results reported in the published literature.

An improved method for measuring wetting front advancing velocity

AbstractThe unsaturated soil hydraulic conductivity function is one of the keys to solving unsaturated seepage issues. The wetting front advancing method (WFAM) is a proposed method for quickly measuring the coefficient of permeability of the unsaturated soils, but it has some demerits and limitations. In this study, a series of capillary rise tests were conducted and the coefficient of permeability of six kinds of unsaturated soils were obtained by using the WFAM. Based on the analyzing of characteristic changes in volumetric water content via soil moisture sensors in the capillary rise test, improvements were made to the method of determining the wetting front advancing velocity. The RMSE evaluation index was used to analyze the deviation of the data obtained by the improved method from the original visual method. The RMSE of the wetting front advancing velocity ranged from 0.0007 to 0.0074 cm s–1, and the RMSE of the coefficient of permeability ranged from 0 to 5.2 × 10–6 m s–1. The results indicated the applicability of the improved method.

Unsaturated Soil Hydraulic Conductivity (Kₕ) and Soil Resistance under Different Land Uses of a Small Upstream Watershed in Mt. Banahaw de Lucban, Philippines

Soil hydraulic conductivity influences hydrologic processes and the ability of watersheds to provide ecosystem services. Like most soil properties, however, it is highly spatially variable at different scales due to biophysical and anthropogenic factors. To quantify spatial variability, the study assessed the influence of land use/land cover (LULC) on soil hydraulic conductivity and compaction, as reflected by soil resistance in a small upstream watershed. Compaction was measured using a soil compaction tester and the unsaturated soil hydraulic conductivity (Kh) was estimated in the field using the inverse auger hole method. Measurements were made across six LULC: agriculture (Agri), coconut with agricultural crops (Coco + Agri), coconut with pasture (Coco + Grass), coconut with forest (Coco + Forest), reforestation area (Refo), and forest (Forest). Measurements were taken at 0–30 cm and 0–50 cm soil depths. Results showed that soil hydraulic conductivity and resistance significantly differed across LULC and soil depths. Soil resistance averages 0.83 MPa for all LULC at 0–30 cm depth, which was significantly lower (p = 1.184e–04) than 0–50 cm depth. Coco + forest recorded the highest soil resistance (1.01 MPa at 0–30 cm and 1.82 MPa at 0–50 cm), while Coco + Agri has the lowest resistance. Pairwise comparison of means also revealed that Forest at 0–30 cm depth had significantly higher (p < 0.01) Kh at 1.18 cm min–1 compared with other LULC across depths. Kh at 0–30 cm depth averages 0.57 cm min–1 for all LULC, which is significantly higher (p = 3.229e–02) than 0–50 cm depth. This indicated a decreasing hydraulic conductivity with increasing soil depth. This implies that strategies for promoting groundwater recharge and sustaining freshwater supply for lowland communities, in general, should be founded on LULC decisions, especially in upstream watersheds.

Observation and simulation of water movement and runoff in a coarse texture water repellent soil

Water repellent or non-wetting soils show poor water infiltration, which leads to an increased runoff in both natural and agricultural setting. Some forest areas of northern Iran are characterized by extreme water repellency, which makes their soil hydrological response highly complex. A field experiment was carried out to study the effect of water repellency on water movement and runoff in coarse-textured soil covered in Pinus teada in northwestern Iran. An adapted version of the popular agrohydrological model SWAP (Soil, Water, Atmosphere and Plant) has been used to evaluate the modelling of soil water flow and runoff generation. Water repellency was accounted for by multiplying the soil water content and the unsaturated hydraulic conductivity of soil with the F factor. This factor is equal to the volumetric fraction of soil occupied by preferential flow paths. Twenty-seven rainfall experiments were carried out under artificial rainfall using soils with various wettability levels and various rainfall intensities (14 mm/h, 27 mm/h and 34 mm/h). The result show that during the summer and early autumn (May to October) the soil became extreme repellent and runoff was formed when the soils were dry (θ<2%) and the rain intensity was more than 27 mm/h. Also, we investigated the role of the repellent layer on evaporation losses. The results of laboratory and simulation data confirm that water repellency reduces evaporation from the soil surface. Comparing modelling results with field data shows that an assumption of uniform flow in water repellent soil leads to an overestimation of soil water content and an underestimation of runoff. After incorporating water repellency in SWAP with the mobile–immobile concept, the agreement between measured and model simulation data considerably improved.

Assessing response times of an alluvial aquifer experiencing seasonally variable meteorological inputs

This study aims to assess the spatiotemporal distribution of the response times of an alluvial aquifer experiencing seasonally variable meteorological inputs and to investigate the aquifer characteristics that influence its response times. The sliding cross-correlogram approach was used in this study to correlate the amount of water infiltrating the subsurface with changes in water table levels that were monitored at six piezometers. The results showed a significant variation of aquifer response times over the study area; this finding indicates that the hydraulic behavior of an aquifer may differ from one location to another within the same aquifer. Aquifer response times were found to be influenced by the variations of the vadose zone thickness. These variations, however, were not observed to be a significant factor controlling the spatiotemporal changes in the aquifer response times. Instead, it was the intensity of the vertical inflow events and the local recharge rate that were found to exert an impact on the spatiotemporal variation of the aquifer response times. It was assumed that large amounts of vertical inflow increase the soil's volumetric water content above the water table, leading to higher values of hydraulic conductivity of unsaturated soils, thus contributing to shortening aquifer response times. A better understanding of the effects of aquifer characteristics on aquifer response times is useful for groundwater resource management.