Estimating the saturated hydraulic conductivity in a spatially variable soil with different permeameters: a stochastic Kozeny–Carman relation

The spatial variability of the saturated hydraulic conductivity (K_s) of a greenhouse banana plantation volcanic soil was investigated with three different permeameters: (a) the Philip-Dunne field permeameter, an easy to implement and low cost device; (b) the Guelph field permeameter; (c) the constant head laboratory permeameter. K_s was measured on a 14×5 array of 2.5 m×5 m rectangles at 0.15 m depth using the above three methods. K_s differences obtained with the different permeameters are explained in terms of flow dimensionality and elementary volume explored by the three methods. A sinusoidal spatial variation of K_s was coincident with the underlying alignment of banana plants on the field. This was explained in terms of soil disturbances, such as soil compaction, originated by management practices and tillage. Soil salinity showed some coincidence in space with the hydraulic conductivity, because of the irrigation system distribution, but a causal relationship between the two is however difficult to support. To discard the possibility of an artefact, the original 70 point mesh was doubled by intercalation of a second 14×5 grid, such that the laboratory K_s was finally determined on a 140 points 2.5 m×2.5 m square grid. Far from diluting such anisotropy this was further strengthened after inclusion of the new 70 points. The porosity (φ) determined on the same laboratory cores shows a similar sigmoid trend, thus pointing towards a plausible explanation for such variability. A power-law relationship was found between saturated hydraulic conductivity and porosity, K_sφⁿ (r²=0.38), as stated by the Kozeny–Carman relation. A statistical reformulation of the Kozeny–Carman relation is proposed that both improved its predictability potential and allows comparisons between different representative volumes, or K_s data sets with different origin. Although the two-field methods: Guelph and Philip-Dunne, also follow a similar alignment trend, this is not so evident, suggesting that additional factors affect K_s measured in the field. Finally, geostatistical techniques such as cross correlograms estimation are used to further investigate this spatial dependence.     

Using the simplified falling head technique to detect temporal changes in field-saturated hydraulic conductivity at the surface of a sandy loam soil

Determining temporal changes in field-saturated hydraulic conductivity (K_fs) is important for understanding and modeling hydrological phenomena at the field scale. Little is known about temporal variability of K_fs values measured at permanent sampling points. In this investigation, the simplified falling head (SFH) technique was used for an approximately 2-year period to determine temporal changes in K_fs at 11 permanent sampling points established at the surface of a sandy loam soil. Additional K_fs measurements were obtained by the single-ring pressure infiltrometer (PI) technique to also compare the SFH and PI techniques. The lowest mean values of K_fs, M(K_fs), were detected in December and January (20.5 ≤ M(K_fs) ≤ 146.2 mm h⁻¹), whereas higher results (190.5 ≤ M(K_fs) ≤ 951.9 mm h⁻¹) were obtained in the other months of the year. The K_fs values were higher and less variable in the dry soil (θ_i ≤ 0.21 m³ m⁻³, M(K_fs) = 340.6 mm h⁻¹, CV(K_fs) = 106%) than in the wet one (θ_i > 0.21 m³ m⁻³, M(K_fs) = 78.4 mm h⁻¹, CV(K_fs) = 185%). Both wet and dry soil were less conductive at the end of the study period than at the beginning one but a more appreciable change was detected for the dry soil (K_fs decreasing by 83.4%) than for the wet one (K_fs decreasing by 63.0%). The simple SFH technique yielded K_fs results similar to the more laborious and time-consuming PI technique (i.e., mean values differing at the most by a factor of two). It was concluded that (i) the soil water content was an important factor affecting the K_fs results obtained in a relatively coarse-textured soil, (ii) the impact of time from the beginning of the experiment on the saturated hydraulic conductivity was larger for a repeated sampling of dry soil than of wet soil and (iii) the SFH technique yielded reliable K_fs results in a relatively short period of time without the need for extensive instrumentation or analytical methodology.     

A multi-step inflow method for estimating hydraulic conductivity at low pressure under the wetting process

The accurate determination of the wetting soil hydraulic properties for a wide range of water contents is essential for studying and predicting infiltration processes. We present a laboratory infiltration method for determining hydraulic conductivity function, K(θ), in the low-to-medium water content range. An initially air-dry soil core is subjected to infiltration from the bottom where the pressure head, ψ_bot, is controlled through a membrane. As soon as the wetting front arrives at the soil surface, the top 0.5-cm layer is sliced for measurement of the water content. The ψ_bot is stepwise increased as the hydraulic equilibrium is nearly attained at each step. The wetting water retention function, ψ(θ), is determined by curve-fitting the equilibrium inflow data and from the independently measured data obtained from vapor equilibrium. The parameter in the K(θ) is estimated inversely using the cumulative inflow and water content of the sliced layer. This method is verified through comparisons with K(θ) obtained by the Boltzmann transform method. Although requiring an additional operation, the slicing procedure is found to be valuable in enhancing the reliability of the optimized parameter. A sensitivity analysis shows that water vapor movement would be negligible under our experimental conditions.     

Soil macropore dynamics affected by tillage and irrigation for a silty loam alluvial soil in southern Portugal

Soil tillage can have a significant effect on soil porosity and water infiltration. This study reports field measurements of near saturated hydraulic conductivity in an undisturbed soil under two tillage treatments, conventional tillage (CT) and minimum tillage (MT). The objective was to determine effective macro and mesoporosities, porosity dynamics during the irrigation season, and their contribution to water flow. Field observations were performed during the 1998 maize (Zea mays L.) cropping season in an Eutric Fluvisol with a silty loam texture, located in the Sorraia River Watershed in the south of Portugal. Infiltration measurements were done with a tension infiltrometer. At each location an infiltration sequence was performed corresponding to water tensions (φ) of 0, 3, 6 and 15 cm. Five sets of infiltration measurements were taken in both treatments in the top soil layer between May and September. One set of measurements was done at the depth of 30 cm at the bottom of the plowed layer in the CT plot. After 5 years of continuous tillage treatments the results show that regardless of the tillage treatment, saturated conductivity values K(φ₀) were several times larger than near saturation conductivity K(φ₃). This indicates that subsurface networks of water conducting soil pores can exist in both CT and MT maize production systems. In CT, the moldboard plow created macro and mesoporosity in the top soil layer while breaking pore continuity at 30 cm depth. This porosity was partially disrupted by the first irrigation, resulting in a significant decrease of 45% in the macropore contribution to flow. Later in the season, the irrigation effect was overlaid by the root development effect creating new channels or continuity between existing pores. In MT macroporosity contribution to flow did not show significant differences in time, representing 85% of the total flow. In both the treatments, macropores were the main contributing pores to the total flow, in spite of the very low macroporosity volumes.     

Amelioration of a hardsetting Alfisol through deep mouldboard ploughing, gypsum application and double cropping. II. Soil-water relations

Deep mouldboard ploughing to a depth of 0.45 m, gypsum (5 t ha⁻¹) and double cropping wheat (Triticum aestivum) with a summer forage crop were applied individually, and in combination, to modify a hardsetting red-brown earth (Alfisol) used for flood-irrigated wheat production. The effects of these treatments upon water infiltration, storage and intake during irrigations, application efficiency, and depletion between irrigations were measured. Gypsum markedly increased saturated hydraulic conductivity (K_s) immediately after being applied in June 1984 but not in 1985. Mouldboard ploughing and double cropping had little effect on K_s. All K_s values were less than 0.5 m day⁻¹. Plant available water content (PAWC) was increased by gypsum application and mouldboard ploughing after the initial irrigation in 1984, and by the three main treatments in 1985. The order in which the main treatments increased PAWC was: gypsum>mouldboard ploughing>double cropping. Water applied and water intake during irrigations were increased by mouldboard ploughing and gypsum during the first irrigation in 1984 by as much as 60 mm. Thereafter mouldboard ploughing, gypsum and double cropping had little effect on water intake, and only gypsum increased the amount of water applied in 1985. The ratio of water intake to water applied during irrigations (application efficiency) was increased by mouldboard ploughing to 59% in 1984; no treatment had values greater than 40% during 1985. Water depletion was significantly increased by mouldboard ploughing in 1984 and by gypsum and double cropping in 1985. Few significant interactions were found between the three main treatments, each of which would improve soil-water relations for crop growth, although the effect of mouldboard ploughing on soil-water relations declined after the first year. Despite substantial improvements after 2 years in some treatments, soil hydraulic properties were inadequate for optimal irrigated cropping.     

Soil quality indicators under continuous cropping systems in the Argentinean Pampas

Over the last two decades, soil cultivation practices in the southern Argentinean Pampas have been changing from a 7 year cash-crop production system alternated with 2–3 years under pasture, to a continuous cropping system. A better understanding of the impact of the period of time a field has been under continuous cropping on a broad spectrum of soil properties related to soil quality is needed to target for sustainable cropping systems. The objectives of this study were to: (i) assess the relationship between physical and chemical soil parameters related to soil quality and (ii) identify soil quality indicators sensitive to soil changes under continuous cropping systems in the Argentinean Pampas.

Correlation analysis of the 29 soil attributes representing soil physical and chemical properties (independent variables) and years of continuous cropping (dependent variable) resulted in a significant correlation (p < 0.05) in 78 of the 420 soil attribute pairs. We detected a clear relationship between hydraulic conductivity at tension h (K_h) and structural porosity (ρ_e); ρ_e being a simple tool for monitoring soil hydraulic conditions.

Soil tillage practice (till or no-till) affected most of the soil parameters measured in our study. It was not possible to find only one indicator related to the years under continuous cropping regardless of the cultivation practice. We observed a significant relationship between years under continuous cropping and K_h under no-till (NT) and wheat fallow (p < 0.001, R² = 0.70). Under these conditions, K₋₄₀ diminished as the number of years under continuous cropping increased.

The change in mean weight diameter (CMWD) was the only physical parameter related to the number of years under continuous cropping, explaining 36% of the variability in the number of years under continuous cropping (p < 0.001) The combination of three soil quality indicators (CMWD, partial R² = 0.38; slope of the soil water retention curve at its inflexion point (S), partial R² = 0.14 and cation exchange capacity (CEC), partial R² = 0.13) was able to explain, in part, the years under continuous cropping (R² = 0.65; p value > 0.001), a measure related to soil quality.     

Different responses in bulk density and saturated hydraulic conductivity to soil deformation by logging machinery on a Ferralsol under native forest

Ferralsols have high structural stability, although structural degradation has been observed to result from forest to tillage or pasture conversion. An experimental series of forest skidder passes in an east Amazonian natural forest was performed for testing the effects of mechanical stress during selective logging operations on a clay‐rich Ferralsol under both dry and wet soil conditions. Distinct ruts formed up to 25 cm depth only under wet conditions. After nine passes the initially very low surface bulk density of between 0.69 and 0.80 g cm^?3 increased to 1.05 g cm^?3 in the wet soil and 0.92 g cm^?3 in the dry soil. Saturated hydraulic conductivities, initially >250 mm h^?1, declined to a minimum of around 10 mm h^?1 in the wet soil after the first pass, and in the dry soil more gradually after nine passes. The contrasting response of bulk density and saturated hydraulic conductivity is explained by exposure of subsoil material at the base of the ruts where macrostructure rapidly deteriorated under wet conditions. We attribute the resultant moderately high hydraulic conductivities to the formation of stable microaggregates with fine sand to coarse silt textures. We conclude that the topsoil macrostructure of Ferralsols is subject to similar deterioration to that of Luvisols in temperate zones. The stable microstructure prevents marked compaction and decrease in hydraulic conductivity under wetter and more plastic soil conditions. However, typical tropical storms may regularly exceed the infiltration capacity of the deformed soils. In the deeper ruts water may concentrate and cause surface run‐off, even in gently sloping areas. To avoid soil erosion, logging operations in sloping areas should therefore be restricted to dry soil conditions when rut formation is minimal.     

Effect of soil compaction on hydraulic properties of two loess soils in China

Soil compaction affects hydraulic properties, and thus can lead to soil degradation and other adverse effects on environmental quality. This study evaluates the effects of three levels of compaction on the hydraulic properties of two silty loam soils from the Loess Plateau, China. Undisturbed soil cores were collected from the surface (0–5 cm) and subsurface (10–15 cm) layers at sites in Mizhi and Heyang in Shaanxi Province. The three levels of soil compaction were set by increasing soil bulk density by 0% (C0), 10% (C1) and 20% (C2) through compression and hammering in the laboratory. Soil water retention curves were then determined, and both saturated hydraulic conductivity (K_s) and unsaturated hydraulic conductivity were estimated for all of the samples using standard suction apparatus, a constant head method and the hot-air method, respectively. The high level of compaction (C2) significantly changed the water retention curves of both the surface and subsurface layers of the Heyang soil, and both levels of compaction (C1 and C2) changed the curves of the two layers from the Mizhi site. However, the effects of compaction on the two soils were only pronounced below water tensions of 100 kPa. Saturated hydraulic conductivities (K_s) were significantly reduced by the highest compaction level for both sampled layers of the Heyang soil, but no difference was observed in this respect between the C0 and C1 treatments. K_s values decreased with increasing soil compaction for both layers of the Mizhi soil. Unsaturated hydraulic conductivities were not affected by soil compaction levels in the measured water volume ratio range, and the values obtained were two to five orders of magnitude higher for the Mizhi soil than for the Heyang soil. The results indicate that soil compaction could strongly influence, in different ways, the hydraulic properties of the two soils.     

Dynamics of soil hydraulic properties during fallow as affected by tillage

There is limited information on the effects of tillage practices on soil hydraulic properties, especially changes with time. The objective of this study was to evaluate on a long-term field experiment the influence of conventional tillage (CT), reduced tillage (RT) and no-tillage (NT) on the dynamics of soil hydraulic properties over 3 consecutive 16–18 month fallow periods. Surface measurements of soil dry bulk density (ρ_b), soil hydraulic conductivity (K(ψ)) at −14, −4, −1 and 0 cm pressure heads using a tension disc infiltrometer, and derived hydraulic parameters (pore size, number of pores per unit of area and water-transmission porosity) calculated using the Poiseuille's Law were taken on four different dates over the fallow period, namely, before and immediately after primary tillage, after post-tillage rains and at the end of fallow. Under consolidated structured soil conditions, NT plots presented the most compacted topsoil layer when compared with CT and RT. Soil hydraulic conductivity under NT was, for the entire range of pressure head applied, significantly lower (P < 0.05) than that measured for CT and RT. However, NT showed the largest mean macropore size (0.99, 0.95 and 2.08 mm for CT, RT and NT, respectively; P < 0.05) but the significantly lowest number of water-conducting pores per unit area (74.1, 118.5 and 1.4 macropores per m² for CT, RT and NT, respectively; P < 0.05). Overall, water flow was mainly regulated by macropores even though they represented a small fraction of total soil porosity. No significant differences in hydraulic properties were found between CT and RT. In the short term, tillage operations significantly increased K (P < 0.05) for the entire range of pressure head applied, which was likely a result of an increase in water-conducting mesopores despite a decrease in estimated mesopore diameter. Soil reconsolidation following post-tillage rains reduced K at a rate that increased with the intensity of the rainfall events.     

Effect of tillage and crop rotations on pore size distribution and soil hydraulic conductivity in sandy clay loam soil of the Indian Himalayas

Tillage management can affect crop growth by altering the pore size distribution, pore geometry and hydraulic properties of soil. In the present communication, the effect of different tillage management viz., conventional tillage (CT), minimum tillage (MT) and zero-tillage (ZT) and different crop rotations viz. [(soybean–wheat (S–W), soybean–lentil (S–L) and soybean–pea (S–P)] on pore size distribution and soil hydraulic conductivities [saturated hydraulic conductivity (K_sat) and unsaturated hydraulic conductivity {k(h)}] of a sandy clay loam soil was studied after 4 years prior to the experiment. Soil cores were collected after 4 year of the experiment at an interval of 75 mm up to 300 mm soil depth for measuring soil bulk density, soil water retention constant (b), pore size distribution, K_sat and k(h). Nine pressure levels (from 2 to 1500 kPa) were used to calculate pore size distribution and k(h). It was observed that b values at all the studied soil depths were higher under ZT than those observed under CT irrespective of the crop rotations. The values of soil bulk density observed under ZT were higher in 0–75 mm soil depth in all the crop rotations. But, among the crop rotations, soils under S–P and S–L rotations showed relatively lower bulk density values than S–W rotation. Average values of the volume fraction of total porosity with pores <7.5 μm in diameter (effective pores for retaining plant available water) were 0.557, 0.636 and 0.628 m³ m⁻³ under CT, MT and ZT; and 0.592, 0.610 and 0.626 m³ m⁻³ under S–W, S–L and S–P, respectively. In contrast, the average values of the volume fraction of total porosity with pores >150 μm in diameter (pores draining freely with gravity) were 0.124, 0.096 and 0.095 m³ m⁻³ under CT, MT and ZT; and 0.110, 0.104 and 0.101 m³ m⁻³ under S–W, S–L and S–P, respectively. Saturated hydraulic conductivity values in all the studied soil depths were significantly greater under ZT than those under CT (range from 300 to 344 mm day⁻¹). The observed k(h) values at 0–75 mm soil depth under ZT were significantly higher than those computed under CT at all the suction levels, except at −10, −100 and −400 kPa suction. Among the crop rotations, S–P rotation recorded significantly higher k(h) values than those under S–W and S–L rotations up to −40 kPa suction. The interaction effects of tillage and crop rotations affecting the k(h) values were found significant at all the soil water suctions. Both S–L and S–P rotations resulted in better soil water retention and transmission properties under ZT.