Soil Water Desorption Research Articles

An adapted Weibull function for agricultural applications

The Weibull function is applied extensively in the life sciences and engineering but underused in agriculture. The function was consequently adapted to include parameters and metrics that increase its utility for characterizing agricultural processes. The parameters included initial and final dependent variables (Y0 and YF, respectively), initial independent variable (x0), a scale constant (k), and a shape constant (c). The primary metrics included mode, integral average, domain, skewness, and kurtosis. Nested within the Weibull function are the Mitscherlich and Rayleigh functions where c is fixed at 1 and 2, respectively. At least one of the three models provided an excellent fit to six example agricultural datasets, as evidenced by large adjusted coefficient of determination (RA2 ≥ 0.9266), small normalized mean bias error (MBEN ≤ 1.49%), and small normalized standard error of regression (SERN ≤ 8.08%). The Mitscherlich function provided the most probable (PX) representation of corn (Zea mays L.) yield (PM = 87.2%); Rayleigh was most probable for soil organic carbon depth profile (PR = 96.4%); and Weibull was most probable for corn seedling emergence (PW = 100%), nitrous oxide emissions (PW = 100%), nitrogen mineralization (PW = 58.4%), and soil water desorption (PW = 100%). The Weibull fit to the desorption data was also equivalent to those of the well-established van Genuchten and Groenevelt–Grant desorption models. It was concluded that the adapted Weibull function has good potential for widespread and informative application to agricultural data and processes.

Moisture-Dependent Wettability of Artificial Hydrophobic Soils and Its Relevance for Soil Water Desorption Curves

Soil wettability is strongly affected by the soil water content. In turn, wettability influences water distribution in soil pores and thereby soil water retention characteristics. In this study, soil water desorption curves (SWDC) for naturally occurring hydrophilic sand, loam, and silt loam samples and for corresponding artificially hydrophobized sand, loam, and silt loam samples were measured in the pF range from 0 to 4.2. Moisture-dependent wettability was described with the contact angle (CA) method and the water drop penetration time (WDPT) test. One objective was to directly measure the phenomenological CA of the soils at different water contents and to compare the results with that from the WDPT test. A second objective was to show differences of soil water desorption curves for the hydrophilic materials as compared to the silanized hydrophobic materials. All experiments were done on soil samples with time-stable water repellency to study directly the impact of water content without further time dependent factors. Soil wettability as a function of water content was generally comparable, indicating that CA and WDPT could be used to indicate the moisture-dependent wettability. The SWDCs were separated into three domains. Domain I was characterized by the Wenzel wetting regime, and Domains II and III were described by the Cassie–Baxter wetting regime. Water repellency showed insignificant impacts on the SWDCs in Domains I and III, but it reduced soil water retention capacity in Domain II, which was supported by the monotonical increase of CA from zero to a maximum as soil water content decreased to a critical point near the permanent wilting point. Water repellency had clear and important influences on soil water desorption curves.

Fractal Analysis of Soil Water Desorption Data Collected on Disturbed Samples with Water Activity Meters

We combined water activity meters and fractal modeling to facilitate rapid, physically based characterization of the soil water retention curve. Desorption data (6 points per sample) were collected in the tension (h) range 2.0 × 102 to 1.5 × 105 kPa using the gravimetric method to measure water content (w) and water activity meters to measure h Thirty‐two disturbed samples from a long‐term nitrogen fertilization and tillage comparison study on a silt loam soil were analyzed. A new form of an established fractal equation was derived: w = ahD−3 − ρw/ρs, where a is a compound parameter including the bulk density (ρb) and air entry tension (ha), D is the mass fractal dimension, ρw is the density of water, and ρs is the particle density. This model was fitted to the measured water retention curves by nonlinear regression analysis. The a and D parameters were estimated, while ρw and ρs were fixed at 1.00 and 2.65 Mg m−3, respectively. Convergence was always achieved and the equation fitted the data extremely well; residual sums of squares ranged from 1.2 × 10−6 to 6.7 × 10−5, with a median value of 2.2 × 10−5 Estimates of a (0.62–0.74) and D (2.948–2.963) were physically reasonable, and sensitive to soil management practices. The a parameter increased, r = 0.80 (P < 0.01), (signifying decreasing ρb and/or ha) whereas D decreased, r = −0.75 (P < 0.01), (signifying more rapid capillary drainage) with increasing soil carbon content. Additional research is needed to test this approach on other soil types, and to assess the influence of soil disturbance and variations in ρs on the model's performance.

Field compaction at different soil-water status: effects on pore size distribution and soil water characteristics of a Rhodic Ferralsol in Western Cuba

The level of compaction induced on cultivated fields through trafficking is strongly influenced by the prevailing soil-water status and, depending on the attendant soil degradation, vital soil hydraulic processes could be affected. Therefore, understanding the relationship between field soil-water status and the corresponding level of induced compaction for a given load is considered an imperative step toward a better control of the occurrence of traffic-induced field soil compaction. Pore size distribution, a fundamental and highly degradable soil property, was measured in a Rhodic Ferralsol, the most productive and extensively distributed soil in Western Cuba, to study the effects of three levels of soil compaction on soil water characteristic parameters. Soil bulk density and cone penetration index were used to measure compaction levels established by seven passes of a 10 Mg tractor at three soil-water statuses corresponding to the plastic (Fs), friable (Fc) and relatively dry soil (Ds) consistency states. Pore size distribution calculated from soil water characteristic curves was classified into three pore size categories on the basis of their hydraulic functioning: >50 μm ( f >50 μm ), 50–0.5 μm ( f 50–0.5 μm ) and <0.5 μm ( f <0.5 μm ). The greatest compaction levels were attained in the Fs and Fc soil water treatments, and a significant contribution to compaction was attributed to the existing soil water states under which the soil compaction was accomplished. Average cone index (CI) values in the range of 2.93–3.70 MPa reflected the accumulation of f <0.5 μm pores, and incurred severe reductions in the volume of f >50 μm pores in the Fs and Fc treatments, while an average CI value of 1.69 MPa indicated increments in the volume of f 50–0.5 μm in the Ds treatment. Despite the differential effects of soil compaction on the distribution of the different pore size categories, soil total porosity ( f Total) was not effective in reflecting treatment effects. Soil water desorption at the soil water potentials evaluated (0.0 to −15,000 cm H 2O) was adversely affected in the f <0.5 μm dominated treatments; strong soil water retention was observed with the predominance of f <0.5 μm , as was confirmed by the high water content at plant wilting point. Based on these findings, the use of field capacity water content as the upper limit of plant available soil water was therefore considered inappropriate for compacted soils.

Comparing observed and simulated water storage during drainage to select hydraulic parameters for volcanic soils

In soil-water modelling, hydraulic parameters must be chosen carefully to simulate water fluxes accurately. This study aimed to parameterise the Soil Water Infiiltration and Movement (SWIM) model for sandy volcanic soils that are irrigated with wastewater from Rotorua, New Zealand. Three plots 1·5 by 1·5 m were dyked, flooded until soils approached saturation, and then covered to prevent evaporation. Water-content measurements were taken to 1 m depth during 20 days of drainage. For each profile, this drainage process was simulated using 9 combinations of hydraulic conductivity [K(θ)] and soil water desorption θ(ψ) parameters. The θ(ψ) parameters were fitted with different ranges of desorption data, using Campbell’s approach. K(θ) parameters were obtained from flow measurements using soil cores or disc permeameters, through use of Campbell’s method, and by applying a unit-gradient analysis to the drainage data. Model output was compared with measured water contents graphically and statistically. Several combinations of parameters provided good agreement between simulated and observed data, but no single combination gave the best prediction for all 3 profiles. Campbell’s approach to determine the slope of K(θ) function provided excellent agreement for 2 of the profiles. In the third profile, a nearly steady water content during drainage at 1 m depth was closely simulated using parameters from the unit gradient approach. Total measured drainage ranged between 8·6 and 15·9 cm amongst the 3 profiles. Results provide 3 parameterised profiles for modelling water movement in these soils, which receive &gt;4000 mm of rainfall and irrigation annually.

Rubberized Asphalt for Sealing Cores of Shrinking Soil

AbstractBecause many soil materials shrink as a result of soil water desorption, it is difficult to seal soil samples into containers for transport experiments when a range of soil matric potentials is to be used. In this note, we introduce the use of a rubberized asphalt product that is capable of changing shape so that a seal between the container and the soil sample can be maintained as the soil shrinks. An experiment was conducted in which the air permeability of two soil materials was measured at a series of soil matric potentials after being sealed with rubberized asphalt or paraffin. The results indicate that the asphalt was able to maintain a better seal than the rigid paraffin as the soil materials dried and shrank. Seal integrity of the asphalt was maintained as matric potentials were reduced from −50 kPa to −750 kPa for soils of moderate and high shrinkage capacity. In contrast, the air‐permeability data indicate that the paraffin failed to provide an adequate seal at −50 kPa, even before the soil was desorbed.

Predicting Seasonal Irrigation Water Requirements on Coarse-Textured Soils

ABSTRACT Awater-balance model has been developed for predicting seasonal irrigation water requirements. The model treats the soil as a multi-layered water storage reservoir. Reservoir capacity is defined by soil water desorption data, rate of root growth, and effective rooting depth. Water withdrawal rate or crop water use is determined from pan evaporation data corrected to potential ET and converted by consideration of plant growth stage. Rainfall is partitioned into runoff, effective precipitation, and deep percolation. Field data from corn and soybeans during the seasons 1979-81 on two soil types were used for verification. The mean absolute difference between predicted and measured seasonal irrigation water requirements was 16%. Simulation data were fitted to frequency distributions, and irrigation requirements are expressed in terms of probabilities.

Physical and Chemical Properties of a Haploxeroll after Fifty Years of Residue Management

AbstractGreater inference about the value of long‐term crop residue management requires that soil chemical properties be measured together with soil physical properties. After 54 yr of several different crop residue management treatments in a winter wheat (Triticum aestivum L.)‐fallow system in the Pacific Northwest, soil bulk density (ρb), saturated hydraulic conductivity (Ksat), soil water desorption curves (SWDC), soil organic carbon, and soil pH were all used to quantify the soil physical and chemical state. All wheat straw, chaff, and manure additions were incorporated by spring plowing in an alternating winter wheat‐fallow rotation. The residue treatments were: (i) straw, (ii) straw plus 22 Mg/ha strawy manure, (iii) straw burned in the fall, and (iv) straw plus 90 kg/ha of N as NH4NO3 broadcast before seeding. Straw return in treatments 1, 2, and 4 depended especially on the N added. Soil ρb at 0.02‐m increments was used to locate compacted layers. Respective tillage pan ρb for treatments 1, 2, 3, and 4 were 1.3, 1.2, 1.3, and 1.2 Mg/m3; respective Ksat, using a double tube method, were 0.34, 0.85, 0.29, 0.23 µm/s. Respective Ksat, above the tillage pan, using an air‐entry permeameter were 1.08, 2.24, 1.52, and 2.36 µm/s. Average ρb and Ksat distinguished between treatments with low vs. high return of residue. SWDCs in the plow layer and tillage pan reflected the degree of soil compaction within and among treatments. No treatment differences in Ksat or ρb were found below the tillage pan. The Ksat in the tillage pan, relative to overlying and underlying layers, indicated that the tillage pan governs water flow in this soil. Soil pH of the Ap horizon was 5.6, 6.1, 5.4, and 4.8 for treatments 1, 2, 3, and 4, respectively; respective percent organic carbon was 1.14, 1.53, 1.07, and 1.25. The Ksat in the tillage pan was maintained at a high level only where residue additions were increased and soil pH maintained above 5.6.

SOIL WATER DESORPTION CURVES ESTIMATED FROM LIMITED DATA

A procedure is described by which the soil water desorption curve, modeled as a power curve, can be approximated from a limited number of measured water contents at specific tensions. Applicability of the model was tested on 90 soils. The results indicated that measurements at the two extremes were the most important, if only two water contents were measured. The best results were obtained when the curve was fitted through three measured water contents, respectively, at saturation, at an intermediate (5.0–35.0 kPa) tension and at a high (1500.0 kPa) tension. Key words: Soil water desorption curves, limited data, power curve

THE DETERMINATION OF SOIL-WATER DESORPTION CURVES FOR SOIL CORES

An improved technique for measuring soil water desorption curves of a large number of soil cores was tested. A procedure for improving contact between the soil and a "tension medium" resulted in rapid extraction of water from 7.6 × 7.6-cm cores at pressure heads from 0 to −500 cm of water. The data for clay and sandy loam cores showed that equilibrium was reached in less than 200 h at all pressure heads. The "tension medium," used essentially as a large porous plate, was carefully chosen with a narrow pore size distribution. This provided a high hydraulic conductivity and high air-entry values, both necessary for efficient desorption over the pressure head range 0 to −500 cm of water. A tensiometer-pressure transducer combination for establishing equilibration time proved more reliable than the traditional weight-loss criterion. The consistency and reproducibility of desorption curves was demonstrated using data for hysteretic loops as well as standard deviations of water contents at each pressure head on similar soils.

Chiseling Influences on Soil Hydraulic Properties

AbstractChiseling is now more frequently used for primary tillage in the drylands of eastern Oregon and Washington. Improved measures of chiseling effects on soil water relations are needed to evaluate water intake and infiltration benefits, especially as related to depth of chiseling. Hydraulic conductivity (K) and soil water desorption characteristic (SWDC) were field measured in the 120‐cm soil profile of a Walla Walla (mesic typic Haploxeroll) silt loam before and after chiseling 43‐cm deep.The test Walla Walla soil is layered. Cation exchange capacity, exchangeable Ca2+, clay content, and organic matter all changed at the 30‐cm depth; dry bulk density decreased with depth above 45 cm and was constant below 45 cm; K was 10 to 1,000‐fold lower in depths above 30 cm; water contents in the SWDC (−50 to −200 mbar range) were lower in upper 30‐cm layer.Chiseling affected both the SWDC and K in the upper 30 cm, especially at 10 and 20‐cm depths, but had no influence on these measurements at 40 cm. Both water potential at constant water content in the SWDC and K were increased especially in the −50 to −300 mbar range. Failure of chiseling to improve water relations in the mild duripan extending from 30 to 45 cm suggests the need for addition of plant residue or chemical amendments into the chisel slots.Water contents and hydraulic heads during drainage showed that chiseling could reduce evaporation by reducing water content and diffusivity. Overall soil profile hydraulic resistances showed relative average K up to 15 times greater as a result of chiseling 43 cm deep, but nearly similar accelerated internal drainages were projected for simulated chiseling to 25‐cm depth vs. chiseling to 43 cm.

Effects of Bulk Density, Aggregate Size, and Soil Water Suction on Oxygen Diffusion, Redox Potentials, and Elongation of Corn Roots

AbstractCorn seeds (Zea mays L. ‘WF9 × 38‐11’) were planted in plastic cylinders of soil at different bulk densities (0.93, 1.03, 1.13, and 1.23 g cm‐3) or aggregates of different sizes (&lt; 0.5, 0.5–1, 1–2, 2–3, and 3–6 mm). Soil water suction during germination was maintained at constant levels (0, 3, 18, 48, and 68 cm of water).Germination began and root elongation rates sharply increased from zero to a maximum in each soil sample as soil water suction increased above the air‐entry value or bubbling pressure. Over these ranges of suction, diffusion of O2 controlled rate of root elongation. Diffusion of O2 in soil was determined primarily by air porosity. Bulk density and aggregate size per se had little effect on diffusion or concentration of O2 at seed depth. Bulk density and aggregate size greatly influenced soil water desorption, the depth of soil to which O2 could diffuse, and the thickness of water films at equal air porosities. Between approximately 0 and 20 cm of suction, thickness of water films limited root elongation. Measured O2 concentrations in soil agreed with concentrations predicted by diffusion theory. Redox potentials were correlated with O2 levels. Root elongation rates decreased in some samples at the highest suctions used, probably because of increased soil strength.