Abstracts

Bagarello, V., Iovino, M., et al. 2000, 'Factors Affecting Measurement of the Near-Saturated Soil Hydraulic Conductivity', Soil Science Society of America Journal, vol. 64, no. 4, pp. 1203-1210.

Different techniques and different technique application procedures are available to determine soil hydraulic conductivity (K). The objective of this study was to determine the dependence of the K values on the selected technique and application procedure. Soil hydraulic conductivity was determined on a sandy loam soil in the near-saturated zone by the pressure infiltrometer, the tension infiltrometer field techniques, and the inverse method applied to laboratory outflow multistep experiments. Other experiments included a study of ascending and descending pressure heads (h) on the K values measured by the tension infiltrometer technique, and the effect of contact material on ponded infiltration rates. Differences between ascending and descending K means and coefficients of variation were <20% for low pressure heads (h ≤-120 mm). Three successive ponding infiltration runs produced mean field-saturated hydraulic conductivities decreasing from 42.7 to 81.8 mm h^-1 to 12.9 to 20.6 mm h^-1, whether or not contact material was present on the infiltration surface. The mean values of K obtained by the selected three methods ranged between 47.1 to 71.9 mm h^-1 at saturation and 0.2 to 0.6 mm h^-1 at h = -120mm. The K(h) relationships obtained by the pressure with tension infiltrometer and multistep techniques were essentially overlapping. The pressure infiltrometer produced K(h) relationships that were different from those determined by the two other techniques. In conclusion, ponded infiltration measurements were not usable for estimating unsaturated soil hydraulic conductivity, but contact material and ascending vs. descending direction had no substantial effect on infiltration rate.

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Chen, C. and Payne, W.A. 2001, 'Measured and Modeled Unsaturated Hydraulic Conductivity of a Walla Walla Silt Loam', Soil Science Society of America Journal, vol. 65, pp. 1385-1391.

There are numerous methods of estimating unsaturated soil hydraulic conductivity [K(Θ)], ranging from direct measurement in the laboratory or field, to models that use only basic soil data, e.g., texture or water release curves (WRC). We evaluated K(Θ) for a Walla Walla silt loam (coarse-silty, mixed, superactive, mesic Typic Haploxeroll) using two field methods and the Mualem–van Genuchten model (MVG). Field methods were the internal drainage method (ID) and the Klaij and Vachaud method (MKV), which was modified to include hydraulic head (H), and used 20 yr of data from a dryland field experiment. Four approaches to estimate WRC were compared for the MKV, and two approaches to estimate parameters of the MVG. For water contents >0.20 m³ m^-3, the MKV gave K values that were two orders of magnitude less than those obtained from internal drainage experiments conducted under wetter conditions. When MVG parameters were predicted by the Rosetta model, which uses pedotransfer functions, K values approached those of the MVK. However, when model parameters were estimated from internal drainage and saturated hydraulic conductivity (K_sat) data, K(Θ) values were closer to those of the internal drainage experiments. The study reaffirms the difficulty of reconciling K(Θ) determined by different methodologies, and of extrapolating to values of Θ outside those measured. It also demonstrates the sensitivity of the MVG and pedotransfer functions to different sources of input values. Advantages and disadvantages of the different approaches to determining K(Θ) are discussed.

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Darusman, A.H.K., Stone, L.R., et al. 1997, 'Water Flux Below the Root Zone vs. Irrigation Amount in Drip-Irrigated Corn', Agronomy Journal, vol. 89, pp. 375-379.

With increasing demands on water resources, greater efficiency is needed in irrigated agriculture. Internal drainage from the root zone is a loss that can be reduced or managed to improve irrigation efficiency. Our objective was to determine the relationship between seasonal water flux below the root zone (1.5 m) and irrigation amount with corn (Zea mays L.). Subsurface drip irrigation systems near Colby and Holcomb, KS, were used to supply water. Driplines were buried at a soil depth of 0.40 to 0.45 m, with a spacing of 1.5 m. The two soils are deep silt loams that formed in loess. Water flux at the 1.5-m soil depth was determined in four irrigation treatments during 1990 and 1991. Tensiometers were placed at soil depths of 1.4 and 1.7 m and at distances from the dripline of 0, 0.4, and 0.8 m. Water flux was calculated using predetermined hydraulic conductivity vs. matric potential (Ψ_m) relationships, Ψ_m data from tensiometers within the corn plots, and Darcy's equation of water flow. Irrigation was applied to four treatmentsuch that irrigation plus rain equaled 125, 100, 75, and 50% of calculated corn evapotranspiration (ET). From a regression analysis relating integrated water flux below the root zone (1.5 m) and in-season irrigation, net upward water flux occurred with in-season irrigation < 296 mm (RMSE = 71 mm), whereas net downward water flux occurred with irrigation > 296 mm. Compared with the 100% ET treatment (full irrigation), the 75% ET treatment had 76% of the in-season irrigation, 25% of the in-season water flux (net downward) below the root zone (1.5 m), and 93% of the grain yield. Near-maximum corn grain yields can be obtained with significant decreases in irrigation amount and internal drainage from the root zone compared with full irrigation.

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Germann, P., Helbling, A., et al. 2007, 'Rivulet Approach to Rates of Preferential Infiltration', Vadose Zone Journal, vol. 6, pp. 207-220.

Preferential infiltration is proposed to occur under atmospheric pressure but in unsaturated soil. Its domain is positioned between the domains of Richards' equation and Darcy's law. Rivulets in the form of tiny water streaks are considered to be the basic units of preferential infiltration. They move under atmospheric pressure and in pores that are filled with air before infiltration. Stokes flow relates variations in soil moisture with velocities of wetting and draining fronts, and with volume flux densities. We show that superposition of rivulets to rivulet ensembles leads to measurable soil moisture variations. The approach was applied to time series of soil moisture that result from sprinkler irrigation experiments and that were recorded with time domain reflectometry (TDR) equipment at depths of 0.1 and 0.2 m. A minimum water content, Θ*, was identified that has to be exceeded at a particular depth before Stokes flow continues.

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Lenhard, R.J. and Meakin, P. 2007, 'Water Behavior in Layered Porous Media with Discrete Flow Channels: Results of a Large-Scale Experiment', Vadose Zone Journal, vol. 6, pp. 458-470.

A meter-scale experimental system (2-m high by 2-m deep by 3-m wide) was used to investigate the behavior of water in a model system consisting of two unconsolidated sediment layers separated by a layer containing discrete flow channels. Stainless steel tubes were inserted vertically through a clayey matrix to represent the discrete flow-channel layer. The experimental system was well characterized, and results from water infiltration experiments were analyzed. A time series of water arrival at a network of 86 probes located in the unconsolidated sediment layers is presented, as well as water pressure histories at specific locations. Some probes were located at opposite ends of the flow channels to assess water migration through the discrete flow-channel layer. Analyses of the experimental results focused on capillary break phenomenon at the interface between the overlying unconsolidated layer and the underlying discrete flow channels. Dissimilar water pressure histories were measured at probes near the upper boundary of the discrete flow-channel layer, suggesting varied and complex water flow behavior. At some locations, a steady or periodic "leaking" of water through the discrete flow channels appeared to occur, contrary to capillary break theory. The authors advocate larger-scale experiments to advance our understanding and ability to model fluid flow across a wide range of spatial and temporal scales.

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Lowery, B., Datiri, B.C., et al. 1986, 'An Electrical Readout System for Tensiometers', Soil Science Society of America Journal, vol. 50, no. 2, pp. 494-496.

Commercially available pressure transducers (PTs) and tensiometers were combined to continuously monitor in situ soil pore-water pressure. The transducers were calibrated over a range of 0 to —76.5 kPa and a temperature range from 5 to 40°C. A single calibration equation corrected for zero offset and a small temperature-induced variation. Field temperature fluctuations between 10 and 30°C had little effect on results from PTs connected to constant pressures of —5 and —11.5 kPa. Wetting fronts were easily mapped with outputs from tensiometers placed at three different depths. From these field data, prior water history and diurnal changes in soil pore-water pressure were easily obtained. Given variable field conditions, results from the PTs agreed well with closely-placed Bourdon gauge tensiometers.

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Moroizumi, T. and Horino, H. 2004, 'Tillage Effects on Subsurface Drainage', Soil Science Society of America Journal, vol. 68, pp. 1138-1144.

Tillage near the soil surface may greatly influence drainage discharge and pressure head values in the subsurface zone. In this study, a controlled comparative experiment was conducted in the field under natural weather conditions, using a tilled and an untilled column, to evaluate the effects of tillage on subsurface drainage discharge and pressure head values. There were no significant differences between the two columns in subsurface drainage before tillage treatment before the study, as checked by a preliminary experiment. After tilling one column of the two columns, cumulative subsurface drainage discharge was larger and occurred earlier for the tilled column than for the untilled column. The measured drainage discharge and pressure head values were evaluated using the water movement model, HYDRUS version 6.0, which numerically solves the Richards equation. The HYDRUS model reproduced measured values well for subsurface drainage discharge and pressure head values after the tillage, as determined by root mean squared error (RMSE), mean bias error (MBE), and R². Therefore, it is concluded that the model, which includes the effects of tillage on the hydraulic properties, can explain the reasons for differences in observed drainage in the two columns. Moreover, the effects of tillage depth on subsurface drainage discharge were simulated. The results implied that the effect of tillage on subsurface drainage were induced by the conditions of pressure head just before the first rainfall, and this effect was equal to, or greater than, the effects of changes in hydraulic properties due to tillage.

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Steinberg, S.L. and Poritz, D. 2005, 'Measurement of Hydraulic Characteristics of Porous Media Used to Grow Plants in Microgravity', Soil Science Society of America Journal, vol. 69, pp. 301-310.

Understanding the effect of gravity on hydraulic properties of plant growth medium is essential for growing plants in space. The suitability of existing models to simulate hydraulic properties of porous medium is uncertain due to limited understanding of fundamental mechanisms controlling water and air transport in microgravity. The objective of this research was to characterize saturated and unsaturated hydraulic conductivity (K) of two particle-size distributions of baked ceramic aggregate using direct measurement techniques compatible with microgravity. Steady state (Method A) and instantaneous profile measurement (Method B) methods for K were used in a single experimental unit with horizontal flow through thin sections of porous medium providing an earth-based analog to microgravity. Comparison between methods was conducted using a crossover experimental design compatible with limited resources of space flight. Satiated (natural saturation) K ranged from 0.09 to 0.12 cm s^-1 and 0.5 to >1 cm s^-1 for 0.25- to 1- and 1- to 2-mm media, respectively. The K at the interaggregate/intraaggregate transition was ~10^-4 cm s^-1 for both particle-size distributions. Significant differences in log₁₀K due to method and porous medium were less than one order of magnitude and were attributed to variability in air entrapment. The van Genuchten/Mualem parametric models provided an adequate prediction of K of the interaggregate pore space, using residual water content for that pore space. The instantaneous profile method covers the range of water contents relevant to plant growth using fewer resources than Method A, all advantages for space flight where mass, volume, and astronaut time are limited.

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Sugita, F. and Nakane, K. 2007, 'Combined Effects of Rainfall Patterns and Porous Media Properties on Nitrate Leaching', Vadose Zone Journal, vol. 6, pp. 548-553.

Laboratory experiments were conducted to investigate the effects of water input properties on the movement of nitrate in three different porous media. A series of artificial rainfall amounts from 4.0 to 26 mm were applied every 24 h in 10 min durations over a weighing lysimeter packed with either fine sand, field soil over fine sand, or fine sand with artificial macropores. Movements of nitrate, chloride, and water in the lysimeter were monitored. Significant nitrate leaching occurred with pistonlike flow caused by the highest applications in all porous media. The nitrate movement coincided with that of chloride in a homogeneous sand medium. In a layered medium with soil cover over sand, nitrate/chloride ratios in the field soil decreased with time, suggesting the presence of degradation processes. Some parts of the soil may be serving as spots for denitrification. High nitrate concentration remained in the topsoil layer under light to intermediate rainfall applications in the two-layered medium, causing large vertical spreading of nitrate under intermediate rains by which infiltration occurs. Double peaks in the concentration profiles were observed in the macroporous medium under 26-mm applications, indicating that part of the nitrate was transported by preferential flow while the rest was transported by matrix flow. We estimated that heterogeneity of the porous medium resulted in large dispersion under intermediate to heavy rains and denitrification under light rains. Heterogeneity also affected pathways of leaching under heavy rainfalls. We estimated that climate change could significantly increase the chance of nitrate leaching by at least 25% in the Pacific East area of Japan through increases of heavy rain frequency and precipitation intensity and decreased time for denitrification in the surface soil.

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Toride, N., Inoue, M., et al. 2003, 'Hydrodynamic Dispersion in an Unsaturated Dune Sand', Soil Science Society of America Journal, vol. 67, pp. 703-712.

Solutes spread relative to the mean displacement position during water flow in soils as a result of meandering through the (partially) saturated pore complex. Spreading is characterized by the hydrodynamic dispersion coefficient in the convection-dispersion equation (CDE). This coefficient has been extensively studied for saturated soils. In this study hydrodynamic dispersion coefficients for nonaggregated dune sand were determined as a function of volumetric water contents, Θ, ranging from saturation to 0.08 cm³cm^-3 in columns of 5-cm diam. and 25- to 40-cm length. Unit-gradient flow experiments were conducted to measure solute breakthrough curves (BTCs) using four-electrode salinity probes at several column depths. Transport parameters for the CDE and the mobile-immobile model (MIM) were determined by optimizing analytical solutions to observed BTCs. A maximum dispersivity, λ, of 0.97 cm was found at Θ = 0.13, whereas for saturated flow λ ≈ 0.1 cm irrespective of pore-water velocity ranging from 208 to 5878 cm d^-1. For the MIM, the mobile water fraction, Θ_m/Θ, gradually deceased from almost unity at saturation to a minimum of 0.85 at Θ = 0.15 followed by a slight increase with further desaturation. The exchange time between the mobile and immobile phases, 1/α, was 0.1 to 0.2 d for Θ > 0.15 presumably because of the relatively homogeneous flow with convective solute mixing. For lower Θ, the exchange became much slower since flow predominantly occurs in water films enveloping sand particles. The Peclet number for molecular diffusion, P_e, decreases as the role of transverse diffusion increases at lower Θ because of smaller v and thinner water films while the resistance increases for solute exchange between mobile and immobile phases. These combined effects lead to a maximum dispersivity value at intermediate water contents in the case of the nonaggregated dune sand.

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Waddell, J.T., Gupta, S.C., et al. 1999, 'Irrigation and Nitrogen Management Effects on Potato Yield, Tuber Quality, and Nitrogen Uptake', Agronomy Journal, vol. 91, pp. 991-997.

Irrigation and N management are perhaps the most important aspects of successful potato (Solanum tuberosum L.) production in central Minnesota. With rising concern about current irrigation and N management practices and ground water quality, a two-year study was undertaken to evaluate the impact of alternative irrigation and N management practices on potato yield, tuber quality, and N uptake. The treatments were different irrigation schemes (sprinkler and drip), irrigation triggers (70 and 40% of available water [AW] remaining), drip tape placements (surface or buried at 25 cm), N sources (urea, turkey manure, sulfur-coated urea [SCU]), and timings of inorganic N fertilizer application. Rainfall in the area was less in 1994 (260 mm) than in 1995 (380 mm). As a result, irrigation applications in 1994 were greater than in 1995. Drip irrigation amounts were less than half of that applied by sprinkler irrigation treatments in both years. Except for the buried drip with fertigation (43.0 and 27.8 Mg ha^-1 in 1994 and 1995) and the control (36.5 and 13.3 Mg ha^-1 in 1994 and 1995), total tuber yields were similar regardless of irrigation and N source treatment. The turkey manure treatments in 1994 had significantly higher marketable tuber yields than the control, averaging 48.3 Mg ha^-1 for both years. Total N uptake in 1994 was nearly 100% of that applied, while in 1995 it accounted for only 60% of the application. Lower N uptake was observed for the control (85.3 and 30.2 kg N ha^-1 in 1994 and 1995, respectively), SCU (165.2 and 111.3 kg N ha^-1 in 1994 and 1995), and BDF (157.2 and 80.1 kg N ha^-1 in 1994 and 1995) treatments. The results showed that the use of unconventional N sources such as turkey manure and SCU are viable alternatives for potato production, provided they are managed properly.

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