Preferential Water Flow Research Articles

Use of ‘Rock Hard’-sodium sulphate tablets for quickly determining preferential water flow routes in soils. Short communication : Renshaw, C E Earth Surf Process Land V14, N5, Aug 1989, P443–446

Use of ‘Rock Hard’-sodium sulphate tablets for quickly determining preferential water flow routes in soils. Short communication : Renshaw, C E Earth Surf Process Land V14, N5, Aug 1989, P443–446

Pesticides pollution of groundwater in the humid United States

Since 1980, many studies have begun to look at the occurrence of pesticides in groundwater; these range from controlled field/plot research studies, to simple monitoring of public water supplies to assess the occurrence of pesticides. The compounds most commonly detected are: (1) mobile and/or volatile soil fumigants and nematicides, used on vegetable or specialty crops; (2) commonly used herbicides from the humid corn-belt regions. The fumigants-nematicides have been the most widely looked for compounds. The herbicides commonly detected are also some of the most widely used of all pesticides, but pesticide properties clearly affect how commonly they are detected. In nearly all corn-belt areas, the herbicide atrazine is the most commonly detected compound. A general overview shows that a total of 39 pesticides have been detected in groundwater from 34 states or provinces. This includes occurrences related to commercial point sources (but not manufacturing sites, or detections exclusively referred to commercial sources) as well as nonpoint sources. From states with appropriate studies, there appears, within the context of pesticide properties, some general proportionality between pesticide use and detections in groundwater; this is also confounded by climatic differences. Kansas studies suggest that 8–10% of private, rural drinking-water supply wells, and 10% of susceptible public water-supply wells may exhibit pesticide contamination. Surveys in Minnesota and Iowa, with more intensive pesticide usage suggest that 20–30% of the more susceptible public water-supply wells, and 30–60% of private wells may exhibit pesticide residues. In contrast however, Illinois, a very high use state, has detected few pesticides in public water-supply wells (except where related to a commercial point source); why such differences occur among adjacent states is not clear. Pesticides are leaching through the soil and into groundwater far more commonly than the preconceptions of a decade ago would have predicted. Point sources may be widespread but are not the sole cause; it is also clear that many pesticides are leaching to groundwater from routine, nonpoint source use. Controlled plot studies show the intermittent, often rapid delivery of many pesticides to shallow groundwaters. The preferential flow of water and solutes through soil and rock materials clearly contributes to the delivery of pesticides to groundwater. The occurrence of preferential flow also implies that notions about the ‘relative susceptibility’ of various soils to pesticide leaching may not be accurate. Some studies show pesticide leaching more commonly in finer-textured soils than coarser, sandy textured soils. Generally, the concentrations of pesticides in groundwater are low, in the 0.1–5.0 μg l −1 range. Even at these concentrations there are concerns for long-term, chronic exposure to a large segment of the public through drinking-water supplies. While concerns with groundwater merit attention, the concentrations of pesticides in drinking water derived from surface waters will typically be greater. Current policy discussions often fail to understand the difference between groundwater protection and drinking-water protection programs and may not provide adequate protection for private rural groundwater supplies. Research and policies to resolve the problems of agricultural impacts on the environment will require a new focus on integrated farm-management systems, that enhance efficiency and reduce off-site impacts.

The use of ‘rock hard’‐sodium sulphate tablets for quickly determining preferential water flow routes in soils

AbstractSubsurface water flow routes have been inferred from gypsum sphere weight loss with minimal disturbance to soil structure or flow nets (Crabtree and Trudgill, 1984). This technique is limited by the relatively slow, fixed dissolution of gypsum. A solution is to use tablets consisting of Durham's ‘Rock Hard Water Putty’ and sodium sulphate. Laboratory tests indicate that tablet dissolution is controlled by the ratio of Rock Hard to sodium sulphate and is proportional to net throughflow yet independent of throughflow pH (between 3·0 and 9·0) and throughflow rate (at rates < 0·04 1 hr−1). The tablets are also mechanically stable, economical, and environmentally safe. Field study results using the Rock Hard‐sodium sulphate tablets are consistent with both the laboratory findings and the results of previous investigators.

A-35 住宅における湯の用途別適温・適流量に関する研究(その1) : 単純集計結果

A-35 住宅における湯の用途別適温・適流量に関する研究(その1) : 単純集計結果

A-36 住宅における湯の用途別適温・適流量に関する研究 : その2 相関行列、クロス集計および数量化1類による分析

A-36 住宅における湯の用途別適温・適流量に関する研究 : その2 相関行列、クロス集計および数量化1類による分析

Modelling soil water dynamics in the unsaturated zone — State of the art

This paper reviews the principles underlying water dynamics in the unsaturated zone and gives an overview of simulation modelling of soil water flow in the unsaturated zone. The basic relations describing kinetics of flow and governing equations of flow in the unsaturated zone are presented in a general form considering unsteady multidimensional anisotropic and nonhomogeneous flow in the presence of sinks/sources. The influence of overburden potential, characterizing swelling soils, on water transport through the unsaturated zone is discussed. Outlines of thermally-induced water flow are presented with an extension to the specific case of frozen soils. Complex relationships describing the hydrological system are reduced into the one-dimensional vertical flow cases, for which the mathematical models are defined. The entire model has the form of a set of partial differential equations together with auxiliary conditions, that describe the system's geometry, parameters, boundary conditions and in case of transient flow also the initial conditions. Simulation is defined as operation with such a mathematical model. Numerical approximations to solve governing equations of unsaturated water flow are emphasized. Recently-introduced numerical methods are discussed pointing out their advantages and limitations. Atmospheric boundary conditions of the modelled system are described with a maximizing procedure of the flux through the soil-air interface. Several options to define lower boundary conditions are discussed and a special type of lower boundary condition is given for the case that unsaturated flow is coupled with a regional groundwater model. Simulation models require data concerning input, calibration and verification. The current status of collecting model parameters is discussed as well as measurement of common verification data, i.e. phreatic surface, matric head, water content and actual evapotranspiration. Preferential flow of water through unsaturated soil has considerable consequences for simulating the field water balance. The attempts to simulate such hydrological systems are also evaluated. Some transient flow problems are affected by hysteresis. Successful attempts to build hysteresis into dynamic simulation models are still scarce, though recently a number of solutions have been presented. Finally, several practical examples of simulation of flow problems are presented. These examples are taken from everyday water management practice and document the wide range of applicability of simulation models.

Effects of Soil Physical Structure on Solute Transport in a Weathered Tropical Soil

AbstractEffects of soil structure on solute transport were studied in a clayey, well‐aggregated Oxic Dystropept under grass and secondary forest at La Selva, Costa Rica. Both fine pores (<0.1 µm) and coarse pores (>30 µm) are abundant, as indicated by a water content of 0.37 m3 m−3 at −1500 kPa matric potential, the shape of the moisture release curve, and an initial infiltration rate (at field capacity) averaging 3900 mm hr−1. Field application of Rhodamine B dye without ponding (simulating heavy rainfall) showed preferential flow along decayed‐root channels, animal burrows, cracks, and other macropores. Dye application to intact cores under conditions of ponded steady‐state flow gave a good correlation between flow rate and total stained area (p <0.01 under forest; p < 0.05 under grass). Solute (CaCl2) breakthrough occurred very rapidly, often after < 0.1 pore volumes had percolated; however, relative concentration of the effluent did not exceed 0.95 even after five pore volumes had percolated. These results indicate that most water flows between aggregates or through macropores (as preferential or channelized flow), even when the fine pores are not fully saturated, and in effect bypasses the fine pore space. When solution inflow was interrupted after ∼1.8 pore volumes, then resumed after 10 min pause, relative concentration of the effluent dropped by 10 to 40%, then rose again. This indicates that solute diffused slowly into the aggregates. Taken together the results suggest that this soil strongly resists leaching. Preferential water flow may serve to prevent nutrient loss from the matrix of all highly aggregated soils and of all noncultivated soils in which animal activity and turnover of woody roots create abundant macropores.

Movement of solutes in tropical alfisols cleared by different methods

Solute movement under saturated water-flow conditions was studied in undisturbed and disturbed soil columns and in the field under four methods of clearing: manual; shear blade; tree pusher; and root rake, and traditional farming. Soil under forest was included as a control. Chloride (Cl −) appeared in the effluent at 0.1 pore volume after its application to the surface of undisturbed soil columns taken from plots cleared manually, with shear blade, or with tree pusher. In columns fron forest and traditional farming plots, no Cl − appeared in the effluent until 0.2 pore volume of solution had passed through the columns. The pore volume required to attain 0.5 relative Cl − concentration was 0.60, 0.60, 0.40, 0.40, and 0.26 for the columns from forest control, traditional farming, shear-blade, tree-pusher and manually cleared plots, respectively. This indicates preferential water flow in the columns from manually cleared plots as compared to others. Nitrate leaching in the undisturbed columns was nearly identical to chloride. Phosphorus appeared in the effluent after the passage of 0.34 pore volume of solution through the columns of forest control and manually cleared treatments, and of 0.7 pore volume in the shear blade treatment. The increase in the relative phosphate concentration was considerably slower than chloride and nitrate. Chloride appeared later in the repacked columns than in the undisturbed columns, indicating the absence of preferential flow pathways in the disturbed columns. In the field, less than 100% Cl − and water were retained in the 0–70 cm depth, indicating some flow of chloride solution to below 70 cm depth.

Leaching of Herbicides Through Undisturbed Cores of a Structured Clay Soil

AbstractThe leaching of the herbicides bromacil and napropamide through large undisturbed cores of a structured Evesham clay (Aquic Eutrochrept) was studied under continuous and discontinuous watering regimes and at different initial moisture contents. A greater proportion of both herbicides was leached from initially dry cores (θ = 0.24) than from prewet cores (θ = 0.35). This was consistent with differences in the amount of herbicide retained in the soil, most of which was retained in the top 0.05 m. Continuous leaching resulted in greater herbicide breakthrough than did the discontinuous treatment, presumably because some herbicide diffused from the conducting channels into the soil aggregates during the quiescent period, where it was less susceptible to subsequent leaching. At a continuous input rate of 12 mm h−1 to initially dry soil, 85% of the applied napropamide (Kd = 17.7 L kg−1) and almost 100% of bromacil (Kd = 1.73 L kg−1) were leached out of the soil by one pore volume of water. Chloride was used as a tracer for water flow through the soil. Chloride breakthrough curves were analyzed using a transfer function equation, which yielded the median travel time of a solute molecule in the soil. It is suggested that preferential flow of water down cracks and channels between soil aggregates led to short travel times and apparently low volumes of soil water participating in solute transport (θst). The θst values were inversely correlated with the percentages of the applied herbicides that were leached.

The movement of nitrate fertiliser from the soil surface to drainage waters by preferential flow in weakly structured soils, slapton, S. Devon

Preferential flow of soil water down structural pathways can be responsible for the rapid movement of a proportion of surface-applied nitrate fertiliser to soil drainage waters. This phenomenon is most marked in cracking clay soils with strong structural development, but it has also been shown for a study site at Slapton in S. Devon, U.K., where weakly structured soils exist. The depth of recovery of surface-applied tracers, moving in preferential flow, was found to increase with increasing rainfall intensity. Soil outflow response was difficult to correlate with observed recovery depths due to the spatially variable nature of preferential flow and the consequent difficulties of effecting an adequate soil sampling programme. Soil outflow response and surface-to-output linkages of preferential flow could, however, be characterised in terms of combinations of antecedent volumetric soil moisture values (θ) and rainfall intensity. At θ > 0.46 cm 3 cm −3 (field capacity), outflow response occurred over a wide range of rainfall intensity; below θ = 0.29, the mobile:retained soil water boundary, there was no response; between 0.29 and 0.46, response occurred mostly above an intensity threshold of 2.5 mm h −1. The management implications of nitrate movement under intense rainfall are briefly discussed.

Soil morphology and preferential flow along macropores

Preferential flow of water along macropores can only be characterized in quantitative terms when the flow system is functionally characterized by using tracers. Standard descriptions of soil structure do not provide adequate information. Expensive micromorphometric techniques allow specific measurements of macropores in terms of size, type, shape and continuity. In expensive macromorphometric techniques are attractive for field use. Preferential flow in saturated soil involves rapid displacement of water from macropores (hydrodynamic dispersion). In unsaturated soil, flow into air-filled macropores (short-circuiting) occurs, which is followed by lateral absorption. Examples are discussed which illustrate the use of soil morphology to characterize preferential flow along macropores: (i) empirical extrapolation of measured data on the basis of macrostructure descriptions. So far, this procedure could only be applied to breakthrough curves of soils with identical textures but very different macrostructures; (ii) using staining techniques which provide essential boundary conditions for newly developed physical flow models. The latter describe macropores in terms of size, type and shape, rather than in terms of relative volumes; and (iii) developing physical interpretations of pedological features, such as mottling patterns. The examples are based on four case studies which deal with practical problems of soil water management. In these studies, morphological methods provided essential data, which could not have been obtained by physical methods.

The nature of passive flows through tightly folded membranes. The influence of microstructure.

Membrane processes such as microvilli and folded structures greatly increase the area for passive transport. A mathematical analysis of flows through folded membrane structures demonstrates that the increase in flows is not proportional to the increase in area. Furthermore, the flows of salt and water do not increase in the same ratio: that of water can be several times the increase in area and that of salt a fraction of the increase in area. A preferential passive flow of water is created by the structure. It is only in the neighborhood of the isotonic state that the effect is significant, but in that region, the effect can be dramatic.

Diurnal Changes in Leaf Resistance to Water Vapor Diffusion at Different Heights in a Soybean Canopy1

AbstractThe effect of soil moisture content and evaporative demand on diurnal variations of leaf resistance to water vapor diffusion were studied at two heights in a soybean (Glycine max L. (Merr.)) canopy. Under soil moisture stress, leaf resistance to water‐vapor diffusion of middle leaves in the canopy increased approximately 2 hr earlier in the day and to a greater extent than the resistance of upper leaves. This indicated a preferential flow of water to upper leaves. It is speculated that the degree of out‐of‐phase change in leaf resistance under stress conditions is related to canopy density.Leaf resistances of middle leaves decreased at a slower rate under adequate soil water supply than under drier conditions. On days with low evaporative demand, leaf resistances of upper leaves tended to be less (0.8 sec/cm) under the low soil moisture than under the high soil moisture supply. The apparent reduction of stomatal activity in leaves under low evaporative demand and high soil moisture supply is believed related to the lack of development of a small leaf‐water deficit necessary for maximal stomatal opening.