No abstract

Although boron (B) adsorption significantly depends on soil texture and pH, few attempts have been made to estimate their influence on B transport behavior. Adsorption and transport of B in three soils of different textures were investigated in batch and column experiments. The B adsorption on the soils in batch experiments was rapid and an apparent adsorption equilibrium was reached over the first several hours. The extent of adsorption on each of the soils was strongly dependent on pH, increasing sharply as the pH increased from 7.0 to 9.2–9.4. The Langmuir equation with the pH‐dependent adsorption coefficient simulated well the B adsorption at equilibrium. Miscible‐displacement experiments were conducted at two water fluxes of 2.9 and 0.19 cm h−1 and at two pH values. The transport of B through the soil columns was retarded. The retardation increased with the increase of clay content and solution pH. The impact of the rate‐limited adsorption on B transport was dependent on water flux and was controlled by mass‐transfer processes. The B breakthrough curves (BTCs) for the loamy sand and sandy loam soils (where Br− transport was ideal) were simulated using the ideal‐transport‐based (local equilibrium‐nonequilibrium [LE‐NE]) model. This model successfully described B transport at different water fluxes and pH values when using the adsorption parameters derived from batch equilibrium experiments. The adsorption rate coefficient values obtained from BTCs were smaller than those obtained from the batch kinetic data. The non‐ideal transport behavior of B in the clay soil was associated with intra‐aggregate and rate‐limited adsorption in the mobile domain. A successful simulation of B BTCs for this soil for the two aggregate sizes (<2 and 4–4.75 mm) was obtained when used the two‐domain, two‐rate (TD‐TR) model. The results indicate that the influence of rate‐limited adsorption on NE B behavior was more pronounced at fast than at slow water flux. However, at slow water flux, the rate‐limited adsorption had a secondary importance in comparison with the rate‐limited mass‐transfer between mobile and immobile water domains.

We have developed a new modeling approach for designing the geometry of trickle irrigation systems: root water uptake rate is treated as the unknown, i.e., sought after, quantity. It is determined from solutions to the linearized, steady‐water‐flow problem that describes the interaction between surface point (or line) sources and subsurface point (or line) sinks. We postulate that a sink that creates a maximum allowable suction defines an upper bound to the relative water uptake, and propose to use this upper bound as a design criterion. In the basic scenarios of a coupled source and sink, as few as three system parameters—the soil sorptive number (inverse of the sorptive length) and the depth and radius of the conceived rooting zone—can be sufficient to characterize the competition between root water uptake and deep percolation processes. In its dimensionless form, the proposed water uptake model is general for all soil types and all source–sink geometries. Evaporation is taken into account in the basic solutions for a source–sink couple with matric‐flux‐potential‐dependent evaporative water flux at the soil surface. Local resistance can be incorporated into the model by adding a skin resistance at the soil–sink interface. In addition to evaluating the relative water uptake rate, we also plotted contours of constant matric flux potential (pressure head and saturation degree) and streamlines, including the dividing streamlines that delineate the water capture zone and separate it from the deep percolation zone and the evaporation zone. Using computations, we elucidated the effects of the above system parameters on the relative water uptake rate and water flow patterns for three‐ and two‐dimensional systems; these computations describe single drippers, drip tapes, and drip lines with closely spaced drippers.

Infiltration of water from arrayed, interacting surface water sources (emitters) and extraction of water by plant roots is of interest in the context of trickle irrigation. In this study, steady flows from subsurface or surface point and line sources in laterally confined .soil domains were analyzed on the basis of the linearized Richards equation written in terms of the matric flux potential (MFP). Analytical solutions (Green's functions) were derived tor the problems of three-dimensional infiltration from point sources into confined strip-shaped, rectangular, and cylindrical domains and for two-dimensional infiltration from a line source into a strip-shaped domain. Incorporating these solutions into a coupled source-sink water flow and uptake model enabled analysis of the effects of lateral confinement and various source-sink (emitter-plant) configurations on relative water uptake rates (RWURs). The exact solutions for infiltration into confined strip-shaped and rectangular domains derived in this study confirmed the accuracy of previously presented RWUR results, by superposition of the solutions for equivalent, arrayed point sources (along a single drip line and in an array of parallel drip lines). An equivalent confining cylinder of the same cross-sectional area as a square was found to yield RWURs in excellent agreement with those tor a square confinement. A bidirectional tcctangulat confinement increased the RWURs more than a unidirectional strip-shaped confinement. Among the various simulated emitter-plant configurations, the lowest RWURs were obtained for a configuration of a single plant row irrigated by two lateral drip lines.Abbreviations: MFP, macric flux potential: RWUR, relative water uptake rate. I nfiltration of water from arrayed, interacting (surface or subsurface) water sources (emitters or drip lines) and extraction of water by plant roots is of interest in the context of trickle irrigation because plant water uptake is, naturally, one of the major components of the irrigation water balance (Dasberg and Or, 1999). These processes are critical for normal plant growth and crop yields, and they also greatly affect the flow of water and solutes in soil-plant systems and thus determine the quantity and quality of percolating excess water. Water uptake by plant roots can be considered at two distinguishable-microscopic and macroscopic-scales (Feddes and Raats, 2004;Raats, 2007). Microscopic models simulate water flow into individual plant roots on the a.ssumption that the root acts as a cylindrical sink of finite radius and infinite (Gardner, 1960;Hillel et al., 1975;Molz, 1975;Passioura, 1988) or finite (Aura, 1996;Personne et al., 2003) length. The flow equation (usually diffusion type) is solved for the appropriate boundary conditions (i.e., a given water potential or given water flux) at the soil-root interface and a prescribed water potential at some distance from the root. Although the microscopic approach provides insights into water uptake by plant roots, its application is quite restricted ...

A quasi‐linear form of Richards' equation, which assumes exponential dependence of the hydraulic conductivity on the pressure head and depth, was used to analyze the flows from a point source and to a point sink in a cylindrically confined soil domain overlying a shallow groundwater table (WT). The evaporation flux at the surface was taken to be proportional to the matric flux potential (MFP). Analytical solutions for the time‐dependent and steady‐state problems were obtained by using integral transforms. The general solution for a point source in a finite‐length cylinder takes the form of double series, which contains the inverse of the finite Hankel transform in the radial direction coupled with the inverse of the generalized Fourier transform in the vertical direction. Numerical evaluation of this solution is straightforward but time consuming for both time‐dependent and steady‐state cases. An alternative solution for a steady point source that involves only the Hankel inverse transform was found to be more practicable. Steady flows from a surface point source toward a WT and from a WT toward a subsurface sink were analyzed by mapping the distributions of the pressure head and the MFP, together with the streamlines. The sink was simulated by a suction zone of prescribed radius whose strength was evaluated from the condition that MFP equals zero at a reference point at either the bottom or the top of the suction zone. Particular attention was paid to the choice of locations of the reference point for evaluating the sink strength, for which the solution for the sink remains physically relevant irrespective of whether or not the surface loses water by evaporation.