Journal of Irrigation and Drainage Engineering

In order to rigorously examine near surface, field to field interactions between irrigation management regimes and a shallow fluctuating water table, an enhanced deforming finite element (DFE) model was recently developed. The enhanced DFE model, through a process of iteration within each time step, avoids making common assumptions regarding the changing geometry of an aquifer free surface. This paper demonstrates the usefulness and effectiveness of the model by employing it to an irrigated region in the western San Joaquin Valley, Calif., where shallow subsurface tile drains have been installed to control shallow water tables. By virtue of the problems created by the need to dispose off the drainage water, this region has been the focus of several important regional scale modeling exercises, which have evaluated the utility of management strategies, such as source control, groundwater pumping, and land retirement. By refining the focus of the analysis, the enhanced DFE model is found to be able to show that both sources control and managed pumping could be more effective drainage control strategies than predicted based on the results of regional models.

Analytical expressions for ramp kernels (new kernels) for an improved convolution for obtaining aquifer responses, viz, groundwater head, rate, and cumulative volume of groundwater flow, to an arbitrary stage, are obtained. The use of the ramp kernels gives accurate aquifer responses and is superior to the conventional convolution in which numerical integration or pulse kernels are used. The extent of improvement in the results with the use of the ramp kernels is discussed and quantified for three examples, where the results are compared to analytical solutions. For the comparisons, the analytical solutions for linear and sinusoidal stream stages are derived. The use of the ramp kernels reproduces accurately the analytical solutions. The concept of ramp kernels can also be used for obtaining an accurate solution of convolution integrals observed in other fields.

This study investigated the characteristics of shallow groundwater quality under agricultural areas dominated by rice paddy fields, vegetable fields, and a residential zone on an alluvial basin by examining spatial distributions of ${\mathrm{Cl}}^{-}$, ${\mathrm{Na}}^{+}$, ${\mathrm{NO}}_3^{-}$, and ${\mathrm{SO}}_4^{2-}$ concentrations. Seasonal changes in spatial distribution of ion concentrations during one cultivation period and the causes of the changes were discussed. Changes in spatial distribution of ${\mathrm{Cl}}^{-}$ and ${\mathrm{Na}}^{+}$ concentrations of the groundwater were mainly affected by seasonal changes in precipitation concentrations. ${\mathrm{NO}}_3^{-}$ concentration was strongly influenced by fertilization of vegetable fields and a following heavy rainfall. Elevated ${\mathrm{NO}}_3^{-}$ concentration was less likely to occur under paddy fields than under vegetable fields. It was suggested that paddy fields might mitigate ${\mathrm{NO}}_3^{-}$ contamination of shallow groundwater possibly because of denitrification in paddy soil in an anaerobic state during ponding. As for ${\mathrm{SO}}_4^{2-}$, it was acknowledged that there could be a specific point source near a particular observation well.

The wastewater total suspended solids (TSS) concentration effects on the saturated hydraulic conductivity, $K_s$, of a clay and a loam soil were investigated on laboratory repacked soil cores by a constant head permeameter. Both municipal wastewater (MW) and artificial wastewater (AW) with different TSS concentrations were used, with the aim to evaluate, by comparison, the effects of biological activity. The development of a surface sealed layer was investigated in loam soil columns supplied with AW and equipped with water manometers at different depths to detect the hydraulic head gradient changes. In the loam soil, $K_s$ reduced to about 80% of the initial value after infiltration of $175\mathrm{mm}$ of MW with $\mathrm{TSS}=57-68\mathrm{mg}{L}^{-1}$. Reductions in $K_s$ were more remarkable in the clay soil. An empirical relationship was proposed to predict the relative hydraulic conductivity, $K_r$, i.e., the ratio between actual and initial hydraulic conductivity versus the cumulative density loading of TSS. Hydraulic head gradients in the top layer $(0-20\mathrm{mm})$ of the soil columns increased during application of AW, as a consequence of the formation of a sealed layer, denoting that the surface pore sealing was the main mechanism responsible for the observed $K_s$ reductions. Laboratory data were gathered in a numerical simulation code specifically created to assess the consequences of $K_s$ reduction on water movement through the soil profile. Simulation of both ponded and sprinkler irrigation with MW resulted in reduced infiltration and increased surface ponding condition compared to the application of fresh water (FW).

Poor uniformity of water application by subsurface drip irrigation has been examined and some explanations are suggested in this paper. Use of soil conditioners for soil structure stabilization around subsurface drip irrigation pipes was suggested by the authors and tested in the laboratory. The conditioners preserve the structure of existing aggregated and may effectively reduce soil clogging. A silt loam soil was uniformly packed in a $1\times 0.8\times 0.15m$ box. Two holes were dilled in the box wall through which two emitters were inserted, one for applying solution of soil conditioner and one as a control. Stabilization was achieved by applying two types of polymer solutions differing by their molecular weights through an emitter buried in a silt loam soil. A measured water volume was injected through the emitters into the soil and, after $48h$ following irrigation, the box was dismantled. Gravimetric soil moisture content and aggregate water stability were measured in vertical and horizontal distances from the emitter. The highest stabilizing effectiveness was obtained with a volume of $1.5L$ polymer solution at $5g/L$ concentration, which was applied to the soil at an initial moisture content of 13%. The volume of stabilized soil increased with the volume of applied solution, but the volume ratio of stabilized soil to applied solution decreased with the increase in solution volume. A polymer of relatively low molecular weight was found less effective since a large portion of the solution was consumed by fast penetration into soil aggregates without improving the soil structure. The proposed method offers a simple and easy means for preparing a stabilized soil envelope around subsurface drip irrigation pipes, which may improve the long-term performance and uniformity of the application of these systems. Yet for some of these aspects, further field evaluation is needed, since the results of the study are from a laboratory experiment limited to one soil only.

Center-pivot laterals are characterized by important friction loss, so therefore the energy consumption is also significant. In this paper, a new, particularly simple, equation is derived to solve explicitly the economic design problem of tapered center-pivot laterals taking into account the energy cost. The appropriate lengths of center-pivot lateral segments with different pipe sizes are calculated directly from the proposed equation in such a way that the total cost of energy and lateral segments is minimized. Friction loss can be calculated using the simple empirical Hazen–Williams formula but also can be calculated using the more general Darcy–Weisbach friction equation. The explicit economic design is applied in two design examples. A detailed economic analysis and comparison between different systems with and without multiple pipe lateral sizes also is performed. A stepwise economic design method determining the most economic pipe diameter at each section of the lateral included between two consecutive sprinklers also is applied. Comparison between the two methods indicates the good performance of the analytical method.

The development of a coupled crop model (Ador-Crop) and solid set sprinkler irrigation model (Ador-Sprinkler) is reported in this work. The crop model incorporates many of the features developed in the well-known CropWat model. Improvements include the use of thermal time and the input of daily ${\mathrm{ET}}_0$. The solid set sprinkler model applies ballistic theory to determine water distribution resulting from water droplets subjected to a wind vector. Regarding the validation of the coupled model (AdorSim), the plot of soil available water versus measured and simulated yield reduction resulted in similar features. AdorSim explained 25% of the variability in measured yield reduction. Most of the unexplained variability is due to the effect of nonwater-related factors affecting crop yield. In a companion paper, AdorSim is used to investigate optimum water management options in the middle Ebro basin in the NE of Spain.

In this work, applications of the coupled solid set sprinkler irrigation and crop model AdorSim introduced in the companion paper are presented. The sprinkler irrigation model is based on ballistic theory, while the crop model is based on CropWat. AdorSim was used to evaluate the effect of sprinkler spacing on seasonal irrigation water use (WU) and crop yield. The most relevant results were related to the characterization of advanced irrigation scheduling strategies. The differences in crop yield and WU derived from irrigating at different times of the day were estimated for two locations strongly differing in wind speed. Irrigation guidelines were established in these locations to relate gross water use and water stress induced yield reductions. Simulations were also applied to estimate adequate wind speed thresholds for irrigation operation. In the experimental conditions, thresholds of $2.0-2.5m{s}^{-1}$ proved effective to control yield reductions and to minimize WU.

Design of a stilling basin downstream of a barrage requires knowledge of various elements of hydraulic jump with known values such as the discharge and the energy lost in the jump. The problem reduces to solution of the implicit equations for sequent depths requiring the tedious method of trial and error. Presented herein are the exact equations for sequent depths.

A dropshaft is an energy dissipator connecting two channels with a drop in invert elevation. The hydraulics of vertical rectangular shafts was systematically investigated in seven configurations. A particular emphasis was on the effects of shaft pool, outflow direction, and drop height, while geometrically similar shafts (scale 3.1:1) were studied using a Froude similitude. The results demonstrate that rectangular dropshafts with 90° outflow are the most efficient energy dissipators. The shaft pool and drop height have little effect on the rate of energy dissipation. Recirculation time results exhibited marked differences between flow regimes and the longest dimensionless residence times were observed at low flow rates. Although basic flow characteristics were similar between model and prototype, observations of dimensionless bubble penetration depths and recirculation times showed some discrepancy, highlighting limitations of the Froude similitude for studies of air entrainment and residence times in dropshafts.

An earth dam can be prevented from a seepage failure due to softening of the downstream slope by providing a rock toe or horizontal drainage blanket. Analytical solutions are not available for determining the length of the filtered drainage blanket and downstream slope cover, though graphical solutions are available for them. Explicit equations have been obtained in the present work for calculating the downstream slope cover and the length of the downstream horizontal drain in homogeneous isotropic and anisotropic earth dams. Similar equations have also been obtained for maximum downstream slope cover and minimum and maximum effective length of the filtered drainage. These equations are nonlinear and representative graphs have been plotted for them covering all the practical ranges of the dam geometry. The numerical example demonstrates that the proposed equations are simple to use, hence the designers may find these equations as an additional check to their design by the conventional flownet method.