Journal of Irrigation and Drainage Engineering

The appearance of pesticides in ground water necessitates the accurate prediction of pesticide transport in ground water. Adsorption and desorption are major mechanisms affecting the movement of pesticides in ground water. Equilibrium and nonequilibrium (chemical and physical) adsorption expressions are reviewed with the merits and disadvantages of each enumerated. The occurrence of desorption and hysteresis of desorption are discussed. It appears that physical nonequilibrium (mass transfer and diffusion limitations) models most accurately describe and predict the pesticide transport experimentally observed. However, the governing equations are the most complex and costly to solve in terms of computer time and memory. Equilibrium adsorption expressions are more easily solved with analytical solutions available for simple conditions. The modeler must thus decide what level of sophistication of the adsorption/desorption processes is merited relative to the level of uncertainty of other soil parameters.

Atrazine and metolachlor are applied preemergent to corn in plots that are either drained (10‐cm plastic tubes 1‐m deep) or undrained. Herbicide concentrations in the soil profile down to 30 cm and in the drain water are measured for 243 days. The pesticides show a steady disappearance from the soil with pseudo first‐order half‐lives (no difference between drained and undrained plots) in the top 2.5 cm of soil of 35–36 days for atrazine and 20–23 days for metolachlor. Total loss of atrazine through soil leaching amounts to 0.038% of the application rate; 97% of this loss occurs within 78 days after application during a period of 530 mm of rain. All (0.13% of that applied) of the metolachlor in the drain water is observed within 59 days after application. The results of the study illustrate the strong correlation between pesticide soil persistence, water solubility, rainfall timing, and pesticide leaching through soil.

The GLEAMS (ground water loading effects of agricultural management systems) model was modified to simulate the generation and degradation of pesticide metabolites. Translocation of the metabolites within and through the root zone is considered. The modifications have been verified and preliminary validation has been made using field data. The paper presents the model concepts for metabolite generation and degradation. Results of model simulations for fenamiphos metabolite concentrations in the root zone are compared with observed concentrations from a research location in the Georgia coastal plain. These additions to GLEAMS will be helpful to model users working with those nonvolatile pesticides that have toxic metabolites with mobility and persistence characteristics differing from those of the parent compounds. Soil samples are taken periodically during the cropping season at depths corresponding to the GLEAMS model computational layers. The samples are analyzed for fenamiphos and its metabolites for comparison with model simulations. Field observations of metabolite mobility and persistence are compared with limited data in published literature.

The literature was reviewed regarding the sorption and transport of a nonionic organic pesticide aldicarb, an extremely toxic and relatively water soluble carbamate insecticide that has been detected in ground water across the United States. Leaching of organic compounds poses a serious threat to ground water. Sorption is a critical factor in determining how rapidly a pesticide will leach through the vadose zone. Aldicarb behaves as a nonionic organic compound being adsorbed primarily by soil organic matter and to a lesser extent by clay minerals. The assumption of a local equilibrium between the soil particles and the pore fluid is not always correct. Aldicarb transport may be controlled by nonequilibrium effects. Certain agricultural management practices may reduce the threat of ground‐water contamination by aldicarb residues.

Carbofuran, alachlor, atrazine, bromide (a tracer), and nitrate are applied to two tile‐drained plots of com, one of which is conventionally tilled and another that is no‐till. Soil water samples are collected from the plots with suction lysimeters and ground‐water samples are collected from wells and tile drains. The no‐till plot first showed low concentrations of atrazine and alachlor ranging from 0.1 to 0.4 μg/L below the root zone one month after application, but the conventionally tilled plot showed no pesticides below the root zone until late fall, when atrazine was detected at 0.4 μg/L. Dye studies indicated that, in the no‐till plots, some of the pesticide bypasses the root zone through surface‐connected ma‐cropores, whereas in the conventionally tilled plot, most of the pesticide is adsorbed within the root zone as a result of greater contact with the soil. Concentrations of bromide (which is not adsorbed) in samples collected from wells and suction lysimeters do not differ between the two plots. Nitrate is found only in that part of the profile that remains unsaturated throughout the experiment.

Renewed interest in the role of delivery networks in farm‐level water‐use efficiency has prompted the development of a transient hydraulic simulation model for improving the operation and design of irrigation canal networks. Numerical solutions derived from surface‐irrigation analyses of the past decade provide fast and accurate applications in a microcomputer environment. The model can simulate the filling and operation of canals, and can handle bulk lateral outflow from turnouts and wasteway weirs. Gate settings can be computed by the model with the objective of maintaining stable flow depths at the downstream end of each canal reach, thus providing a basis for improved canal operation and more efficient water management. A description is given of the adaptation of surface‐irrigation hydraulic modeling theory for use in branching canal networks, and model verification results are presented.

A method is presented by which the proportions of a watershed that contribute surface runoff in different storms and at different times during the same storm can be determined by analysis of rainfall and runoff records. The method is a modification of a method described in an earlier paper, which eliminates the need for subjective selection of particular runoff events for analysis. The method is illustrated using data from a 16.8‐ha watershed in Queensland, Australia, and the estimates of runoff from the different source areas are compared with actual records of runoff from the whole watershed. Runoff occurred from the entire watershed area on only three occasions in the 15‐yr study period, about 10% of runoff events. In about two‐thirds of runoff events, runoff came only from the 15% of the watershed that has the smallest surface storage capacity.

The storage coefficient and transmissivity are important aquifer parameters that are useful in assessing the ground‐water potential of an aquifer. For a nonleaky confined aquifer, these parameters can be determined with the help of a pump test. The present practice of aquifer parameters determination uses approximations of well function valid for small arguments only. In the absence of a full‐range equation for the well function, it is not possible to utilize the entire set of pump test data. In this paper a high accuracy expression for the well function valid for the entire range of its argument has been developed. The present practice of parameter estimation uses least square method. This gives undue importance to large errors. A criteria function capable of ignoring large observational errors has been suggested to replace the least square method. Minimizing the cumulative criteria function, the aquifer parameters can be estimated with a high degree of accuracy. A criteria function capable of ignoring large observational errors also has been developed. Minimizing the cumulative criteria function, the aquifer parameters can be estimated with a high degree of accuracy.

Various analytical and numerical solutions are available for the steady‐state form of Boussinesq's equation describing the phreatic surface on sloping lands. Except for the experimental results of Luthin and Guitjens (1967), however, no analytical or numerical solution is available for the unsteady‐state drainage of highly sloping lands. A finite difference solution has been obtained for the nonlinear unsteady‐state differential Boussinesq equation for drainage of sloping lands. The unsteady‐state solutions for higher slopes of up to 70% are presented and compared with the Luthin and Guitjens solutions. The proposed solution compares reasonably well with experimental solutions for up to 30% slope, but beyond that, the solutions tend to deviate substantially.

Flow in surface irrigation is subcritical and downstream conditions can propagate upstream. The shooting or initial‐value method started from the downstream end and proceeded upstream against the flow. Saint Venant hydrodynamic equations were solved cell by cell for flow area and flow rate in the upstream direction, given the advance increment of the wave front and an estimate of the time required to achieve that advance. This was in contrast to the two‐point boundary‐value solution of the full hydrodynamic model where the process started at the upstream end and swept downstream and then upstream during each iteration. Flow area and discharge were solved simultaneously for all nodes and advance distance was calculated for the given time step.

The paper presents guidelines that are designed to aid in the correct and successful crossing of drainline systems that are intersected by pipeline construction. The paper covers procedures, considerations, general criteria, alternatives, and numerous other elements that should be considered in the rehabilitation process. The purpose of the guidelines is to provide assistance to drainline owners, pipeline companies, and government entities that become involved in drainline systems crossed by pipeline construction. These guidelines are not intended to be used as complete specifications or replace the site‐specific engineering that may be required, but are to be used as a guide to reasonable and acceptable procedures to follow for the rehabilitation of any drainline system crossed by pipeline construction.