Evidence for deviations from Darcy's law during hydraulic flow through fine-grained soils is reviewed. Abnormal water properties, electrokinetic coupling, fabric changes under the action of seepage forces, and experimental errors are considered as possible causes for the observed behavior. Data are presented to suggest deviations from Darcy's law in saturated kaolinite and in saturated, compacted silty clay. No evidence for a threshold gradient was found for these materials. The behavior of the compacted specimens was such as to indicate that particle migrations are more likely causes for non-Darcy flow than are abnormal water properties. Support for this conclusion was provided by the results of tests wherein pore pressure distributions along the length of specimens were determined during flow. In this way it was possible to determine the variations in hydraulic gradient with time at different points in the sample. The effects of non-Darcy flow behavior in soil mechanics problems are discussed. If high gradient tests are used in the laboratory, the results may bear little relationship to the behavior in the field where gradients seldom exceed unity. Data are presented illustrating the variation of excess pore pressure with time at several points throughout the height of a consolidating clay specimen. The results indicate, contrary to other evidence in the literature, that there could have not been a threshold gradient for flow. Because of the considerable experimental uncertainty associated with permeability measurements on fine-grained soils, laboratory test results must be interpreted with caution.
Bioremediation of fuel components seems to be proceeding along two fronts, in situ biodegradation and landfarming. This chapter first determines the state-of-the-art regarding landfarming of waste fuels, petroleum sludges, and pesticide-contaminated soils. It reviews the potential of biostimulation to be used in conjunction with landfarming of pesticides. The chapter also examines some results from a recently reported experiment that assessed the feasibility and safety of landfarming soil contaminated with the herbicide alachlor. A review of the literature has indicated that landfarming of fuel and pesticide wastes has great potential, but further study is needed to develop practical guidelines and ensure protection of surface and groundwater. The literature also suggests that biostimulation by addition of organic matter may enhance the landfarming process. The feasibility of landfarming soil contaminated with the herbicide alachlor was tested in small field plots.
ABSTRACT A comprehensive relationship for the splash erosion process was derived from dimensional analysis. The processes which occur in both the drop-solid and the drop-liquid-solid domains were described with analytical relationships and simplified models. The models con-sider raindrop, sediment and system parameters affect-ing splash erosion. The effects of drop mass and impact velocity, slope gradient, water depth and time were evaluated using experimental data available.
ABSTRACT A fundamentally-based, soil erosion and sediment transport model which is incorporated with a distributed-system hydrologic model (ANSWERS) was developed. It is a single-event model and considers typical upland erosion and sedimentation processes of splash erosion, flow erosion, sediment transport, and deposition. Channel erosion is also included. Erosion rates are related to hydrologic and hydraulic variables; soil characteristics; and surface and geographic condi-tions. The model was tested with data from two small agricultural watersheds of less than 10 ha in area. The model simulated sediment discharges which were com-parable to observed data.
ABSTRACT The storage volume of tilled soil surface roughness was considered a controlling factor in the form of the boundary and in damping out turbulence due to rainfall impact and overland flow. Four surfaces of different roughness were created using different tillage operations on field plots. An element composed of several micro-storages within a partial area of the plot was considered as a system. Hydraulic performance of the elements that effectively produced runoff was identified by using a coefficient of storage of the tilled layer. The coefficient of storage, or the degree of hydrostatic pressure increase on the tilled layer, during the rainfall-runoff process was evaluated. This parameter and other factors affecting surface roughness storage such as rainfall rate, porosity of the tilled layer and initial soil moisture condition were evaluated to define the time response of micro-storage. Rainfall-runoff processes on elements of a plot were examined using videotapes recorded during the events. Computed values of three physical models were statistically compared to observed values. The models predict time responses of depression and detention storage, and ponding using an evaluation of the coefficient of storage.
A physically based model was used to simulate runoff in agricultural watersheds with tile drainage systems.The TOPMODEL, which is based on the detailed topographical information provided by a digital elevation model (DEM),was modified for this simulation study. Nine possible flow generation scenarios were used in the development of themodel. The model can identify the portions of the hydrograph resulting from tile flow, subsurface flow, and surface runoff.The performance of the model was tested through a calibration, sensitivity analysis and validation process. The simulatedhydrologic response was designed to produce several components of the outflow hydrograph which were associated withthe various possible flow generation scenarios. The results of this simulation study show that the model describes thephysical system well and provides a better insight into the hillslope hydrology of agricultural watersheds with subsurfacedrainage systems.
Drainage of excess water from agricultural lands is essential for crop growth. In the Midwestern U.S., Illinois, Indiana, Iowa, Ohio, Minnesota, Michigan, Missouri, and Wisconsin are some of the states that are highly drained with intensive subsurface drainage systems. Illinois alone has a total drained area of approximately 4 million ha. Many watersheds in east-central Illinois have less than 1% surface gradient and poorly drained soils, yet subsurface drains have made these lands some of the most productive farmlands in the world. Subsurface drainage enhances productivity and reduces sediment transport and phosphorous losses from fields; however, it increases NO3-N delivery to the receiving water bodies. Nitrate-N is mobile and can be lost from the soil profile by leaching and through subsurface drains (tile drains). Nitrate-N concentrations in drinking water reservoirs in many Midwestern states frequently exceed the EPA's maximum contaminant level (MCL) of 10 mg L-1. Several studies show NO3-N concentration data of more than 10 mg L-1 in discharges from subsurface drains. This article presents a history of subsurface drainage in Illinois, summarizes the results of decade-long field and watershed-scale research on subsurface drainage, and provides information on some innovative practices that are currently being evaluated to meet water quality challenges. It has been observed that infiltration is the predominant hydrologic process, and surface runoff rarely occurs from these watersheds. It has also been observed that base flow contributes significantly higher nutrient loadings into the streams than the subsurface drains from these watersheds.
The effectiveness of wetlands to cleanse eventdriven agricultural drainage water in eastcentral Illinois wasstudied. A wetland was constructed at the outlet of a subsurfacedrained agricultural field in a cornsoybean rotation.Hydrology data from the wetland inlet and outlet, including precipitation and evapotranspiration data, were used to developa water budget for the system. Water quality data were collected from the wetland inlet, outlet, and the pond section of thewetland and analyzed for nitratenitrogen (NO3N), orthophosphate (PO4P), and nine common Midwestern herbicides:trifluralin, atrazine, alachlor, metolachlor, ethalfluralin, butylate, clomazone, cyanazine, and pendimethalin. Constituentmass loads were calculated at the inlet and outlet, and both concentration and mass load data sets were statistically analyzed.Results indicated variable performance based primarily on seasonal processes and individual chemical constituent. Overall,NO3N mass load assimilation was approximately 174 kg (32.9%) over the course of the study, although assimilation rateswere seasonally dependent. PO4P and herbicide concentration and mass load assimilation were not significant.
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