Browsing by Author "Redden, D. L."
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Item Contaminant Transport in Hydrogeologic Systems(Texas Water Resources Institute, 1981-03) Redden, D. L.; Chin, C.Contaminant transport in hydrogeologic systems requires knowledge of transmissivity, storage coefficient, and dispersivity. Techniques for evaluating transmissivity and storage coefficient under field conditions are well known. However, the evaluation of dispersivity under field conditions is a costly and time consuming job. The process of transporting a specific conservative ion species in an aquifer is analogous to the transport of heat in the system. Because of this analogy, the original objective of this research project was to evaluate the use of low-grade thermal water to measure aquifer dispersivity. However, available thermal models of groundwater aquifers proved difficult to use for evaluating the thermal properties (and dispersivity) of an aquifer. Therefore, additional objectives were developed to (1) derive analytical solutions describing the steady and unsteady temperature distribution around a well with a finite caprock thickness and (2) establish a technique for determining the thermal properties (including thermal dispersivity) of an aquifer using field measurements of temperature distribution within the aquifer. Analytical models of hot water injection into groundwater aquifers were developed in this study. Available analytical models of this problem assume that the caprock overlying the aquifer is of infinite thickness. However, many groundwater aquifers have caprock thicknesses of only a few meters. This paper shows two mathematical models which were developed to examine the influence of a caprock with finite thickness on the thermal response of an aquifer. In both models, the horizontal heat conduction and heat convection in the aquifer plus the vertical heat conduction in the caprock are considered. The first model (Model I) assumed that the vertical temperature gradient in the caprock is linear, which can be approached in a caprock with a relatively small thickness. The second model (Model II) removed this restriction and allowed the vertical temperature gradient in the caprock to be nonlinear. For Model I, a steady state and an unsteady state solution for the water temperature distribution surrounding an injection well were obtained. For Model II, a steady state and two unsteady state solutions for the water temperature distribution surrounding an injection well were obtained. One of the two unsteady state solutions is for a short-time period and the other one is for a long-time period. A graphical technique was developed for determining four pertinent aquifer thermal properties: (1) the horizontal thermal conductivity of the aquifer (thermal dispersivity), (2) the thermal capacity of the aquifer, (3) the vertical thermal conductivity of the caprock, and (4) the thermal capacity of the caprock. Dimensionless type curves are constructed from the steady state solution and the unsteady state solution for short time periods in Model II, respectively. Using field data, one curve is constructed using long-term temperature observations (approaching steady state) from several observation wells, and a second curve is constructed using short-time temperature observations from any one of the observation wells. These curves are then matched with the dimensionless type curves, respectively, and values of the four aquifer thermal properties evaluated. Since the steady state condition is difficult to attain in the field, an approximate graphical technique for evaluating the thermal parameters is developed without using the steady state field date. In this approximate method, the vertical thermal conductivity of the caprock is assumed equal to the horizontal thermal conductivity of the aquifer, and the thermal capacity of the caprock is assumed equal to the thermal capacity of the aquifer.Item Improved Water and Nutrient Management Through HighFrequency Irrigation(Texas Water Resources Institute, 1981-03) McFarland, M. J.; Redden, D. L.; Newton, R. J.; Brown, K. W.; Howell, T. A.High frequency irrigation implies the uniform, frequent application of water to crops. The fequency may range from several irrigations per week to daily irrigation to even several irrigations per day in greenhouse and nursery settings. Most of the high frequency irrigation in the United States is through necessity; i.e., the limited water holding capacity of the soils or a limited water supply make irrigation application of more than a few centimeters impractical. Irrigation of field crops in sandy soils (such as in the Nebraska Sand Hills) with traveling or outer pivot sprinkler systems is a classic example of high frequency irrigation dictated by a limited water holding capacity. Another widespread use of high frequency irrigation is found in the various low pressure systems such as drip, trickle, bi-well, and bubbler. These systems deliver relatively small amounts of water to the root zone as a consequence of factors such as limited water, shallow soils, limited water holding capacity, and high erosion potential. The increased frequency of irrigation is not commonly a goal in itself, but several advantages of high frequency irrigation have been identified (Rawlins and Raats, 1975; Howell, et. al., 1976). These include: 1. Improved plant internal water balance, 2. Decreased drainage from the root zone, 3. Decreased runoff from the crop, 4. Decreased importance of soil hydraulic characteristics, 5. Improved salinity control, 6. Increased enhancement of rainfall utilization, 7. Reduction of high temperature stress, and 8. Reduction of nutrient leaching The results of these advantages are usually increased crop yield or quality, decreased water use, and decreased pollution from drainage and runoff. The yield expected under high frequency irrigation may not be signsficantly increased over well-managed conventional irrigation, but increased efficiency of water, energy, fertilizer, and labor make even modest yield increases important. Nutrient management is critical with high frequency irrigation. Nitrogen in particular is susceptible to loss from the root zone by leaching, so high frequency irrigation systems require frequent, light nitrogen applications. This is facilitated by distribution of nutrients through the irrigation system. Other chemicals such as soil fumigents for nematode control, systemic insecticides, and herbicides can also be injected into the irrigation system and applied very uniformly to the crop. This research was directed toward defining the best management practices for irrigation timing and fertilizer applications under high frequency irrigation. The specific objectives were to: 1. quantitatively determine plant nutrient requirements for specific crops grown under high frequency irrigation, under optimum soil-water metric potential, and 2. evaluate the impact of high frequency irrigation on water quality and water and energy consumption.