Browsing by Author "Brown, K. W."
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Item Comparison of Methods for Determining Soil Hydraulic Characteristics(Texas Water Resources Institute, 1980-12) Humphreys, K. B.; Newton, R. J.; Brown, K. W.; Reddell, D. L.; McFarland, M. J.; Howell, T. A.An adequate description of soil moisture movement is necessary for solution of agriculturally oriented problems such as irrigation, drainage and runoff control. Three approaches for determining the hydraulic properties of soil are in situ measurements, laboratory measurements and theoretical models. Field measurements, though representative, have the disadvantages of being costly and time consuming. Laboratory and mathematical processes are more practical but require extensive comparison to field results for evaluation. The purpose of this study was to determine the principle hydraulic properties of a soil of the Norwood Series utilizing the three approaches and to compare the results. The laboratory method selected was centrifugation (Alemi, et al., 1972). Soil cores were centrifuged and the redistribution of water was measured as change in weight with time. Inconsistent results and limited data obtained with this method, consequently, prevented adequate conclusions from being made. Hydraulic conductivity was obtained by measurement of hydraulic head and moisture content of the soil profile in situ with tensiometers and neutron probe, respectively. The theoretical procedure utilized water retentivity curves in conjunction with values of saturated hydraulic conductivity for computing hydraulic conductivity as a function of water content. Saturated hydraulic conductivity was measured in the field using Bouwer's (1961) double-tube method. The pressure-water content curves were obtained with disturbed soil samples for 30 to 80 cm depths and with soil cores for O to 15 cm depths using pressureplate extractors. A combination of laboratory and field measured values for these curves was also used for comparison. The field measurements yielded several relationships between hydraulic conductivity and water content, varying with soil depth. Comparison of calculated values with field data using only the laboratory water retention curves gave mediocre results for the 30 to 80 cm soil depth. However, when the field and laboratory data were combined and the resulting water retention curve was used to calculate hydraulic activity, the correlation was greatly improved. The O to 20 cm soil depth showed good results with both curves. Thus, it appears that this theoretical technique is applicable to soils of the type studied, but the accuracy of the calculated values is quite sensitive to the shape of the water retention curve, the saturated water content value and the saturated hydraulic conductivity value. Thus, accurate measurement of these parameters is necessary for its successful use.Item Determining the Transpiration Rate of Peach Trees Under Two Trickle Irrigation Regimes(Texas Water Resources Institute, 1980-12) Reeder, E. L.; Van Bavel, C. H. M.; Rodrigue, P. B.; Newton, R. J.; Brown, K. W.; Reddell, D. L.; McFarland, M. J.; Howell, T. A.The scientific design and management of a modern irrigation system requires that the designer or manager have knowledge of site and plant criteria such as infiltration, drainage, soil fertility, plant water needs, and plant production under varying conditions. With modern trickle systems water control is very precise and thus precise information on irrigation needs of a crop allow for the optimal use of water supplies. Work has been conducted on the effects of trickle irrigation on peach trees in North Central Texas. Initial data relating trickle irrigation amounts to total production, peach size, and plant growth have indicated that trickle irrigation may provide benefits that would offset costs of the irrigation system and water. Previous work however has related these benefits only to the amount of water applied through irrigation and did not consider the total water use of the tree. Research was undertaken to determine the transpiration rate of peach trees under two trickle irrigation regimes. To determine the transpiration rate a volume of soil around the test trees was instrumented with neutron access tubes. Soil moisture depletion was measured weekly. A soil water balance was conducted equating evapotranspiration to the sum of the change in the soil moisture content (a decrease being positive) plus irrigation applied, plus any rainfall that occurred in the period. For this work runoff and flux across the measurement zone boundaries was assumed zero. Estimates of evaporation from the soil surface were made using a two-stage evaporation process along with values of potential evapotranspiration made with the Penman (1956) equation. The estimates of evaporation from the soil surface were subtracted from total evapotranspiration to give estimates of the transpiration of the peach trees. Estimates of transpiration were not consistent from one measurement period to the next. Errors in the estimation of evaporation from the soil surface directly affect the estimate of transpiration. During latter stages of a rain-free period an estimate of transpiration was made which should not have been influences by the low values of evaporation from the soil surface that existed. This method of estimating transpiration has many errors and can be much improved upon by using a method such as a lysimeter to estimate transpiration more accurately.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.Item Response of Peanuts to Irrigation Management at Different Crop Growth Stages(Texas Water Resources Institute, 1980-12) Dahmen, P.; Newton, R. J.; Brown, K. W.; Reddell, D. L.; McFarland, M. J.; Howell, T. A.Past irrigation research on peanuts has shown that when the plant is exposed to soil moisture stress at different crop growth stages, different responses seem to exist between the Spanish and the Florunner peanut varieties. The Spanish peanuts appear more susceptible to soil moisture stress during the blooming and pegging stage, while the Florunners seem more susceptible during the late maturation stage. The objective of this experiment was to determine the optimum irrigation schedule for peanuts at different crop growth stages for the Spanish and the Florunner varieties. The yield of the two varieties was evaluated under seven different irrigation treatments including a "no stress" check treatment and a dryland treatment. Each treatment had a different schedule of either irrigating or stressing the peanut plant during one or more of three crop growth stages. The three crop growth stages were: (1) pegging; (2) early maturation; and (3) late maturation. Rainfall during the vegetative and blooming stage ensured adequate moisture for both of the crop growth stages. Evapotranspiration was monitored throughout the life cycle for both peanut varieties. The evapotranspiration was determined using a soil moisture balance equation. Plant growth in the form of dry matter accumulation and leaf area index was also studied for the Spanish variety. No significant differences in the leaf area index existed between the treatments. The dry matter growth analysis showed that an irrigation during the pegging stage resulted in a faster pod weight accumulation during the early maturation stage than if no irrigation occurred during that stage. The yield and evapotranspiration results showed that differences existed between the two peanut varieties. First, for the Spanish variety, the results indicated that soil moisture is needed during the pegging stage to obtain near maximum yields. Treatments with an irrigation during the pegging stage had a greater evapotranspiration and larger yields, than the treatments without an irrigation during this stage. Second, if an irrigation is made during the pegging stage, an additional irrigation during the early maturation stage is unnecessary. Third, an irrigation during the late maturation stage will increase yield if dry climatic conditions normally exist during this stage. In the case of the Florunner variety, the yield results indicated that moisture stress should occur in no more than one of the crop growth stages if yield reductions are to be minimized. Also, an adequate supply of soil moisture during the late maturation stage is absolutely necessary in order to obtain maximum yields for Florunner peanuts. Treatments which had an irrigation during the late maturation stage had a steady evapotranspiration rate during this crop growth stage and had near maximum yields. Treatments which showed a decrease in the evapotranspiration rate during the late maturation stage produced a significantly lower yield.