Monitoring Subsurface Drainage Flow at Remote Locations

Laboratory evaluations and field results are presented for a slotted weir used to measure discharge from subsurface drains. The head–discharge curve for the vertical slot is a simple power function with an exponent of 1.5. There was excellent agreement (r2 \u3e 0.99 and 1:1 slope) between predicted and observed discharge in laboratory testing of 12 test weirs representing five slot widths. The primary advantages of the vertical slot weir are its simplicity, ease of maintenance, and ability to measure small flow rates. Disadvantages include a tendency for the slot to close a small amount over time as a result of creep when using a PVC pipe and the possibility for material to become clogged in the slot. The use of a spacer in the slot eliminated the tendency for the slot to close

Comparison of Water and Temperature Distribution Profiles Under Sand Tube Irrigation

Drip irrigation is one of the most efficient systems in delivering water to the plant root zone. Research has shown that the saturated, or nearly saturated, surface beneath the emitter may increase evaporation thereby reducing the irrigation efficiency. To increase the efficiency of surface applied drip irrigation on permanent tree crops a sand tube irrigation (STI) method was developed and tested. The sand tube method consists of removing a soil core beneath the emitter and filling the void with coarse sand. A weighing lysimeter was designed and instrumented to directly measure temporal evaporation during irrigation and for a period of three days after irrigation ceased. Thermocouples were used throughout the soil profile to detect the temperature variation and also to determine temporal movement of the wetting front. The results indicated that for the surface applied drip irrigation method, approximately 30% of the applied water evaporated during the four-day period after irrigation. The STI method resulted in approximately 4% of the applied water being evaporated. The STI method allowed more water to remain in the soil profile thereby increasing the irrigation efficiency

Evaporation Reduction Potential in an Undisturbed Soil Irrigated with Surface Drip and Sand Tube Irrigation

The efficiency of drip irrigation is highly dependent on evaporation losses occurring from the constantly saturated soil beneath emitters. Advent of subsurface drip irrigation is in part an approach to curb this inefficiency. An irrigation method, Sand Tube Irrigation (STI), is proposed to increase the efficiency of “Normal” surface applied drip Irrigation (NI method) on permanent tree crops without the need for burying the irrigation tubing. The sand tube consists of removing a soil core beneath the emitter and filling the void with coarse sand. A weighing lysimeter was constructed in the laboratory and instrumented to directly measure temporal evaporation from large, undisturbed soil columns, 0.7 m in diameter and 0.8 m in height. Experiments were performed on six replicated soil monoliths to compare the two methods. The results indicated that, for four consecutive days after irrigation, there was a significant difference at the 95% confidence level between evaporation occurring from the NI and STI methods. After four days of evaporation, comparison of water contents indicated that a higher amount of water existed between the depths of 0.2 to 0.55 m in the STI versus the NI method. Although drainage occurred from the macropore structure of the undisturbed soil monoliths, the STI method showed potential in retaining more water in the micropore structure of the lower depths, that would be available for plant use rather than potential evaporation

Hydrologic Properties of Pervious Concrete

Pervious concrete is concrete made by eliminating most or all of the fine aggregate (sand) in the concrete mix, which allows interconnected void spaces to be formed in the hardened product. These interconnected void spaces allow the concrete to transmit water at relatively high rates. The main objective of this project was to conduct research on the potential application of pervious concrete in agricultural settings, specifically for use in animal feed lots, manure storage pads, animal manure and bedding compost facilities, or floor systems in animal buildings. Laboratory tests were conducted on replicated samples of pervious concrete formed from two rock sources (river gravel and limestone) for coarse aggregates and different size fractions to determine hydrologic relationships. Linear relationships were found between density and porosity, density and permeability, porosity and permeability, and porosity and specific yield. The results suggest that properties such as permeability, porosity, and specific yield are not significantly affected by different aggregate types. However, density and porosity can be effective methods for predicting porosity, specific yield, and permeability. In addition, t-tests were conducted to determine the effect of aggregate types on the solid/liquid separation properties of the pervious concrete after adding composted beef cattle manure and bedding to the surface of the specimens. The amount of composted beef cattle manure and bedding retained within the specimens was significantly less (p = 0.012) when samples constructed of #8 river gravel were used rather than the other aggregates. The #8 river gravel also had significantly less reduction in permeability compared to other aggregates. Although the #8 river gravel had a different effect on the compost retained and the reduction in permeability for the specimens, all four aggregates exhibited a significant reduction in the permeability after the compost was applied

Simulation of Daily and Monthly Stream Discharge from Small Watersheds Using the SWAT Model

The Soil and Water Assessment Tool (SWAT) was evaluated and parameter sensitivities were determined while modeling daily streamflows in a small central Kentucky watershed over a two-year period. Streamflow data from 1996 were used to calibrate the model and streamflow data from 1995 were used for evaluation. The model adequately predicted the trends in daily streamflow during this period although Nash-Sutcliffe R2 values were –0.04 and 0.19 for 1995 and 1996, respectively. The model poorly predicted the timing of some peak flow values and recession rates during the last half of 1995. Excluding daily peak flow values from August to December improved the daily R2 to 0.15, which was similar to the 1996 daily R2 value. The Nash-Sutcliffe R2 for monthly total flows were 0.58 for 1995 and 0.89 for 1996 which were similar to values found in the literature. Since very little information was available on the sensitivity of the SWAT model to various inputs, a sensitivity analysis/calibration procedure was designed to evaluate parameters that were thought to influence stream discharge predictions. These parameters included, drainage area, slope length, channel length, saturated hydraulic conductivity, and available water capacity. Minimization of the average absolute deviation between observed and simulated streamflows identified optimum values/ranges for each parameter. Saturated hydraulic conductivity, alpha baseflow factor, drainage area, channel length, and channel width were the most sensitive parameters in modeling the karst influenced watershed. The sensitivity analysis process confirmed die trace studies in the karst watershed that a much larger area contributes to streamflow than can be described by the topographic boundaries. Overall, the results indicate that the SWAT model can be an effective tool for describing monthly runoff from small watersheds in central Kentucky that have developed on karst hydrology however calibration data are necessary to account for solution channels draining into or out of the topographic watershed

Response of Runoff Diazinon Concentration to Formulation and Post-Application Irrigation

Pesticides used in urban environments can be transported in runoff to downstream waters and cause adverse environmental consequences. This experiment assessed the effects of post-application irrigation depth (0, 6.4, and 12.7 mm) and formulation (liquid and granular) on concentration and transport of diazinon (a pesticide commonly used for lawn insect control) in runoff from “tall” fescue (Festuca arundinacea Schreb.) plots. The post-application irrigation was applied using rainfall simulators immediately following diazinon application. The rainfall simulators were again used approximately 2 h after diazinon application to apply the equivalent of a heavy rainfall (64 mm/h for approximately 1.5 h) to generate runoff. Runoff was sampled and analyzed for diazinon using the enzyme-linked immuno-sorbent assay method. Post-application irrigation depth had no effect on diazinon concentration but increased diazinon mass transported off the plot by increasing plot runoff. Flow-weighted mean runoff diazinon concentration for the liquid formulation of diazinon was roughly double that of the granular formulation (0.59 vs 0.29 mg/L), attributed to the higher solubility of the liquid formulation relative to the granular formulation. The results indicate that post-application irrigation can increase runoff losses of diazinon for heavy rainfall occurring soon after application, but that these losses can be reduced by use of the granular formulation

Runoff from Fescue Plots Treated with TRIMEC

Runoff of herbicides can promote adverse impacts in receiving waters. The objective of this study was to assess the effects of rainfall delay, herbicide application rate, rainfall intensity, and pre-application rainfall on runoff of TRIMEC (a combination of 2,4-D, dicamba, and mecoprop), a herbicide that is commonly used in central Kentucky. The levels of rainfall delay were 0, 2, and 4 d following application; and the levels of herbicide application rate were 0, 0.5, 1 and 2 times the recommended rate. Simulated rainfall was applied at intensities of 64, 102, and 140 mm h-1; and the depths of water applied prior to TRIMEC application were 0, 13, and 25 mm. Flow-weighted composite runoff samples were analyzed by gas chromatography. Maximum concentrations in runoff for treatment combinations studied were: 2,4-D, 45.5 µg L-1; dicamba, 1.59 µg L-1; and mecoprop, 212 µg L-1. The rainfall delay affected both 2,4-D and dicamba concentrations but not mecoprop concentration, suggesting that its foliar half-life might be longer than suggested. As anticipated, runoff concentrations of all TRIMEC constituents were significantly (p \u3c 0.05) affected by herbicide application rate. Rainfall intensity affected only the concentration of mecoprop, with concentrations at the highest intensity being significantly (p \u3c 0.05) greater than those at the two lower concentrations. Pre-application rainfall had no significant effects on runoff concentrations. Mass transport averaged 1.51, 0.38, and 14.8% of amounts applied for 2,4-D, dicamba, and mecoprop, respectively, reflecting differences in degradation rates, wash-off characteristics and other factors. Mass transport was in no case significantly affected by the treatments. The findings of this study suggest that when TRIMEC is applied at the recommended rate under comparable soil, vegetation and weather conditions, the potential for 2,4-D to exceed the maximum contaminant level of 70 µg L-1 in runoff is low

Atrazine and Alachlor Dissipation Rates from Field Experiments

Chemical transport is being monitored in the root zone of three agricultural management systems at the Ohio Management Systems Evaluation Area (OMSEA). Atrazine and alachlor concentration data from soil cores taken to a depth of 0.9 m and partitioned into the increments of 0.0 to 0.15, 0.15 to 0.3, 0.45 to 0.6, and 0.75 to 0.9 m show the herbicides remained in the top 0.15 m of the profile during the 1991 and 1992 growing seasons. The slow movement of herbicides was partly due to below normal rainfall during the period. Since the herbicides have not been transported out of the soil profile, dissipation rates could be determined from the field observations. The data collected follow first-order kinetics in the dissipation of atrazine during the 1991 and 1992 growing season and of alachlor during the 1991 growing season for the two- to three-month period following chemical application. The computed rate constant, k, was 0.02 d–1 and half-life, t1/2, was 35 days for atrazine for both years. A rate constant of 0.04 d–1 and half-life of 17 days were computed for alachlor. The degradation rates became slower with residence time in the soil as a result of decreased availability from sorption/binding in the soil

Comparison of Daily Water Table Depth Prediction by Four Simulation Models

The Agricultural Drainage And Pesticide Transport (ADAPT) model was compared to the water management simulation models DRAINMOD, SWATREN, and PREFLO. SWATREN and PREFLO are one-dimensional finite-difference models while ADAPT and DRAINMOD are one-dimensional mass balance models. ADAPT, an extension of the computer model GLEAMS, also provides chemical transport information. All four models were tested against field data from Aurora, North Carolina. Observed water table depth data were collected during 1973 through 1977 from a water table management field experiment with three subsurface drain spacing treatments of 7.5, 15, and 30 m. Both the standard error of estimate and the average absolute deviation were computed between measured and predicted midpoint water table depths. For the five-year period ADAPT, DRAINMOD, SWATREN, and PREFLO had standard errors of estimated water table depth of 0.18, 0.19, 0.19, and 0.18 m and absolute deviations of 0.14, 0.14, 0.14, and 0.14 m, respectively. The results show good agreement between the models for this experimental site and encourage the further adoption of ADAPT to predict chemical transport

Modeling Surface and Subsurface Pesticide Transport Under Three Field Conditions Using PRZM-3 and GLEAMS

Contaminant transport models should be evaluated over a wide range of conditions to determine their limitations. The models PRZM and GLEAMS have been evaluated many times, but few studies are available in which predicted movement in runoff and percolate were simultaneously evaluated against field data. Studies of this type are essential because pesticide leaching and runoff are mutually dependent processes. For this reason, PRZM-3 and GLEAMS were evaluated for their ability to predict metribuzin concentrations in runoff, sediment, subsurface soil, and pan lysimeters under three field conditions (yard waste compost amended, no-till, and conventional-till) on a Lowell silt loam soil. Sensitive input parameters were either site specific (climatic, soil, and chemical) or calibrated (K-factor, C-factor, curve number). In general, both models under-predicted metribuzin concentration in runoff water, runoff sediment, subplow layer soil (15-75 cm), and pan lysimeter water (75 cm). Contrary to field data, both models predicted that a large percentage (\u3e 50%) of metribuzin would move below the “mixing zone” (top 1 cm) during the first rainfall event after application. Relatively little metribuzin was predicted to move beyond the plow layer (top 15 cm) into the pan lysimeters or subsurface soil throughout the simulation period, possibly due to the lack of a macropore component in the models. High metribuzin concentrations in sediment (field data) indicated that relatively little metribuzin moved below the “mixing zone”, possibly because of hysteresis but much of the metribuzin that did move was quickly transported into the pan lysimeters, probably due to macropore flow. GLEAMS more accurately predicted pesticide concentration in sediment and PRZM predicted subsurface soil concentration somewhat more accurately than GLEAMS. Little difference in accuracy was detected between models on metribuzin concentration in runoff or metribuzin concentration in percolate. Although both models generally under-predicted metribuzin concentration in runoff, runoff transport (mass of metribuzin in runoff) for the study period was over-predicted by both models which emphasizes the importance of accurately predicting herbicide concentration and runoff volume soon after application when the surface pesticide concentrations are highest