Journal of Irrigation and Drainage Engineering

In this paper the writers report the results of an experimental investigation on flow-resistance law in small-diameter plastic pipes. The experiments were carried out for a wide range of Reynolds number values obtained by varying the discharge and the water temperature. The experimental results are analyzed by both a purely empirical and a semitheoretical approach based on the hypothesis that the velocity distribution has a power-law form. The analysis shows that both the empirical and the semitheoretical approach allow accurate head loss per unit length estimate. In most cases (96–97% of runs) the estimate errors are less than 5%.

The Parshall flume was developed nearly 70 years ago to measure open-channel discharge accurately for agricultural applications. When the flume is subjected to numerous wet-dry, freeze-thaw, and heating-cooling cycles, the flume becomes susceptible to settlement. As the flume settles, the measurement accuracy becomes suspect. Commercially available Parshall flumes were purchased and assembled in accordance with the manufacturer's instructions and tested in the laboratory. A series of 383 experiments was performed to determine the accuracy of the Parshall flume when subjected to an array of settlement and submergence conditions. The results indicated that although significant difference between apparent and measured discharges exists, the apparent discharge can be corrected. An analysis of the submergence test results determined that for submergence of less than 90%, the flow can be corrected to within ±3%. However, flow conditions with submergence of 90% or greater can only be corrected to within ±5%. A comprehensive procedure is presented for adjusting the apparent discharge for settlement and submergence conditions.

This study determined whether termiticides (insecticides that are formulated and applied for termite control), applied according to label directions to the soil surrounding basement walls, can leach into foundation drainpipes. A licensed applicator injected silt loam and loamy sand soils adjacent to full-scale basement walls with technical chlordane and chlorpyrifos, according to label directions, using standard rod-injection equipment and procedures. During the rod injection, the termiticide flowed directly into foundation drainpipes at a rate of 300 mg technical chlordane and 180 mg chlorpyrifos per meter length of drain pipe. More termiticide was injected into the soil at deeper depths than at shallower depths. Over an eight-month period, 2 mg technical chlordane and 16 mg chlorpyrifos per m length of drain pipe were leached into the drainage system. It is concluded that, when applied according to label directions, termiticide can leach from foundation drainpipes, especially during injection. Recommendations include modifying equipment and the rodding technique to eliminate direct injection into drainpipes, increasing the number of injection holes to reduce termiticide volume per hole, and modifying injection protocol when shrinkage cracks are present.

The probable maximum flood (PMF) is a function of numerous factors, both meteorological and hydrologic. An accurate estimate of the PMF, required for high-hazard design problems, requires finding the optimum combination of factors. The PMP/UH (probable maximum precipitation/unit hydrograph) method is the currently accepted method to estimate the PMF. Difficulties with the current trial-and-error process used to estimate the PMF include: (1) Finding the optimum combination of factors; and (2) assessing the relative effect and precision of the underlying assumptions. The first difficulty may lead to an inadequate design, and the second difficulty may result in an inaccurate estimate of the risk involved in the design. An experiment was designed to perform a sensitivity analysis of the PMF to its contributing factors. A set of guidelines were developed for assessing the effects of causative factors on the PMF, so that: (1) The optimum value of the PMF is more likely to be computed; and (2) the effort to find the optimum PMF is reduced. Preliminary verification of the results was conducted using Seneca Creek, Maryland, as a case study.

A methodology to assess the relative merits of using evaporation-pan networks and semiempirical functions to estimate reference-crop evapotranspiration (ET) is presented. Estimation-error variance is proposed as a basis for comparison. In the case of pan networks, ordinary kriging and universal kriging are viable options for determining estimation-error variance. In the case of semiempirical ET functions, a formulation is developed that expresses the estimation-error variance in terms of the uncertainties in extrapolating meteorological data from measurement locations. The proposed methodology is demonstrated within a 44,000 km ² area of southern Florida. The results show that universal kriging is the preferred method for estimating reference-crop ET from pan measurements, and the Penman-Monteith method is the preferred estimation method from meteorological data. The pan-based universal kriging approach yields a lower estimation-error variance than the Penman-Monteith function, indicating that pan-based estimates of reference-crop evapotranspiration are preferable to semiempirical evapotranspiration functions in south Florida.

A simulation model of rainfall under center pivots was used to elevate center-pivot testing procedures. The model calculates rainfall parameters such as weighted mean and uniformity coefficient for various scenarios including the case of a voluntarily disrupted machine. Observed variations in rainfall parameters suggested modifications to standard procedures in order to improve accuracy. Those modifications include staggering rain-gauge position on the two radii of measure and a uniformity coefficient derived from a coefficient of variation calculated on weighted values, theoretically more sensitive than existing approaches. Such a procedure has been proposed as an improvement over the draft international standard ISO/TC23/SC18/DIS 11545, “Agricultural Irrigation Equipment—CentrePivot and Moving Lateral Irrigation Machines Equipped with Sprayer or Sprinkler Nozzles—Determination of Uniformity of Water Distribution.”

Although widely used for field measurements of soil moisture, problems continue with the use of the neutron probe in clay soils comprised of over 40% clay fractions, and in the determination of the intensity of field sampling necessary to characterize a site. We examine issues associated with field practice and calibration, data analysis and estimation of variance, and error in soil moisture measurements for silty clay soils in the Imperial Valley, California, using large calibration data sets from several different layouts of neutron access tubes. Our analysis indicates that for field calibrations: (1) bulk density corrections of the count ratio data do not necessarily improve the standard errors of the water content measurements; (2) soil sampling for calibration within the access tube hole at the time of tube installation results in smaller measurement errors; (3) calibrations improve as the average soil moisture of the profile at the time of the original sampling decreases; and (4) field-wide calibrations improve, and the sampling intensity required to characterize soil moisture content decreases with an increasing soil depth. We also address aspects of the neutron probe use associated with soil salinity and polyvinyl chloride (PVC) access tubing (chloride interference), inclusion of shallow sampling data in calibrations, and instrument, calibration, and spatially related standard variances affecting field measurements.

In the absence of irrigation, water from a water table below the root zone can sometimes sustain crop growth. This study was conducted to determine whether alfalfa grown without irrigation, but in the presence of a water table below the root zone, can give normal yields. Three irrigation treatments were applied: irrigating during the first two growth cycles, irrigating during the first three growth cycles, and no irrigation restrictions for the four growth cycles of the irrigation season. Water-table depths midway between parallel tile drains were evaluated. Yields from the midway position between drains were compared with yields nearer the drains. The influence of occurrence/nonoccurrence of irrigation on yield was also tested by regression analysis. Water tables tended to be deeper in the absence of irrigation but differences were small among the three treatments. Yield was dependent on the occurrence of irrigation but not on water-table depth. Most important, nonirrigation caused an 80% drop in yield.