Journal of Irrigation and Drainage Engineering

A series of laboratory experiments were carried out in a flow-measurement flume of a rectangular compound cross section to investigate the effect of the throat width and step height on the values of discharge coefficient, approach-velocity coefficient, and modular limit. For this reason, nine different models made of Plexiglas were tested in a horizontal laboratory flume for a large range of discharges. In each test, the head, h₁ , over the crest elevation at depth-measurement section and head, h₂ , over the crest elevation in the tailwater channel were measured. The calculated discharge coefficients, modular limits, and approach-velocity coefficients were related to relevant parameters. The consistency between aforementioned quantities and those given in the literature are found to be quite good. Modular limit values as high as 95% are obtained.

There are relatively few investigations of the free overfall in a trapezoidal channel and its practical use as a flow-measuring device. The flow over a fall in a trapezoidal channel is simulated by that over a sharp-crested weir, taking into account the streamline inclination and curvature at the brink. A general end-depth–discharge relationship, for both subcritical and supercritical flow, is obtained. Discharges predicted using this general relationship are compared with experimental data and with those obtained from other theoretical methods. The surface profiles for the zone between the brink and an upstream section with hydrostatic pressure distribution are also investigated using the energy equation for both subcritical and supercritical flow.

The conventional approach for determining drag coefficient in the ballistic theory fits well for simulating water-drop movement emitted from circular sprinkler nozzles. This approach was checked for square and double-rectangular nozzles. The conventional approach does not work. Therefore the concept of an “apparent drag coefficient C _d ” was introduced, and a new approach for determining C _d was developed. It was verified that the spray distance for a specified droplet diameter calculated by the new approach fitted the observed one very well. The effect of droplet diameter; nozzle-shape, nozzle-size, and discharge coefficient; and pressure on C _d was analyzed, and showed that C _d is very sensitive to droplet diameter and nozzle-shape coefficient. The new approach was applied to investigate the influence of a sprinkler's trajectory angle on spray distance, and showed that both circular and noncircular nozzles reached their largest spray distances when the trajectory angle was around 37°. The effect of nozzle shape on the direction of droplet terminal velocity was also investigated.

Water-resource projects require a correct estimation of evapotranspiration for their successful planning, design, and operation. The accuracy of evapotranspiration is particularly important in, for example, water-quality models. Evapotranspiration estimation is very sensitive to the degree of forecasting error and even small errors can produce highly misleading results. Unfortunately, the time-dependent behavior of evapotranspiration makes it very hard to be represented by simpler expressions and forecasting based on such relations will naturally be inaccurate. In this study, the inherently stochastic process of reference evapotranspiration is modeled in a dynamic regression environment, also known as transfer-function-noise (TFN) modeling. TFN models have the capability to relate different time series and the capacity to allow delays between system inputs and responses, which make them quite attractive. In this study, various single input-single output TFN models are developed for reference evapotranspiration. It is shown that evapotranspiration can be adequately represented and forecasted by TFN models.

A laboratory investigation was conducted to study the effects of soil compaction and soil strength on the erodibility of a silty sand soil. A 6.1-m-long flume was designed and constructed to measure erosion. Erosion rates were determined at four soil bulk densities ranging from 1.28 to 1.52 Mg/m ³ , eight shallow-flow depths ranging from 5.5 to 17.5 mm, three slopes ranging from 0.005 to 0.03 m/m, and eight flow rates ranging from 0.11 to 0.48 m ³ /(m$\dot{s}$min). Soil shear strengths were measured using a Swedish fall-cone penetrometer. Erosion rates increased when soil air was entrapped and not allowed to escape into the atmosphere. Water velocity profiles over eroding soils were measured using a computerized pitot tube and pressure transducer apparatus. Velocity near the soil bed increased with greater bulk density. As a result, erosion rates increased as bulk density became larger.

A downstream control algorithm was developed for the control of multireach canal systems. The objective of the control is to maintain a target water level at the downstream end of each pool by adjusting the control gate at the upstream end of the pool. If the measured water level differs from its target value, the gate is adjusted, aiming to eliminate the deviation as soon as possible. The method is based on an explicit solution procedure of the St. Venant equations. Adjustments of each control gate are calculated according to water levels and flow rates at the information nodes downstream of the gate and the water level immediately upstream of it. Unless the pool is extremely long, an effective control can be obtained with only two information nodes in each pool (at the most upstream and downstream ends of the pool). If flow rates at information nodes are estimated using the slope stage method, four water levels are measured in each pool.

Vapor-pressure deficit ( VPD ) affects evapotranspiration, water-use efficiency, and radiation-use efficiency of crops. VPD calculation methods were evaluated for a semiarid environment in the Southern Great Plains. Air temperature and relative humidity were measured near Bushland, Texas, during 1992 and 1993. Temperature and relative humidity were measured at 0.17 Hz (6 s), averages were recorded for each 15-min period, and daily (24-hr) maximums, minimums, and averages were recorded. VPD , actual vapor pressure, and dew-point temperatures were computed and averaged for each 15-min period and day. Methods that used mean daily dew-point temperature to compute daily actual vapor pressure performed well, and methods that used hybrid calculations based on maximum and minimum air temperature and relative humidity performed the worst. Methods using one-time-of-day dew-point temperatures as recommended by the 1990 ASCE Manual No. 70 should be used with caution in this environment. Weather data sets containing maximum and minimum temperatures and daily mean dew-point temperature should provide the most accurate calculations of VPD in this environment.

The management of water resources under drought conditions has received added emphasis recently because of prolonged droughts affecting all regions of the United States. The American Society of Civil Engineers Irrigation and Drainage Division has supported a task committee to review this topic. This paper summarizes some of the more important findings of this task committee. The paper provides an overview of drought-response practices by federal and state agencies and discusses specific experiences by the U.S. Bureau of Reclamation, the U.S. Army Corps of Engineers, the Tennessee Valley Authority, and the states of Colorado and California. The experiences of these organizations show the importance of having a coordinated, interagency, well-defined response that is flexible enough to adapt to the unique nature of a situation.

The objective of this study is to investigate the effects of changes in precipitation and temperature on runoff from the Wabash River basin. A water-balance model is used in the study. The investigation is conducted using data from the Wabash River watershed in Indiana. A monthly water-balance model is used to simulate the monthly runoff. The calibrated water-balance model is used to perform sensitivity analyses of runoff under varying climatic conditions. Then it is used to investigate the effects of changes in temperature and precipitation on runoff in the Wabash River basin in Indiana. A significant conclusion from the study is that changes in rainfall have larger effects on runoff. The effect of changes in temperature on runoff is not as large.