Journal of Irrigation and Drainage Engineering

A new approach to modeling canopy resistance is presented as an alternative to the Food and Agricultural Organization of the United Nations Penman-Monteith method with the constant canopy resistance. The evapotranspiration (ET) model is based on the “big-leaf” approach and a variable canopy resistance. The model's input requires standard meteorological data as in the Penman-Monteith combination approach. The model was validated using weather and grass lysimeter data measured on an hourly basis at Davis, Calif., and on a daily basis at Policoro, Southern Italy. ET estimates from the model were compared with the results of ET values obtained by the Food and Agricultural Organization of the United Nations Penman-Monteith approach using the constant canopy resistance r_c = 70 s m⁻¹. The results showed a very convincing performance of the model for estimating reference ET on both an hourly and daily basis. This work confirms that the canopy resistance depends on climate, and that a variable r_c is recommended for ET models. The proposed model does not introduce any empirical parameter, does not require calibration for the two sites tested or for different time scales, and it is simple enough for direct practical application.

The nonlinear Boussinesq unsteady-state differential equation used for evaluating drainage of sloping lands with drains lying at a distance above the impermeable layer was solved. A combination of explicit and implicit difference methods was used to obtain a finite-difference solution for a linearized system of equations of Graute-Nicolson type on two time levels, ensuring the stability of the solution. The maximum height of the water tables was obtained as a function of time for different slopes varying from 0 to 70%. Model results were compared with the available experimental solutions of Luthin and Guitjens and Chauhan et al. as well as the numerical solution of Moody and were found to be in reasonable agreement.

Large-scale nitrate contamination of ground water is often observed in irrigated areas where the soils are generally permeable, and nitrate plumes have been reported to occur at shallow depths in unconfined aquifers. It has been hypothesized that these nitrate plumes can be captured by the pumping action of irrigation wells and recycled at the field site. This paper presents results from a field and modeling study investigating the distribution of nitrate in an unconfined sand aquifer in an irrigated system. Results from tracer tests under both natural and forced gradients indicate that irrigation pumping has minor but measurable effects on solute transport. Hydrological and solute transport modeling using various pumping schedules suggests that, in most cases, the influence of irrigation pumping on solute transport will be minimal, even on local scales.

Water reuse and ground-water recharge can be used to meet the growing demands for water, particularly in arid regions. Ground-water recharge using fresh water or treated wastewater is most often accomplished by infiltration from surface basins. The water percolates through the unsaturated soil region to an underlying aquifer for storage and future use. In the case of wastewater, additional treatment occurs as the effluent flows through the soil. The system hydraulics of recharge basins have been examined through a combination of field and laboratory investigations. These studies indicate that infiltration rates and soil aquifer treatment of wastewater are influenced by soil type and soil profile characteristics, surface clogging material, pond depth, and wetting/drying cycle times. The surface-clogging layer was found to be susceptible to consolidation and to associated reduction in hydraulic conductivity under seepage forces.

The volume balance model was modified to allow explicit solution for the advance distance in surface irrigation. In a dimensionless formulation, the advance distance equation was compared with the correct form of the Hall technique, considered the most accurate numerical volume balance solution. The results indicated that the presented equation can be used to determine the advance distance with insignificant errors. A furrow irrigation example was used to demonstrate the solution of the present method. For comparison with the zero inertia model, a border example was used, and the relative error of the proposed method was less than 4%.

Analytical equations for friction correction factors for center-pivot laterals without end guns are developed. This work illustrates a discrepancy when earlier equations are applied to center-pivots with small numbers of outlets. Earlier equations were also limited to center-pivots with constant outlet spacing. Equations presented in the current work are developed for center-pivots with constant outlet spacing and also for center-pivots with constant outlet discharge. When the equations developed in the current work are applied to center-pivots with a large number of outlets, the results are in good agreement with previous work for center-pivot laterals with an infinite number of outlets. When applied to a smaller number of outlets the equations presented here provide a more precise estimate of the friction correction factor. Using the current equations, the friction correction factor for center-pivots with constant outlet spacing was found to be very similar to the friction correction factor for center-pivots with constant outlet discharge. Useful simple equations are also presented for calculating the discharge of each outlet or for calculating the spacing between outlets.

A new method for control of unsteady flow in open channels is presented. Control is achieved using an iterative approach, the Broyden, Fletcher, Goldfarb, and Shanno quasi-Newton method that utilizes an adjoint sensitivity method to efficiently compute gradient information. The adjoint equations are derived from the differential form of the full shallow-water equations in one dimension and the resulting sensitivies allow a wide class of flows to be controlled. In addition, the adjoint sensitivity method permits both scheduled and adaptive flow control to be achieved. Application of the method to scheduled flow control is presented.

Sewer sideweirs in a combined sewer system are used for diversion of excess discharge during rainfalls. Their hydraulic performance depends significantly on the approach Froude number, the relative weir height, and the overflow length. The throttling pipe is a simple device to limit the discharge to treatment facilities. An experimental work was conducted to establish the main flow features of the converging sewer sideweir. The effect of the throttling pipe was determined. It was concluded that Abwassertechnische Vereinigung of Germany or the European regulations for side overflows can only be satisfied if the weir height is larger than about 70% of the approach diameter. Also, the approach Froude number should be smaller than about 0.7. Using the energy equation, relations for the free surface profile were developed that agree with experimental observations. Further results include the spatial discharge and velocity distributions, the lateral outflow characteristics, the excess overflow, the end depth, and equations for the discharge in the throttling pipe. Previous results of Gisonni and Hager for the short sewer sideweir without throttling pipe are confirmed, and photographs illustrate the complex spatial flow patterns.

Experimental investigations have been carried out on the sediment removal efficiency of settling basins. Laboratory data on removal efficiency from the present and earlier studies were first used for checking the accuracy of the existing empirical and analytical methods for determination of the sediment removal efficiency of settling basins. The existing relationships were not found to yield satisfactory results over the whole range of data. Therefore, reanalysis of these data was done and a new relationship developed. The effect of continuous flushing of a sediment-water mixture from the settling basin on its removal efficiency was also studied through analysis of experimental data.