Journal of Irrigation and Drainage Engineering

Very limited citrus and avocado orchard evapotranspiration data are available, and practically no published information is available considering citrus/avocado evapotranspiration ET_c on steeply sloping fields in coastal climates. Often the data available are from measurements based on limited time periods, and season-to-season orchard water use variability is unknown. The objective of this project was to obtain such long-term-field ET_c data. Of particular interest was determination of the monthly variability of this orchard water use through years of drought and heavy winter rains. ET_c and ET_o measurements (using Bowen ratio and evaporation pan techniques, respectively) and soil-moisture monitoring (using neutron scattering methods) combined with the analysis of 12 more years of water balance data were used to determine citrus/avocado orchard water use at a commercial coastal orchard located northwest of Ventura, Calif. All three approaches indicated that mature citrus/avocado trees utilize approximately 60% of ET_o. The greatest variability in water use occurred during the early irrigation season (April through June) because the orchards drew upon residual soil moisture available from winter rains. Such use reduced early irrigation season water applications to substantially less than that indicated by ET_o.

During 1995, daily evapotranspiration (Et) from a flooded rice field was calculated with a water balance equation using measured values of water level, precipitation, irrigation, seepage, and tailwater runoff. Stagewise Et was 6.3, 8.1, and 6.8 mm/d for the vegetative, flowering, and yield formation stages, respectively; average daily Et was 6.8 mm/d. Grass reference Et (Et_r) combination models Penman-Monteith, FAO-Penman, and 1963 Penman were evaluated for their capabilities to predict rice Et using daily weather data. Daily Et_r was also calculated by summing up hourly values with the Penman-Monteith method. The Penman-Monteith (daily) method had a coefficient of determination (R²) of 63.7%, as compared with 62.9, 60.0, and 61.7% for Penman-Monteith (hourly), FAO-Penman, and 1963 Penman methods, respectively. Crop coefficients (K_c) using the Penman-Monteith (daily) model were 1.39, 1.51, and 1.43 for the vegetative, flowering, and yield formation stages, respectively. Developed K_c values were verified using limited rice Et data in 1996. The K_c values underpredicted rice Et by 3.7%, which was acceptable for water management.

The analytical solution to the problem of transient drainage without replenishment for a two-layered soil, by applying Girinsky's potential, has been presented. The effect of various parameters on drainage has been evaluated, and a set of relationships has been developed that may help in the design of drainage in layered soils. The proposed analytical solutions have been experimentally investigated on a vertical Hele-Shaw model in which the two-layered soil conditions were simulated. The comparison of results of proposed analytical solutions with the experimentally observed ones in the Hele-Shaw model showed that the experimentally observed and predicted midpoint water tables were found to be in good agreement for the entire duration of the experiment. The proposed theoretical model may be utilized for transient design of a drainage system in a two-layered soil.

Measurement of unsaturated hydraulic conductivity is needed for precise control of water and solutes in the vadose zone. Because of the spatial variation of soils, a large number of surface and subsurface measurements are needed to characterize a field. In this work, permeameters were developed and tested for estimating subsurface unsaturated hydraulic conductivity. The permeameters apply water under tension; they are easy to use and have adequate accuracy. Unsaturated hydraulic conductivity was determined by measuring the steady flow rates for various values of negative pressure. Tests using a soil of known hydraulic conductivity showed that the permeameters provided valid measurements. Two types were used, a porous cloth model that was inflated against the soil and a porous ceramic cup that was rigid. The field testing determined that a rigid design using a ceramic cup coupled to the soil by a layer of fine sand was easier to use, was reliable, and provided good results.

The minimum area section is a thoroughly investigated problem in the hydraulics literature. However, because of the complexities of the analysis, the design of a minimum seepage loss section has not been attempted as yet. In this investigation, using previously derived results, simplified algebraic equations for computation of seepage loss from triangular, rectangular, and trapezoidal canals have been presented, which replace accurately the cumbersome evaluation of complex integrals. Using these seepage loss equations and the general uniform flow equation, explicit equations for the design variables of minimum seepage loss canal sections have been obtained for each of the three canal shapes by applying nonlinear optimization technique. The optimal trapezoidal section has the least seepage loss and cross-sectional area among the three optimal sections. A step-by-step design procedure for rectangular and trapezoidal canal sections has been presented. The analysis also includes the sensitivity of the seepage loss to design variables around the optimum value.

A model of solute transport in overland flow is developed and applied to the simulation of surface fertigation. Water flow is simulated using the depth-averaged, 1D shallow water equations. Solute flow is represented by an advection-diffusion model. The resulting set of three partial differential equations is sequentially solved at each time step. First, water flow is computed using the explicit two-step McCormack method. Based on the obtained velocity field, solute transport is explicitly determined from the advection-diffusion equation using the operator split technique. Four field experiments involving fertigation events on an impervious free-draining border were performed to validate the proposed model and to obtain estimates of Kx, the longitudinal dispersion coefficient. A value of Kx = 0.075 m² s⁻¹ satisfactorily reproduces the field experiments. The model is also applied to the simulation of a fertigation event on a pervious border. A sensitivity analysis is performed to assess the dependence of fertilizer distribution uniformity on the value of Kx. Finally, the proposed model is compared with a previous model based on pure advection.

This paper presents the physical basis of the FRACTURE submodel for simulating infiltration of precipitation/irrigation water into relatively dry, cracked, fine-textured soils. The FRACTURE submodel forms part of the HYDRUS-ET variably saturated flow/transport model. Infiltration into the soil matrix is formally divided into two components: (1) Vertical infiltration through the soil surface; and (2) lateral infiltration via soil cracks. The first component is described and solved using the 1D Richards' equation. Excess water that does not infiltrate through the soil surface is either considered to be runoff, if no soil cracks are present, or routed into soil cracks from where it may laterally infiltrate into the soil matrix. Horizontal infiltration from soil cracks into the soil matrix is calculated using the Green-Ampt approach and incorporated as a positive source/sink term S_f in the Richards' equation describing flow in the matrix. In addition to the hydraulic properties of the soil matrix, the FRACTURE submodel requires parameters characterizing the soil cracks, notably the specific crack length per surface area l_c and the relationship between crack porosity P_c and the gravimetric soil water content w. An example problem shows that infiltration from soil cracks can be an important process affecting the soil water regime of cracked soils. A comparison with the more traditional approach, involving surface infiltration only, indicates important differences in the soil water content distribution during a rainfall/irrigation event. This extension of the classical approach to include crack infiltration significantly improves the identification and prediction of the soil water regime.

Supercritical flow in a bend manhole for combined sewer systems is considered for a typical relative curvature and deflection angles of 45° and 90°. The typical features of bend flow in a U-shaped channel are determined, including the surface profiles along the inner and the outer bend walls, impact flow at the manhole outlet into the downstream sewer pipe, air entrainment characteristics, and wave development in the downstream sewer. Also, typical velocity distributions along the manhole are presented, and the capacity of the conventional bend structure is determined. To increase the discharge capacity across a bend manhole, the so-called bend cover is introduced. This cover element finished in steel can be removed for maintenance and inspection purposes. It limits the wave height close to the bend outlet to 90% of the sewer diameter and allows air entrainment into the downstream sewer. The performance and capacity of bend flow may significantly be increased, and the novel element may easily be added to existing manholes.

Analytical equations are presented for two average pressure correction factors developed for linear displacement laterals with or without outflow at the downstream end. The average correction factor for laterals without downstream outflow, when applied to a relatively large number of outlets is in good agreement with earlier work. For a relatively small number of outlets, the average correction factor presented is more accurate. The average correction factor for laterals with outflow reduces to that for laterals without outflow when the outflow ratio is reduced to zero. For a relatively large number of outlets, this average correction factor is primarily a function of the outflow ratio. For both large outflow ratios and large outlet numbers, the average correction factor is almost a constant. To apply the average correction factor to design a tapered lateral, an expression relating lateral inlet head to required average head and friction head loss has been developed. The expression can be applied to a lateral with any number of reaches with different diameters. A practical application has been demonstrated through an example.

The discharge and yield of a flowing well can be computed by existing complex solutions or by Duhamel's technique using kernel coefficients. The mathematical complexities of these methods can be very much simplified through numerical methods without loss of accuracy. In the present study simple equations for both the well discharge and well production functions are presented. The equation for the well discharge function has been used to find aquifer constants for known well discharge and drawdown through the error minimization method. The results are compared in order to demonstrate the relative simplicity of the proposed equations.

For channels with composite roughness, an equivalent uniform roughness coefficient and flow geometric elements are used in an optimal design method using the Manning equation. The optimal design problems are formulated in a nonlinear optimization framework with the objective function being a cost function per unit length of the canal. Constraints are the Manning equation, positive values for design variables, and specified values of side slopes or top width. The constrained problem is transformed into an unconstrained problem using the Lagrangian multipliers. To obtain an optimal solution for the resulting unconstrained problem, the first-order necessary conditions for optima are applied. The resulting simultaneous nonlinear equations are solved using the computational methodology developed. This technique is applied to illustrative numerical examples. The evaluations establish the potential applicability of the developed computational methodology for optimal design of open channel cross sections with composite roughness.