Irrig Sci (1994) 15:55-56 �9 Springer-Verlag 1994

The past two decades have witnessed new and exciting de- velopments in irrigation science and engineering. Many of these developments are now possible because of the easy access to virtually unlimited computing capability. An ac- curate simulation of the entire irrigation cycle is now within our reach; design of surface irrigation systems is much improved as a result. In the near future, we will wit- ness simulation of farm irrigation systems in conjunction with delivery systems and crop systems, subject to simu- lated antecedent moisture conditions, required fertilizer in- puts, and drainage requirements. With increasing concern for environmental quality, environmental consequences will have to be factored in the design of irrigation systems. This special issue grew out of a desire to provide a focus on some aspects of surface irrigation and a sample of some of the developments that are in progress. For want of space, the contributions to be included in this special issue are limited to six papers that touch upon different aspects of surface irrigation: (1) dimensional analysis in surface irri- gation, (2) numerical simulation of irrigation cycle, (3) structure of surge front, (4) kinematic-wave modeling of furrow irrigation, (5) optimization of furrow irrigation system design parameters, and (6) accuracy of kinematic- wave and diffusion-wave approximations.

Strelkoff and Clemmens revisit dimensional analysis in surface irrigation. Their paper describes theoretical bases for dimensional studies of surface irrigation in borders, both graded and level. It compares several different choices for expressing pertinent variables in dimensionless form and enumerates the advantages and implications of each. These, in turn, lead to generalized results that permit ra- tional choices to be made for flow simulation techniques such as the kinematic-wave or zero-inertia theory. Strel- koff and Clemmens lay the groundwork for extension of the dimensional analysis technique to sloping border and furrow irrigation systems.

The paper by Sakkas, Bellos, and Klonaraki presents a mathematical model based on the complete hydrodynamic equations of open-channel flow for simulation of the com- plete surface irrigation cycle. Also included in the simula- tion is the unsteady unsaturated flow in a porous medium.

An explicit two-step numerical scheme is employed for the simulation. The subsurface flow is simulated first, leading to a series of infiltration depth values as a function of time. Then the surface flow is modeled. For furrow irrigation, however, the two types of flows can be simulated simulta- neously. The model can be employed to develop surface irrigation design and operational criteria that will result in an optimum overall irrigation efficiency under the exist- ing physical, operational, and management conditions.

Katopodes develops a model for surface irrigation that permits determination of the vertical structure of the ve- locity profile in the vicinity of the wave front, without as- suming that the pressure is hydrostatic or that the free sur- face profile is of a particular shape. The model is based on turbulent Navier-Stokes equations. The solution employs a two-dimensional finite element model in the vertical plane and the kinematic condition for determining the po- sition of the free surface. It also includes a numerical tech- nique for describing surface penetration and wave break- ing. The model provides information on vertical mixing, settling and suspension of contaminated solids commonly occurring in surface irrigation applications.

Reddy and Singh describe a kinematic-wave model for furrow irrigation. Through linearization of the finite dif- ference equations of the kinematic-wave model, an explicit algebraic scheme is employed for computation of advance and runoff rate. The results obtained with model are com- pared with those yielded by the nonlinear scheme and the zero-inertia model and with field data, and close agreement is reported. The paper concludes with the derivation of an error differential equation for the kinematic-wave model by comparison with the zero-inertia model. This differen- tial equation and two-dimensionless terms are used to de- fine limits for applicability of the kinematic wave model in furrow irrigation.

The paper by Reddy presents an optimization procedure for design of furrow irrigation systems with due consider- ation of runoff and drainage water quality. The generalized geometric programming technique is used to achieve the optimal values for design variables that maximize the net benefits from a furrow irrigation system under a given set

56

of soil, crop, and economic conditions. The procedure sim- plifies the design problem and reduces the time required to obtain the optimal design.

Singh, Aravamuthan, and Joseph derive errors of kine- matic-wave and diffusion-wave approximations by com- parison with hydrodynamic equations for time-indepen- dent flows in infiltrating channels. The upstream bound- ary condition is one of finite depth and discharge. The ac- curacy of the kinematic-wave and diffusion-wave approx- imations varies with the product of the kinematic-wave number and the square of the Froude number. Both the kin- ematic-wave and the diffusion-wave approximations are found to be in good agreement with the dynamic-wave model.

It is hoped that this special issue will stimulate further interest in surface irrigation systems. There can be little ar- gument that future planning, design, operation, and man- agement of irrigation systems will have to be based on more advanced and accurate models of surface irrigation. Con- ventional design approaches to surface irrigation systems will have to be either modified or replaced by their more comprehensive counterparts. Irrigation systems will have to be treated as a part of the overall agricultural and eco- system environment.

V. P. Singh, Baton Rouge, Louisiana, USA