Theodor Strelkoff

Estimation of Soil and Crop Hydraulic Properties

Some two dozen methods for estimating infiltration and roughness parameters from field measurements of test irrigations are reviewed in this paper. They differ in their assumptions, ease of analysis, quantity of field data required, and accuracy. They are divided into two broad categories, depending upon the basic approach to determine infiltration. One features direct application of mass conservation, expressed in terms of the infiltration parameters and then inverted in some way in order to extract those parameters. The other involves repeated simulation with a sequence of values of the infiltration parameters, coupled to some kind of search procedure—an optimization—to minimize differences between simulation and measurement. A new one-point technique is proposed, along with suggestions for extending existing methods.

Nonuniform and Unsteady Solute Transport in Furrow Irrigation. I: Model Development

A cross-section-averaged advection-dispersion equation ADE model was developed to simulate the transport of fertilizer in furrow irrigation. The advection and dispersion processes were solved separately at each time step by implementing a method of characteristics with cubic-spline interpolation and a time-weighted finite-difference scheme, respectively. The upstream boundary condi- tion was a prescribed concentration. Downstream, a zero-flux boundary condition during advance and a concentration gradient following completion of advance were prescribed. Local pseudosteady state was assumed in order to apply Fischers longitudinal dispersion equation under nonuniform and unsteady furrow flow conditions. Statistical parameters were used to evaluate the ADE model performance. DOI: 10.1061/ASCEIR.1943-4774.0000106 CE Database subject headings: Solutes; Fertilizers; Finite difference method; Furrow irrigation; Models. Author keywords: Fertigation; Advection-dispersion equation; Longitudinal dispersion; Finite differences; Method of characteristics; Cubic-spline interpolation.

Structured Application of the Two-Point Method for the Estimation of Infiltration Parameters in Surface Irrigation

The two-point method is one of the best known procedures for estimating empirical infiltration parameters from surface irrigation evaluation data and mass balance, mainly because of its limited data requirements and mathematical simplicity. However, past research have shown that the method can produce inaccurate results. This paper examines the limitations of the method, reviews alternatives for improving two-point method results based on data that are collected or can easily be collected as part of a two-point evaluation, and suggests strategies for estimation and validation of results for different levels of evaluation data. Results show the limitations of formulating the estimation problem with advance data only and the benefits of using instead an advance and a postadvance mass balance relationship in the analysis. Because different combinations of parameters can satisfy the mass balance equations, the estimated function cannot be extrapolated reliably beyond the times used in formulating those relationships. While results can be used with confidence to characterize the performance of the evaluated irrigation event, they need to be used carefully for operational analysis and design purposes.

Field Properties in Surface Irrigation Management and Design

Field properties—topography, hydraulic resistance, and infiltration—play an important role in the performance of surface irrigation systems, and appropriate characterizations of these are required as data input to simulation or design software. The EWRI/ASCE Task Committee on Soil and Crop Hydraulic Properties has been charged with preparing a guide for practitioners faced with such data entry. The result is this special section of the Journal of Irrigation and Drainage Engineering in which this paper is the first in the series presented. It describes the characteristics of these field properties and notes a series of caveats to be considered when dealing with them in the course of analyses or designs of surface irrigation systems.

Zero-Inertial Recession for Kinematic-Wave Model

Kinematic-wave models of surface irrigation assume a fixed relationship between depth and discharge (typically, normal depth). When surface-irrigation inflow is cut off, the calculated upstream flow depth goes to zero, because the discharge is zero. For short time steps, use of the kinematic-wave model can create an incorrect water-surface profile. Issues include difficulty in solving the continuity equation in the first upstream cell and a calculated adverse water-surface profile. The kinematic-wave model produces a recession time at the upstream boundary that is too small, that can lead to incorrect calculation of recession times when water continues to recede from the surface. This problem becomes more severe for smaller values of bottom slope. In this paper, we present a zero-inertia approximation to the water-surface profile at cutoff, which can be extended to the start of recession, after which calculations continue with the kinematic-wave model.

Simulation of Unsteady Flow and Soil Erosion in Irrigation Furrows

AbstractThis study developed a one-dimensional numerical model for the simulation of unsteady flow and the resultant soil erosion in irrigation furrows. The model solves a modified version of the Saint-Venant equations that consider the loss of mass and momentum attributable to infiltration and sediment transport. The transport rate of fine sediment was predicted with a modified Laursen formula that treats the tractive shear stress as a function of both Reynolds number and the particle size. The modified Laursen formula was verified by using the erosion data measured in the field and in a laboratory flume. The model accurately predicted flow advance times and outflow hydrographs in comparison with data measured in irrigation furrows at Kimberly, Idaho. Sediment discharge predictions were less accurate.

Improved Surface Volume Estimates for Surface Irrigation Volume-Balance Calculations

Abstract This article reviews procedures for estimating surface storage in surface irrigation volume balance calculations. Those procedures are based on the assumption of a power law relationship for flow depth as a function of distance along the stream. The analysis uses zero-inertia simulation and a system of dimensionless variables to examine how the depth profile varies as a function of hydraulic conditions when infiltration is given by the empirical extended Kostiakov equation. Alternatives for approximating the exponent of the depth profile power law (β ) are suggested. The magnitude of the resulting errors relative to zero-inertia model predictions is quantified. Results show that the range of variation for the parameter β increases with field slope, with increasing advance length relative to the maximum advance distance, and when infiltration rates are relatively constant with time during the irrigation event. Estimating β as a function of advance distance is most challenging under these condition...

Optimal and postirrigation volume balance infiltration parameter estimates for basin irrigation.

Engineering analysis of surface irrigation systems is predicated on reasonably accurate estimates of a field’s infiltration properties. Optimal estimation methods pose multiple volume balance equations at various stages of an irrigation event and are assumed to produce the most accurate results among volume balance based procedures. They have the disadvantage of requiring surface volume determinations, which may be difficult to obtain in practice under many field conditions. This study contrasts infiltration solutions from optimal and a simpler postirrigation volume balance method and examines the implications of those solutions on the performance of management strategies with zero-slope and low-gradient basins. With those types of systems, there is little benefit in using optimization over postirrigation volume balance due to the nonuniqueness of solutions and uncertainties of inputs required by the estimation procedures. In addition, system hydraulic characteristics mitigate the insensitivity of the dis...

External Iterative Coupling Strategy for Surface-Subsurface Flow Calculations in Surface Irrigation

Coupling the unsteady open-channel flow equations of surface irrigation with the equation of variably saturated porous media flow is a computationally complex problem because of the dependence of infiltration on surface-flow depths. Several models of this coupled process have been developed, all of which solve the surface and subsurface flow iteratively at each time step of the surface-flow solution. This study presents an alternative strategy, in which stand-alone surface and subsurface-flow models are used to conduct the calculations sequentially and iteratively at the time level of the irrigation event. At each iteration, the subsurface-flow results are computed using the surface-flow depths generated by the surface-flow model at the current iteration. Infiltration results computed at selected computational nodes are fitted to an empirical infiltration function, and then fed back to the surface-flow model. The proposed strategy, labeled external iterative coupling, was prototyped for border and basin irrigation systems using the WinSRFR and HYDRUS-1D models. The proposed procedure produced irrigation performance results comparable to those generated with an internally coupled model, even when using a single representative location to calibrate the empirical infiltration equation used by the surface-flow model. In comparison with models that iterate at each time step, the proposed coupling strategy reduces the computational effort and improves convergence. The approach provides a practical alternative for coupling existing and future surface and subsurface flow models.

Development of Management Guidelines for Efficient Irrigation of Basins on Sandy Soils

Level and graded-basins are widely used to irrigate citrus (Citrus sp.) and alfalfa (Medicago sativa L.) crops grown on sandy soils of the Yuma Mesa Irrigation and Draniange Districts (YMIDDs) of southwestern Arizona. Historically, irrigation application efficiencies in the YMIDDs have been low (generally <40%). Owing to rising demands for water from urban and industrial interests and heightened water quality concerns, raising irrigation efficiencies have become a high priority in the Lower Colorado River Region. Thus, the objective of this study is to develop a management package (management tools and guidelines) for increased irrigation performance of basins in the YMIDDs. The study consists of field experimentation, modeling (model calibration, model verification, and the development of management tools by simulation), and outreach-educational components. Results show higher application efficiencies (Ea) and distribution uniformities (DU) can be obtained in level and graded-basins through the proper selection of unit inlet flow rate (qo) and cutoff length (Lco) or cutoff time (tco). In addition, for the relatively small net irrigation requirements and long basisn of the YMIDDs, higher application efficiencies are obtained with level basins compared to graded-basins. Since implementation of the proposed management tools does not require reconfiguration of the physical infrastructure, it has the potential to be smoothly adopted by growers. Finally, some aspects of an on-going outreach program are highlighted.