Journal of Irrigation and Drainage Engineering

In an effort to improve plant growth and to achieve high yield and/or quality, irrigation scheduling (IS) seeks to provide plants with appropriate quantities of water at appropriate times. To better understand irrigation scheduling’s main processes and principles, its four most common methods of operation—(1) evapotranspiration and water balance (ET-WB), (2) soil moisture (Θ) status, (3) plant water status, and (4) models—along with their pros and cons are introduced and compared. Irrigation applications, including software, programs, and associated controllers are introduced. Given that some of these methods focus on Θ or plant responses to soil moisture, the determination of target soil moisture levels, along with estimates (either calculated or measured) of current soil moisture status are key to both scheduling irrigations, and the precise replenishment of soil moisture to target levels. Accordingly, factors in the soil-crop-atmosphere system affecting soil moisture must be considered in the scheduling process. As all four types of IS methods focus on soil water content, which serves as a bridge between irrigation management and crop water requirements for growth, future scheduling methods should focus on the management of soil moisture based on an advanced understanding of its effects on crop growth either by the integration of existing IS methods or the development of new models, using intelligent algorithms. Using these approaches, more practical, accurate, and easily adaptable IS applications should be developed for real-time farming operations. Weather station networks and online data access should be enhanced to better serve these IS applications.

Waters flowing over rounded weirs experience a rapidly accelerated flow region near the crest. The head-discharge relationship of a half-round crested weir was tested physically under carefully controlled flow conditions for a weir design with a small radius of curvature. The experiments were undertaken for a wide range of discharges, over two orders of magnitude, and the results were compared to the literature on circular weirs. The nappe was not aerated. With increasing discharges, the nappe was initially attached to the weir’s downstream wall, until some nappe detachment occurred. With a further increase in flow rate, the detached nappe reattached at large flow rates. The transitions, i.e., both nappe detachment and reattachment, were characterized by large instabilities, change in flow properties, and sometimes loud noise, and the processes were subject to some hysteresis. The finding demonstrated the complicated features of an unaerated round-crested weir overflow. The importance of careful experimental procedure is discussed.

Currently, discharges passing measurement devices and structures in unsteady flow conditions are determined based on relationships developed for steady flow condition due to lack of sufficient data. In this study, the operation of triangular and rectangular sharp-crested weirs in unsteady flow conditions was experimentally investigated. The compiled data indicated the presence of looped rating curves, but with transposition of the rising and falling limbs compared to that commonly observed in stream gauge ratings. The presence of such looped rating curves was justified, and the validity of the steady rating curve relationships, which are presently being applied to unsteady flow conditions, was questioned. Accordingly, relationships for evaluating unsteady discharge passing different rectangular and triangular sharp-crested weirs were presented. The proposed formulas considerably improve the discharge measurements accuracy in unsteady flow conditions and could be incorporated in hydrodynamic models.

Evapotranspiration estimation and forecasting is a key step in water management projects, especially in water-scarce countries such as Iran. Seasonal autoregressive integrated moving average (SARIMA), support vector machine (SVM), and group method of data handling (GMDH) models were developed and assessed to find an appropriate model for short and long-term forecasting of monthly reference evapotranspiration in the Guilan Plain, northern Iran. Monthly meteorological data gathered from four weather stations (Anzali, Astara, Manjil, and Rasht) were used to calculate monthly reference evapotranspiration in the period of 1993–2014 using the FAO-56 Penman–Monteith (FAO-PM) equation. The evapotranspiration data from 1993 to 2012 were used to fit SARIMA models and calibrate SVM and GMDH models, and the monthly evapotranspiration rates for the years 2013 and 2014 were forecasted using the calibrated models. The developed models were assessed using RMS error (RMSE), the Pearson correlation coefficient ($R$), the Nash–Sutcliffe model efficiency coefficient (NS), and percent bias. Taylor diagrams also were used to compare the accuracy of forecasts produced by the models. For the whole forecasting period (2013–2014), the RMSE of the calibrated SARIMA, SVM, and GMDH models were, respectively, 8.796, 9.830, and $9.547\mathrm{mm}/\mathrm{month}$ for Anzali weather station; 8.136, 9.057, and $7.808\mathrm{mm}/\mathrm{month}$ for Astara weather station; 9.454, 8.947, and $8.876\mathrm{mm}/\mathrm{month}$ for Manjil weather station; and 9.301, 10.509, and $10.138\mathrm{mm}/\mathrm{month}$ for Rasht weather station. In other words, in two weather stations under study (Anzali and Rasht), the best results were obtained from SARIMA; however, for Astara and Manjil weather stations, GMDH generated the best forecasts. Furthermore, at different forecasting horizons (1–24 months), the SARIMA models generally outperformed the SVM and GMDH models.

Based on the structural force analysis of a lap-assembled parabolic concrete channel, it is thought that the longitudinal tensile stress is the main factor that influences channel damage. The locations where the channel is most likely to be damaged were determined, and an optimization model was established with the tensile-stress value of the structure most prone to damage area as the objective function. The calculation method of the optimization model parameters is proposed, and the calculation formula of the lap assembled parabolic concrete channel structure is derived. The multi-objective optimization model is transformed into a single-objective model by using the weighted sum method, and the resulting model was solved with the Fmincon function in MATLAB R2010a. The example analysis shows that the channel with the optimum structure section obtained by optimization is better than the channel of the practical economic section in hydraulic performance and structural force performance, and the breadth–depth ratio is reduced by 48.51%, which greatly reduces the occupied land area. In a comparison with the optimum hydraulic cross section, the hydraulic performance of the optimum structure section was slightly worse, but the breadth–depth ratio decreased by 22.98%. Regarding the structural force performance, the transverse maximum tensile stress of the channel with the optimum structure section increased by 6.02%, the longitudinal tensile stress decreased by 5.63%, while the longitudinal tensile stress was the primary influencing factor of channel damage. Therefore, the optimum structure section is better than the optimum hydraulic cross section, which indicates that the optimization method is feasible and can provide significant guidance for the structural optimization of an assembled channel and similar structures.

This paper describes a computationally efficient numerical model for unsteady flow computations in open channel networks using the four-point Preissmann implicit scheme. The proposed algorithm achieves improved convergence and computational efficiency by: (1) adopting a junction point water stage prediction and correction technique based on recurrence coefficients in the double sweep algorithm for the Preissmann scheme; (2) incorporating an adaptive relaxation technique in the iterative loop; and (3) adopting a subtiming framework for time stepping. Subtiming strategy requires small computational time steps for only those channels of the network where temporal variations in flow conditions are significant, while larger time steps are taken in other channels. The subtiming strategy minimizes computational inefficiency due to temporal overdiscretization associated with schemes using uniform time step size throughout the domain. The application of the proposed algorithm is illustrated for three test cases. For a large network of channels, the proposed algorithm increased the computational efficiency by a factor of two, as compared to the conventional junction point water stage prediction and correction algorithm presently available in literature.

Quantifying surface and subsurface water dynamics is essential to design and manage furrow irrigation systems. Many furrow irrigation models estimate infiltration using empirical functions without considering the variable ponding depth and the initial soil moisture. Other models, such as the detail coupled models, which describe subsurface flow with the 2D Richards equation, are complex and difficult to apply in practical use. In this paper, we propose a modified furrow irrigation model (MFIM) that couples the approximate furrow infiltration model with the zero-inertia model. The finite difference method and an iterative procedure were used to couple computations of surface flow and infiltration. The influence of time resolution on simulating results was also examined. Results showed that the simulated water advance of the MFIM agreed reasonably well with that of the detail coupled model. More accurate simulation results were obtained by using smaller time step size. The proposed model has shown high computational efficiency. The MFIM may be an effective alternative tool for furrow irrigation design and management.

Conventional trash screens are widely used to prevent debris from being transported to downstream reaches of waterways and to allow safe passage along the same waterways. However, debris accumulation at the screens can significantly affect their hydraulic performance, resulting in negative economic and environmental consequences. Thus, this study aims to find applicable solutions for a range of screen arrangements and blockage ratios to reduce the negative hydraulic impacts caused by using the classical screen. This paper reports the results of a laboratory investigation of trash screen models in which four screen angles to the flow direction, four blockage ratios, and five flow discharges are studied. A novel curved screen shape ($\alpha <90°$) and circular bars were developed, methodically investigated, and compared to classical screens with respect to their head losses and the downstream screen velocities. The results demonstrate that the curved screens are more advantageous than the classical screens used in open channels because of the appreciably reduced head loss coefficients and improved hydraulic performance. New head loss relationship prediction formulas are developed with the tested parameters, which are strongly interrelated, for optimum engineering solutions.