Journal of Irrigation and Drainage Engineering

A mixed-integer program is presented for scheduling canal irrigation among a group of users where the duration of flow of each outlet and a target start time is specified by the users. Two models are developed. The first is a single-period model which uses as input a minimized demand at the head of the canal. This will allow the discharge at the head of the canal to be set once at the beginning of the irrigation period. Within this constraint, the single-period model minimizes the sum over all outlets of the difference between the scheduled start time of flow to each user and the requested target start times. The second model is a multiperiod model. This model favors those users who had been disadvantaged in the previous irrigation periods by giving them priority in scheduling their actual start times with target start times for the subsequent irrigation period. The proposed models can be used to support decisions in irrigation schemes operating on an arranged demand.

This paper describes a simple model for the desorption and transport of soluble reactive phosphorus (SRP) to subsurface drains. The model assumes first-order kinetically rate-limited desorption in a soil surface mixing layer over a soil profile layer that rests on an underlying, shallow restricting layer. Input data include precipitation, soil hydraulic properties, drain outflow, free water surface fluctuation, sorbed P concentrations for the mixing layer and profile, desorption rate and equilibrium soil-SRP partitioning. Model results are compared to data on flow and SRP concentrations in drain outflow collected during natural rainfall events under field conditions. The concentration time series simulated follow the sharp rise, peak, and gradual recession of the observed field data. Predicted event mass loads resulting from observed and simulated tile discharges differ from the observed load by +8.2% and −9.7%, respectively. Sensitivity analysis indicate that equilibrium assumptions would not provide satisfactory results and that mass transfer limits SRP release to the tile drain.

Soil water and nitrogen dynamics under a corn-crop field in Northern Greece were simulated during the 1996 growing period and the subsequent noncropped period. The soil water and nitrogen simulations were evaluated using a one-dimensional model based on the Galerkin finite-element method. The simulations describe dynamic environmental conditions, irrigation schedule, and inorganic N applications. They were carried on two plots of the field that differed in the amount and timing of nitrogen fertilizer applications. Inadequate irrigation water was applied, resulting in low availability of water in the root zone. The qualitative and quantitative procedures for model evaluation showed that there was good agreement between the simulated and the measured values of water content and inorganic species of N at different soil depths and the cumulative N uptake by plants. The average error was 0.006 cm³/cm³ for water content and ranged from −1.06 to 0.052 μg/g soil for NH₄-N and from −0.107 to 2.753 μg/g soil for NO₃-N. Different procedures for getting the characteristic curves result in differing water contents and nitrogen concentrations in the soil.

The EXCEL design procedure for a simple hydraulic flap gate for automatic upstream canal water level control is provided. Basic configurations were developed in The Netherlands in the 1920s and have recently been used in Indonesia, the Dominican Republic, and Nigeria. Four irrigation districts in the San Joaquin Valley of California have constructed and installed over 60 properly functioning flap gates. The gates can be installed within 2 h, but require free discharge conditions and in practice are limited to controlling water depths of about 1 m or less.

A process-based erosion model is used to study parameterization problems of sediment entrainment equations in overland flow areas. One of the equations for entrainment by flow is developed based on a theory of excess stream power, while the other two relate to excess hydraulic shear. The investigation is conducted in two steps. The first step examines parameter optimization for simulated data sets where the parameter values are known. In the second step, parameter optimization for the most robust equation is examined using experimental data from rainfall simulator plots. Results demonstrate that although the model is capable of estimating total sediment yields with relatively small errors in parameter estimates, the converse is true when the optimization is performed for sediment concentrations. Although sediment yields calculated from simulated sediment concentrations match well with observed data, the parameter estimates generally underestimate sediment concentrations on the rising limb of the sediment graphs, and they overestimate them on the falling limb. This difficulty might be related to structural problems in the model, and unique solutions for parameter estimates cannot be obtained.

The flow in a junction manhole is analyzed using hydraulic modeling and a semiempirical approach. A large number of experiments was conducted to obtain insight in the complex hydraulic features of flows in either one or both of the approach junction branches. Given their significance in applications, in addition to supercritical flow conditions, junction flow with mixed approach flows were also considered. The results include information regarding the main wave structure, with detailed discussions of locations and heights of so-called waves A, B, and C. Further, the swell height at the downstream manhole end was determined and it was found that choking of the supercritical flow structure occurs mainly because of a capacity limit at this outlet. Then, both the minimum and maximum flow limitations were determined. If the discharge is smaller than the minimum discharge, there is a transition from supercritical to subcritical junction flow. If, on the other hand, the discharge is larger than the maximum discharge, supercritical junction flow breaks down and a pressurized two-phase flow is established, which can be responsible for dangerous phenomena such as manhole geysering.

This paper presents a simplified approach to determine the end depth of a free overfall in horizontal or mildly sloping circular channels. The flow over a free overfall in a circular channel is simulated by that over a sharp-crested weir to calculate the end-depth ratio (EDR). The theoretical model is calibrated by the experimental data, making the coefficient of velocity as a free parameter. The EDR, related to the critical depth, varies almost linearly from 0.72 to 0.74 for a critical depth-diameter ratio up to 0.86. The method of estimation of discharge from the known end depth is also presented.

Analytical solutions exist for the problem of determination of seepage loss from polygonal sections such as a triangle, a rectangle, and a trapezoid. In this investigation, seepage from circular and exponential sections has been calculated by a finite-difference-based numerical solution of the differential equation governing the seepage flow. The phreatic boundaries of the flow domain were described in terms of two parameters that are estimated by a minimization process. Such seepage computations were performed for a large number of independent dimensionless variables of the section geometry. Subjecting the computed seepage to regression analyses explicit equations for seepage loss from canals having circular and exponential cross sections have been obtained. Using these seepage-loss equations, the design variables for minimum seepage loss of circular and exponential canal sections have been obtained. The use of the design equations has been illustrated by design examples.

Bentonite, a commercially available clay mineral, can be used for increasing the water holding capacity of the soil and as a sealant material. Laboratory studies were conducted with the amended soil using bentonite as an ameliorant. The mixtures were prepared with the additives on a volume basis. The chemical, physical, and hydraulic properties of the bentonite and native red soil (alfisol) mixtures were determined. Available water contents improved significantly in soil-bentonite mixtures. The pH of bentonite and soil mixtures increased from neutral value to alkalinity with the increasing proportion of bentonite in the mixtures. The same trend was also observed for cation exchange capacity. Bulk density increased from a 1:50 mixture to a 1:5 mixture and afterward decreased with the increase in the proportion of bentonite. Plastic and flow limits of the mixtures increased with the increase in bentonite proportion. Soil erodibility (dispersion ratio) as well as surface cracking increased with the addition of bentonite to native soil.