Journal of Irrigation and Drainage Engineering

The effects of wear on the ability of hand-move and side-roll sprinkler irrigation systems to maintain the designed water application rate and uniformity are of concern with regards to crop performance, water use efficiency, and environmental impact. Six hand-move and six side-roll sprinkler irrigation systems used under commercial crop production in Lane County, Oreg., were evaluated for equipment wear and performance. Individual sprinkler nozzle size and discharge rates were measured for each system and used in a computer model to estimate field application rate and uniformity. New nozzles were installed on six systems to compare potential application rate and uniformity with existing performance. Despite reducing the coefficient of variation in discharge between sprinklers from 10 to 2%, little increase in water application uniformity was attained. A 13% decrease in mean water application rate was documented after nozzle replacement. Ignored overapplication due to worn or mismatched nozzles results in overirrigation in both rate and total amount, which gives rise to the potential for increased surface redistribution and deep percolation, resulting in water and nutrient losses.

Optimal resources allocation strategies for a canal command in the semiarid region of Indian Punjab are developed in a stochastic regime, considering the competition of the crops in a season, both for irrigation water and area of cultivation. The proposed strategies are divided into two modules using a multilevel approach. The first module determines the optimal seasonal allocation of water as well as optimal cropping pattern. This module is subdivided into two stages. The first stage is a single crop intraseasonal model that employs a stochastic dynamic programming algorithm. The stochastic variables are weekly canal releases and evapotranspiration of the crop that are fitted to different probability distribution functions to determine the expected values at various risk levels. The second stage is a deterministic dynamic programming model that takes into account the multicrop situation. An exponential seasonal crop-water production function is used in this stage. The second module is a single crop stochastic dynamic programming intraseasonal model that takes the output of the first module and gives the optimal weekly irrigation allocations for each crop by considering the stress sensitivity factors of crops.

The adjustment of structures along irrigation free-surface systems aims to achieve, or maintain, a targeted status of the canal network. The interaction between structures and reaches is an important aspect of canal control and the process of the operation itself can generate perturbations. These perturbations depend on the frequency and mode of operation as well as the sensitivity to the setting of the structures. Highly sensitive structures generate relatively large changes in their outputs for small input changes. Therefore, highly sensitive structures should be operated with more precision than others to minimize deviations between the actual and targeted states. Knowing which are the sensitive structures along a canal and how to operate them is essential for a cost-effective operational procedure. This paper presents a set of sensitivity indicators that enable the identification of sensitive structures and an assessment of their operational tolerance.

A priori identifiability (i.e., identifiability under perfect data) is a necessary condition for posteriori identifiability (i.e., identifiability under real data), and by implication a priori nonidentifiability is a sufficient condition for posteriori nonidentifiability. Therefore, it is important to prove a priori identifiability before attempting to estimate model parameters through nonlinear regression with real noisy data. This paper investigates the a priori (also called classical, structural, or deterministic) identifiability of soil parameters using Richards's equation with perfect distributed pressure data and prescribed initial and boundary conditions. The study of a priori identifiability is made possible through the concept of linear independence of vectors. As expected, it is shown that the unsaturated soil parameters are not a priori identifiable, and thus not posteriori identifiable, with either zero flow pressure data or steady-state flow pressure data. In addition, it is shown that models with more than two parameters are not a priori identifiable with transient pressure data. Therefore, models with more than two parameters are not posteriori identifiable with real pressure data regardless of the quantity and quality of this data. However, it is found that two parameter models are a priori identifiable with transient pressure data. Hence, the necessary condition for the posteriori identifiability of two parameter models is proven.

Two-dimensional flows near cavities were studied by superposition of the Philip solution for an isobaric cylindrical cavity in an infinite unsaturated soil and vertical infiltration. Depending on the radius of the cavity and the intensity of the net infiltration, the cavity irrigates the surrounding soil forming a plume of seeping moisture (source regime), drains the descending infiltration forming a capture zone (sink regime), or admits water through its upper section and releases water through its bottom (dipole regime). The separatrices, i.e., the lines, which divide the incoming, outgoing, and passing-by portions of the flow, were found. For the dipole regime, the values of flow rates through the admitting and releasing sections of the cavity and streamlines were calculated. The supercritical flow regime is characterized by a permeable blunt upstream section of the cavity and watertight downstream segments. For elongated cavities, this regime was modeled as a combination of a permeable isobaric segment and a part of the adjacent separatrice. Analogies with saturated flows in confined aquifers near a circular constant head boundary and with contaminant transport from a circular isoconcentric source were discussed.

A series of experimental runs on the performance of stilling basin models with different configurations in terms of shape, size, and location of appurtenances was carried out to develop an efficient and economical stilling basin for circular pipe outlets. Test results from the experimental studies conducted on the outlets of 7.5- and 10-cm diameter, with inflow Froude number ranging from 1.70 to 5.50, are reported. A performance criteria obtained from scour parameters are described to compare the performances of the different basin models tested with the same erodible bed and a constant run time. A wedge-shaped splitter block was found to be very effective in spreading the jet of water over the width of the stilling basin within a shorter length, and resulting in a better energy dissipation. A set of experiments was also carried out to obtain the suitable size and the location of the wedge-shaped splitter block. The new stilling basin model with a wedge-shaped splitter block along with an impact wall, an intermediate sill, and a rounded end sill compares well in terms of performance with the USBR impact type VI stilling basin and is 25% shorter in length.

Addition of a strong acid to irrigation water could enhance sorption of boron and arsenic in a Lahontan Valley field. Approximately 90 soil samples, taken from a field irrigated with mildly brackish drainage water (TDS ≈ 1,250 mg L⁻¹), were analyzed for soluble constituents. When the pH of the saturated soil paste was between 7.2 and 8.8, dissolved arsenic and boron concentrations increased, possibly indicating decreased sorption on oxyhydroxide precipitates of manganese and aluminum, and goethite, inferred from equilibrium aqueous speciation modeling. In the pH range of 7.2–8.8, linear regression models indicate a unit decrease in pH may result in a log[B] reduction of 1.18 and a log[total As] reduction of 1.58, when the concentrations are expressed in mg L⁻¹. Geochemical modeling indicates that a reduction in pH in the observed range should not solubilize carbonates that could counteract acidity increase. Addition of HCl or H₂SO₄ to reduce soil water pH may be an effective way to minimize deep percolation of boron and arsenic.

This paper reports the results of an investigation carried out to establish the stage-discharge relationship for a flow simultaneously discharging over and under a sluice or a broad-crested gate. The stage-discharge relationship is deduced by a theoretical analysis, based on the application of the Π-theorem of the dimensional analysis and the incomplete self-similarity theory, coupled with an experimental investigation carried out by using a laboratory flume.