Journal of Irrigation and Drainage Engineering

Abstract

This paper presents the results of experimental measurements and numerical simulations on the hydromechanics of trapezoidal asymmetric piano key weirs and compares the performance of piano key weirs and labyrinth weirs with the same crest lengths and layouts. The results show that gradually increasing the sidewall angle from 0° to 6° for labyrinth weirs continuously increases the discharge capacity of the weir. In contrast, for piano key weirs, increasing the sidewall angle from 0° to 3° significantly increases the efficiency of the weir at low to medium heads, but further increasing the sidewall angle does not significantly change the efficiency of the hydraulically optimal asymmetric piano key weir. The high angle of the lateral spilling jet to the outlet keys, along the downstream half of the side crests and the widening of the outlet keys increase the discharge efficiency of the trapezoidal piano-key weir with 3° of sidewall angle and 4 unit keys, compared to the identical weir with 6° of sidewall angle. Despite the wider outlet keys at the downstream of the trapezoidal piano key weir with 6° of sidewall angle, the narrower width of the inlet key, results in a higher streamwise inertia and thus the lower discharge at the outlet keys. Discretization of the discharge along the crest of the tested weirs confirms these results.

Practical Applications

In this research, the discharge capacity of trapezoidal piano key and labyrinth weirs were examined under various conditions and their performance was compared. The authors suggest an optimal ratio of weir height to unit width for rectangular piano key weirs, which increases the discharge coefficient by 6%. This finding can be applied to new dam projects to design cost-effective and highly efficient weirs. The study also showed that increasing the sidewall angle from 0° to 6° in trapezoidal labyrinth weirs improves the discharge coefficient by an average of 10.3%. However, in trapezoidal piano key weirs, increasing the sidewall angle from 0° to 3° has a significant impact on discharge coefficient, but further increase reverses the trend. Overall, piano key weirs perform better than their labyrinth counterparts.

Surface irrigation is still by far the most common method adopted worldwide. For many developing countries, shifting to modern pressurized irrigation is hampered by system and energy costs. Gated pipes are an improvement on furrow irrigation. They offer an affordable modernization option for traditional surface systems. Designed to operate at low pressure, gated pipes have the potential to reduce conveyance losses, improve application uniformity, and minimize runoff, which is often responsible for spreading waterborne diseases and lowering water quality. However, very little pertinent data are available on their performance. In this paper, a new model (OptGate) for the design and performance analysis of self-compensated (SC) and conventional rectangular (CG) gated pipes is described and field validated. OptGate proved its reliability in simulating the discharges along the gated pipe with RMSE = 0.29 and $0.119{\mathrm{m}}^3/\mathrm{h}$ for CG and SC, respectively, under a range of streaming head pressures ranging from 2 to 10 m with a 2-m step. The proposed model can provide users with the ability to predict system performance under different pressures, spacings, gate shapes and behaviors, pipe diameters, and land topography scenarios.

Abstract

Accurate reference evapotranspiration (${\mathrm{ET}}_o$) estimation is crucial for water irrigation management and sustainable agriculture planning. The difficulty in obtaining several data requirements for employing the recommended Food and Agriculture Organization Penman-Monteith method (FAO-PM) for reliable estimation of ${\mathrm{ET}}_o$ has led to the development of many empirical models. This is particularly crucial for Iraq, located in West Asia (29°15′00″–38°15′00″ N; 38°45′00″–48°45′00″ E), where meteorological data are often limited or missing. The objectives of the present study were to assess the performance of 40 ${\mathrm{ET}}_o$ empirical models (13 radiation-based, 13 mass-transfer-based, and 14 temperature-based) against the FAO-PM model and identify alternative models with the minimal available data in three major climatic zones of Iraq: the Mediterranean climate (MCZ), semiarid (SCZ), and arid desert (ACZ). The recent ERA5 data set was adopted. The results indicate that (1) the Rohwer mass-transfer method is the best for estimating ${\mathrm{ET}}_o$ for two-thirds of Iraq with a mean correlation coefficient ($R^2$) of 0.97, mean Kling-Gupta efficiency (KGE) of 0.84, mean percent bias (PBIAS) of $-8.92\%$, mean Nash-Sutcliffe efficiency coefficient (NSE) of 0.92, and root mean square error (RMSE)-observations standard deviation ratio (RSR) of 0.27, followed by the Penman ($R^2=0.90$, $\mathrm{KGE}=0.75$, $\mathrm{NSE}=0.77$, $\mathrm{RSR}=0.46$, and $\mathrm{PBIAS}=6.36\%$) and Caprio ($R^2=0.90$, $\mathrm{KGE}=0.66$, $\mathrm{NSE}=0.54$, $\mathrm{RSR}=0.58$, and $\mathrm{PBIAS}=24.64\%$) models; (2) Caprio is the best radiation-based model for estimating ${\mathrm{ET}}_o$, mainly in the ACZ, whereas Kharrufa is the best temperature-based model for estimating ${\mathrm{ET}}_o$, primarily in the SCZ and ACZ. Overall, the mass-transfer-based models performed better than other-based models for ${\mathrm{ET}}_o$ estimation. The outcomes of this study provide a scientific reference for accurate ${\mathrm{ET}}_o$ estimation using empirical models under limited data sets, which is valuable for irrigation management in Iraq.

Practical Applications

Accurately estimating ${\mathrm{ET}}_o$ is vital for effective water irrigation management and sustainable agriculture planning. However, the recommended method for estimating ${\mathrm{ET}}_o$, the FAO-PM method, requires various data inputs that may not always be readily available, especially in regions like Iraq. Therefore, this study assessed the performance of 40 empirical ${\mathrm{ET}}_o$ models, categorized into radiation-based, mass-transfer-based, and temperature-based models, against the FAO-PM model in three major climatic zones of Iraq: the Mediterranean climate, the semiarid region, and the arid desert. The study found that the Rohwer mass-transfer method showed the best performance in estimating ${\mathrm{ET}}_o$ for two-thirds of Iraq. The Penman and Caprio models also performed well in estimating ${\mathrm{ET}}_o$ in specific areas. The study revealed that the choice of the ${\mathrm{ET}}_o$ model varied depending on the climatic zone. The Caprio model performed best for radiation-based estimation in the arid desert, whereas the Kharrufa model was most effective for temperature-based estimation in the semiarid region and arid desert. Overall, the mass-transfer-based models outperformed other types of models in ${\mathrm{ET}}_o$ estimation.

Open-channel confluences are important junctions in natural and human-made channel networks. Different controls of the confluence hydrodynamics have already been studied intensively, often in schematized geometrical configurations. The T-shaped planform with branches having rectangular cross-sections of equal width is a popular geometry in lab experiments and numerical modeling. However, limited research has been conducted concerning the influence on the flow features of widening or narrowing the downstream branch in such a confluence. Therefore, four geometrical cases with a different downstream channel width were studied with large eddy simulations, while keeping the tributary-dominant discharge ratio, the width of the upstream branches, and the tailwater depth constant. The effect of increasing the downstream-to-upstream branch width ratio was analyzed with regard to the water surface shape, the stagnation zone location, the tributary inflow angles at the interface with the junction, the separation zone dimensions, the flow acceleration toward the vena contracta, the secondary flow patterns and intensity, the turbulent kinetic energy in the shear layers, the bed shear stresses and the backwater effects in the upstream branches. When comparing the numerically predicted upstream-to-downstream water depth ratios with those predicted by the single analytical model applicable to confluences with unequal channel widths, it is found that both models capture the reduction of the upstream-to-downstream water depth ratios when widening the downstream branch, but benchmark cases show that the numerical model predicts the correct relative magnitude of the water depth ratios in the main channel and the tributary channel, contrary to the analytical model.