Journal of Irrigation and Drainage Engineering

Many studies have investigated easy methods to design drip laterals, as well as the best resistance equation to use, which is fundamental to accurately account for friction losses. This paper addresses both the features of lateral design relationships and the influence of the friction-loss equation on the design variables. First, simple closed-form solutions of the energy balance equation for both sloped and horizontal drip laterals are derived with the simplified Darcy-Weisbach resistance formula by assuming the Darcy friction factor as invariant versus the Reynolds number. Second, an error analysis is performed assuming the friction factor as constant in the design, which is compared to using the Blasius resistance equation that accounts for Reynolds number variation. Third, once these closed-form solutions have been derived, their extension to account for friction-factor variation versus the Reynolds number is demonstrated by introducing an average friction-factor relationship, with errors in computing the design variables that do not exceed 0.27% (i.e., almost the exact solutions obtained when the Blasius equation is used).

Inclined rectangular weirs, also named pivot weirs, are used both to adjust the upstream water level and to increase the flow capacity compared with that of normal rectangular weirs. However, the experimental studies available in the literature present contradictory conclusions for the inclined weir discharge capacity. In this paper, new experimental runs and numerical simulations were performed to investigate the effect of the rectangular weir inclination angle on the stage-discharge formula. The experiments were carried out using inclination angles ranging from 30° to 90°. Buckingham’s theorem of dimensional analysis was used to obtain an equation for the flow magnification ratio (the discharge through the pivot weir divided by the corresponding discharge over the vertical weir) as a function of the weir inclination angle. The experimental observations revealed that the flow magnification ratio increases with the inclination angle to a maximum of 1.082 for 30° of inclination. Bidimensional (2D) numerical analysis using the OpenFOAM computational fluid dynamic toolbox was also used. The flow magnification ratios obtained numerically confirmed those obtained experimentally.

Free overfalls offer a simple approach to determining a discharge using a single depth measurement at the end section (brink) of a channel. Fundamental equations based on the sharp-crested weir theory, both continuity and energy, were used to determine the end-depth ratio (EDR) and the end-depth discharge relationships for generalized trapezoidal free overfalls in both subcritical and supercritical flow regimes. Streamline curvature and the brink pressure effects were also incorporated into the aforementioned equations to better describe the flow phenomenon. In subcritical flow regimes, the EDR is the ratio between the water depth at the brink and the critical depth. For trapezoidal channel cross sections, the EDR cannot be expressed as a constant value even when the flow is in a subcritical regime, unlike cases for rectangular ($y_b=0.714y_c$) and triangular ($y_b=0.800y_c$) channel cross sections. In supercritical flow regimes, the EDR is the ratio between the water depth at the brink and the normal (uniform) depth. Hence, the upstream Froude number (${\mathsf{F}}_n$) was involved as a predictive variable. The conventional form of Manning’s equation was used to express the discharge as a function of the depth of water at the brink and the geometric characteristics of the channel cross section. Simple empirical discharge equations were generated for practicing engineers. Verification of the theory with existing theoretical and experimental data was also performed.

Reference evapotranspiration (${\mathrm{ET}}_{\mathrm{o}}$) estimates in greenhouses require the measurement of several meteorological variables. These measurements are not always conducted by water managers. The ${\mathrm{ET}}_{\mathrm{o}}$ estimates also depend on the aerodynamic conditions in the greenhouse. The objective of this study is to evaluate the use of atmometer-based evapotranspiration estimates (${\mathrm{ET}}_{\text{gauge}}$) in greenhouses under ventilated and unventilated conditions as a surrogate for the reference evapotranspiration estimated by two grass-reference ${\mathrm{ET}}_{\mathrm{o}}$ equations: the Penman-Monteith equation (${\mathrm{ET}}_{\text{oPM}}$) and the Hargreaves-Samani equation (${\mathrm{ET}}_{\mathrm{oH}}$). Two versions of the Hargreaves-Samani equation were compared with the ${\mathrm{ET}}_{\text{gauge}}$; one used the measured solar radiation (${\mathrm{ET}}_{\text{oHm}}$), and the other used the calculated solar radiation (${\mathrm{ET}}_{\text{oHc}}$). The results showed that the ventilation had a great impact on the relationships between the ${\mathrm{ET}}_{\text{gauge}}$ and the reference equations. Under ventilated conditions, the daily ${\mathrm{ET}}_{\text{gauge}}$ and both the ${\mathrm{ET}}_{\text{oPM}}$ and ${\mathrm{ET}}_{\text{oHm}}$ were linearly correlated. The daily ${\mathrm{ET}}_{\text{gauge}}$ measurements were 13% lower than the ${\mathrm{ET}}_{\text{oPM}}$ and 57% higher than the ${\mathrm{ET}}_{\mathrm{oH}}$. Under unventilated conditions, the ${\mathrm{ET}}_{\text{gauge}}$ measurements were higher than the ${\mathrm{ET}}_{\text{oPM}}$ and ${\mathrm{ET}}_{\mathrm{oH}}$ by 68 and 50%, respectively. With the proposed air temperature–based and relative humidity–based adjustments for the ${\mathrm{ET}}_{\text{gauge}}$ readings, the performance of the atmometer inside the greenhouse improved (the relative error decreased by 40% on average). Our findings indicate that atmometers can be used in greenhouses in lieu of the calculated Penman-Monteith reference evapotranspiration (but not the Hargreaves-Samani’s) if the proper calibration is performed, especially under unventilated conditions.

The performance of Class A pan data and six commonly used pan coefficient ($k_p$) approaches were evaluated with respect to the reference evapotranspiration ($E{T}_0$) in a typical farm setup of a hot arid environment. To assess the possible impacts of different microclimates and periods of the year, data from two locations on the farm and two periods of the year were considered. The results show a low agreement between modeled and real $k_p$ with a coefficient of determination ($R^2$) ranging from 0.60 to no correlation. Reference evapotranspiration from pan evaporation ($E{T}_{0,\mathrm{pan}}$) was evaluated, and $E{T}_0$ estimates delivered better results with $R^2$ ranging from 0.94 to 0.90. However, comparing daily $E{T}_0$ based on hourly $E{T}_0$ values with $E{T}_0$ based on daily averages of weather data yielded significant differences in model performance. Furthermore, different periods of the year and different microclimatic conditions also affected model performance and influenced the ranking of the models. Due to the sensitivity of the $k_p$ coefficient models against these impacts, it was not possible to identify a robust “best” model that delivered nonbiased $E{T}_0$ estimations within an acceptable range of accuracy. These findings indicate that the drivers of pan evaporation are not sufficiently reflected in easy-to-measure weather data used by the $k_p$ coefficient models.

Multiple-inlet rice irrigation (MIRI) decreased irrigation applications by an average of 24% over an 86-day flood when compared to single-inlet (SI) rice flood distribution. Even in the absence of rainfall, MIRI required 22% less irrigation than SI when generalized water-balance equations were used to simulate irrigation use in a 16-ha, clay soil, straight-levee field typical of the Lower Mississippi River Valley (LMRV). The majority of MIRI savings was due to improved distribution uniformity that results in less runoff rather than increased rainfall capture. Using 260 site-years of rainfall from nine LMRV locations, these results also suggest that the wide variability in irrigation use observed in MIRI field trials is not due to variations in rainfall alone. Rather, other factors such as management style and field conditions likely impact irrigation use as much, if not more than, year-to-year variations in rainfall. Taken together, these results reinforce university extension efforts that educate farmers on the importance of irrigation management in realizing the water-conserving benefits of MIRI. Further, the results support efforts to automate irrigation monitoring and pump control by showing that significant irrigation savings occur with MIRI when pumping is halted in a timely manner.

A substantial amount of work has been done concerning the use of siphons to automatically start and stop the discharge of fluid in many applications, except for application as a water supply system for smallholder drip irrigation systems. Thus, the main aim of this study was to develop a siphon apparatus and apply it to regulate the low-flow-rate water supply from a solar-powered pump as intermittent doses to pressurize small-scale drip irrigation farms in Sub-Saharan Africa (SSA). A siphon apparatus was assembled from polyvinyl chloride (PVC) pipes and installed in a 210-L tank. A drip irrigation system covering an area of $500{\mathrm{m}}^2$ was connected to the tank containing the siphon, and water was lifted into the tank by a low-capacity 12-V pump that was powered by a 50-W solar panel. The siphon apparatus in the tank was tested over extended periods of time and was shown to work reliably and consistently by automatically and intermittently discharging water to pressurize the drip irrigation laterals, thus being able to irrigate while largely unattended. Test results showed that the inflow rate of $4.75\mathrm{L}{\min }^{-1}$ successfully caused the siphon to start, and the average coefficient of discharge for the siphon was found to be 0.35. Results from water application uniformity tests calculated using the method established by the American Society of Agricultural and Biological Engineers (ASABE) showed average to good discharge uniformity from emitters. The setup cost of the system was USD 415, whereas the net farm income obtained by accounting for all variable costs and annualized capital cost of the irrigation setup was USD 69. We conclude that it should be fairly easy and economically feasible to adapt the dosing siphon, low-flow-rate pump, and solar panel to other small-scale drip irrigation systems that are currently being promoted in SSA.

In many coastal areas, high water tables are present, complicating installation of some stormwater best management practices (BMPs) that rely on infiltration. Regional estimates of the seasonal high water table (SHWT) often rely on sources such as soil surveys taken over a decade ago; these data are static and do not account for groundwater withdrawals or other anthropogenic impacts. To improve estimates of the SHWT, we developed a GIS-based methodology relying on surface water elevations. Data sources included a 1.5-m (5.0 ft) resolution Lidar-derived digital elevation model (DEM), aerial imagery, and publicly available shapefiles of water boundaries. Twenty-six groundwater monitoring wells were screened to eliminate well locations influenced by pumping, yielding 22 wells. In coastal Virginia, tidal water bodies and ditches form terminal boundaries for discharge from the water-table aquifers and permit water table elevations to be fixed at the landward boundaries of surface water bodies. Water table elevations interpolated from well data and boundary elevations were used to create a triangulated irregular network representing the water table elevations for November 2012, which was the date of the DEM. An adjustment factor, calculated from the highest recorded April water table depth from long-term groundwater monitoring data, was added to estimate the SHWT elevation. SHWT elevations were subtracted from the DEM to yield SHWT depth, which was compared with long-term monitoring well data, yielding an $R^2$ value of 0.91. Residual errors were random, although the method underpredicted the highest expected SHWT and overpredicted the median SHWT. The SHWT depth map was validated by using water table depths from 57 soil borings at 10 different sites, and consistently matched observations better than available soil survey estimates. The SHWT depth map could be useful for BMP siting and feasibility studies in similar hydrogeological settings.

Climate change (i.e., fluctuating precipitation and increasing temperature) is extremely likely to challenge water systems through an increased water demand, potentially impacting water use for irrigation. In China, over 40% of wheat planting areas need to be irrigated to ensure crop yield, especially in semiarid and semihumid regions. To quantitatively simulate the effects of climate change on the net irrigation requirements (NIR) for winter wheat across central-eastern China, we use historical observations to drive a crop-specific water balance model with crop-specific potential evapotranspiration (${\mathrm{ET}}_c$) and effective precipitation ($P_e$) and then apply it to future climate scenarios. Specifically, the future climatic variables are based on three concentration pathway (RCP2.6, RCP4.5, and RCP8.5, respectively) scenarios of a general circulation model (GCM) from phase five of the Coupled Model Intercomparison Project (CMIP5). The results show that at the mean annual scale, during the last few decades, high ${\mathrm{ET}}_c$ and NIR values have mainly been distributed at the junction of Hebei, Henan, and Shandong Provinces. The days of the growing period (DGP) and $P_e$ values are high in the northwest and southern areas of the study region. The historical values of DGP, ${\mathrm{ET}}_c$, $P_e$, and NIR are characterized by significant downward trends, accounting for 91.8, 49.5, 29.9, and 26.8% of all stations, respectively. For the future period, DGP, ${\mathrm{ET}}_c$, and NIR will likely decline in most regions. In comparison with the baseline period, the average change rates of NIR in the three scenarios are $-11.3$, $-16.8$, and $-23.9\%$ in the 2020s, 2050s, and 2080s, respectively. Moreover, the future NIR will increase in Hebei Province, Shanxi Province, and northern Jiangsu Province.