Journal of Irrigation and Drainage Engineering

This work presents a simple, cost-effective, and operational approach to monitor crop water requirements at the regional scale for water management and monitoring purposes. The recommended Food and Agricultural Organization of the United Nations methodology (FAO-56) calculates crop evapotranspiration using crop-specific coefficients $\left(K_c\right)$ , which vary according to the crop type, health, and phenological stage. This approach, though widely applied for irrigation planning, cannot always match the appropriate crop coefficient with the actual crop phenological stage and health condition, especially in anomalous situations. Previous research demonstrated that crop coefficients and spectral vegetation indexes are correlated. Recent studies have used this relationship with high-resolution satellite data from different sensors to provide information to irrigation advisory services. However, high-resolution data are not feasible for an operational and routine monitoring of water consumption and needs. This paper tests the usefulness of time series of coarse resolution satellite data such as those collected by the moderate-resolution imaging spectroradiometer (MODIS) sensor, to monitor crop coefficients temporal and spatial variability and therefore crop water needs at the regional scale taking advantage of the peculiar characteristics offered by MODIS in terms of high temporal resolution and preprocessed products availability. The outlined methodology takes into account the actual growing stage of the crops and nearly real-time vegetation variations, overcoming some limitations of the traditional FAO approach while preserving the maximum operability. The analysis was carried out in the South Milan agricultural area on data referring to 2003 and 2004. The results agreed with those of other studies and proved to be able to account for the anomalous conditions of the summer in 2003. These results were then compared with those obtained using the traditional FAO crop coefficient curves built with data collected during field campaigns in the same years in rice fields. Constraints, limitations, and possible uses are discussed.

Estimation of evapotranspiration (ET) is important for monitoring crop water stress and for developing decision support systems for irrigation scheduling. Techniques to estimate ET have been available for many years, while more recently remote sensing data have extended ET into a spatially distributed context. However, remote sensing data cannot be easily used in decision systems if they are not available frequently. For many crops ET estimates are needed at intervals of a week or less, but unfortunately due to cost, weather, and sensor availability constraints, high resolution $\left(<100\text{m}\right)$ remote sensing data are usually available no more frequently than 2 weeks. Since resolution of this problem is unlikely to occur soon, a modeling approach has been developed to extrapolate remotely sensed inputs needed to estimate ET. The approach accomplishes this by combining time-series observations from ground-based radiometers and meteorological instruments with episodic visible, near infrared, and thermal infrared remote sensing image data. The key components of the model are a vegetation density predictor and a diurnal land surface temperature disaggregator, both of which supply needed inputs to a surface energy balance model. To illustrate model implementation, remote sensing and ground-based experimental data were collected for cotton grown in 2003 at Maricopa, Ariz. Spatially distributed cotton canopy densities were forecasted for a 22-day interval using vegetation indices from remote sensing and fractional cover from ground-level photography. Spatially distributed canopy and soil surface temperatures were predicted at 15-min time steps for the same interval by scaling diurnal canopy temperatures according to time of day and vegetative cover. Considering that the predictions span a rapid growth phase of the cotton crop, comparison of spatially projected canopy cover with observed cover were reasonably good, with $R^2=0.65$ and a root-mean-squared error (RMSE) of 0.13. Comparison of predicted temperatures also showed fair agreement with $\text{RMSE}=2.1°\text{C}$ . These results show that combining episodic remotely sensed data with continuous ground-based radiometric data are a technically feasible way to forecast spatially distributed input data needed for ET modeling over crops.

The distribution and trends in reference evapotranspiration $\left({\text{ET}}_o\right)$ are extremely important to water resources planning for agriculture, and it is widely believed that rates of ${\text{ET}}_o$ will increase with global warming. This is a big concern in China, where water deficits are common in the North China Plain (NCP). In this study, Penman-Monteith reference evapotranspiration at 26 meteorological stations during 1961–2006 in and around the NCP was calculated. The temporal variations and spatial distribution of ${\text{ET}}_o$ were analyzed and the causes for the variations were discussed. The results showed that: (1) the NCP was divided into two climatic regions based on aridity values: a semiarid region that accounts for 69% of the area and subhumid regions that made of the remaining area; (2) over the entire NCP, the highest annual ${\text{ET}}_o$ occurred in the central and western areas and the lowest total ${\text{ET}}_o$ was observed in the east. Comparing the mean monthly ${\text{ET}}_o$ and annual ${\text{ET}}_o$ distributions, the high ${\text{ET}}_o$ values from May through July mainly determined the annual ${\text{ET}}_o$ distribution; (3) for the whole NCP, annual ${\text{ET}}_o$ showed a statistically significant decrease of 11.92 mm/decade over the 46 years of data collection in the NCP or approximately a 5% total decrease compared to the ${\text{ET}}_o$ values in 1961; (4) to determine which variable has the greatest effect on the decrease in ${\text{ET}}_o$ , decadal changes were observed for daily values of maximum air temperature $\left(+0.16°\text{C}\right)$ , minimum air temperature $\left(+0.35°\text{C}\right)$ , net radiation $\left(-0.13\text{MJ}{\text{m}}^{-2}\right)$ , and mean wind speed $\left(-0.09\text{m}{\text{s}}^{-1}\right)$ . These results indicate that the decreasing net radiation and wind speed had a bigger impact on ${\text{ET}}_o$ rates than the increases observed by the maximum and minimum temperatures.

A modified form of Horton’s infiltration equation was used to model advance trajectory of the wetting front as well as for predicting cumulative infiltration in a furrow-irrigated system using experimental data obtained in a Yolo loam soil (Typic Xerorthents) planted to a furrow-irrigated tomato crop. Two distinct cases were considered: (1) furrows with deposional crusts at the bottom (uncultivated) and (2) furrows in which a crust breaking device was used to remove the deposional crust (cultivated). The model was used to successfully predict infiltration parameters, which are important for design and management of furrow irrigation systems. Simulation studies were also conducted to determine cumulative infiltration along a furrow.

The flow exponent greatly determines the hydraulic performance of drip emitters. The objective of this study is to reveal the change of flow exponents with different water pressures. Laboratorial experiments of the relationship between discharges and water pressures were conducted with five types of drip emitters used for surface drip irrigation systems. The regression for calculating flow exponents was done with different segment pressures. The results showed that the flow exponent reduced gradually with the increase of the pressure segment except a brief increase in the early stage of pressure increasing due to the channel expansion. The eddy drip-arrow is most suitable for the pressure ranges of 2–8 and $8–14{\text{mH}}_2\text{O}$ . The effect of the use of the small diameter drip-tube is best in the high-pressure range of $17–25{\text{mH}}_2\text{O}$ . The relation between average flow velocities and water pressures is characterized by the flow exponent, the same as that between emitter discharges and water pressures. The eddy drip-arrow and the in-line drip-tape with a low discharge but a high flow velocity can meet the requirements of both prominent anticlogging and long-distance use. The Reynolds numbers of the five types of drip emitters range from 200 to 1,800, below the critical value of the turbulent transition of a conventional scale flow. The small diameter drip-tube needs the lowest Reynolds number required for full turbulence transition which assures a lower flow exponent when the emitter runs with a relatively low discharge.

The hydrant pressure head in an on-demand water distribution system can be subject to high fluctuation depending on the discharge flowing inside the pipes, with consequent impacts on the performance of on-farm irrigation systems. In this work, an Italian water distribution system was analyzed using the AKLA model at upstream discharges of 1,200 and $600\text{L}\cdot {\text{s}}^{-1}$ to estimate the range of hydrant pressure variation. A computer model was developed, calibrated, and used to evaluate the performance of a drip irrigation system by relating the on-farm network with the hydrant characteristic curve at a certain operating status. The flow regulator within the hydrant played an important role in stabilizing the performance of the network at hydrant pressures higher than 27 m. At lower hydrant pressures, to apply the same amount of water, irrigation time must be extended by 17 and 95% for pressure heads of 20 and 12 m, respectively. These approaches described have great utility to ensure adequate irrigation management when water is delivered by pressurized on-demand systems.

In this study, a method for designing paired laterals that meet with required water application uniformity on sloped fields was developed using the energy gradient line approach based on the definition of the best submain position locations in which the same minimum pressure exists in uphill and downhill laterals. The best equation form of best submain position was determined. Also, the solution procedure was introduced to get the final solution to avoid the phenomena of no convergence or slow convergence. In this method, the required water application uniformity was used directly as a computational parameter in designing. When the designed emitter discharge, required water application uniformity and one parameter (either length or diameter) of a paired lateral are provided; the system developed here enables another parameter and the best submain position to be determined for any field slope conditions. Taken together, the results of this study show that final solutions can be obtained quickly and reasonably.

A procedure is proposed for calculating the spatial and temporal variation of drawdown due to pumping a well tapping two aquifers separated by an aquitard, using convolution and MODFLOW. It can take into account the unsteady pumping discharge and cross flow through the intervening aquitard. A discrete pulse kernel method based on superposition/convolution is used to account for the unsteady pumping discharge. The discrete pulse kernels are calculated using MODFLOW. The contributions of the aquifers to the pumped discharge are accounted implicitly and not required to be specified explicitly. Available numerical models (e.g., MODFLOW) require the aquifer contributions that are implicitly controlled, to be specified explicitly. The use of the suggested procedure is illustrated using examples. The contributions of the aquifers are found not in proportion to their transmissivities but vary with time, when the diffusivities of the aquifers are not equal. Applying the new procedure, the numerical models, such as MODFLOW can be used to correctly model the transient pumping from two aquifers with cross flow; thus, it opens up the possibility of numerically accounting for the aquifer heterogeneity while dealing with the flow to a well tapping two aquifers under a transient pumping, which would be otherwise difficult to account for analytically.