WaterSubject

On-demand pressurized irrigation systems are designed to deliver water with the flow rate and pressure required by the farm irrigation systems, sprinkling or micro-irrigation, and respecting the time, duration and frequency decided by the farmers. Due to the variation in farm demand along the season and the day, a large spatial and temporal variability of flow regimes occurs in these systems, which may affect the performance of the farm systems and the yields of the irrigated crops. Therefore, there is a need to analyse those systems to identify and solve performance problems. In this research, two simulation models for the analysis of irrigation systems operating on-demand, ICARE and AKLA, are used and compared to assess the hydraulic performance of the irrigation network of the Lucefecit Irrigation System, in Southern Portugal. ICARE assesses the global performance of the irrigation system through the indexed characteristic curves, while AKLA provides for the identification of the relative pressure deficit and reliability at every hydrant. Both models adopt a flow-driven analysis approach, performing the analysis for multiple flow regimes. To support the hydraulic characterization of the system and for calibration of the steady-state hydraulic model, field measurements were performed at selected nodes of the network, including four hydrants. The analysis with ICARE does not provide for a sufficient identification of problems. In fact, poor performance is indicated when a few hydrants operate below the minimum pressure set at design. Differently, the analysis with AKLA, applied at the hydrant level, shows that the performance of the Lucefecit system is generally acceptable. AKLA identifies which hydrants operate below the required pressure and, therefore, allows to support any eventual related improvement. Results show that the performance of the system highly improved when changing the piezometric elevation from 260 to 265 m a.s.l. However, this improvement is not sufficient because three hydrants still have high relative pressure deficit and low reliability. Solutions for those hydrants require increasing diameters of network pipes supplying them.

Pressurized irrigation systems operating on-demand deliver water with the flow rate and pressure required by farm irrigation systems, sprinkling or micro-irrigation, and with time duration and frequency decided by the farmers. Therefore, they allow farmers to operate their irrigation systems with a large freedom with respect to other types of delivery schedules. The water delivery nodes of a pressurized irrigation network consist of hydraulic valves, usually designated as hydrants, which are generally equipped with flow and pressure regulators. The nominal discharge at a hydrant is then supposed to be independent of the pressure. Hydrants’ discharges are set at the design phase according to the size of the field, the type of on-farm irrigation systems, crop water requirements and, more recently, taking into consideration the variable decisions of farmers relative to the time duration and frequency of irrigations and the farmer’s behaviour. Systems are designed to assure that the pressure at all hydrants is equal or greater to the minimum pressure set at design, to assure appropriate functioning of the on-farm systems.

Sizing these systems requires careful selection of design discharges and pressure regulation and an optimization of the pipe network. Discharges flowing into each section may be computed by using probabilistic approaches in which the Gaussian distribution of discharges is hypothesized. With these methods a risk threshold is accepted, i.e. during the operation of the system, discharges higher than those assumed at design may occur with low probability. Alternatively, discharges may be generated through the simulation of the demand by performing the water balance at level of each hydrant combined with a stochastic approach to take into consideration the farmers’ behaviour, i.e. that farmers’ irrigation decisions vary relative to the assumptions made at design. Then design may be performed using several flow regimes as proposed.

Usually just one flow regime, computed from the First Clément formula, is used to design collective irrigation systems operating on-demand. This approach does not permit to take into consideration the variety of flow regimes occurring in a collective irrigation system. In fact, hydrant pressure depends on the conditions at the upstream end – discharge demand and upstream piezometric head – and on the combination of the hydrants operating simultaneously, what is usually referred to as hydrants’ configuration. demonstrated that, even when the design discharges are not exceeded, very low hydraulic performance could occur in the system during its operation due to the seasonal and daily variation in farm demand. Consequently, a large spatial and temporal variability of pressure and discharge available at the hydrants may occur and affect network performance and even crop yield. Consequently, there is a need to examine the performance of on-demand pressurized irrigation systems.

A few models have been developed to analyse the performance of pressurized water distribution networks, either assuming steady-state flow conditions, such as the ICARE and the AKLA models, or considering unsteady flow, like the FLUCS model and EPANET, this one after convenient adaptations. Their application requires a detailed characterization of the network and data concerning the discharges and piezometric elevations at the upstream end of the network relative to the peak demand period, when the variation of flow regimes is higher and insufficient pressure at hydrants is more likely to occur.

Both ICARE and AKLA models assume the flow rate delivered by each hydrant within an irrigation system is known and constant (flow-driven analysis). This is true when the hydrants are equipped with flow limiter and pressure reducing valve; and when the total discharge of the hydrants operating simultaneously is smaller than the discharge at the upstream flow limiter. When these assumptions are not met, a pressure-driven model is required for analysis. Field studies were, therefore, performed to verify if the pressurized network of the Lucefecit system, Southern Portugal, satisfies those flow assumptions, and then apply the ICARE and AKLA models to analyse the network performance, which is the main objective of this study.