SW—Soil and WaterExploring energy saving scenarios for on-demand pressurised irrigation networks
Introduction
Recently, with water scarcity, irrigated areas in arid and semi arid regions have been facing the challenge of improving efficiency in the use of irrigation water. One way to achieve this has been the replacement of the obsolete open channel distribution networks by on-demand pressurised networks. This change appears to be quite effective. Conveyance efficiencies are significantly improved from typical values of 60–70% for open channels to values close to 100% for pressurised networks (Rodríguez Díaz et al., 2008). Furthermore, these new systems allow farmers to use more efficient on-farm irrigation systems such as trickle irrigation or sprinklers since they receive water at their hydrants at suitable pressures.
Another advantage is demand flexibility. Open channel flow delivery provides pre-arranged demand, where users request water some time in advance with limited flexibility in its duration and flow (Plusquellec, 2009). Pressurised networks, however, are usually arranged to be on-demand, so water is continuously available to farmers (Rodríguez Díaz et al., 2007a, Pulido Calvo and Gutierrez Estrada, 2009). As a result they can apply the right amount of water when required and do not have to wait for an irrigation schedule. Also these systems can be easily automated and give farmers the possibility of remote scheduling. This has led to water consumption being dramatically reduced in Southern Spain where their introduction has reduced water consumption up to 50% (Rodríguez Díaz et al., 2008).
But with this change, there is an implied increase in the use of another limited resource, energy. In Spain, previous work has reported that, on average, modernised irrigation districts require 2 kW ha−1 and yearly energy consumption typically ranges from 600 to 1600 kW h ha−1 (IDAE, 2008, Blanco, 2009). With the modernisation process, irrigation districts are moving from an inefficient system in the use of water that is very efficient in its use of energy, to a more efficient use of water but clearly with reduced energy efficiency. Blanco (2009) reported that with pressurised networks energy costs can average 25% of the total management, operation and maintenance (MOM) costs, but in some specific cases this ratio can rise to 50%. Total MOM costs move from average values of 0.02 € m−3 for open channel networks to more than 0.10 € m−3 for pressurised networks because of not only their energy requirements but also their higher maintenance, operation and amortisation costs (Rodríguez Díaz et al., 2008). In the traditional open channel systems energy costs were not significant as only small elevations of water were required for the distribution channels (Rodríguez Díaz, 2004). This increased charge is becoming a problem for farmers and sometimes these improvements in infrastructure are seen as creating a difficulty rather than an advantage.
Some recent research has highlighted the necessity for optimising simultaneously both water and energy efficiency. In 2001, the California Energy Commission (CEC) launched the “Agricultural Peak Load Demand Program” with the main objective of reducing peaks in energy consumption in irrigation districts (ITRC, 2005). In Spain, the Institute for Diversification and Energy Savings (IDAE) developed a protocol for auditing simultaneously the use of energy and water resources in pressurised networks (IDAE, 2008). Moreno et al., 2007, Moreno et al., 2009 focused on the improvement of energy efficiency at pumping stations and the determination of optimal pump curves. Pulido Calvo et al. (2003) developed a pump selection algorithm for reducing energy costs in irrigation districts and Vieira and Ramos (2009) introduced a water turbine in the network in order to use any excess available hydraulic energy.
These previous studies appear to confirm that on-demand irrigation implies an important expenditure of energy, since the network must be designed in order to ensure a minimum required pressure at the hydrant with higher pressure requirements, which means that when the water is supplied many other hydrants receive excessive pressure which must be reduced in valves located throughout the network.
This work focuses on the evaluation of energy needs in pressurised networks and the development of possible management alternatives for improving the energy efficiency. Also, measures to reduce energy requirements while keeping current levels of water consumption are tested and the potential energy savings are calculated. A method for analysing energy requirements in irrigation networks based on the EPANET 2.0 engine (Rossman, 2000) was developed and applied to the particular case of Fuente Palmera Irrigation District, Southern Spain.
Section snippets
Study area
The Fuente Palmera Irrigation District, Córdoba, Spain has a total irrigated area of 5611 ha (Fig. 1). The climate is Mediterranean with annual average rainfall of 550 mm and average temperature of 17.9 °C, with July being the hottest month with an average temperature >36 °C (Pulido Calvo and Gutierrez Estrada, 2009). In the irrigation season studied, 2007, the annual rainfall was 523 mm and reference evapotranspiration was 1323 mm (Carrillo, 2009). The irrigation district is devoted to extensively
Analysis of pressure in the network
Table 1 shows the pressures registered (Pmax, Pmin and Pave) in the network for all the scenarios studied and the all the ranges of open hydrants considered. Results for every probability in each scenario were the average of 1500 iterations. For low probabilities, the average flows demanded at the pumping station were reduced.
There were no major differences among scenarios 1 and 2, mostly for the high probabilities of open hydrants. But when the probabilities were smaller, differences started
Discussion
On-demand irrigation gives farmers the maximum degree of flexibility but, as is shown in this work, it can require significant amounts of energy for its operation. In this work, management alternatives, that do not imply any improvement of the existing infrastructures, have been evaluated using simulations. Results show that savings of more than 20% in energy could be achieved in the peak demand period by losing one degree of flexibility, operating by sectors and concentrating the irrigation
Conclusions
On-demand irrigation represents a step forward in flexibility for water users but it implies a significant expenditure in energy. It is probably time to reflect on this and examine if this improvement represents a clear benefit in terms of global sustainability and the economic viability of irrigated agriculture. It could be that we should lose one degree of flexibility in order to improve the overall sustainability of irrigated agriculture.
This work shows that significant energy savings can be
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