6. Plan Intake Structures, Conveyance and Distribution
The principal engineering challenge of any irrigation system consists in withdrawing water from a source, delivering it to land in due time and in the required amount, distributing it among individual farms and crop-rotation fields, and providing soil moisture needed for plants on fields. All this requires energy to move water, maintain pressure and ensure quality.
The operation of the system should offer enough flexibility to supply water to the crop in variable amounts and schedules that allow the irrigator some scope to manage soil moisture for maximum yields as well as water, labour and energy conservation.
Water may be supplied on a continuous or a rotational basis in which the flow rate and duration may be relatively fixed. In those cases, the flexibility in scheduling irrigation is limited to what each farmer or group of farmers can mutually agree upon within their command areas. At the preliminary design stage, the limits of the water supply in satisfying an optimal irrigation schedule should be evaluated (see Section 1).
The 'intake structure' is used for water withdrawal from an irrigation source and delivery to an irrigation network. These can be gravity and water-lifting types.
Pumps powered by solar energy can be used both for surface water and groundwater withdrawal. There are two main types of pumps: centrifugal pumps and positive displacement pumps. Both can be used for SPIS.
Solar-powered pumps have to be oversized to meet peak demand, which means they tend to be underutilised during the off-season. To a certain extent, this seasonal variability in water demand can be balanced by adapted crop rotations (including permanent crops) and irrigation management.
The performance of solar-powered pumps depends on the crop water requirements, size of water storage, head (m) by which water has to be lifted, volume of water to be pumped (m³), PV array virtual energy (kWh), energy at pump (kWh), unused PV energy (kWh), pump efficiency (%), system efficiency (%), and diurnal variation in pump pressure due to change in irradiance and pressure compensation. All this needs to be considered when designing the SPIS and is best done by an expert.
The fluctuations in solar irradiance, the accumulation of dust on PV modules and high air temperatures affect the performance of the PV systems and hence the pump. Spraying clean water on the PV modules results in cleaning the dust as well as cooling of modules improves the module efficiency and hence the water flow rate. Therefore, PV modules should be easily accessible for maintenance purposes.
The DESIGN – Pump Sizing Tool can be helpful in ensuring that the pumping system is designed to purpose and to avoid unnecessary pressure loss.
Conveyance and Distribution
Once the water has entered through the intake structure, it needs to be delivered through conveyance and distribution systems. Typical conveyance and distribution systems are diversion dams, lined or partially lined canals and ditches, pipelines, hydrants and other means.
A distinction can be made between water provision for lands of a single (on-farm irrigation system) or several (inter-farm irrigation system) farms, associations of farms and agricultural enterprises, and even several administrative centres.
A badly planned conveyance and distribution system can lead to high water losses, poor irrigation efficiency and much smaller areas than planned being irrigated.
Design software is available for irrigation system planning. For example, GESTAR is a software developed by the Faculty of Fluid Mechanics at the University of Zaragoza and can be used to size medium to large-scale irrigation schemes. GESTAR tools and methods are specifically designed for pressurized irrigation (such as sprinkler and drip irrigation). Planning tools specific to an irrigation method also exist.
What are the Implications of Solar-powered Irrigation for Enregy?
SPIS can provide a reliable and affordable source of energy in rural areas, potentially reducing energy costs for irrigation and reducing greenhouse gas emissions associated with fossil fuel pumping systems.
Irrigation systems use energy to lift water from a well or reservoir, to pressurize water to overcome friction losses in pipes and to distribute water evenly over the soil. Pumps are typically powered by diesel or electrical energy, with the latter supplied from the grid, or by decentralized energy sources.
- Energy efficiency: How efficiently irrigation systems use water and energy is determined primarily by the type of system and the way it is operated, maintained, and managed. When specifying pump size and designing water distribution systems, engineers consider the distance the water has to be lifted and transferred, the depth from which water needs to be transported, and the friction caused within pipes and channels as determined by layout, diameter and operating pressures. They should also consider the system resilience to future climate scenarios and changes in groundwater levels that may occur through widespread implementation of SPIS.
Energy savings can be made through efficient design (e.g. pipe layout), appropriately sized pumps, and optimised equipment (e.g. variable speed drives). A further consideration is the trade-off between water application efficiency and energy efficiency. For instance, forcing water through a drip irrigation network will use more energy than running it through channels and furrows, but this type of system will apply water more efficiently than a more energy-efficient centre pivot irrigation system.
- Energy costs: Pressurised systems tend to be more efficient, but have higher energy requirements and thus higher energy costs. These costs depend on the source of energy, energy price per unit as well as other factors, such as the depth of the aquifer from where the water is pumped. As such, the energy costs can potentially undo any cost savings that were anticipated when investing in making irrigation systems more efficient. It leaves scope for interventions at technical and management level to improve both water and energy use efficiency and to reduce operating costs.
Solar-powered photovoltaic systems can provide an economically viable alternative source of energy without emitting greenhouse gas emissions. They also have the advantage of not depending on the availability and costs of fossil fuels.
Nevertheless, it requires some prior knowledge of how to set-up and use solar pumps optimally. In contrast to motor-driven pumps, the dimensioning of PV irrigation systems is a critical strategic decision for farmers, given the initially higher investment costs and the complexity in designing, operating and maintaining the system. Usually this is done by technical experts. Training of farmers is necessary to operate the PV system for maximum benefits.
Although costs have decreased significantly in recent years, the economic viability of PV systems varies, especially for small-holder farmers for whom a solar pump represents a substantial investment. Thus, the economic viability of such an investment needs to be assessed to understand whether the introduction of PV pumps is economically viable.
The INVEST – Payback Tool can be useful in assessing the costs of solar systems vs. other types of energy systems for irrigation.
- Understanding of the different aspects of an irrigation system
- Overview of how to size pumps and other parts of a SPIS
- Recognition of the long-term cost savings from installing a SPIS
- Understanding of the need to design in resilience and adaptability to the system
- Water volume and pressure required
- Pump size, pump cost, electricity requirements
- PV system size requirement and cost
- Ancillary structures and systems, sizes, and costs
- Irrigation system planners
- Irrigation managers, water user groups or farmer organization
- Careful life-cycle cost-benefit analysis should be carried out
- Resilience and adaptability should be built into the system by design
- A poorly designed system can be damaging to the environment and other watershed users
- Technical expertise are required for the sizing, installation, and maintenance of such systems