5. Select Field Irrigation Method
The vast majority of the total global irrigated area is equipped for full control irrigation. Full control irrigation methods differ in the way water is distributed (AQUASTAT).
Once the crop requirements, water resources, and impacts of implementing a SPIS are understood, the appropriate irrigation methods can be selected.
What Irrigation Methods are There?
Surface irrigation uses gravity to move water over land. This category can be subdivided into small channels (furrows), strips of land (borders) and basin irrigation (including submersion irrigation of rice).
Surface irrigation is widely utilised and therefore a well-known system, which can be operated without any high-tech applications. In general, it is more labour intensive than other irrigation methods. When designing a surface irrigation system, the soil type (texture and infiltration rate), slope and levelness of the field, stream size and length of run should be taken into account. It is generally more difficult to obtain a high uniformity of water distribution in long fields on coarse textured soil (gravels and sand) than on fine textured soils (loams to clay). Levelling the field and building water ditches and reservoirs may be expensive, but once this is done, costs are low and the farmer has greater capacity to respond to changing demand for irrigation.
Sprinkler irrigation consists of a pipe network, through which water moves under pressure before being delivered to the crop via sprinkler nozzles. The system basically simulates rainfall in that water is applied through overhead spraying.
The pump is usually a centrifugal pump, which takes water from the source and provides adequate pressure for delivery in the pipe system. Mainline and sub-mainline pipes deliver water from the pump to lateral pipes. The laterals then deliver water to the sprinklers. They can be permanent, but more often they are portable and made of light material (e.g. aluminium) that is easy to carry.
Rotor-type sprinklers operate by rotating streams of water over an area of land. This includes impact and gear-drive sprinklers, producing streams of water, and spray nozzles that discharge water in patterns.
Due to the high-capital investment costs, the more elaborate systems (e.g. centre pivots, side roll systems etc.) are typically used for high-value crops, such as vegetables. A higher-level of expert knowledge is needed to operate the systems, even though the overall labour requirement is low due to the high level of automation. Motors, pipes and other mechanical components all need to be systematically maintained to avoid damage and high repair or replacement costs.
Sprinkler irrigation is suited for most row, field and tree crops, and water can be sprayed over or under the crop canopy. However, large sprinklers are not recommended for delicate crops, such as lettuce, because the large water drops produced by the sprinkler may damage the crop.
Localized irrigation consists of “water being distributed under low pressure through a piped network, in a predetermined pattern, and applied as a small discharge to each plant or adjacent to it” (AQUASTAT 2016).
A typical drip irrigation system has a pump unit, a control head, mainlines and sub-mainlines, lateral lines and emitters or drippers. It may also feature reservoir tanks, filters and fertigation devices.
With drip irrigation, water is applied more frequently (usually 1-3 times a day) than with other methods, providing a favourable high level of soil moisture. As long as the water application rate is below the soil’s infiltration capacity, the soil remains unsaturated and no free water stands or runs over the surface. This way water resources can be used very efficiently and water losses can be reduced to a minimum.
Moreover, fertiliser and nutrients can be used with high efficiency as water is applied locally and leaching is reduced. Weed growth is reduced as water and nutrients are supplied only to the cultivated plant.
Nevertheless, drip irrigation has a high initial investment cost as well as it requires a high-level of technical knowledge and regular investments to replace equipment, which is vulnerable to clogging and dysfunction, especially when water quality is not optimal. There also is a risk of rising soil salinity.
The method is suitable for most soils. On clay soils, water must be applied slowly to avoid surface water ponding and run-off. On sandy soils, higher emitter discharge rates will be needed to ensure adequate lateral wetting of the soil.
Drip irrigation is most suitable for row crops, such as vegetables and fruits, tree and vine crops. Given the high investment, drip irrigation tends to be used for high-value crops.
Other types of irrigation include:
Equipped lowland areas, such as (i) cultivated wetland and inland valley bottoms that have been equipped with water control structures for irrigation and drainage; (ii) areas along rivers where cultivation occurs making use of structures built to retain receding flood water; (iii) developed mangroves and equipped delta areas.
Spate irrigation, using floodwaters of ephemeral streams and channelling it through short steep canals to where cropping takes place. Dams are often built in the streams to be able to store the water whenever it arrives.
What Irrigation Methods is Best Suited for Solar-Powered Systems?
PV water pumping systems have shown significant advancements in the last decade. The limitations in the design of solar pumps from the 1970s – such as terminals that did not readily provide good electrical connection or overheated electronic circuitry - have been overcome, and solar pumps are now much more efficient and reliable.
Nowadays, they can support drip, sprinkler, pivot or flood irrigation technologies. Systems range from sophisticated computer controlled setups with high start-up costs to moderate-cost systems that include bubblers, mini-sprinklers and drip irrigation.
Generally, the size – and cost – of the solar pumping system is determined by the water and pressure requirements of the irrigation system. Methods working at comparably low operating pressures are often the preferred option in combination with PV pumps.
Sprinkler irrigation requires relatively high water pressure to operate, which demands a specific SPIS composed of high capacity solar modules and an integrated energy storage capacity (battery). In contrast, drip irrigation requires low pressure and have the potential to apply water more efficiently.
Drip irrigation – also known as micro-, localised, or trickle irrigation – uses networks of pipes and tubes to apply water directly to the soil surface or root zone of plants. It has the potential to reduce the ‘drop per crop’ water consumption through minimising non-productive evaporative losses (e.g. Narayanamoorthy, 2004; Rijsberman, 2006). Another advantage is that moderately saline water may be used for irrigation. Marginal plots of land may be used productively as drip irrigation techniques can deliver required water and nutrients directly to the plants.
Drip irrigation is ideal for high-value crop production such as vegetables and fruits, tree and vine crops, and due to its high efficiency, the solar pump can be quite conservatively sized. Nevertheless, drip irrigation comes at a high initial capital cost and requires sufficiently good quality water (to avoid clogging of the emitters) or a pre-treatment system. Moreover, good irrigation management is needed to operate the system effectively, apply fertigation and maintain equipment.
Table 3: Suitability of irrigation methods for solar pumps, adapted from “Manual and Tools for promoting SPIS – Stocktaking and Analysis Report” (2015).
| Irrigation method
|| Typical application efficiency
|| Suitability for use with solar pump
|| Depends on local conditions
| Open channels
|| Depends on local conditions
Beyond these technical considerations, there are other factors determining the suitability of irrigation methods regardless of the source of energy powering the pumps. These include natural conditions such as:
- Soil type, determining water storage capacity and infiltration rate;
- Slope of land, influencing water drainage and whether land needs levelling;
- Climate, including winds (e.g. that may disturb spray from sprinklers), sun radiation and precipitation patterns, and temperature;
It is also important to consider the type of crop grown both from an economic as well as from an agronomic point of view. Due to the higher capital investment costs per hectare, sprinkler and drip irrigation are commonly used for high-value cash crops, such as vegetables, fruit trees and spices. Drip irrigation is better suited for individual plants, trees or row crops.
There are also other socio-economic aspects to consider when selecting the irrigation method. Labour input is one such factor. The construction, operation and maintenance of surface irrigation often requires higher labour input than sprinkler or drip. Surface irrigation requires accurate land levelling, regular maintenance and a high level of farmers’ organisation to operate the system. Another aspect to consider are unexpected complications when introducing new irrigation methods. Getting farmers to change practices and servicing the equipment may be challenging.
When selecting an irrigation method, these aspects needs to be weighted and a cost/benefit analysis of the available options needs to be made. Costs include the capital investment, construction and installation as well as operation and maintenance, including energy. These costs should be compared to the expected benefits, including yields, market prices, avoided operational costs, and labour savings. This cost benefit analysis is explained in further detail in the SPIS Toolbox section on financing.
- Understanding the different irrigation methods and their respective advantages / disadvantages
- Ability to incorporate the natural conditions that affect irrigation into the choice of irrigation method
- Appropriate application of the cost benefit analysis
- Understanding the trade-offs involved in different irrigation methods regarding capital and operational costs, water efficiency, and increased agricultural production and income.
- Water pressure
- Seasonal water availability (sustainable abstraction allowance)
- Water quality
- Soil type
- Slope of land
- Capital, operational, and maintenance costs
- PV efficiency and power available
- Power requirement of different irrigation methods
- Policy makers
- Irrigation advisors/ planners
- Irrigation managers, water user groups or farmer organization
- Different irrigation methods exist that provide different benefits and drawbacks
- The ultimate decision of which irrigation method to use should be a balance of the financial and environmental costs / benefits over the life of the asset