7. Irrigation Efficiency Tips
Irrigation water is a precious resource. This not only applies directly to agricultural production, but also indirectly to the ecosystem at large. The wise use and conservation of irrigation water is hence essential. A number of best practices and tips can reduce overall water consumption, improve plant growth and lead to higher yields.
Map it Out
Review the layout of the land and map the optimal placement of irrigation piping, with attention to reducing the number of fittings used (prone to leakages). Keep in mind that slopes would cause an uneven distribution of water and could result in run-off. Hence levelling and terracing is advised when using flood or sprinkler irrigation (drip lines should run horizontal to the slope). Sprinkler irrigation disperses water in a circle around the central nozzle. Determine the radius and place sprinklers in such a fashion that overlaps are kept to a minimum, while still ensuring that a maximum area is covered (i.e. few dry areas remain).
Ideally, large trees should remain in the crop area. Not only do they provide moving shade, but certain species (e.g. acacias) support nitrogen-fixing bacteria which enhance soil fertility.
The ten agro-ecological principles outlined by the FAO highlight aspects that relate to water resources, use, reuse, governance, and rights. These principles also provide a means to look beyond the farm level intervention to impacts in the wider community and food system.
The PROMOTE & INITIATE – Impact Assessment Tool incorporates, to some extent, these principles.
Sedimentation layers in the soil jar test (Source: K Blumenthal)
The Soil Texture Triangle (Source: United States Department of Agriculture)
Soil moisture available to plant roots is dependent on soil type. The soil type can be determined in the laboratory using a particle size analysis. Sand, silt and clay have different diameters, by sieving them their distribution delivers information about the soil type. Another way to determine the soil type is the "Jar Test":
- Remove soil from the zone to be irrigated
- Place the soil sample into the jar (halfway) and fill it with water (until two-thirds full)
- Shake the jar and let it sit for two hours, the particles will settle to the bottom, in their different layers
- Measure the total height of all layers, then the height of each layer.
- Divide the height of each layer by the total height which gives you the percentage of clay, silt and sand.
Putting in the obtained percentages of clay, silt and sand, a soil texture triangle, as seen in the figure below, shows the type of soil present.
The IRRIGATE– Soil Tool describes the different soil type properties and allows for establishing a soil-based irrigation schedule according to different crops.
Scheduling irrigation based on soil-plant or atmosphere measurements can decrease water use while improving yields. Software programs can collect weather data including local temperature, rainfall, humidity, and crop evapotranspiration to provide recommendations for optimal irrigation scheduling (see FAO Irrigation and Drainage Paper 56). The IRRIGATE – Soil Tool allows for establishing a soil-based irrigation schedule according to different crops.
Mulching is an effective technique to reduce evaporation of soil moisture, insulate against cold weather and gradually enhance the organic composition of soils. It helps to prevent soil compaction, acts as a soil conditioner, and encourages the presence of natural aerators, like earthworms. It adds nutrients by contributing to the availability of potassium and can add nitrogen, phosphorus, and trace elements to the soil. Furthermore, it is an ideal way to make use of recycled crop waste.
Mulching comprises the layering of organic (straw, wood bark, leaf litter, maize stalks) or inorganic (PVC sheeting) materials over the crop area, through which the crops can grow. Mulching can also be achieved through intercropping, by for instance planting ground creepers (water melon, pumpkin) between rows of maize. Important considerations regarding mulching include:
- Once you begin mulching stay with it. Removing a layer of mulch will dry out the soil and potentially injure the roots below.
- Mulching against a tree’s trunk can lead to bark rot, disease, and insect problems. Thus leave several centimeters of space between the base of the tree and the mulch layer.
- Avoid over applying mulch. Spreading mulch too thickly can cause roots to grow shallow and make them more susceptible to dying during extended dry periods. As a general rule the mulch layer should not exceed 5 cm.
- Use woody or bark mulches in areas where little digging is required, e.g., around trees and in flower beds. Lighter mulch materials such as straw, which is easily worked into the soil, is better suited for seasonal crops and vegetable gardens where replanting is regular.
- Before applying a new layer of mulch, rake through and mix the older mulch layer. Mulch, especially from woody materials, can compact over time and thus prevent soil aeration and water penetration.
Intercropping is a multiple cropping practice involving growing two or more crops in proximity. The most common goal of intercropping is to produce a greater yield on a given piece of land by making use of resources or ecological processes that would otherwise not be utilized by a single crop (Ouma, George; Jeruto, P (2010)). The elements of intercropping (Wikipedia, “Intercropping”, January 2018) include:
- Resource partitioning: Careful planning is required, taking into account the soil, climate, crops, and varieties. It is particularly important not to have crops competing with each other for physical space, nutrients, water, or sunlight. Examples of intercropping strategies are planting a deep-rooted crop with a shallow-rooted crop, or planting a tall crop with a shorter crop that requires partial shade
- Mutualism: Planting two crops in close proximity can especially be beneficial when the two plants interact in a way that increases one or both of the plant's fitness (and therefore yield). For example, plants that are prone to tip over in wind or heavy rain (lodging-prone plants), may be given structural support by their companion crop. Climbing plants can also benefit from structural support. Some plants are used to suppress weeds or provide nutrients. Delicate or light-sensitive plants may be given shade or protection, or otherwise wasted space can be utilized. An example is the tropical multi-tier system where coconut occupies the upper tier, banana the middle tier, and pineapple, ginger, or leguminous fodder, medicinal or aromatic plants occupy the lowest tier. Intercropping of compatible plants can also encourage biodiversity, by providing a habitat for a variety of insects and soil organisms that would not be present in a single-crop environment. These organisms may provide crops valuable nutrients, such as through nitrogen fixation
- Pest management: There are several ways in which increasing crop diversity may help improve pest management. For example, such practices may limit outbreaks of crop pests by increasing predator biodiversity. Additionally, reducing the homogeneity of the crop can potentially increase the barriers against biological dispersal of pest organisms through the crop.
There are several ways pests can be controlled through intercropping:
- Trap cropping involves planting a crop nearby that is more attractive for pests compared to the production crop, the pests will target this crop and not the production crop.
- Repellant intercrops have a repellent effect to certain pests. This system involved the repellant crop masking the smell of the production crop in order to keep pests away from it.
- Push-pull cropping, this is a mixture of trap cropping and repellant intercropping. An attractant crop attracts the pest and a repellant crop is also used to repel the pest away.
Agricultural extension officers and advisors should be able to give guidance regarding intercropping and companion planting.
Ensuring that rainwater does not run-off, but percolates into deeper soil layers avoids top soil erosion and can improve groundwater recharge, while improving soil moisture at depth. Strategically placed furrows can trap rainwater and divert it to crop areas (or the pump well), while roof gutters leading to water storage tanks can build up reserves for the dry season.
Monitoring water consumption and soil moisture levels regularly, ensures a deeper understanding of the water resources necessary for healthy crops. Water flow meters and hand-held soil moisture meters are important devices through which data is collected and recorded for analysis.
Numerous techniques exist to optimize water flow through furrows. These may include covering furrows with plastic PVC sheeting or rock slabs to reduce evaporation, lining or firming main furrows to reduce water infiltration rate at the head end of the field (then additional water is available to advance further down the furrow. The result is faster advance time to the end of the field and improved water distribution) or optimizing slope angles to ensure efficient water flow.
Evaporation from uncovered water storage and water conveyance systems implies a direct loss of water resources to the atmosphere. This loss has financial implications where costs were incurred to pump the water from a well or procure it from a service provider. Stopping evaporation is a question of restricting the solar energy available to the water (to energise water molecules) and lowering its exposure to dry air. When water evaporates it forms a moist layer of air over the surface, lowering the capacity of the air to accept more water molecules from the liquid. Moving air draws water vapor away from the area over the surface of the water and replaces it with drier air, increasing evaporation. Using sealed tanks or covering open storage tanks and canals is advisable. For larger irrigation reservoirs or dams floating covers can be considered, along with wind breaks (e.g. hedges and trees) around the perimeter. The latter might also contribute towards shading the water surface, thus reducing the kinetic energy available to water molecules.
In principle it is best to irrigate during the morning hours starting just before sunrise. Cooler air and lower wind speeds reduce evaporation losses, while crops are assured an adequate water supply at the root zone in preparation for higher daytime temperatures. Watering in the late afternoons and evenings is not advised, as crops cannot absorb the available water and stagnant water offers a breeding ground for pests and fungi.
- Understand some practical approaches towards reducing irrigation water demand
- Information on intercropping and companion planting
- Soil properties in crop areas
- Agricultural extension officers and advisors
- Irrigation planners and service providers
- Horticulture and permaculture specialists
- Irrigation efficiency can only be ensured through active and regular monitoring. Any improvement measure should be scrutinized carefully before implementation and baseline information captured (e.g. amount of water consumed, amount of fertilizer added). Comparing the baseline information with the new post-measure data, allows to assess the success or failure of the improvement. This deepens understanding.