Irrigation

Did you know that when you water your plants using a garden hose or sprinklers, you are practising irrigation? Does this surprise you? Perhaps it does. Often when we think about the term irrigation, we might picture a more sophisticated system operating on a commercial farm rather than on the lawn in your back garden. For this explanation, we are going to focus on commercialised and large-scale irrigation, but it's still interesting to think about smaller-scale irrigation too. So, what exactly is the definition of irrigation? Are there different types or methods? What benefits does irrigation bring? Let's find out!

Irrigation Definition

Irrigation is an important component of contemporary agriculture, particularly for food production. So, how do we define irrigation?

Irrigation is typical in areas that don't have enough water to support plant growth, perhaps due to the seasonality of rainfall, drought, or other climatic conditions. Irrigation is also common in areas that have soils with high salinity levels (amount of salt in the soil), commonly found in arid or semi-arid regions, or a result of poor agricultural methods and improper drainage. Irrigation may be carried out even in areas with moderate levels of rainfall to ensure the maintenance of consistent soil moisture levels.² It is likely that the significance of irrigation in agriculture and food production will continue to grow, particularly as global warming and climate change continue to be major concerns, which will cause changes in rainfall patterns across the world.

Fig. 1 - An example of irrigated agricultural land in the desert in Pinal County, Arizona, USA

Irrigation Water Sources

The water that is used for irrigation purposes comes from a variety of sources. These include surface water sources, e.g., rivers, lakes and groundwater sources (springs or wells). Irrigation water is also sourced from storage ponds, which have been specifically designed to collect water for irrigation. Desalinated water is another source of water used for irrigation. The water is transported from the source to the cropland via pipes or channels.

Types of Irrigation

There are two main types of irrigation, with different methods of irrigation used within both. We'll talk more about these different methods later.

Gravity Powered Irrigation

Gravity Powered Irrigation speaks for itself. This is an irrigation method powered by gravitational forces. This means that water is moved across the land by gravity, following its natural path. This can be seen with irrigation infrastructures like pipes or field furrows (the ploughing lines often seen on fields).

Pressure Driven Irrigation

Pressure Driven Irrigation is a more controlled form of irrigation. This is when water is forced onto the land via pipes, e.g., sprinkler systems. Pressure irrigation is said to be more efficient, as less water is lost from the water running off the land, seeping into the land (percolation), or being evaporated.

Four Methods of Irrigation

Although there are multiple different methods of irrigation, we'll take a look at four in more detail. Each of these methods shows a different way of artificially watering the land. Some are gravity powered, whilst others are pressure driven.

Surface Irrigation

Surface irrigation is a gravity-powered irrigation system. Also known as flood irrigation, surface irrigation involves water spreading across the surface of the land. There are four different types of surface irrigation.

Basins

For this type of surface irrigation, the crops are within an enclosed basin. Water can spread across the whole basin and infiltrate into the soil; the basin acts a bit like a pond, where the water collects. The basin is surrounded by levees to stop the water from flowing out. Certain crops are more suited to basin irrigation than others; they specifically need to be able to withstand heavy waterlogging. The best example of a crop that would thrive in these conditions is rice. Rice fields are often flooded and offer prime conditions for crop growth.

Borders

Border surface irrigation is similar to basin irrigation, except the flow of water is changed due to the presence of ridges. Instead of the water being stationary like in a basin, the water flows through strips of land, separated by these ridges, which divide the basin. There is a drainage system at the end.

Uncontrolled flooding

This is a kind of free flooding irrigation method without any border control for the water. The water is fed onto an area of land and allowed to flow anywhere without restriction. The main issue with this is that a higher amount of irrigation occurs at the water entry point on the field, and at the other end of the field, irrigation will be lower. There are no excess costs in preparing the land with other irrigation infrastructures like borders. However, this can be quite a wasteful method of irrigation; without the presence of barriers, the water just runs off the field into neighbouring areas.

Furrow

With these other forms of irrigation, the land is typically completely flooded. With furrow irrigation, this is not the case. Furrowing creates small downward-sloping channels in the land where the water can flow through. This kind of surface irrigation is much better for crops that are planted in rows.

Fig. 2 - furrow irrigation on sugar cane in Australia

Sprinkler Irrigation

Sprinkler irrigation occurs with heavy machinery that can spray large amounts of water onto the land. These sprinkler systems can either be long pipes with sprinklers running along them, or there can be a central sprinkler system in the middle of a field that rotates. These are highly pressurised irrigation systems. However, this form of irrigation is relatively inefficient; much of the water evaporates into the air or is even blown away by the wind.

Fig. 3 - sprinkler irrigation sprays water onto crops through a pressurised piping system

Drip/Trickle Irrigation

Drip or trickle irrigation is similar to sprinkler irrigation, however, it is much more efficient. These are less pressurised systems (low-pressure irrigation systems). Instead of sprinklers jetting the water far into the air, in drip systems, the water is aimed more directly onto the crops. Water is provided closer to the roots through holes in the pipes. This is also known as micro-irrigation.

Fig. 4 - drip irrigation watering a banana plant

Subsurface Irrigation

Subsurface irrigation systems are not pressurised irrigation systems. This type of irrigation involves pipes that are buried beneath the surface of the land and underneath the crops. Artificial subsurface irrigation comes from the pipes that are buried underground. There are small openings in these pipes, allowing water to flow out and irrigate the crops. This method is significantly more efficient than sprinkler or drip irrigation, as less water is evaporated. However, this method is typically much more expensive.

Benefits of Irrigation on Agriculture

As can be expected, irrigation has a significant number of benefits for agriculture. Let's explore some of these.

• Water is vital for crop growth. Irrigation helps during water deficiencies caused by a lack of rainfall, which is particularly important during times of drought or lower-than-usual precipitation.
• Irrigation can increase crop yields; when the correct amount of water is provided for crops, this can aid in their growth productivity.
• If irrigation is done efficiently, it allows farmers to grow the same amount of crops using less water.
• The use of irrigation expands the areas which can be farmed by increasing water availability in drier regions. This will be especially significant as the world's climate gets warmer.

Irrigation and Landscape Alterations

Irrigation can actually alter the landscape dramatically. This can have both a positive and a negative effect.

• When land is watered regularly, it can cause crop roots to extend deep into the soil and create a large root system. This can help to make the soil deal with drought more efficiently.
• The landscape can be altered to accommodate irrigation strategies. We already mentioned that farmers can make the land more level to improve irrigation efficiency. Digging furrows or creating dykes also affects the natural landscape.
• Over-irrigation can cause adverse effects on the soil; with too much irrigation, soil can be leached of essential nutrients due to becoming waterlogged, making the soil have poorer quality for crop growth.
• Some areas even experience degradation of environmental landscapes and soil quality due to over-irrigation and also human activity on the landscape, such as creating furrow canals or deforesting the land for crop growth.