Can salt water be used for irrigation?

Saline water is water that contains a high concentration of dissolved salts. The salinity of a sample can be measured by electrical conductivity and is given in the units, deciSiemens per metre (dS m^-1).

In most countries across the globe, freshwater is used for agricultural purposes. This has led to the overexploitation of groundwater resources in some countries, resulting in the intrusion of saline water into freshwater aquifers. This makes the water unsuitable for human consumption thus, placing restraints on the amount of fresh water available for domestic use.

Saline water often has negative impacts on the yield of crops due to the osmotic potential being lower in the soil solution. This prevents water uptake by plant root cells, therefore inhibiting maximum growth. Salinity is often a major issue in hot, dry regions such as the Sahel Region, where salt accumulates as a result of evaporation. An arid climate paired with a lack of rainfall will lead to the build-up of salts, known as secondary salinisation [Karlberg, 2005]

Despite negative impacts, research has been undertaken to investigate the possibilities of using saline water for irrigation in Africa. To use saline water, investment into specific technologies and management procedures must be made. According to Karlberg, 2005 and Selim et al, 2019, drip-irrigation is the most effective method of irrigation in comparison to furrow irrigation and sprinkler irrigation. The benefits of drip irrigation include high water application efficiency and high frequency of water delivery [Karlberg et al, 2007]. Karlberg states that drip-irrigation is 80%-90% efficient in water application compared to surface irrigation schemes which only have an efficiency of 50%. In addition to this, the high frequency of water delivery prevents the dehydration of soils thus, preventing fluctuations and peaks in the soil salt concentration.

Research conducted by Karlberg et al, 2007 on the use of saline water drip-irrigation in southern Africa, shows that the yield of crops can be increased with the use of saline water. Three different levels of saline water (0 dS m^-1, 3 dS m^-1,6 dS m^-1) were used to test the effect on the growth of tomatoes. The results showed that even with  the most saline water sample (6 dS m^-1), the yield was significantly greater than the average marketable yield for South Africa of 31.4 Mg ha^-1. Furthermore, the addition of plastic mulch to soils prevented evaporation of water from the soil, increasing yields further to an average of 75 Mg ha^-1.

Other research shows that optimal yields can be obtained when the salinity level is 11 dS m^-1. This means that drip-irrigation technologies can be utilised in countries such as Tunisia where groundwater salinity levels are below 3 dS m^-1 [Selim et al, 2019].

In conclusion, with the correct technologies, saline irrigation is an ideal method of irrigation as not only does it increase crop yields, it increases the availability of fresh water for domestic consumption. This is especially beneficial in Africa where 85% of its inhabitants rely on agriculture to make a living and freshwater is scarce [You et al, 2010]. Nevertheless, various crops respond to and uptake saline water differently, meaning irrigation mechanisms must be suited to the crop planted. As a result of this, initial set-up costs may be high, causing farmers to produce cash crops to make a profit. Small-holder farmers may, however, be able to access simple drip-irrigation kits that are affordable for small garden size plots.

Methods of irrigation

In my previous blog post, I discussed the economics of irrigation development in Africa. The aim of that blog was to identify the factors influencing the feasibility of irrigation in a given area. Today, in this blog, I would like to explore the various types of irrigation that exist in Africa with reference to case studies.

Irrigation by definition is the artificial application of water to land to produce crops[1]. Although irrigation has developed over the years due to modernisation and advances in technology, some farmers still use traditional approaches. This is due to these methods being reliable and relatively inexpensive. Examples of traditional irrigation methods include water harvesting, flood plain irrigation, swamp irrigation and groundwater irrigation.

Water harvesting is a method that has been used extensively for centuries. Rainwater is collected and channelled into a small growing area. An area where water harvesting is heavily used in the lowlands of Tanzania where rainfall is between 600 – 900mm and run off naturally collects at the bottom of the valleys. This provides optimal conditions for paddy rice cultivation and research shows that this system contributes to 35% of the total rice production in Tanzania [FAO,2001]. Figure 1 shows the tanks used for rainwater harvesting in Tanzania.

 

Figure 1 shows the tank used for rainwater harvesting in Tanzania. [Source]

Groundwater irrigation, unlike water harvesting, uses water within the Earth, in the form of medium to deep aquifers. It is believed that the use of groundwater will alleviate seasonal water scarcity as it is a reliable water resource that is less susceptible to evaporation and contamination in comparison to surface water resources.

In areas such as southwest Egypt, groundwater irrigation is ideal due to the region’s aridity and lack of surface water. The Nubian Sandstone Aquifer System within this region has a reasonable amount of groundwater that is being heavily exploited by surrounding inhabitants. Results from research conducted by Ebraheem, 2003 indicate that large-scale irrigation cannot be supported in this area as the annual recharge of the aquifer is too low. Ebraheem further stated that the present value of extraction must decrease from 203 million m³ yr^-1 to 93 million m³ yr^-1 to avoid depletion.

In contrast to traditional methods of irrigation, modern technologies are costly and usually require the presence of a specialist upon construction and installation which may incur additional charges. The High Aswan Dam is an example of a full control dam built to provide sufficient water for agricultural and other purposes. The dam was built on the border between Egypt and Sudan, successfully ceasing floods and droughts in regions around the Nile River. Figure 2 shows the location of the dam along the Nile River. The dam supplies Egypt’s annual quota of 55.5 milliard cubic meters of water and has been effective in doing so since 1970. However, the cost of construction neared $1 billion which cannot be afforded by rural farmers and smaller countries with lower GDP.

Figure 2 shows the location of the High Aswan Dam along the Nile River. [Source]

Although modern technologies may seem disadvantageous, the benefits in some cases outweigh the negatives. For example, modern technology such as motor-driven pumps can be used to reduce the drudgery of lifting water. This is will increase the efficiency of farming thus, allowing more plants to be grown over a larger area. In addition, distribution technologies such as trickle sprinkle irrigation and piped water supplies will aid the reduction of water wastage. Water can be fed directly to the root of the plant at regular intervals, allowing optimal root moisture to be maintained. This will reduce the stress on the plant, enabling the maximum growth rate, therefore increasing the profits made by farmers. Despite perennial irrigation providing new opportunities for more intensive crop cultivation, issues may build up in the long run, reducing the efficiency of production. These include waterlogging, salt builds up in the soil and fluctuations in the water table [El Gamal F, 2000].

Due to the global market, the prices of cereal crops have dropped over the years, meaning it would be unreasonable for a small-holder farmer to invest in costly, large-scale irrigation infrastructure. I believe it would be more ideal to use modern technology that has been modified to become low-cost such as a treadle pump. This compromise will ensure the farmer makes enough money for a living as well as increasing the efficiency of their food production.

Overall, I think the use of irrigation for crop production is beneficial, especially in areas with variable rainfall and an arid climate. However, the type of irrigation used must be suited to the growing area and the farmers income.