Salt water irrigation

I recently came across an article that mentioned one of the many strategies to tackle water shortages in South Africa is desalination on water. As much help as this strategy would not only bring to South Africa and many other parts of Africa and the world I do not find it logical nor sustainable because of its high cost and how energy-intensive it is. Instead, I wondered, has there ever been any research on irrigating with saline water? Would that work?

I found out one of the first major success stories of salt water use was in China. The recent success story in China shows how this approach can grow crops efficiently, reduce the dependence on freshwater for irrigation, and reduce hunger in the country. Scientists in China developed several types of rice able to grow in high salinity seawater and expected to produce around 4.5 tonnes of rice per hectare, but to their surprise, in some cases, more than 9.3 tonnes were produced (Kentish, 2017). There are millions of square kilometers of land in China that are unsuitable for agriculture because of high salinity in the soil and the success of this story shows that some of this land could be used to grow rice in the future, providing food to more than 200 million people (Kentish, 2017).

This is important because one of the biggest challenges many parts of Africa faces today is access to safe water. Around 80% of the freshwater in Africa is used for agricultural crops and already more than 13 countries are facing water stress and scarcity (Karlberg, L. and Penning de Vries, F., 2004). One of the main reasons for this is the water demand for industrial growth, irrigation in agriculture, and growing populations. As water sources are scarce alternative options are important to avoid conflicts between urban growth and irrigation for agriculture. Using saline water for irrigation that is unsuitable for domestic use opens up a new source of water for farmers to grow crops. The use of saline water may come as a surprise as already ⅓ of the irrigated land in Africa is badly affected by salinity (Karlberg, L. and Penning de Vries, F., 2004).

Salt Water Irrigation on rice fields in China

A study was done in South Africa with saline water as an irrigation technique to find ways to relieve the pressure on land and limited water resources due to increasing population growth, which has been one of the reasons for the downward spiral poverty trap. For now, rainfed agriculture can feed around 20-30 people in South Africa, but the population is currently more than 45 million (Villiers, M.C., Nell, P, J., Barnard, O, R. and Henning, A., 2003). Around 60% of the country’s residents are familiar with food insecurity and ⅓ regularly suffer from hunger (Villiers, M.C., Nell, P, J., Barnard, O, R. and Henning, A., 2003). The problems are land mismanagement (not land scarcity) and limited water resources, which have caused the semi-arid land of South Africa to become not suitable for agriculture. 

Drought in South Africa

One of the main problems associated with hot and dry climates, such as in South Africa, is the salinization of irrigated land due to evapotranspiration. The concentration of salts from irrigated water remains in the soil and increases when water from the soil evaporates, is taken up by plants or is not leached from the soil. The important tactic about salt water irrigation is that salt levels in the soil have to be kept below a certain level, which is already to some extent regulated by the plant, where less water is taken up if the salinity of the soil increases and left to leach the salts out of the soil. 

Evapotranspiration

That’s why if saline water is applied with appropriate irrigation management techniques, long term sustainability in crop production can be achieved. Such irrigation technique is drip-irrigation, which has been used in combination with salt water. According to Karlberg, L., Rockstrom, J., Annandale, G, J. and Martin Steyn, J. (2007), the 3 main benefits obtained using saline water and drip irrigation are:

1. High application efficiency - less water is used and lost resulting in less salt deposited on the fields

2. It helps to avoid the soil from drying out in between irrigation events, which results in more even salt concentrations in the soil 

3. Leaf burn is avoided 

To increase the benefits of drip irrigation and the use of saline water, plastic mulches could be used to prevent the water from evaporating from the soil and preventing it from drying out (Karlberg, L., Rockstrom, J., Annandale, G, J. and Martin Steyn, J., 2007). 

Typical drip irrigation system diagram

The study in South Africa took place in Hathfield Experimental Farm from 2003-2004 and included two low-cost drip irrigation systems to irrigate two different types of tomatoes (Lycopersicon esculentum Mill. cv.‘‘Daniella’’) with 3 different salinity levels (0, 3 and 6 dS m¹) during two growing seasons (autumn and spring) (Karlberg, L., Rockstrom, J., Annandale, G, J. and Martin Steyn, J., 2007). The experiment also included the use of plastic mulch and was compared to a field without one. The study found that an average yield of 75 Mg ha¹ was recorded with salt water irrigation for all seasons, which is more than twice compared to the average yield with freshwater irrigation of  ~31.4 Mg ha¹ (Karlberg, L., Rockstrom, J., Annandale, G, J. and Martin Steyn, J., 2007). Even at the highest level of salinity in the water, a higher yield was achieved than the yield with freshwater irrigation and no mulch (Karlberg, L., Rockstrom, J., Annandale, G, J. and Martin Steyn, J., 2007)., showing the success and benefits of using saline water with drip-irrigation. 

The layout of the saltwater irrigation systems for tomatoes in South Africa

To my surprise, the use of salt water irrigation with drip irrigation has been a success. Not only is it cheap and easy to use, but there is also plenty of access and availability to salt water and this system is already used by many small-scale farmers already in Kenya, Tanzania, Uganda, and South Africa (FAO, 1997), helping local farmers to grow food while having access to water resources. However, the only question that remains is the access of saline water to landlocked countries and the additional costs it would bring.