Large-scale irrigation - not to be ignored!

I may not have praised large-scale irrigation schemes in my previous blogs, but the reality is that large-scale schemes are getting more and more popular in Africa. In the last ten years alone, more than 22 million ha of land in Africa has been leased out to large-scale land acquisitions for agricultural purposes (Johansson et al. 2016).  This has led to increased pressure on freshwater sources. With further population growth, industrialisation and urbanisation, there is intense competition for water with other water-intensive sectors such as the fishing and energy industries. However, the agricultural industry is still the sector that consumes the most water, using 70% of all global freshwater withdrawals (Steduto et al. 2012).

Water used in agriculture can be categorised as either blue water or green water. Green water is the water that is stored in soils and taken up by plants by evapotranspiration, whilst blue water is water that is extracted for agricultural production from surface or groundwater sources (Falkenmark and Rockstrom 2006).  On a global average, only one third of precipitation becomes runoff that discharges into surface water sources (e.g. rivers) and recharges groundwater sources. This is blue water. The remaining two thirds of precipitation enter the soil, which is the green water, and is constantly being returned into the atmosphere as water vapour (Hoff et al. 2010). The issue is that blue water can be taken from non-renewable sources as well as non-local sources in order to allow for agricultural production. This is unsustainable and taking water from non-local sources can reduce water availability downstream or elsewhere. According to Falkenmark and Molden (2008), increasing agriculture leading to increased water withdrawals has resulted in the 'closure' of a rising number of river basins.  A river basin is said to be 'closed' when 'committed outflows from a sub-basin ('including flows required to meet downstream allocations to meet societal needs, dilute pollution, meet environment flow needs including sustenance of estuarine and coastal ecosystems, flushing sediments and controlling saline intrusion') cannot be met for an entire year (Falkenmark and Molden 2008: 202), and currently, 1.2 billion people in the world are living in areas undergoing river closure. The whole concept of green and blue water can help us understand the additional amount of freshwater needed besides precipitation to enable agricultural production, which in turn will aid understanding of the extent that it is sustainable. 

Transnational agricultural investors have rushed to Africa for cheap land and labour costs, and have been welcome by African national governments in hopes that investment will spur agricultural modernisation. Land contracts do not specify any restrictions on water usage, so investors usually tend to choose the cheapest irrigation schemes which have very inefficient water uses.

Figure 1

Figure 1 shows a graph, taken from the paper written by Johansson et al. 2016, depicting the green and blue water requirements for different types of crops grown on 95% of the large-scale land acquisitions in Africa. The size of each plot (bubble) is dependent on the total water demand according to the national level for each crop (countries labelled on plot), with the darker grey area showing crops with relatively low water requirements, and the lighter grey area showing crops with relatively high water demand. The graph shows us that water demand for each crop varies by country, even if it is the same crop. For example, sugarcane grown in Madagascar has a higher water demand than sugarcane grown in Zimbabwe. Another example is in corn production, where corn grown in Egypt has a higher blue water demand than corn grown in Kenya. This is due to variations in temperature and rainfall between countries (Johansson et al. 2016). Where there are high temperatures and low rainfall rates, soil moisture content will be low, therefore green water availability will be low, requiring more blue water inputs. In theory, the solution to reducing water usage would be to grow crops where there is an abundance of green water (reducing the volume of blue water needed) and to choose crops that do not need as much water inputs, like corn. Albeit, in reality, the choices of crops grown are seldom made on the basis of how much water they require, and more to do with their prices and demand. This is where irrigation is necessary.

According to (Johansson et al. 2016), irrigation will double the yields for crops compared to solely rainfed management.  Sadly, as mentioned, many irrigation schemes using blue water on these large-scale plots are not efficient. As a result, some areas will face increased water pressure and scarcity if they continue to operative using the same irrigation methods.

Figure 2

Johnansson et al. 2016 identify what they call ‘blue water hotspots’, which are areas where more than 50% of the water demand is for blue water sources, as shown in Figure 2. The map on the left shows areas where more than 50% of the water demand can be met by precipitation. According to this data, 35% of all the area under contracts and investment would be blue water hotspots. Rather than unrealistically saying that large-scale irrigation needs to be prevented or that green water should be the only water input allowed for crop growth, a more feasible solution would be to increase irrigation (and hence blue water) efficiency.

I found the paper written by Johansson et al. particularly interesting as it differentiated between the types of water required in crop production to give a clear explanation as to why water demand varied spatially. The spatial variation in green and blue water demand is a key point to consider, as certain areas will be more adapted for sustainable irrigation than others. In a growing bid to increasing food security and meeting rising food demands, it is important that that suitable areas which require less blue water inputs are identified quickly, and investment is encouraged where blue water demands are low.