Increasing green water efficiency through conservation agriculture

In a previous blog post, I highlighted the need to focus agricultural activities in areas with high green water availability and low blue water needs. Much of eastern and southern Africa have relatively low green water use efficiencies of only 15-30% due to high rainwater runoff, large evaporative losses and poor soil water storage (TerAvest et al. 2015: 285). As green water is water that is stored in the soil, I wondered if unsustainable farming in parts of Africa has in fact led to decreases in green water availability through soil reductions in soil water capacity, and whether this can be controlled.

A study by Nyberg et al. (2012)  showed how land use changes from forest to agricultural land in Western Kenya has reduced infiltration rates, where median infiltration rates on agricultural land decreased by 15% compared to original infiltration rates in the forest. Not only is it essential to maintain soil infiltration so that plants can uptake water, but infiltration is also necessary for groundwater recharge. Intensive agricultural practices are partly to blame for the decline in infiltration capacities. For example, the moldboard plough has been a popular tool in agricultural practices in southern Africa, but intensive over-usage of the plough has led to soil degradation and increased run-off, reducing soil moisture and water infiltration (Thierfelder et al. 2013).  A reduction in the capacity of the soil to hold water reduces green water availability for crops, making it necessary to withdraw more water from blue water sources which can be unsustainable. 

When looking at what can be done to reduce erosion whilst still being agriculturally productive, I came across a concept that encourages and encompasses water efficiency, known as ‘conservation agriculture’. Conservation agriculture is a set of soil management practices, defined as ‘minimal soil disturbance, year-round ground cover, and crop rotations being promoted in a way to sustainably improve water-use efficiency, reduce soil erosion and boost crop production’ (TerAvest et al. 2015: 285). Parts of Australia, North America and South America have adopted conservation agriculture, albeit the agricultural context of farming is very different in these countries compared to Africa – that is, Africa has a less mechanised, smaller-scale agricultural system. However, based on the success of conservation agriculture in Australia and the Americas, international donor communities have encouraged the adoption of the approach in Africa (Thierfelder et al. 2013).  There are many practices that make up conservation agriculture, but I will focus on a practice that has directly impacted soil-water levels, known as residue retention, or mulching. Mulching involves using crop residues (dead or live, such as stalks and leaves) to increase ground cover, which lowers evaporation, and decreases erosion (Thierfelder et al. 2013).  Not only does mulching improve soil-water content and quality, but also maintains soil temperature throughout the night (Stauffer and Spuhler [no date]). Studies on maize growth in Africa generally showed that soil water infiltration improved when mulch was added to the maize fields (Brouder and Gomez-Macpherson 2014).

A groundnut farm in Malawi following conservation agriculture. Mulch in the form of corn stoves can be seen in between the rows (source). 

However, there is also controversy surrounding the potential for conservation agriculture in improving small-scale agriculture in Africa, and currently, its adoption in Africa has not been widespread, albeit, conservation agriculture practices have been increasing in Zambia, Malawi and Zimbabwe (Wall et al. 2013 cited in TerAvest et al. 2015). Whilst research has shown that conservation agriculture generally improves soil moisture, soil moisture content is essentially dependent on the soil type. And if soil type is not accounted for, conservation agriculture can have devastating impacts. For example, conservation agriculture can increase the risk of waterlogging if it is carried out on sandy granitic soils (Thierfelder and Wall 2009).  Academics have also ignored the different socio-economic contexts that constrain the adoption of conservation agriculture. I talked about mulching as a method to reduce soil degradation, but mulching requires crop residue, which farmers use for other purposes, such as for animal bedding, livestock food, construction (fencing and thatching), and as a source of fuel (Thierfelder et al. 2013). It is definitely not a resource that is available in abundance to small-scale farmers in areas where crop-livestock systems are common.  One solution I came across to address this issue is the use of inorganic mulches, such as plastic sheets, stone or shredded rubber, which can act as a replacement for organic mulch. Regardless, for the majority of farmers who are small-scale, inorganic mulching may not be affordable, and the implementation of inorganic mulch often requires machinery and additional knowledge, as the type and colour of inorganic mulch required is dependent on what is being grown. Inorganic mulch may be more effective in preventing evaporation but may not be the best for increasing infiltration.  I personally prefer the use of organic mulching as this makes the best use of the available resources and does not entail as much technical knowledge. Nonetheless, organic mulching can have its downfalls too. In addition to not there being enough crop residue for mulching, too much organic mulch can lead to rotting and encourage pests to accumulate causing crop damage. Furthermore, if carbon rich materials such as stalks are used, there will be competition between plants and decomposing microorganisms for nitrogen, which can also impair crop growth (Stauffer and Spuhler [no date]).

Even though I only touched upon a very small part of conservation agriculture, I believe it is not a solution on its own to address Africa’s soil degradation. I do not believe that conservation agriculture should be applied blindly wherever possible, as the effectiveness of conservation agriculture can evidently vary between different contexts. Farmers may also not be interested conservation agriculture given the risks associated with increased mulching, and if the associated risks cannot be mitigated to a level that gives farmers a sense of security, then it should not be forced upon small-holder farms. Conversation agriculture does however, hold one of the answers to reducing blue water usage if applied carefully and can be part of a wider solution to address inefficient agricultural water usage in Africa.