Don't map it all at once!

A very recent study conducted by Lebel et al (2015) evaluates the performance of In Situ Rainwater Harvesting (RWH) techniques for Maize production in Africa as an adaptation strategy to Climate Change.

Changes in rainfall due to climate change can have a detrimental effect on food production in Africa since it is the main influence on crop productivity (Muller et al 2011). Farmers in Africa rely on crop production not only as a means of income but also to survive as food for the family. Maize is the most extensively grown crop, for example it occupies 50% of harvested area in South Africa alone (Portmann et al 2010). From this alone we can tell how important this crop, amongst others, is to the livelihoods of African citizens.

In Situ RWH techniques aim to reduce the variability in crop yields, giving the farmers more security (Fox and Rockstrom 2000). They do this by reducing soil degradation from water erosion and therefore ensures a better environment for the crops to grow more successfully. Techniques such as planting pits or stone bunds allow the soil to store water in the form of soil moisture which would have otherwise been runoff, making better use of the limited water resources (Lebel et al 2015).

The study Lebel et al (2015) carried out used continental scale GCMs outputs to see which areas, in Africa, would benefit from RWH under specific changes in climatic conditions. They concluded that overall RWH is a viable solution which should be considered. It provides an alternative to irrigation from the scare water resources under current and future modelled climatic conditions in 2050. They found it could increase maize production by up to 50% in the 2050s by alleviating water deficits.

Not only can RWH be used to directly better water use efficiency by storing water, but it can also improve crop yields indirectly through improvements in soil fertility. By reducing surface runoff and the volume of water reaching the soils, it means there is less soil erosion and more nutrients remain in the soil. Breman et al (2001) found that this alone can increase water use efficiency for crops by 3-5 times.

Figure 1- The performance of RWH in different regions of Africa. Showing changes by 2050s from the 1990s. Using three GCMs as named above each map of Africa. (Lebel et al 2015).

From these results the study seems promising. It frames RWH as a go to solution for the water deficits facing Africa, but, there are still hurdles to be jumped. The study is on a continental scale- this means the details of smaller regions are ignored. This is especially alarming for a continent like Africa where the climate varies extensively, from the tropical rainforest to desert- each region receives different amounts of rainfall and so needs to be mapped in finer detail. They (Lebel et al 2015) did use GCMs, as shown in figure 1, to look at the performance of RWH techniques in different regions. For example, this compared predictive conditions in 2050 to data from the 1990s to show RWH will not work as well in countries like Zambia as oppose to Ethiopia where it works well. 

Furthermore, the implementation of these techniques requires government, NGO and community participation, again, each of which differ from one country to the next. RWH is a cheap and low skilled option compared to other technology ridden, top- down schemes but this does not erase the fact that it still requires training and financial backing.

Grouping Africa as a whole is a no go. You need tailored research and proposals for each region to get a better understanding of whether RWH is their go to or not.

Food for thought when sizing tanks...

A comment on a previous blog post raised the questions of tank sizes for Rainwater Harvesting (RWH). How large would the tanks need to be to collect and store water for 6-8 months of the year? Well there is no simple answer. In this blog post I will be detailing some of the factors, as mentioned by Kahinda et al (2007), which should be considered when sizing the RWH tank.

There is obviously a limit to the amount of water that a tank can supply in a day or week. This depends on its size and location. It needs to be sized properly to get the full benefit from it so its use can be optimised for the location and climatic conditions it is in.

To find the optimum size the following conditions need to be considered: 

• How much water is available and when; if the rainfall is high intensity in a short period then things like seepage and high evaporation rates need to be minimised.
• The space available for the tank; some areas, such as peri- urban areas, may be too crowded. Any alternative water supplies; if there are alternatives the tank may not need to be as big. 
• Technical constraints such as lack of labour and rocks in the ground which make digging and underground storage difficult.  
• The roof type; rural houses usually have thatched roofs which allow less runoff than corrugated iron roofs  
• The soil type; above ground tanks should not be built on clay soils which expand.
• Socio- economic limiting factors such as the amount of labour available or regular upkeep of the tank which require financial input. For example in South Africa, 67% of rural households cannot afford the purchase of a tank, never mind about the maintenance (Kahinda et al 2007).

 

Therefore, we can see that RWH is not as simple as one may first think. With sizing there are many aspects which need to be considered and there needs to be extensive cooperation between governments, financers, communities, users, NGOs…the list can go on. It is not only physical aspects that need to be considered but socio- economic aspects too. 

The success needs to be evaluated in terms of locality and so in subsequent blogposts I will be looking at a particular country to see rainwater harvesting in practice.

Rainwater Harvesting: What would you choose?

If you were told you could give a rural family, in Africa, a tank for Rainwater Harvesting (RWH) and it could be any size for the same price, most people would go for the largest tank. But there are many dangers in going with this decision!

 Figure 1- A larger tank is not always a better tank

 

Figure 1 shows that the benefits of a tank do not uniformly increase with size. This is because a smaller tank will be filled and emptied (cycled) but larger tanks are cycled less rugularly. Therefore, a smaller tank can save time especially in the riany season when footing is wet and slippery. Demand satisfied within communities begins to level off after a particular tank size. The size can be different for different locations and uses. Look out for the next blog post which will detail a few of the many aspects taken into account when sizing tanks!