Water harvesting techniques- a possible solution?

This post will highlight some of the suggestions put forward by Rockstrom and Falkenmark (2015) in retaining and storing green water. Rainwater harvesting is the 'small- scale concentration, collection, storage and use of rainwater runoff for productive purposes' (Kahinda et al 2007: 1050).

Green water can be stored or retained in various ways. Firstly, the run off can be collected such as in ponds or tanks (Figure 1). This is ideal in the light of climate change when rainfall events are likely to get heavier and more infrequent. When a rainfall event is high intensity it means that crops can get damaged or soils worn away of nutrients. Therefore, collecting this water will not only reduce this impact but it will also mean there will be water to use as the rainfall gets more infrequent. 

Figure 1- Rainwater harvesting using a water tank.

Another way of harvesting green water is by using Fanya juu terraces like those in Machakos, Kenya (Graham 1991). This technique is used to improve arable land where soil erosion has damaged the productivity of crop fields. The soil is mounted upwards from a ditch to form an earth embankment. Several of these are made across one field. The rainwater is conserved between the mounts and this keeps rain in the area and keeps the soil in the field. Therefore, there is better moisture for the crops and nutrients in the soil are not worn away. Although it is labour intensive, it is low- skilled and does not require much training.

The final suggestion by Rockstrom and Falkenmark (2015) is using ditches to channel runoff into fields. I, however, think this is very problematic. The Hjulstrom curve (1935*) (figure 2), amongst many other things, shows that fine particles are easily transported whilst coarser particles are less easily transported from the area of erosion to the sedimentation site. Thus, the channel water is likely to erode the ditch or silt up with coarse material. There would need to be regular maintenance of the channel which can be expensive and be a source of conflict.

 Figure 2- The Hjulstrom curve.

Consequently, Rockstrom and Falkenmark (2015) have some promising suggestions but as shown above some can be problematic.


*Please note Hjulstrom's thesis, from which the graph was first used, is unavailable online.

Lack of green water in Sub- Saharan Africa? Let's savour what we can

A recent paper by Rockstrom and Falkenmark (2015) begins by painting the harsh but realistic truth; one fifth of the current 1 billion people in Sub- Saharan Africa face water shortages (Rockstrom and Falkenmark 2015). The population of Sub- Saharan Africa is set to double by 2050 to almost 2.5 billion (Gerland et al 2014). That is a staggering 150% larger than current numbers. If so many are facing water problems now, what kind of picture will be painted then?

Due to these water shortages prospects of agriculture are limited.

The arid desert and semi- arid savannahs receive too little surface runoff to grow staple crops such as maize, rice, corn millet etc (figure 1). These kinds of crops need around 400mm of water annually but the region receives just under 100mm (Rockstrom andFalkenmark 2015).

 

 Figure 1- Rainfall and runoff in Sub- Saharan Africa is inadequate for growing staple food crops (Rockstrom andFalkenmark 2015).

Green Water is precipitation that is held in the soil and 95% of Sub- Saharan Africa depends it. Future rainfall could drop by 25% in many semi- arid regions (Schewe et al 2013).  Thus, management of green water, in my opinion, is essential. Aside from reductions in rainfall, precipitation is likely to be unpredictable and varied. We are already seeing signs of this inconsistency. In 2000 Kenya experienced an early onset of rain followed by 6-9 weeks of no rain. This prevented staple crops growing and caused 4 million people across the country to experience food shortages (Rockstrom andFalkenmark 2015). As can be seen these inconsistencies, variability and altogether lack of rainfall has detrimental effects on the agriculture of the region.

So what can be done? I think Rockstrom and Falkenmark (2015) make a good suggestion: water harvesting. It is small- scale, cheap and arguably practical for many subsistence farmers across Sub- Sahran Africa so keep a look out for the next post!

Farmers reach for groundwater: a possible solution?

We know from the last blog post that water resources in Africa are highly variable and unreliable. With a growing population, reducing the amount of agricultural produce in Africa would certainly not be a viable solution to these issues. This continent needs to up keep levels of food production (if not increase them!) but how is this possible with such inconsistent water supplies?

Groundwater. The groundwater stores in Africa, in my opinion, could be the future for increasing agricultural output and allowing Africa to not only meet national food demands but increase exports too. Africa rests on extensive stores of untouched water supplies which could potentially, if utilised in the right way, alleviate their water scarcity. But what way is the right way?

MacDonald et al (2012) estimated Africa to have around 0.66 million Km³ of groundwater storage. Figure 1 shows many areas to have substantial stores, the most significant being the large sedimentary aquifers in North Africa. This water may not be evenly distributed, however there are significant stores all across Africa- it is the most widely distributed store of freshwater in Africa. So why aren’t these areas using these stores?

Figure 1- Groundwater stores across Africa (MacDonald et al 2012)

The type of aquifer makes it unavailable for abstraction. The hardrock systems are low yielding (MacDonald et al 2012). However, despite this, even these are significant because they are considerably more than the volume abstracted manually using hand pumps by the widely distributed small hold farmers across Africa. They also act as a buffer against the variations of water supply because the groundwater is stored for hundreds of years and can be abstracted for use during drier periods (MacDonald et al 2009). 

Africa is characterised by many small scale farmers sparsely distributed all across the continent. In a similar way groundwater is distributed all cross the continent as shown in figure 1. Unlike large rivers this means that farmers remote from the river or lake can still have access to the water.  Furthermore, they do not pump water as intensely as the large irrigations schemes and so groundwater stores, which may be small in volume, will be sufficient for unintensive use by the small hold farmers. Groundwater also reduces political tensions amongst users. This is because if the ground water is to be intensively pumped the rule of the ‘cone of depression’ means that only water stores from immediate and similar geology will be used and so distant farmers will not be affected. This reduces the potential for conflict arising with larger rivers such as the Nile where countries go to war over water rights and access to the source/ river. Therefore, the over abstraction by one farmer would only be detrimental to themselves and this solution erases issues of transporting water over long distances and the conflicts that come with sharing one large source over many national boundaries.

Furthermore, as explained in the previous blog post, climate change is likely to cause less frequent but more extreme rainfall events. This is favourable for the replenishment of groundwater sources. Taylor et al (2013) conducted research on groundwater resources in Makutupora, Tanxania and showed that recharge occurs during heavy rainfall events. This is to do with higher evapotranspiration rates resulting from warmer temperatures which means a larger volume of rainfall is needed to induce recharge.

So groundwater seems like a viable option. It is accessible all across Africa, it works with the projected changes in precipitation, and reduces potential for conflict. But although I think it is the most suited solution for now, there are still questions that remain. Can local farmers afford the hand pumps and the maintenance? What is the quality of water like? There is still a lack of detailed knowledge and understanding about groundwater as a solution.