Agriculture and Climate Change; Groundwater, a Hidden Hero?

Before I get into the nitty gritty of today’s blog, I just want to level with you for a bit. I know last week’s blog on groundwater was pretty damning, but this week I want to talk about how it might be the key to Africa’s future in the face of climate change. I know what you’re probably about to say; “Joe, how can you rebuke groundwater so badly last week and then talk about it being the future this week?” I know it’s weird, but after a lot of soul-searching (and literature-searching), I’ve had somewhat of a change of heart ok, so just leave me alone and read the blog.

Now after being alive for at least the last decade or so, I know you are no stranger to the concept of global warming. It’s becoming somewhat irrefutable nowadays that the planet is becoming warmer and that this is having innumerable knock on effects on all sorts of global systems. Today I’d like to focus on the effect this is having on water availability in Africa, and how groundwater may just step up to save the day.

Africa is a bit of a special case when it comes to global warming. The IPCC (2014) tell us that temperatures in Africa are supposedly going to rise faster than the global average, with many areas exceeding the 2C warming expected by the end of the 21^st century. This is also supported by Carter and Parker (2009) who suggest that temperatures for much of Africa are expected to rise by 3.2-3.6^oC.

This warming is due to have profound effects on precipitation and water availability in Africa. Precipitation across the continent is already characterised by extremely high variability between years, with many countries experiencing pronounced periods of significantly lower or higher rainfall. This is perfectly illustrated by the figures below from Carter and Parker (2009), which detail rainfall variability in African countries.

This table, from Carter and Parker (2009), shows the rainfall range as a percentage of the mean for 5 African countries. These figures are huge, as high as 45% variation in Niger and only going as low as 16% for Uganda. The dramatic variability of rainfall makes successful agriculture seemingly impossible.

This table, also from Carter and Parker (2009), shows in more detail how Mean Annual Rainfall (MAR) can vary for a country such as Niger. The rainfall changes by huge amounts between years, and shows little if no consistency. Planning for next years crop under such conditions would be nigh on impossible.

 

 

As you can see, this variability is massive, and has pronounced effects on agriculture. As the majority of Africa’s agriculture is rainfed (as revealed by our good friend Giordano (2006) last week), this means that a bad year for rain generally means a bad year for yields, and many in a row can be disastrous and dramatically reduce food security. Crops grow best when rainfall is reliable and consistent, but the precipitation patterns of Africa rarely offer such conditions.

Unfortunately, due to climate change, this temporal variability of rainfall is only due to get worse. Due to increases in atmospheric humidity because of temperature rise, sensitivity of tropical rainfall to total moisture content will be increased. This means that we will see far less frequent rainfall and a rise in extreme rainfall events. In essence, as we heat the already hot air, it gains significant capacity to hold more water vapour, and dew point is ever harder to reach. This means it will rain less, but when it does, far greater amounts of water vapour will have built up in the atmosphere,  making rainfall events all the more extreme (Taylor 2013).

This is the Clausius-Clapeyron curve. The y-axis shows the water vapour pressure that can be held by air, and the x-axis shows how this varies by temperature. It is important to note that it is an exponential curve, so if you make a hot tempaerature (say, 25C) even hotter, the capacity of air to hold water vapour increases significantly. This is what is happening in Africa due to climate change.

According to the IPCC (2014) in their AR5 report, this potentially will have dramatic effects on the face of African agriculture. Maize-based systems are identified as being the most vulnerable, with projected yield losses of around 22% across Sub-Saharan Africa and losses of up to 30% in some areas such as Zimbabwe. They also estimate around a 35% reduction in wheat yields for the whole of Africa. Even crops such as Cassava, which will initially benefit from increased CO² levels, will decline. And this is just the tip of the iceberg.

Things aren't looking good for wheat

However all hope is not lost. As Taylor and Tindimugaya (2009) reminds us, the people of Sub-Saharan Africa have been turning to groundwater to help them through periods of rather half-hearted rainfall. He also reminds us that the groundwater resources available in Africa are estimated to be 3 times greater those of China, and 6 times greater those of India. Is groundwater the way forward then? Is it the ray of hope amid the turbulent storm of climate change?

It might well be. As Carter and Parker (2009) remind us, groundwater recharge is dependent on what is known as Hydrologically Effective Rainfall (HER), which is rain that produces direct runoff and groundwater recharge when precipitation is greater than evapotranspiration. And as we are seeing a growth in extreme rainfall events, wherein precipitation dwarfs evapotranspiration, climate change may actually benefit groundwater recharge and make it a more reliable and sustainable resource.

This is exactly the phenomena which Taylor (2013) explores in his study that examines data from a 55-year record of groundwater level observations from the Makutapora Wellfield aquifer in central Tanzania. Interestingly, Taylor finds no linear relationship between rainfall and recharge, noting that it is only the extreme rainfall events caused by ENSO and Indian Ocean Dipole forcing that show any real potential for recharge. These extreme events happen on almost decadal scales, putting a stop to multiannual recessions in groundwater levels, as detailed in the graph below.

The top graph details a time-series of groundwater levels from 6 monitoring wells. The second shows monthly rainfall from 1955 to 2010, and the bottom graph shows monthly abstraction. Note that the most significant jumps in groundwater levels occur during periods of >200mm monthly rainfall (the most significant of which occurs around 1997-1998), however when rainfall is lower groundwater is shown only to be decreasing (Taylor 2013)

In fact, for nearly 2/3rds of the 55-year long record, very little if any recharge is observed, and it is only when monthly rain exceeds 200mm (a very rare event that happens in the >95^th percentile of records) or if seasonal rainfall is greater than 670mm (again very rare, only occurring in the third quartile of observed records) does significant recharge occur. Taylor (2013) concludes with the observation that more intensive monthly rainfall clearly benefits recharge, and that groundwater may in fact present itself as a viable alternative to surface-water resources in times of climate change.

It is clear then, that although the future of agriculture in Africa is uncertain, groundwater-shaped hope exists. However, as we have observed, groundwater isn’t always easy to access. Whatever mitigation against climate change groundwater will be able to offer will, at least at current rates, be limited to the few areas in Africa that have capitalised on the hidden resource. For a greater proportion of Africa’s agriculture to be able to turn to groundwater in the next few decades, mass innovation in groundwater usage across Africa is needed, and significant investments need to be made. Questions also exist about the sustainability of the resource. Without a doubt, drastic action needs to be taken if groundwater is to be considered as an alternative in times of global climate upheaval.