Clausius-Clapeyron giving hope

We’ve seen that the Earth is getting warmer – and extremely so on the African continent. The evapotranspiration robs moisture. To assess impacts on the continents 'blue water' formation, however, we must take into account a physical relation that affects a different part of the hydrological cycle.

The Clausius-Clapeyron relation describes the relationship temperature has with the atmosphere’s potential to hold moisture. With rising temperatures, the atmosphere can hold more moisture (see Figure 1), leading to longer moisture retention before a rainfall event (longer drought events or dry seasons) and more intense rainfall once it does occur. This intensification of the hydrological cycle is more extreme in warmer regions (such as those semi-arid/arid regions of Africa) due to the Clausius-Clapeyron relation progressing in an exponential manner. Reviewing a wide range of climate modelling studies, a 2-3 degree rise in global temperatures will intensify the water cycle by 16-24%.

Figure 1. Clausius-Clapeyron relation, showing that the saturated water vapor pressure increased exponentially with increasing temperatures. (source)

The expected in-situ effect of this change on precipitation is different across Africa, especially as changes in global circulation further complicates prediction. Additionally, modelling of rainfall is severely limited in certainty by the lack of continuous observational data across the continent (Niang et al., 2014).

The intensification of rainfall can have beneficial consequences for the formation of 'blue water', by helping to maintain the ephemeral surplus in moisture despite a rise in evapotranspiration by higher temperatures. I want to relate back to my post 2 (which assumed no change in precipitation): the expected rainfall pattern intensification will likely result in the increase of moisture supply in a shorter period of the wet season - where the threshold (P-ET) is surpassed - and blue water resources can be formed. This is obviously a theoretical perspective and in reality it is the subtleties of the relative changes in ET and P at a location that will determine climate change's impact on 'blue water' formation.  

Thoughts on the implications on the ground:

While the potential for 'blue water' formation by this intensification of rainfall is clearly possible, the benefit of this to local populations and environments depends on the potential of water storage and retention to last through the extended dry seasons (that are also an effect of the intensification). The adverse effects of extended droughts and higher time-variable availability of water resources may often not be relieved by the 'hope' for this source of excess moisture. Even given that yearly sum 'blue water' does not change, the amplified variability of supply poses threats especially to subsistence households, farmers that live off rain-fed agriculture and highly adapted ecosystems. 

From causing disastrous floods (guardian.com) to dramatically improving groundwater storage (Taylor et al., 2013) everything is possible, depending on catchment characteristics, its geophysical properties and land-use.