Evapotranspiration stealing the blue water

I want to start, by referring to my first blog post, with map number 2 showing the intrinsic variability in rainfall across the African continent. We can distinguish different patterns of seasonality, ranging from highly seasonal rainfall in arid environments (e.g. Sudan) to seasonally humid environments with most months experiencing precipitation (e.g. Malawi).

Adding to the annual rainfall variability, it is also important to acknowledge the year-to-year rainfall variability in Africa, and the rainfall variability within a certain period of year (especially onset of wet season).

Blue water resources: renewable freshwater resources, such as river discharge of a permanent river.

I want to extrapolate the occurrence of “blue water resources” on this rainfall variability by examining the influence of evapotranspiration at present and under climate change.

Blue water resources are only possible to be formed when the rainfall exceeds the evapotranspiration – thus, in all African regions, only certain months of the year contribute to their maintenance. In the tropical regions and the Sahel, temperatures throughout the year are relatively warm and stable and therefore potential evapotranspiration rates do not vary as greatly as in higher latitudes (temperate climates). Rainfall, however, as I have outlined previously, varies greatly in its distribution and sums.

Sudan’s mean rainfall throughout the year never significantly exceeds the constant evapotranspiration (ET) loss threshold and this explains the lack of blue water in the Sahel.  A warming planet, and the resulting rise in potential evapotranspiration will only increase this threshold further, and reduce the number of years (and months in a year) rainfall exceeds this.

It is interesting to show the potential of a rise in this ET threshold on regions with blue water resources, such as in the seasonally humid tropics of Africa. For simplicity, and due to the uncertainty of GCM model output in predicting future rainfall, the precipitation pattern is left unchanged. In the diagrams below, the shaded blue area represents potential renewable freshwater resources at present (ET) and under rising temperatures (ET-CC).

 

                                                                                           

With only a slight rise in evapotranspiration throughout the hydrological year, we see a dramatic reduction of potential freshwater resources, even if the annual rainfall is not altered.