Cape Town Drought: Forecast shows retreat, but is it soon enough?

31 January 2018

"YOUR FORECAST SAYS WHAT?"
With media attention focused on the dire water situation in Cape Town, South Africa - the city is currently expected to run out of water on 16 April - some of our followers are asking why our most recent blog post and maps didn't echo the alarm. Our 12-month forecast ending September 2018 (below) shows merely "moderate" water deficits ahead for the region, something you'd expect to see once every 5 to 10 years. Certainly no alarming red blobs indicating "exceptional" water deficits, those that might occur only once in 40 or more years. How can that be, given the desperate and very real situation in Cape Town right now?

First, the map above is a forecast (things to come), actually a composite that uses three months of observed data - conditions that occurred - and nine months of forecast data - what ISciences Water Security Indicator Model (WSIM) expects to occur. So it's an image of what we project will happen, not what is happening now. We'll come back to forecast analysis, but let's take a look at the history of the Cape Town drought.

HISTORY OF THE CAPE TOWN DROUGHT
The drought has been solidly entrenched in Western Cape for a long time!  Cranking the dial on our water security model to go back in time three years from December 2017 (below) to see what actually happened, the red blobs of exceptional water deficit in the region practically scream. Going back two years shows some minimal improvement in South Africa but not in the western part of the country. And the situation for the past calendar year -  still no relief for Cape Town.

IT'S HARD TO FILL THOSE RESERVOIRS FAST ENOUGH
Reservoirs levels are reflective of the difference in water supply and demand over long periods of time. In order to understand the supply side of the equation, it is best to look at how long-term drought conditions will likely evolve over the next nine months. This is best done with our 12-month forecasts as depicted in the series of maps below. 

Notice how colors fade in South Africa as we shift emphasis in the three images below. First a blend of 9 months observed and 3 months forecast. Second, a blend of 6 and 6. And third, a blend of 3 observed and 9 forecast. As we move past history and into the future, that future slowly begins to look brighter. Based on these data, we expect the water supply in Cape Town to be an issue at least through March 2018 and possibly through June 2018. While conditions improve after June, we still project a moderate long-term drought with a 5-10 year return period.

It will be awfully hard to refill reservoirs at any restorative pace under conditions of even moderate persistent water deficit. No surplus water anomalies are on the horizon that might provide rescue. So, the lingering long-term deficits are just too much to overcome even if short-term conditions improve.

WHAT DOES IT ALL MEAN?
What we can surmise it means is that Capetonians are in for a long haul, not unlike what residents of another megalopolis - São Paulo, Brazil - faced a few years ago. Water shortages are likely to linger in Cape Town longer than people expect and will require extraordinary response measures.


Banner image attribution: Banner image by Andrew Massyn (Own work) [Public domain], via Wikimedia Commons

Comment

Many analyses reported in ISciences-authored blog posts are based on data generated by the ISciences Water Security Indicator Model (WSIM). Other sources, if used, are referenced in footnotes accompanying individual posts. WSIM is a validated capability that produces monthly reports on current and forecast global freshwater surpluses and deficits with lead times of 1-9 months at 0.5°x0.5° resolution. This capability has been in continuous operation since April 2011 and has proven to provide reliable forecasts of emerging water security concerns in that time-frame. WSIM has the ability to assess the impacts of water anomalies on people, agriculture, and electricity generation. Detailed data, customized visualizations, and reports are available for purchase.

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