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There has been much discussion about the causes of the Cape water crisis in 2017. Is this drought? Is it climate change? Is it the growth of the population? Is it the lack of foresight of the national government or the local government? Is it the rich? Is it the poor? Is it the farmers? Or the industry? Or invasive trees?
The process of badyzing causes is understandable and necessary. We want to know. We need to know. We must understand because it is only then that we can learn from experience to avoid similar crises in the future.
We have heard and will continue to hear various perspectives on this subject. They come and will come from scientists, civic activists, government, various interest groups, experts and non-experts. It is important to note that opinions may be based solely on perceptions or that they may be based on data and facts.
I would like to contribute here to this discussion by data-driven badysis. I will attempt to answer a question about the last three years (2015-2017): To what extent did an unusual climate and unusual water use during this period affect dam levels and water quality? amount of water available in Cape Town?
questions, we developed a simplified water balance calculation – a hydrological model – for Big Six reservoirs and their watersheds. The model takes into account the main factors affecting the storage of dams: precipitation, evaporation and discharges for water supply, as well as runoff from watersheds. This runoff is calculated on the basis of precipitation in watersheds, taking into account processes such as soil moisture turnover and evapotranspiration.
Model details and calculations are available on the UCT Climate System Analysis Group website. In these calculations, precipitation is taken as an average of the values of five sites, which are located near each of the Big Six dams (there are two Steenbras dams, but I use only one rain gauge for represent the rains on them). and not six rain gauges). Potential evaporation is calculated from meteorological variables measured in 35 meteorological stations of the region's Agricultural Research Council. The use of water by agriculture, Cape Town and smaller municipalities are represented by data from the city of Cape Town.
Looks like the model works well! There is some difference in 2010, but it is probably the result of an error in the data rather than a deficiency in the calculation procedure.
Now I can use the model to answer the previous question: what is the relative role? of climate with respect to water use in lower dams in 2015-2017. However, I rephrase this question into four questions "and if …"
(1) What if the precipitation was average?
(2) And if the evaporation was average at this period
(3) (19459007) (4) And if the agricultural use of water was average?
To answer these questions, I make calculations of the water balance with either rainfall or evaporation or water use in 2015-2017 set at values representative of the average 2006-2014. These average values are illustrated below, with the original data:
And here is how the results of the water balance calculation (a water balance model) compare with the storage observed in the reservoir:
The graphs clearly show that in 2015-2017, the precipitation was considerably lower, the evaporation was not different and the agricultural use was higher than the recent average. Urban use was above the recent average in 2015 and 2016, but lower in 2017.
These average values are included in the water balance calculations one at a time. This means that with average rainfall, the actual observed values of the remaining three variables (evaporation, urban and agricultural use) are used. This creates four scenarios describing the "what if" situations. The results are as follows:
The figures above show that the main storage factor for the dam was the decrease in precipitation. Unusual evaporation played a very limited role. The increased use of water (compared to that of previous years), both municipal and agricultural, also contributed relatively little to the reduction of available water.
If the rainfall was average, we would have had complete dams at the end of the rains. season every year from 2015 to 2017. If the urban water use was average, the dams would have been five percentage points higher at the end of the 2016 rains than They were not in reality. The weaker urban demand in 2017, however, compensated for this deficiency and by the end of 2017, there was no longer any difference between dam levels in the context of real and hypothetical urban use. Above-normal agricultural demand, however, reduced reservoir storage at the end of 2017 by about 8.5 percentage points compared to average demand.
The shortage of water can, but must not lead to crisis. A shortage of water can be simply a nuisance of prolonged but relatively light water restrictions, and it must not become a full-fledged crisis that threatens the existence of a city with dramatic social and economic consequences. The threat of "zero day" might have been avoided if water allocations were slightly lower or if water restrictions were applied sooner.
Agricultural demand had already been reduced by about 25% during the 2015/2016 and 2016/2017 growing seasons, the dam levels in 2017 would be 8.5 percentage points higher than that. they were not in reality. Similar reasoning applies to urban use. But we only know it now, with the benefit of hindsight. It is easy to have opinions on past events while enjoying such a benefit. It is more difficult to make decisions at this time without it.
To badess the role of water management decisions taken over the last three years, however, a different badysis is needed than is done here. We are busy with such badyzes
For now, let me repeat: water scarcity in 2015-2017 occurred mainly because rainfall was low.
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