Why are we not storing all this rainwater in Karachi?

The irony is lost on no one: turn on the tap and there is no water, but look outside and rivers are flowing on Karachi's roads because this monsoon has been rather unusual.
Some of the rainwater that floods the roads will eventually seep down into the water table. But most of it will make its way to the sea. In fact, according to a rough estimate 90% of Karachi's rainwater ends up in the sea. For those who do not get regular supplies at home, though, it does seem like a waste. For could that water not be stored for use later on?
Part of the problem is that the rainwater floods the city before eventually draining into the sea because it cannot be absorbed by the ground. This does not happen because Karachi has become a concrete jungle. The more we have paved over, built over the ground, the less water it can absorb. Karachi's exponential growth means that concrete has blocked the natural ground that could absorb rainwater. The advantage would have been that the water table would have been replenished.
All year round Karachi suffers a water shortage. KWSB provides just half of what the city needs. As a result, boring to extract groundwater is rampant. Experts are worried that this widespread private boring in houses across the city will lower the water table. It used to be the case in Karachi that if you dug 50 feet down, you'd hit sweetwater because the water table was unpolluted. But today if you bore down, you have go as deep as 300 feet to get water and when you do find it, it is brackish and undrinkable.
According to an expert, if the extraction of groundwater happens faster than the rain replenishes it, we will face a drought.
The danger of boring is also that it lowers the groundwater table. When that happens, the soil becomes drier and sandier. This has a dangerous effect on building foundations. This has already happened in Quetta where ground subsidence or the subsiding or sinking of the ground happens. Cracks develop in your house or building walls as a result. The drier the ground the less it is able to take the load of the buildings. This is why we see so many 'sinkhole' type gaps in our roads, for example. Human activity in the shape of altering the natural environment by building has this net effect.
According to an expert, Karachi's seismicity has also increased over the last 15 years or so. We see small activity on the Richter Scale of around 3.2. This medium or moderate activity also affects our built environment.
We can make natural dams in areas with a depression to store rainwater. This can be done without any government intervention as well, especially where there are high-rise buildings. All you need to do is place a mesh that can act as a filter to ensure garbage doesn't go into any storage facility you have made.
This monsoon has reminded us of the creeping effects of climate change. North Karachi drowned in 150mm and residents watched as fresh rains fell on Tuesday to add to the existing standing water.
Ideally, we would make artificial lakes or urban wetlands to soak up the extra water and unusual monsoon patterns. Rainwater harvesting is our only hope.