Why does depletion of underground water levels happening, if the Earth is covered by nearly 78% water, and there is also water in rivers and lakes?
How can we still lack groundwater?
How can we still lack groundwater?
That 71% of the Earth's surface is covered by oceans is completely irrelevant to the issue of fresh groundwater depletion. Salty groundwater is useless for drinking or irrigating crops.
The issue at hand is the depletion of those underground reserves of fresh water. This depletion can result from natural or human causes. The Sahara is an example of natural depletion. The Sahara was significantly wetter than it is now several thousands of years ago. Rains over the course of a few thousand years created a number of underground reservoirs of fresh water in the Sahara. Rain stopped falling on the Sahara about 7000 years ago. The few remaining oases in the Sahara are places where groundwater still comes close to the surface as a result of that roughly 3000 year long wet spell.
Humans can also cause groundwater depletion. It takes several centuries for rainfall to percolate down to the water table. The replenishment of aquifers is very slow. This is balanced by a very slow trickle from a natural spring here, another one there. The end result is a more or less steady state where the slow replenishment balances the slow outflow. It's very easy to disrupt that balance. Climate change (both natural and anthropogenic) can change the replenishment rate, while artificial wells can drastically increase the depletion rate.
Groundwater is formed from precipitation of rain into the ground. It flows towards the oceans through the soil as the oceans have a lower surface. Often it flows into river and lakes on the way.
If the precipitation decreases or water is pumped from the ground for e.g. irrigation, the groundwater table level sinks.
If the groundwater becomes lower than the sea surface, salty seawater will flow from the sea under land and destroy potable wells.
I hope this answers your question.
Think about elevation.
The bottom of the lowest/deepest part of the Ogalala Aquifer is still hundreds of meters above sea level.
I will try to explain it with an analogy.
In one pot pee.
In the next pot is water you can drink.
Now take a table spoon and move the pee from the pee pot to the drink pot, taking time to sanitize it and turn it back to drinkable water.
It doesn't matter how big the pee pot is, 7,000,000 gallons of pee is still only turned into drinkable water one table spoon at a time, so the max you can take from the drink pot is one table spoon (at the same rate your converting your pee). If you take more then that then your diminishing your drinkable water.
The earth is much like our pee pot and drinkable pot example. There is a massive amount of salt water (and non-drinkable fresh water). That's our pee pot. The process of converting that into drinkable water is slow and limited. It takes quite some time to evaporate "dirty" water rain it, for that water to pool, collect and filter, and finely become drinkable.
No matter the size of the pool of undrinkable water, the drinkable water is only replenished at a very slow and limited rate. If we "use" more drinkable water then is being produced, then it's kinda like drinking our entire drinkable water pot, and peeing back in the pee pot. We still don't have anything to drink.
That is a very simple view, and I tired to use a funny, but "gross enough" example to help illustrate. But the fundamentals are the same.
As professor A.S.Phillipe says:
The cycle of water, this marvelous pump which work with solar energy and affords the alimentation of continent with around 40 000 km3/year.
When seawater evaporates, it becomes fresh and then it's poured on earth it's filtrated by the soils to form an underground reservoir of freshwater neglecting any form of human pollution.
I just want to elaborate a little on groundwater reserves. Below the watertable, the porous rocks within the earth are saturated with either (1) fresh water (2) brine water (contains many dissolved ions and is too salty to drink, sometimes even much saltier than the ocean water), (3) oil, (4) natural gas. The aquifers with potable water are located at shallow depths and get recharged by rain, snow, glacial ice melt etc. It takes many thousands of years for water in these aquifers to accumulate.
When an oil well is drilled down to 1-3 km depth, it encounters many aquifers (porous rocks that have pore-spaces filled with water) that are separated by aquicludes (typically mudstones, tight rocks with low permeability that prohibit fluid flow). The shallowest aquifers are typically potable and deep aquifers contain brine water that is not suitable for drinking. The point is that not all underground water is drinkable.
Potable water regulations are different for different places, for example cattle/farm-animal/irrigation water standards are lower than for human drinking (irrigation water is allowed to be a bit more saltier).
The issue is that municipalities extract water from these shallow, good quality aquifers at a much greater rate than they are replenished. Some cities treat and reuse their wastewater (unfortunately this is rare, because people are too squeamish to drink water that once contained pee), while others just keep pulling new water out of the ground/lakes/rivers.