I think the water table in our garden in Warwickshire has risen to the actual surface of our garden. There has been so much rain here – and crazy hailstorms – over the past week or so that the ground is saturated. The lawn goes *squidge* when you walk on it.
This is the first of (probably) several posts about water and how cool it is. H2O is a marvellous, and profoundly odd, substance. But more about that later. It is the stuff of life, and knowing where and how it is stored and how it moves around is pretty important.
So: it rains. (At the moment, it seems to do nothing but rain, but that’s by-the-by.) Water falls onto the Earth – but then what? Some of it falls directly into water courses and drainage channels (streams, rivers, etc.). Some of it falls onto trees and plants; some of that evaporates or reaches the ground.
Only about a third of precipitation results in runoff (also known as streamflow or discharge). Runoff is the water that flows over the land and increases flow in rivers, and is measured in cumecs (cubic metres per second).
Then it starts to get complicated, with flow diagrams, code letters and fiddly rivulets finding their way into nooks and crannies unseen by you and me.
In a nutshell, here is where the rain goes:
- Into rivers and streams (channel precipitation) OR over the land (overland flow)
- Infiltrates into the ground below the water table as groundwater flow OR above the water table as throughflow
- The infiltrated water becomes subsurface runoff
- Subsurface runoff, channel precipitation and overland flow come together in channel flow (i.e. a river or stream)
- The mouth of the river spits out the total runoff.
It’s pretty important to keep an eye on these processes, and be aware of how topography, soil type and land use will affect water courses, because that’s what helps with flood preparation.
…lots of drops to drink
What I find interesting is what happens to the rain when it disappears beneath the surface.
You’ve got the ground surface, then soil water, then unsaturated rock, then the capillary fringe, then the water table, then saturated rock. That’s quite a lot of groundwater, and much of it is available for our purposes.
Capillary action means that water is drawn into the spaces between rock grains – and into cracks and fissures – and is held there by surface tension. The larger the surface area of the rock particles, the higher the surface tension and the more water can be potentially be stored. Hold the edge of a sugar cube in a cup of hot chocolate and see what I mean. The liquid will creep up the sugar lump.
So, the amount of groundwater stored in rocks depends on their porosity, which can be calculated by dividing the volume of void space by the total volume of rock, then multiplying by 100.
Vp/Vtot x 100
Unconsolidated rocks – not compacted or cemented – are the most porous, while consolidated rocks and dense crystalline rocks are usually less porous. But just because a rock is very porous, doesn’t mean it is necessarily permeable…
Rocks with large pores, or with pores that join together, make it easier for water to flow through them. Sandstones and gravels are permeable rocks. But clay, which has high porosity (due to lots of very small pores) holds its water virtually immobile because of the high surface tension.
Porous, permeable rocks are great natural water storage vessels. They are aquifers, and most of those in the UK are found in the south-east of England. The Chalk – the White Cliffs of Dover and the rest of this particular limestone formation – has a network of fine cracks making it very permeable to water. A single borehole in the Chalk can yield enough water to provide for around 70,000 people per day. That is a LOT of water, because we use way more than we should.
There are two types of aquifer:
- Unconfined: the aquifer sits on a floor of impermeable rock and soaks up rain. When it becomes saturated, marked at the top by the water table, the water flows out as a spring.
- Confined: the aquifer lies between two layers of impermeable rock, trapping water at a higher pressure than atmospheric pressure. Boreholes – artesian wells – drilled into the aquifer allow the water to rise until the pressure equalises.
What is the potentiometric surface?
If several boreholes were drilled into a confined aquifer, the water level in each would rise to a maximum. An imaginary line can be drawn linking these maximum water levels – and this is the potentiometric surface.
If the potentiometric surface is above the ground surface, water will flow freely from the well; if it is below the surface, then water must be pumped from the well. Essentially, it is the level at which the weight of each column of water balances the water pressure in the aquifer.
Oases in the desert are often portrayed as magical, mystical things – how, in such an arid and inhospitable place, does water flow? Aquifers; that’s how. Porous and permeable rocks lying underground can transport water many miles from a source of precipitation to the middle of a desert.
You can see why people were mystified by oases. Now we understand how they work, the appearance of water is no less wondrous.
I don’t know about you, but the more I know, the more I want to know…