For groundwater to exist precipitation must exceed evaporation. Light rain falling over a long period of time infiltrates more than a heavy short-lived downpour.
On flat and gently sloping areas rainwater has ample time to infiltrate because it remains in one place for a long time. On steep areas, there is low infiltration since a lot of water turns into a runoff because of getting less time to percolate.
Nature of Rocks
There is a greater possibility of the existence of groundwater where there are permeable surface rocks.
Groundwater exists where impermeable rock overlies an impermeable one so that when water infiltrates and percolates underground it’s trapped by impermeable rock and accumulates above it.
Aquifer – permeable rock which is permanently saturated with water.
Rocks that allow water to pass through them.
a) Porous – Those with pores/airspaces between rock grains through which water passes e.g. sandstone, limestone and chalk.
b) Pervious – Ones with cracks fractures and joints through which water enters and passes e.g. granite, limestone and chalk.
Ones that don’t allow water to pass through them.
Aquifuge – Impenetrable impermeable rocks e.g. gabbro, shale and slate.
Aquiclude – Porous rocks which absorb water and expand narrowing air spaces between grains preventing water to percolate downwards e.g. clay.
CoverPlants break the speed of raindrops causing drops to hit the ground gently giving rainwater ample time to percolate. On bare surfaces, most of the precipitation flows away as runoff.
Level of Saturation of Ground
Infiltration is more on dry ground because it has wide open-air spaces while and less on ground whose air spaces are saturated with water.
Water that infiltrates and percolates into the ground fills air spaces creating zones of saturation whose upper levels are called water tables.
Zones of Saturation
Zone of Non-saturation
Zone of permeable through which water passes but doesn’t remain in the pores for a long time. Well sunk to this zone doesn’t contain any water.
Zone of Intermittent Saturation
Zone which during the rain season the rocks are saturated with water while during the dry season they are unsaturated.
Temporary water table – Upper level of groundwater in the zone of intermittent saturation.
The zone of non-saturation and that of intermittent saturation are called a vadose zone.
Zone Of Permanent Saturation
Zone where pores spaces are permanently filled with water.
Permanent water table – Upper level of groundwater in the zone of permanent saturation.
The presence of groundwater leads to the formation of springs, wells boreholes, and artesian basins.