Hydrology
Chalk deposits are very porous, so the height of the water table in chalk hills rises in winter and falls in summer. This leads to characteristic chalk downland features such as dry valleys or coombes, and seasonally-flowing streams or winterbournes. The modern practice of extracting water from this 'reservoir', in order to satisfy demand for water, may be putting some of these streams under extreme stress.
In the valleys below the downs there is typically a clay soil, and at the interface between the two a springline can occur where water emerges from the porous chalk. Along this line, settlements and farms were often built, as on the higher land no water was available. This is demonstrated very clearly beneath the scarp of the White Horse Hills, above the Vale of White Horse. In many chalk downland areas there is no surface water at all other than artificially created dewponds.
Read more about this topic: Downland