Climate has a great influence on soil moisture conditions. The engineering behaviour of soils is governed in large measure by their moisture contents. Most clay subsoils exist in an approximate state of moisture equilibrium that may be seriously disrupted by construction, drainage or irrigation. Lawn watering in urban areas can have results equivalent to a substantial increase in rainfall - by amounts estimated to be in excess of 25 per cent for at least one Canadian city. This practice often results in a marked increase in subsoil moisture and volume and brings about new problems for shallow foundations.
Vegetation also plays an important role in affecting soil moisture content.
It may be visualized as a very efficient pump capable of extracting great volumes of water from relatively impermeable soils. Competition of shallow and deep-rooted plants in the same soil profile tends to accelerate soil moisture depletion. The physical dimensions, or volume, of coarse-grained soils, such as sand, are governed solely by loading stresses. The weight of overburden and foundation loads are the principal external forces acting on a subsoil. The internal stresses arise from physico-chemical forces which increase as the soil dries.