This must equal the sum of evaporation at sea and evapotranspiration on land. Thus on any given day, water molecules in the atmosphere have been there an average of 11.4 days. Driving Forces Solar Energy causes water to evaporate. Fuels plants to transpire Gravity drives liquid water to flow downhill to the ocean. Above the water table is the unsaturated zone.
Porosity is affected by both the size of particles and how the particles are arranged.
Permeability is somewhat influenced by porosity but not always. Think of Swiss cheese, it has high porosity but very low permeability. Hydraulic conductivity is a property of the porous-fluid system together. It is a measure of the resistance to flow of a certain fluid through a certain porous medium. It is called the coefficient of permeability in Skinner and Porter.