What does “driving force” mean?
All water movement responds to some driving force that causes water to move in some particular direction. Gravity pulls water downward through the soil profile. The adhesion 1 of water to soil particles surfaces and cohesion 1 of water (the forces holding a liquid together) draws water from the filled pores of moist soil into the empty pores of dry soil. This can cause water to move against the pull of gravity. Water vapor diffuses from outward from soil pores or the interior of leaf tissue, where humidity is always 100%, into the atmosphere above the soil surface if the humidity in the air is lower than 100%.
