Pressure

Pressure variations in fluids are reviewed, with a particular emphasis on water. Variations under both hydrostatic and flowing conditions are described, and impacts of density stratification on pressure distribution are discussed. Calculations of forces due to pressure acting on submerged objects are derived, both for static and moving fluids, and the relationship between buoyancy and pressure is shown. Additionally, the assumption of hydrostatic pressure variation in the vertical direction is shown to be generally suitable for development of models of water motion in many natural systems, specifically in lakes where water velocities are generally slow. This assumption is related to the condition of barotropic flow, in which the gradients of pressure and density are closely aligned. A discussion of baroclinic flows, where this condition is not the case, is beyond the scope of this chapter. Several examples of the impact of pressure and buoyancy on biological processes are presented.