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These definitions have been compiled from various sources, including the USGS's Water Resources Data, Weather Service Operations Manual, Chapter E-90, and Glossary of Meteorology (AMS, 1959).

peak discharge

Rate of discharge of a volume of water passing a given location. Usually expressed in cubic feet per second.

percolation

The vertical movement of water within the soil.

perennial stream

A stream which flows all year round. Compare intermittent stream.

piezometer

An instrument for measuring pressure head in a conduit, tank, soil, etc. It usually consists of a small pipe or tube tapped into the side of the container, the inside end being flush with, and normal to, the water face of the container, connected with a manometer pressure gage, mercury of water column, or other device for indicating pressure head.

piezometric level (or surface)

Confined groundwater is usually under pressure because of the weight of the overburden and the hydrostatic head. If a well penetrates the confining layer, water will rise to this level, the piezometric level, the artesian equivalent of the water table. If the piezometric level is above ground level, the well discharges as a flowing well, artesian well, or a spring.

pool height

The height of the water behind a dam. Various datums may be used and various pool height may be used, e.g., conservation pool, flood control pool, etc.

porosity

The ratio of pore volume to total volume of the soil. Sandy soils have large pores and a higher porosity than clays and other fine-grained soils.

potential evapotranspiration

Water loss which will occur if at no time there is a defiency of water in the soil for use by vegetation and for evaporation. Varies by season.

pressure head Energy contained by fluid because of its pressure, usually expressed in a depth of fluid.