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Potential natural water loss
The water loss during years when the annual precipitation greatly exceeds the average water loss. It represents the approximate upper limit to water loss under the type and density of vegetation native to a basin, actual conditions of moisture supply, and other basin characteristics, whereas potential evapotranspiration represents the hypothetical condition of no deficiency of water in the soil at any time for use of the type and density of vegetation that would develop.
Potential Pareto Improvement Criterion
The policy objective that gainers from a policy change (or project) could compensate the losers from the change and still be better off. In particular note that a policy that passes this criterion does not need to include the compensation, the compensation merely has to be possible.
Potential rate of evaporation
See Evaporativity.
Practical Sustained Yield
The rate at which groundwater can be continuously withdrawn without lowering water levels to critical stages, exceeding recharge, or causing undesirable changes in water quality (Walton, W.C., 1970, Groundwater Resource Evaluation). If sustainability is defined as “the ability to meet the needs of the present generation without compromising the ability of future generations to meet their needs,” value judgments may be required to define “meeting the needs” (Wood, W., 2001, Water Sustainability - Science or Management?, Ground Water, 39(5): 641). Similar subjective judgments may be involved in determining “unacceptable consequences.” Similar to Safe Yield,and Optimum Yield, the ISWS will no longer use this term. Instead, the ISWS elects to use the term Yield and explicity state all conditions and assumptions that stand behind the determination of it.
Prairie potholes
A type of wetland that is at the center of a shallow depression characteristic of glaciated areas in the Upper Midwest (North Dakota especially). Many potholes are wet during only a portion of the year, usually early spring. They provide important nesting habitat for migratory waterfowl, and were designated as a national priority area by the Secretary of Agriculture under the Conservation Reserve Program.
Precession
The tendency of the Earth's axis to wobble in space over a period of 23,000 years. The Earth's precession is one of the factors that results in the planet receiving different amounts of solar energy over extended periods of time.
Precipitation
Water vapor in the atmosphere that condenses, falls to, and reaches the earth in various forms (e.g., rain, snow, hail, sleet, etc.).
Precision farming
Farmers use global positioning (GPS) technology involving satellites and sensors on the ground and intensive information management tools to understand variations in resource conditions within fields. They use this information to more precisely apply fertilizers and other inputs and to more accurately predict crop yields.
Prescribed burning
Deliberate setting and careful control of surface fires in forests to help prevent more destructive fires and to kill off unwanted plants that compete with commercial species for plant nutrients; may also be used on grasslands.
Present Value
Value today of a sum to be paid or collected in the future to buy a good or service.

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