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Hydrology

1. The science encompassing the behavior of water as it occurs in the atmosphere, on the surface of the ground, and underground. 2. The science that relates to the water of the earth. 3. The science treating of the waters of the earth, their occurrence, distribution, and movements. 4. In practice the study of the water of the oceans and the atmosphere is considered part of the sciences of oceanography and meteorology.

Hydroponics

The growing of plants without soil by using an inert medium such as sand, peat, or vermiculite and adding a nutrient solution containing all the essential elements needed by the plant for its normal growth and development. Water culture, when plant roots are suspended in a liquid medium containing the nutrient solution while their crowns are supported in a thin layer of inert medium, is true hydroponics. Often called soilless culture, it also includes aeroponics where plant roots are suspended in a dark chamber and sprayed with the nutrient solution.

Hydropower

Electrical energy produced by falling or flowing water. See hydroelectric power plant.

Hydrosphere

The part of the Earth composed of water including clouds, oceans, seas, ice caps, glaciers, lakes, rivers, underground water supplies, and atmospheric water vapor.

Hyetograph

Graphical representation of rainfall intensity against time.

Hypolimnion

See Thermal stratification

Hypothetical Bias

Difference in actual willingness to pay and willingness to pay revealed in a survey arising from the fact that in actual markets purchasers suffer real costs, while in surveys they do not.

Hypoxia

A low oxygen condition in the water that may occur where a nutrient-laden free-flowing body of water (like a river) enters a lake or ocean. The high nutrient content promotes rapid growth of plankton/phytoplankton that subsequently die and, in the process, consume large amounts of oxygen (see biochemical oxygen demand). While fish and shrimp can migrate away from a hypoxic area, less mobile bottom-dwelling organisms are unable to escape. A Mississippi River/Gulf of Mexico Watershed Nutrient Talk Force under the leadership of the Environmental Protection Agency (comprised of representatives from the scientific, economic, ecological and agricultural communities) is investigating the recurring and increasingly large hypoxia problem at the mouth of the Mississippi River. Currently, scientists believe nitrogen making its way into tributaries that flow into the Mississippi River, and eventually the Gulf of Mexico, causes the hypoxia condition.

Hypoxie zone

An area in the Gulf of Mexico off the mouth of the Mississippi River covering about 6,000 square miles where there is not enough oxygen to support fish and shellfish populations. The oxygen depletion is caused by an excessive amount of nutrients that are brought together from throughout the Mississippi River watershed. Many of these nutrients are believed to originate from agricultural activities, and the largest portion, over 30%, has been traced to the upper Mississippi drainage, according to research prepared by the U.S. Geological Survey.

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