Hydrological cycle See water cycle

hydrology The study of the water cycle: the properties, distribution, and circulation of water in the atmosphere, biosphere, lithosphere, and hydrosphere, and how this changes with time. It encompasses geology, meteorology, and oceanography.

Hydrology and Water Resources Program (HWRP) A program of the World Meteorological Association that aims to collect and analyze hydrological data as a basis for assessing and managing freshwater resources, e.g. for human consumption, sanitation, irrigation, hydroelectric power generation, and water transport, and for flood forecasting and the prediction of droughts.

hydrophilic Describing a molecule or surface that has an affinity for water. Such molecules are usually polar, for example proteins. Compare hydrophobic.

hydrophily pollination in which water carries the pollen from anther to stigma.

hydrophobic Describing a molecule or surface that has no affinity for water. Such molecules are nonpolar, e.g. benzene. Compare hydrophilic.

hydrophyte A plant that grows in water or in extremely wet areas, for example arrowhead (Sagittaria) and water lilies. Hydrophytes show certain adaptations to such habitats, notably development of aerenchyma, reduction of cuticle, root system, and mechanical and vascular tissues, and divided leaves. Large intercellular air spaces in leaves, stems, and roots allow oxygen to diffuse through the plant. Hydrophytes with floating leaves can exchange gases with the atmosphere; many submerged hydrophytes, such as spiked water milfoil (Myriophyllum spicata), have no stomata, absorbing water and gases over their entire surface. Compare meso-phyte; xerophyte. See also Raunkiaer's lifeform classification.

hydroponics (water culture) The growth of plants in liquid culture solutions rather than soil. The solutions contain the correct balance of all the essential mineral requirements. The method is used commercially, especially for glasshouse crops, and also in experimental work in determining the effects of mineral deficiencies.

hydropower See hydroelectric power.

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