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Glossary beginning with H


Hydrologic Model

A hydrologic model is a simplified, conceptual representation of the terrestrial hydrologic cycle. Two major types of hydrologic models can be distinguished: data-based and process-based. Models based strictly on data use mathematical and statistical concepts to link a certain input (for instance rainfall) to the model output (for instance runoff). Commonly used techniques are regression, transfer functions, time series analysis, and artificial neural networks.

Models based on process descriptions try to represent the physical aspects of the hydrologic cycle observed in the real world. Given a description of watershed characteristics (e.g. soil characteristics, vegetation cover, and elevation), input (e.g. precipitation), and initial conditions, process-based models utilize a set of equations representing physical processes to explicitly simulate the various hydrologic components (e.g. soil moisture, snow accumulation, evapotranspiration, and runoff).


The component of the climate system comprising liquid surface and subterranean water, such as oceans, seas, rivers, fresh water lakes, underground water, etc.

[IPCC Fourth Assessment, Working Group I, Appendix I Glossary, p. 947]