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Hydrologi på 1-2-3

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Figur 1: The water cycle (Source: USGS )

Hydrology and hydrological models

Quite often people ask me about my field of study in parties or activity groups out of geo department. When I answer: “hydrology”, there is a large chance that a big question mark shows on their face with a “not sure” nod. Sometimes people just reply directly: “what is that?” I just say “water resources”, and then everybody is happy. Of course hydrology does not simply equal water resources. What is hydrology? There are two explanations:

Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability —— from Wikipedia.

Hydrology is broadly defined as the geoscience that describes and predicts the occurrence, circulation, and distribution of the water of the earth and its atmosphere —— from Dingman 2002,

In hydrology we study the movement of water on earth. This movement is known as the hydrological cycle (Figure 1). It is also named the water cycle and is the circulation of water travels from the atmosphere to the land (surface, subsurface, groundwater), to the sea and back to the atmosphere again. During the trip, the water goes through all the phases of liquid (rainfall), solid (snow, ice) and gas (evaporation) (Figure 1). We can imagine this cycle more vividly if we “follow a drop of water from where it falls on the land, to the stream, and all the way to the ocean—— R.M. Hirsch, USGS.”

In reality we can’t follow a single water drop. It is extremely complex to understand all the details of the hydrologic cycle. Therefore we need a tool or abstraction to help us understand the aspects that control the water cycle. That’s how the hydrological model was born.

Hydrological model is based on the river watershed (Figure2). The hydrological model is an approximation of the real processes in a watershed hydrologic system, i.e. evaporation and streamflow, operating in a catchment.

Figure 2: the watershed as a hydrologic system (from Chow et al, 1988)

The models are primarily used for understand those processes, hydrological prediction and facility design. For example, how high should we build a dam? It will be a huge waste of money if it is too high or it will be useless if it is too low and not contain a big flood. In recent decades, we have increasingly used hydrological models as the main assessment tool for global/regional water resources. Many large-scale hydrological models have been developed to estimate present and future water resources at large scales for the purposes of, for example, climate change impact and water resources assessment.

Next time, I go to party, maybe someone will read this article and I won’t see confused face again.










Dingman, S.L., 2002. Physical Hydrology. Long Grove, IL: Waveland.
Chow, V.T., et al., 1988. Applied Hydrology. McGraw-Hill Series in Water Resources.

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