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Life depends on water. Our entire way of life is based on accessibility to water resources and water abundance. In order to be able to harvest and produce food, people have for thousands of years established societies and cultures around water resources. Hydrology is the study of water, with focus on movement, distribution and quality of water. Hydrology is divided into several domains, where each domain has its own environmental identity, including hydrometeorology, surface hydrology, hydrogeology, management of drainage basins and assessment of water quality.

To understand the movement of water in our environment, a fundamental step is to acknowledge how water circulates in the world in which we live. To establish the origin of the rainfall and the source of the lakes and rivers that never stop flowing down the hills, we have to realize that water circulates both in the atmosphere and below Earth's surface. The circulation is named the hydrological cycle and is sketched in the figure below. In its simplest form, this is described as the movement of water, as it evaporates from the surface of both sea and land to the atmosphere. Water vapor is transported in the atmosphere until it condenses (clouds) and, subsequently, may dissolve over land in the form of precipitation. The precipitation that falls on the surface area is partly collected into streams and rivers by surface runoff that eventually runs back into the sea. The remaining part of the rainfall, however, is infiltrated into the soil.

The infiltrated water is stored in the subsurface, also called vadose zone. The area below land surface is roughly divided into the subsurface and the groundwater. You distinguish between the two parts by means of the watertable, which is an elevation of saturation for the water stored below surface. From the soil in the vadose zone the water flows as a subsurface flow into streams and lakes, or by gravitational forces percolates further into the ground and recharges the groundwater. The groundwater flow diverts the water towards lakes, streams and the ocean. The precipitated water can also be intercepted by vegetation and, subsequently, return to the atmosphere by the so-called transpiration; a process similar to evaporation, but assigned to loss of water vapor from plants, flowers, etc.

Hydro Cycle

Hydrological Models

Hydrological models are a simplified descriptions of parts of the hydrological cycle. The purpose is to gain such information about the hydrological process that it can be used for predictions for states of the hydrological system. The objective in hydrological modeling is essentially to determine a description for the flow as it passes from the input to the output, i.e. to obtain an acceptable input-output representation for the hydrological system. The methods of flow routing depend on knowledge about storage capacities in the hydrological system and, in general, either deterministic models or stochastic models are used to evaluate the storage.

The deterministic models (or the white box models) are very detailed descriptions of the hydrological system, and are usually based on physical knowledge of the system dynamics, only. The system parameters are obtained from hydrological surveys related to the system's characteristics. Thus, they are predefined in the model structure without any uncertainty. The deterministic models tend to become fairly complicated, because the model accuracy is optimized by improving the system with inclusion of as many processes and sub-processes as possible in order to minimize the output error. In contrast, stochastic hydrological models refer to statistical models (or black box models), i.e. the model structure is obtained by correlating the available input and output data series for the hydrological system in question. Processes in hydrology that are categorized as stochastic models are usually models with the primary goal of making short-term predictions, e.g. rainfall-runoff and flood forecasting.

To accomplish better predictions and, consequently, improved water resources management systems, the mainstream in modern hydrological modelling is focused on gaining a more general understanding of the behavior of the hydrological systems.

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