The availability of high-speed computers has resulted in a widespread use of computer models in the analysis and prediction of hydrologic variables for research as well as for practical design and management purposes. These models vary greatly in type and complexity, from simple computer implementations of methods previously based on manual calculations to attempts to solve the nonlinear partial differential equations describing surface and subsurface flow processes that require much computation. All have their practical limitations.

The simpler models treat the catchment as a single “lumped” (or undifferentiated) unit. It is clearly not possible to describe hydrologic processes in detail in such a model, and most processes are represented as empirical functional relationships between inputs and outputs. Some lumped models do not refer to the internal hydrologic processes of the catchment at all but use systems analysis techniques to relate inputs to outputs. The parameters of such computer models are calibrated by fitting the model to simulate a known discharge record. It is consequently very difficult to interpret parameters derived in this manner in a physically meaningful way or to extend the use of the model to sites where there are no discharge records. Parameter values for ungauged sites can sometimes be estimated from empirical relationships between catchment characteristics and parameter values derived from fitting a model at a number of gauged sites. The uncertainties in such a procedure, however, are high.

The more-complex computer models attempt to analyze the internal processes of the hydrologic system in greater detail, taking into account the spatial nature of the catchment, its topography, soils, vegetation, and geology. These are “distributed” models, usually formulated in terms of flow equations for each hydrologic process considered to be important. Some processes such as channel flows and groundwater flows can be described in a reasonably satisfactory way. In the case of other processes, as, for example, flow through the soil and evapotranspiration, hydrologists cannot be so sure of their descriptions. Distributed models tend to have many parameters. In principle, many of these parameters can be measured in the field or can be estimated from the physical characteristics of the catchment. In practice, such models have proved difficult to apply and have not been shown to provide more accurate results than simpler models in spite of their theoretical rigour.

Most models for hydrologic forecasting in practical use today are deterministic; that is to say, given a sequence of inputs to the model, the outputs are uniquely determined. In a probabilistic description of catchment hydrology, the effects of uncertainty in the model inputs, parameters, or descriptive equations must be reflected in a degree of uncertainty in the outputs. Such a model is known as a stochastic model.

Water quality

Natural water is a dilute solution of elements dissolved from Earth’s crust or washed from the atmosphere. Its ionic concentration varies from less than 100 milligrams per litre in snow, rain, hail, and some mountain lakes and streams to as high as 400,000 milligrams per litre in the saline lakes of internal drainage systems or old groundwaters associated with marine sediments.

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Earth sciences: Hydrologic and atmospheric sciences

Water quality is influenced by natural factors and by human activities, both of which are the subject of much hydrologic study. The natural quality of water varies from place to place with climate and geology, with stream discharge, and with the season of the year. After precipitation reaches the ground, water percolates through organic material such as roots and leaf litter, dissolves minerals from the soil and rock through which it flows, and reacts with living things from microscopic organisms to humans. Water quality also is modified by temperature, soil bacteria, evaporation, and other environmental factors.

Pollution is the degradation of water quality by human activities. Pollution of surface and subsurface waters arises from many causes, but it is having increasingly serious effects on hydrologic systems. In some areas the precipitation inputs to the system are already highly polluted, primarily by acids resulting from the combustion of fossil fuels in power generation and automobiles.

Other serious causes of pollution have been the dumping of industrial wastes and the discharge of untreated sewage into watercourses. Salt spread on roads in winter has resulted in the contamination of subsurface drinking water supplies in certain areas, as, for example, in Long Island, New York. Excess water resulting from deforestation or irrigation return flows that leach salts from soils in semiarid areas are major sources of pollution in the western United States and Western Australia.