Modeling catchment hydrology
- Related Topics:
- hydrology
- oceanography
- hydrometeorology
- glaciology
- paleohydrology
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.
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.
Study of lakes
Limnology is concerned with both natural and man-made lakes, their physical characteristics, ecology, chemical characteristics, internal energy fluxes, and exchanges with the environment. It often includes the ecology and biogeochemistry of flowing freshwaters. The study of former lakes is known as paleolimnology. It involves inferring the history of a former lake basin on the basis of the evidence contained in the sediments of the lake bed.
Lakes may be formed as a result of tectonic activity, glacial activity, volcanism, and by solution of the underlying rock. Man-made lakes or reservoirs may result from the building of a dam within a natural catchment area or as a complete artificial impoundment. In the former case the reservoir may be filled by natural flow from upstream; in the latter the supply of water must be piped or pumped from a surface or subsurface source. The use of reservoir water for water supply, river regulation, or hydroelectric power generation may cause rapid changes in water levels that would not normally occur in a natural lake. In addition, water is usually drawn from a reservoir at some depth, resulting in a shorter residence time relative to an equivalent natural lake.
The history of lakes
A newly formed lake generally contains few nutrients and can sustain only a small amount of biomass. It is described as oligotrophic. Natural processes will supply nutrients to a lake in solution in river water and rainwater, in the fallout of dust from the atmosphere, and in association with the sediments washed into the lake. The lake will gradually become eutrophic, with relatively poor water quality and high biological production. Infilling by sediments means that the lake will gradually become shallower and eventually disappear. Natural rates of eutrophication are normally relatively slow. Human activities, however, can greatly accelerate the process by the addition of excessive nutrients in wastewater and the residues of agricultural fertilizers. The result may be excessive biomass production, as evidenced by phytoplankton “blooms” and rapid growth of macrophytes such as Eichhornia.
The physical characteristics of lakes
The most important physical characteristic of the majority of lakes is their pattern of temperatures, in particular the changes of temperature with depth. The vertical profile of temperature may be measured using arrays of temperature probes deployed either from a boat or from a stationary platform. Remote-sensing techniques are being used increasingly to observe patterns of temperature in space and, in particular, to identify the thermal plumes associated with thermal pollution.
In summer the water of many lakes becomes stratified into a warmer upper layer, called the epilimnion, and a cooler lower layer, called the hypolimnion. The stratification plays a major role in the movement of nutrients and dissolved oxygen and has an important control effect on lake ecology. Between the layers there usually exists a zone of very rapid temperature change known as the thermocline. When the lake begins to cool at the end of summer, the cooler surface water tends to sink because it has greater density. Eventually this results in an overturn of the stratification and a mixing of the layers. Temperature change with depth is generally much smaller in winter. Some lakes, called dimictic lakes, can also exhibit a spring overturn following the melting of ice cover, since water has a maximum density at 4 °C.
A second important characteristic of lakes is the way that the availability of light changes with depth. Light decreases exponentially (as described by Beer’s law) depending on the turbidity of the water. At the compensation depth the light available for photosynthetic production is just matched by the energy lost in respiration. Above this depth is the euphotic zone, but below it in the aphotic zone phytoplankton—the lowest level in the ecological system of a lake—cannot survive unless the organisms are capable of vertical migration.
Patterns of sediment deposition in lakes depend on the rates of supply in inflowing waters and on subsurface currents and topography. Repetitive sounding of the lake bed may be used to investigate patterns of sedimentation. Remote sensing of the turbidity of the surface waters also has been used to infer rates of sedimentation, as in the artificial Lake Nasser in Egypt. In some parts of the world where erosion rates are high, the operational life of reservoirs may be reduced dramatically by infilling with sediment.
Water and energy fluxes in lakes
The water balance of a lake may be evaluated by considering an extended form of the catchment water balance equation outlined above with additional terms for any natural or artificial inflows. An energy balance equation may be defined in a similar way, including terms for the exchange of long-wave and shortwave radiation with the Sun and atmosphere and for the transport of sensible and latent heat associated with convection and evaporation. Heat also is gained and lost with any inflows and discharges from the lake. The energy balance equation controls the thermal regime of the lake and consequently has an important effect on the ecology of the lake.
An important role in controlling the distribution of temperature in a lake is played by currents due to either the action of the wind blowing across the surface of the lake or the effect of the inflows and outflows, especially where, for example, a lake receives the cooling water from a power-generation plant. In large lakes, Earth’s rotation has an important effect on the flow of water within the lake. The action of the wind can also result in the formation of waves and, when surface water is blown toward a shore, in an accumulation of water that causes a rise in water level called wind setup. In Lake Erie in North America, increases in water level of more than one metre have been observed following severe storms. After a storm the water raised in this way causes a seiche (an oscillatory wave of long period) to travel across the lake and back. Seiches are distinctive features of such long, narrow lakes as Switzerland’s Lake Zürich, when the wind blows along the axis of the lake. Internal seiche waves can occur in stratified lakes with layers of different density.
