- Hydrological transport model
An hydrological transport model is a
mathematical modelused to simulate river or stream flowand calculate water quality parameters. These models generally came into use in the 1960s and 1970s when demand for numerical forecasting of water quality was driven by environmental legislation, and at a similar time widespread access to significant computer power became available. Much of the original model development took place in the United Statesand United Kingdom, but today these models are refined and used worldwide.
There are dozens of different transport models that can be generally grouped by pollutants addressed, complexity of pollutant sources, whether the model is steady state or dynamic, and time period modeled. Another important designation is whether the model is distributed (i.e. capable of predicting multiple points within a river) or lumped. In a basic model, for example, only one pollutant might be addressed from a simple point discharge into the
receiving waters. In the most complex of models, various line sourceinputs from surface runoffmight be added to multiple point sources, treating a variety of chemicals plus sedimentin a dynamic environment including vertical river stratification and interactions of pollutants with in-stream biota. In addition watershed groundwatermay also be included. The model is termed "physically based" if its parameters can be measured in the field.
Often models have separate modules to address individual steps in the simulation process. The most common module is a
subroutinefor calculation of surface runoff, allowing variation in land usetype, topography, soiltype, vegetative cover, precipitation and land management practice (such as the application rate of a fertilizer). The concept of hydrological modeling can be extended to other environments such as the oceans, but most commonly (and in this article) the subject of a river watershed is generally implied.
In 1850, T.J. Mulvany was probably the first investigator to use mathematical modeling in a
stream hydrologycontext, although there was no chemistry involved. [T.J. Mulvany, "On the use of self registering rain and flow gauges", Proc. Instiute Civ. Eng. 4(2) 1-8 (1850)] By 1892 M.E. Imbeau had conceived an event modelto relate runoff to peak rainfall, again still with no chemistry. [M.E. Imbeau, "La Durance: Regime. Crues et inundations", Ann. Ponts Chausses Mem. Doc. Ser. 3(I) 5-18 (1892)] Robert E. Horton’s seminal work [R.E. Horton, "The role of infiltration on the hydrologic cycle", Trans. Am. Geophys. Union 145: 446-460 (1933)] on surface runoffalong with his coupling of quantitative treatment of erosion [R.E. Horton, "Erosional development of streams and their drainage basins: Hydrological approach to quantitative geomorphology", Bull. Geol. Soc. Am. 56:275-330 (1945)] laid the groundwork for modern chemical transport hydrology.
Types of hydrological transport models
Physically-based models (sometimes known as deterministic, comprehensive or process-based models) try to represent the physical processes observed in the real world. Typically, such models contain representations of surface runoff, subsurface flow, evapotranspiration, and channel flow, but they can be far more complicated. The first model to integrate all the required submodels for basin chemical hydrology was the Stanford Watershed Model (SWM). [N.H. Crawford and R.K. Linsley. "Digital simulation in hydrology: Stanford Watershed Model IV", Technical Report No.39
Stanford University, Palo Alto, Ca. (1966)] The SWMM ( Storm Water Management Model), the HSPF (Hydrological Simulation Program - FORTRAN) and other modern American derivatives are successors to the SWM.
In Europe a favoured comprehensive model is the Système Hydrologique Européen (SHE), [M.B, Abbott, J.C. Bathurst, J.A. Cunge, P.E.O’Connell and J. Rasmussen, "An Introduction to the European System: Systeme Hydrologique Europeen (SHE)", Journal of Hydrology 87: 61-77 (1986)] [Vijay P. Singh,, "Computer Models of Watershed Hydrology, Water Resource Publications", pgs. 563-594 (1995)] later named
MIKE SHE, a watershed-scale physically based, spatially distributed model for water flow and sedimenttransport. Flow and transport processes are represented by either finite difference representations of partial differential equations or by derived empirical equations. The following principal submodels are involved:
Evapotranspiration: Penman-Monteith formalism:* Erosion: Detachment equations for raindrop and overland flow:* Overland and Channel Flow: Saint-Venant equations of continuity and momentum:* Overland Flow SedimentTransport: 2D total sediment load conservation equation:* Unsaturated Flow: Richards equation:* Saturated Flow: Darcy's lawand the mass conservation of 2D laminar flow:* Channel Sediment Transport 1D mass conservation equation.
This model can analyze effects of
land useand climate changes upon in-stream water quality, with consideration of groundwaterinteractions.
Worldwide a number of basin models have been developed, among them RORB (
Australia), Xinanjiang ( China), Tank model ( Japan), ARNO ( Italy), TOPMODEL ( Europe), UBC ( Canada) and HBV ( Scandinavia), MohidLand ( Portugal). However, not all these models have a chemistry component. Generally speaking, SWM, SHE and TOPMODEL have the most comprehensive stream chemistry treatment and have evolved to accommodate the latest data sources including remote sensingand geographic information systemdata.
These models based on data are black box systems, using 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, neural networksand system identification. These models are known as stochastic hydrology models. Data based models have been used within hydrology to simulate the rainfall-runoff relationship, represent the impacts of antecedent moistureand perform real-time control on systems.
urface runoff modelling
A key component of a hydrological transport model is the
surface runoffelement, which allows assessment of sediment, fertilizer, pesticideand other chemical contaminants. Building on the work of Horton, the unit hydrograph theory was developed by Dooge in 1959. [J.C.I. Dooge, "Parameterization of hydrologic processes", JSC Study Conference on Land Surface Processes in Atmospheric General Circulation Models, 243-284 (1959)] It required the presence of the National Environmental Policy Actand kindred other national legislation to provide the impetus to integrate water chemistry to hydrology model protocols. In the early 1970s the U.S. Environmental Protection Agency(EPA) began sponsoring a series of water quality models in response to the Clean Water Act. An example of these efforts was developed at the Southeast Water Laboratory, [C.M. Hogan, Leda Patmore, Gary Latshaw, Harry Seidman et al. "Computer modeling of pesticide transport in soil for five instrumented watersheds", U.S. Environmental Protection Agency Southeast Water Laboratory, Athens, Ga. by ESL Inc., Sunnyvale, California (1973)] one of the first attempts to calibrate a surface runoff model with field data for a variety of chemical contaminants.
The attention given to surface runoff contaminant models has not matched the emphasis on pure hydrology models, in spite of their role in the generation of stream loading contaminant data. In the United States the EPA has had difficulty interpreting [Steven Grant, I K Iskandar , "Contaminant Hydrology", CRC Press (2000) ISBN 1-56670-476-6] diverse proprietary contaminant models and has to develop its own models more often than conventional resource agencies, who, focused on
floodforecasting, have had more of a centroid of common basin models.
Liden applied the HBV model to estimate the riverine transport of three different substances,
nitrogen, phosphorusand suspended sediment[Rikard Liden, "Conceptual Runoff Models for Material Transport Estimations", PhD dissertation, Lund University, Lund, Sweden (2000)] in four different countries: Sweden, Estonia, Boliviaand Zimbabwe. The relation between internal hydrological model variables and nutrienttransport was assessed. A model for nitrogensources was developed and analysed in comparison with a statistical method. A model for suspended sediment transport in tropicaland semi-aridregions was developed and tested. It was shown that riverine total nitrogen could be well simulated in the Nordic climate and riverine suspended sediment load could be estimated fairly well in tropical and semi-arid climates. The HBV model for material transport generally estimated material transport loads well. The main conclusion of the study was that the HBV model can be used to predict material transport on the scale of the drainage basinduring stationary conditions, but cannot be easily generalised to areas not specifically calibrated. In a different work, Castanedo et. al applied an evolutionary algorithm to automated watershed model calibration [F. Castanedo, M.A. Patricio, J.M. Molina: "Evolutionary Computation Technique Applied to HSPF Model Calibration of a Spanish Watershed". IDEAL 2006, 216-223. http://dx.doi.org/10.1007/11875581_26 ] .
The United States EPA developed the
DSSAM Modelto analyze water quality impacts from land useand wastewatermanagement decisions in the Truckee Riverbasin, an area which include the cities of Reno and Sparks, Nevadaas well as the Lake Tahoebasin. The model ["Development of a dynamic water quality simulation model for the Truckee River", Earth Metrics Inc., Environmental Protection Agency Technology Series, Washington D.C. (1987)] satisfactorily predicted nutrient, sediment and dissolved oxygen parameters in the river. It is based on a pollutant loading metric called "Total Daily Maximum Load" (TDML). The success of this model contributed to the EPA’s commitment to the use of the underlying TDML protocol in EPA’s national policy for management of many river systems in the United States. [USEPA. 1991. "Guidance for water quality-based decisions: The TMDL process", EPA 440/4-91-001. U.S. Environmental Protection Agency, Office of Water, Washington, DC.]
The DSSAM Model is constructed to allow dynamic decay of most pollutants; for example, total nitrogen and phosphorus are allowed to be consumed by benthic
algae in each time step, and the algal communities are given a separate population dynamic in each river reach (e.g. based upon river temperature). Regarding stormwater runoff in Washoe County, the specific elements within a new xeriscapeordinance were analyzed for efficacy using the model. For the varied agricultural uses in the watershed, the model was run to understand the principal sources of impact, and management practices were developed to reduce in-river pollution. Use of the model has specifically been conducted to analyze survival of two endangered speciesfound in the Truckee Riverand Pyramid Lake: the Cui-ui sucker fishand the Lahontan cutthroat trout.
* [http://www.springerlink.com/(iaxnfy45xo4roa45unxg3lih)/app/home/contribution.asp?referrer=parent&backto=issue,6,14;journal,62,221;linkingpublic HBV model applied to climate change in the Rhine River basin]
* [http://www.eng.newcastle.edu.au/~ncwe/ncweAJWR/Vol4No2Yr2000.htm RORB model applications in Australia] .
* [http://www.ncl.ac.uk/wrgi/wrsrl/projects/nirex/nirex.html Overview of the SHE and SHETRAN models]
* [http://www.wiz.uni-kassel.de/model_db/mdb/topmodel.html TOPMODEL characteristics and parameters]
* [http://www.iwaponline.com/nh/036/nh0360175.htm Xinanjiang model and its application in northern China]
* [http://www.springerlink.com/content/a40255n85rv52242/ Evolutionary Computation Technique Applied to HSPF Model Calibration of a Spanish Watershed]
* [http://www.toolkit.net.au Catchment Modelling Toolkit]
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