WHAT DOES GIS BRING TO HYDROLOGIC MODELING?

By Caroline Frey


 

Contents

Hydrologic modeling
Flow transport processes in hydrology
View of motion in hydrology and GIS
Coupling hydrologic modeling with GIS
Limits to effective coupling
Conclusions
  

 

 


Hydrologic modeling


Hydrologic modeling has existed for about 150 years but really developed with computers. The major issues are:
- Water utilization;
- Pollution control and mitigation;
- Flood control and mitigation.

David Maidment defines it as a: "mathematical representation of the flow of water and its components on some part of the land surface or subsurface environment."

There are 4 categories of spatial components used in hydrologic modeling that can usually be simplified from 3D to 2D:

  1. Watersheds
  2. Pipes and stream channels
  3. Aquifers
  4. Lakes and estuaries
The USGS has a web site from which their water resources software can be downloaded.

Flow transport processes in hydrology

All spatial components in hydrology governed by same principles:
  1. Principle of mass conservation:
    Continuity equation: Rate of storage = inflow – outflow 

  2. Momentum principle: (Newton's 2nd law of motion)
    If an unbalanced external force acts upon a body, its motion will change in proportion to the magnitude of the force and in the direction of the external force. 

  3. Energy principle: (contained in 1st law of thermodynamics)
    Change in energy of a body = amount of heat input – amount of work
    (work = force applied by body * distance it moves)
Modeling of 1 and 2 factors is well understood. 3D variation is more difficult and still experimental. GIS can help progressing in 3D modeling because it solidifies the treatment of spatial variation.
 

View of motion in hydrology and GIS

In hydrology, modeling is usually done using the Eularian view of motion, which focusses on a fixed frame through which motion occurs. The mass, momentum and energy equations can all be written for the Eularian view.
Fluid modeling can be done in GIS using the Eularian view of motion, because GIS presents fixed frames in space through which fluid passes, as required by the Eularian view.
 

Coupling hydrologic modeling with GIS

GRASS is a public domain modeling system commonly used. The software and information about its applications can be found at:

http://www.baylor.edu/~grass/

There are 5 levels of hydrologic modeling in association with GIS:

  1. Hydrologic assessment:
    Definition: mapping in GIS of hydrologic factors that relate to some situation, usually in order to assess a risk, for example the groundwater contamination potential. 


    2. Hydrologic parameter determination:
    This is the most active area in GIS related to hydrology. The goal is to determine the parameters needed for hydrologic modeling by analysis of the terrain and land cover features. These parameters are usually extracted from raster GIS systems (channel length, land surface slope, land use, soil characteristics).

    The following schematic explains how data is acquired by GIS: 

    This web site shows how new parameters can be extracted from the different layers of a GIS model.

    For this kind of modeling, terrain needs to be represented by grids or TINs to have automated, simple delineation and description routines.

  2. Hydrologic modeling within GIS:
    Modeling is possible directly within GIS if time is not considered. It is possible to avoid the problem if annual averages are considered. An example of application is the modeling a pollutant plume: 

  3. Time can also be eliminated if you consider an event (peak flow condition), and assume same conditions throughout the system (limited view).

  4. Linking GIS and hydrologic models:
    Also a very active area of research, especially for groundwater flow (surface water flow is more difficult because of the important time variation).

    5. Hydrologic models are linked to GIS in order to compute flow and display results (ex: piezometric head surface or contaminant plumes). 

  5. Object-oriented linkage:
    Several steps are necessary to represent the real world through GIS. Semantic modeling goes beyond tabular modeling. It tries to capture the functional interrelationship of the things located on the map. It is used a lot in hydrology.

Limits to effective coupling

Several differences between GIS and hydrologic modeling limit the effectiveness of their coupling:
  1. GIS is made for processing a vast amount of data, and hydrologic modeling is rather concerned by very precise and detailed analysis of a small area. This difference makes that GIS is often not complete enough to process hydrologic data.
  2. Hydrologic models are time varying, particularly for surface water flow, and GIS has no explicit representation of time in the data structure. This is why GIS is mainly used for data input and output.
  3. GIS and hydrologic modeling don't use the same idea of a relationship:
  4.  In GIS, a relation is a simple association between two sets of data using a key item common to both.
  5. In hydrology, it is a complex mathematical equation.
 Relations seem weak in GIS systems compared to hydrologic models. GIS is not capable of handling the complex physical laws that control hydrologic processes.
 

Conclusions

  1. Accomplishments:
    Linking GIS and hydrologic modeling allowed:
  • models to be more effective and efficient; 
  •  to better represent the surface environment used to build up the input data for hydrologic modeling easier; 
  • the output from hydrologic models to be displayed in a good environment. 

 
 
  1. Future improvements:
    The efficiency and accuracy of GIS and hydrologic modeling coupling could be improved in the following way:
  • Instead of averaging spatial properties,  a hydrologic model should be built, that really accounts for the different geologies and soils based on polygon coverage in GIS. 
  • The use of GIS coupled with models of soil saturation should allow developing more realistic models of stream flow generation than those assuming Hortonian overland flow. 
  • Surface water and groundwater are obviously linked, but in hydrologic modeling, they are rarely linked, which gives an incomplete view. GIS can be used for adding more layers to the model in order to allow the coupling of surface and groundwater. 
 

 
 
 

Questions?  Send me e-mail at:    caroline_frey@hotmail.com


 
 
 

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