Wednesday, August 31, 2016

Software Scenario Functions: Watershed modelling

Watershed is a concept in hydrology that refers to the topographical boundary dividing two adjacent catchment basins. A watershed is an area of land that catches rain and snow and drains or seeps into a marsh, stream, river, lake or groundwater. Homes, farms, cottages, forests, small towns, big cities and more can make up watersheds. They come in all shapes and sizes and can vary from millions of acres, to a few acres that drain into a pond.

Modelling is the process of representing a real world object or phenomenon as a set of mathematical equations.

Watershed models study natural processes of flow of chemicals and microorganisms while determining the impact of human activities on these processes. Watershed modelling is an important tool to focus efforts to solve watershed based water resource, environmental, social and economic problems.

A watershed model can be used for:

  • Water resources planning, development, design, operation and management
  • Flooding
  • Droughts
  • Upland erosion
  • Stream bank erosion
  • Coastal erosion
  • Sedimentation
  • Non point source pollution
  • Water pollution from industrial, domestic and agricultural sources
  • Migration of microbes
  • Deterioration of lakes
  • Desertification and degradation of land
  • Irrigation of agricultural lands
  • Conjunctive use of surface and groundwater, etc

Watershed models are classified into

  • Black Box models that mathematically describe the relation between variables. 
           Ex: Unit hydrograph approach, ANN, Rational formula etc.
  • Lumped models that lie between the Black Box models and Distributed models. 
           Ex: Stanford watershed model, etc
  • Distributed models that are based on complex physical theory on the solution of real governing equation.
           Ex: St. Venant equations for watershed modelling, etc
GIS plays an important role in watershed modeling.
The areas in which GIS is applied in watershed modeling are:

  • Hydrologic assessment
  • Model setup
  • Parameter determination and
  • Modeling

Hydrologic assessment involves using GIS for the analysis of various hydrologic factors for the purpose of risk assessment or susceptibility to pollution, flood, drought, erosion, etc.

Model setup involves defining topography, boundaries and drainage networks of a watershed so as to form the basic framework for applying both lumped and distributed watershed models. DEM is the main data structure used for this work.
In the context of hydrologic assessment and model setup, GIS provides several valuable tools for data creation and management, automated feature extraction and watershed delineation.

Data creation is done by collecting elevations using GPS or digital contour maps to generate new DEMs where no data exists for the aera of interest. Sometimes, contour data on paper-based maps can be converted to digital format using GIS digitizing tools.

Automated feature extraction is performed by various GIS software packages that offer automated routines for delineating watershed boundaries and draining divides. GIS software can also be used for extracting surface drainage channel networks and generating other hydrography data from DEMs.   Ex: WMS and Archydro.

The application of watershed models with GIS requires data from a variety of sources in different formats into a common coordinate space for efficient processing or display. Most GIS software provides tools that assist transforming datasets into a common coordinate space.

An important aspect of modeling watershed processes is to determine parameter inputs. The Watershed Modeling System (WMS) is capable of processing both vector and raster data for land use, soil type, rainfall zone and flow path networks to develop important modeling parameters.