An integrated hydro-topographic model for Viti Levu Gau

Conway Pene

2012 Pacific GIS&RS Conference27-30 November 2012, Suva

Hydro-topo integration

• Water flows downhill... • Hydro – surface water, stream network• ... plus ...• Topo – surface shape, slope and aspect• ... equals ...• A catchment model

Test site

• Gau island, central Fiji

The big picture

1. Create a Digital Elevation Model (DEM)2. Process the DEM to find breaklines– Streams and ridges

3. Create hydro network– Logical network – junctions, streams, catchments

4. Integrated model for hydro flow analysis

Step 1 - DEM using terrain-specific tools

• Traditional terrain data– Contours, spot heights,

cliffs– PLUS…

• Hydrological data– Streams, lakes, sinkholes– Hydrology easy to follow

on air photos, thus higher accuracy

– hydrology ‘enforce’ the terrain shape i.e. breakline

Need to do some cleaning first...

Completed DEM

Step 2 – Process DEM

• DEM• Flow direction from each cell to next (slope)• Flow accumulation• Stream segments (accumulation threshold)• Catchment boundary

Flow directions

Flow accumulation

Stream segment

Catchment boundary

Step 3 –Hydro network

• Convert raster from DEM processing– Stream segment to vector line network– Catchment boundary to vector polygons– Points created at stream junctions

Stream segments

Catchment polygons

Network junctions

Flow tracing using the model

Runoff calculation

• Predict volume of water at end of catchment– Amount of rainfall– Size, shape, slope of catchment– Surface characteristics (vegetation, soil)

Amount of rainfall

• WorldClim climate raster

Catchment characteristic

• Curve number – hydrological parameter– 0 – 100% of rainfall runoff

• Function of land cover and soil type• Land cover – vegetation layer• Soil type – made up (no soils layer for this

island)

Vegetation + soils

Curve number

Curve number x rainfall = runoff

Clip runoff to a catchment

What next?...

• Apply to larger island (Viti Levu)– Heavy data load for desktop computer

• Calculate realistic curve numbers• Make dynamic runoff query tool– Hover over map and pop-up runoff value