How to stratify watersheds?

For the regional-scale studies we need to identify the upper / middle / lower regions of each watershed.

There are many ways to stratify watersheds into sub-regions. For this stage, we are interested in a simple method that does not rely on many variables. For example, using land cover as an input to define upper / mid / lower watersheds would mean that we could not later examine the relationship between forest cover and watershed region.

Later in the analysis, we could develop a more sophisticated watershed classification/typology that would be a project output, this is a totally different product.

Maps prepared by the

Spatial Information and Analysis Group, DECRG, World Bank

September 2001Data sources, too numerous to mention…..sorry!

Terrain in Central America

0 100 200 300 400

Kilometres

Watersheds in Central America

These example maps are from the GTOPO30 1km elevation, and HYDRO1k watershed data sets.

The final product will use higher resolution data

1) Elevation by regionStratify the entire region into 3 elevation classes,

Simple method One variable used to define the regions.Produces contiguous areas, but implies that some watersheds will not have all three regions within them.

Upper

Middle

Lower

2) Elevation by watershedStratify each watershed into 3 regions based on the elevation range within each watershed

Simple method One variable used to define the regions.Now each watershed has three regions, but leads to non contiguous areas.

Upper

Middle

Lower

3) Terrain Typology (slope/elevation) by regionBased on the Meybeck et al. paper, simplify the 15 terrain types (plains, plateaus, hills, mountains, etc.) into 3 classes

.

Simple method Based on elevation and slope, Produces contiguous areas, but implies that some watersheds will not have all three regions within them.

Upper

Middle

Lower

4) Water Flow Characteristics by regionCalculate flow-length from the output point (river mouth) of each watershed back to the source. Classify into 3 regions based on flow-length..

Not so easy method Based on elevation and derived surface flow characteristics, Produces contiguous areas, some watersheds will not have all three regions within them. Larger watersheds have larger flow lengths.

Upper

Middle

Lower

5) Water Flow Characteristics by watershedCalculate flow-length from the output point (river mouth) of each watershed back to the source. Classify into 3 regions based on flow-length range within each watershed..

Not so easy method Based on elevation and derived surface flow characteristics, Each watershed now has 3 regions but produces non-contiguous areas

Upper

Middle

Lower

How to stratify watersheds?

Each method has pros and cons

Do we want/need the watershed stratification to be contiguous?

Or can each watershed be stratified individually?

Variables to define the stratification could include

elevation / slope / flowlength / roughness / … / others?

Which method is best for us?

Answers on a postcard to anelson@worldbank.org