Digitizing Watersheds

Digital Elevation Models DEM DEMs with 100m and 30m resolution Hand Digitizing Watersheds Digital Elevation Model D8 model: FLOWDIRECTION command Eight Direction Pour Point Model Aspect Direction of Steepest Descent Flow Direction Grid Grid Network Flow Accumulation Grid FLOWACCUMULATION Contributing Area > 10 Cell Threshold Stream Network for 5 Cell Threshold Drainage Area Display Flow Accumulation NOT like the default Rather like this DEM creation usually results in artificial pits in the landscape
A pit is a set of one or more cells which has no downstream cells around it
Unless these pits are filled they become sinks and isolate portions of the watershed
Pit filling is the first thing done with a DEM Filling in the Pits FILL Display: Filled DEM minus original DEM Threshold Value Selection One contributing area threshold value does not fit all watersheds.

When does channelisation begin?

Where does concentrated flow occur? Stream Order All upstream tributaries are assigned an order of 1, (first order).

When two first order links intersect, the downslope link is assigned an order of 2.

When two second order links intersect, the downslope link is assigned an order of 3, and so on.

This is the most common method. Command: STREAMORDER STRAHLER (1952): STRAHLER: SHREVE: All links with no tributaries are assigned a magnitude (order) of 1.

Magnitudes are additive downslope.

When two links intersect, their magnitudes are added and assigned to the downslope link. Command: STREAMORDER SHREVE (1967): First create a raster with a threshold cell value
STR5000 = CON(FLOWACC > 4999,1)
Then use StreamLink command to create a raster with individual stream numbers.
Then use that raster as pour point raster to delineate watersheds for each individual stream. Delineating a Watershed You can define a watershed from a flowdirection grid by using a grid with one or more pour points (locations of lowest watershed grid cell) The alternative BASIN command does this automatically by using pour points along the edges of the DEM. *!Disadvantage!* Another Alternative is to: