Demo #1 – Creating a Moisture Index

GEOG 491

Fall 2007

For this demonstration, we’ll again use Glacier National Park(GNP) as our study area.

Intro

Moisture availability is one of several key factors determining what type of vegetation grows in a particular natural area, especially in areas where rainfall is not abundant year round. The amount of moisture available in a given area depends on more than just rainfall, especially in highly mountainous areas like GNP.

What landscape factors might affect moisture availability in a highly mountainous area? (Hint: Which way does water flow?)

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In Lab 2, you’re constructing an index of fire risk using a digital elevation model and several other GIS layers.

Today, we’re going to look at another example of an index – a moisture index called the topographic convergence or wetness index (TCI or TWI). This index creates a raster of potential moisture availability based on slope steepness and flow accumulation.

TCI = ln(α/tanβ) ,

where α is upslope flow accumulation and β is slope steepness in degrees. Tanβ is equivalent to slope percent.

We’ll then try to relate this index to the vegetation types in our land cover layer for GNP.

We can create this index raster using our digital elevation model (elev raster) for GNP and the Spatial Analyst tools in ArcMap.

To begin, map drives to both your student work directory and the course data directory (directly to the GNP subfolder, if you like). In ArcMap, open the project file, TCI_demo.mxd, in the Demo1 subfolder of the GNP folder. This project contains the GNP elev and landcover layers.

What layers will you ultimately needin order to calculate TCI? What might be your first step toward calculating TCI?

In ArcToolbox, scan the topics under Spatial Analyst Tools. Do any of them seem relevant to the layers we need for TCI?

Explore the tools under each topic that seems relevant until you find a tool or tools that might be useful.

An in-depth explanation of calculation of flow accumulation in ArcMap is beyond the scope of this course. With some brief explanations, now we’ll proceed through a guided demo of the process of deriving flow accumulation and calculating TCI in ArcMap so that we have time to compare our TCI to landcover.

Demo…

Do you see any general similarities in pattern between the distribution of vegetation types in landcover and the distribution of TCI values?