Exercise 4. Wetness Index, local saturation deficits, and variable source areas
Due Date: March 31, 2012
Report: ex4_youronyen_report.doc in your personal working directory
Objective
In this exercise, you will learn how to generate estimates of the variable source area of a
watershed based on topographic information and mean soil moisture levels. The work will
include generating maps of the wetness index, the mean value of the wetness index, and the
extent of the variable source area (saturated region as a proportion of total watershed area).
You will also explore the sensitivity of these results to the m parameter, that controls the
distribution of the local saturation deficit, Si, from the mean saturation deficit, 𝑆̅.
You will be working with Coweeta using the 10m DEM data. You will be using Terrain
Analysis System (TAS).
Task. Wetness Index
Wetness index is defined as:
WI = ln(/tan )
Where, WI is wetness index,  is specific catchment area, and  is the local slope. It measures
the effects of topography on the water saturation level of soils at a location. It helps to identify
the time-varying portions of a watershed that can produce surface runoff (saturation overland
flow). The mean wetness index is λ, and the local saturation deficit is computed from
knowledge of the mean saturation deficit as:
𝑆𝑖 = 𝑆̅ + 𝑚(𝜆 − 𝑊𝐼𝑖 )
You will need to compute the wetness index map, and the mean wetness index for Morgan
Creek.
1.
Copy the directory exercise_2012/ex4 into your student space.
2. Open TAS and set the working directory, and display dem_10m.dep. REMEMBER!!!!
Before you display any image you need to choose a color palette! If you don’t, TAS has the
annoying feature of crashing….
3. Preprocess the DEM to remove pits – choose to breach pits.
4. Terrain Analysis, Compound Terrain Attributes provides a menu, including the wetness
index. Use D∞. Note that the lakes may show up as -99 (zero slopes) so you will need to
adjust the minimum in the colorbar.
5.
Using Statistical Analysis and the mask of the dem_10m_br_wat – 1 is the watershed,
background is 0), compute the mean wetness index.
6. For this watershed, previous simulation work suggests an optimal value of the parameter
m is 0.4517 (m-1) . The range of the mean saturation deficit is from 2.5~4 m, mean: 3.19 m.
Using the GIS Analysis, Raster Calculator and a range of values for the mean saturation deficit
(choose at least 5 values between 2.5~4 m create the following maps:
a.
Maps of Si
b. Maps of the saturated areas
c.
Maps of return flow (remember for any value with an Si <0, the return flow, seepage,
is the absolute value).
7.
Compute the variable source area (total saturated area/total catchment area) for each value of
the mean saturation deficit, and then plot the variable source area against mean saturation
deficit. To compute the total area of the watershed, fist extract map of the saturated areas by
watershed boundary, “cwt_mask.” Then go to Raster Calculator and choose “Area” in
Functions and follow the Syntax instruction.
8. Choose another value for m and repeat to test the sensitivity of the saturation area dynamics
with this critical parameter.
9. Turn in maps of wetness index, Si, saturated areas, return flow for both values of m and the
graph of the variable source area against the mean saturation deficit.
10. Compare the saturation dynamics in terms of the expansion of the saturated area as the
saturation deficit drops, and between the two values of m.