LTHIA and Online Watershed Delineation- Tale of a DEM consumer

Larry Theller ,Bernie Engel, and Tong ZhaiPurdue UniversityAgricultural and Biological Engineering Dept.theller@purdue.edu

March 13, 2007Indiana GIS conference Workshops

Presentation Overview

L-THIA (Long-Term Hydrologic Impact Assessment) modelWWW watershed delineationBehind the scenes for Region 5Issues encountered

L-THIABased on the rainfall – land cover – runoff analysis

method already used in many communities (TR55)

Input: Land Use + Soils Information

Process: Daily Runoff and Pollutant Loading Calculations (30 years of local rainfall)

Output: Average Annual Runoff and NPS loads for each specific land use pattern

Watershed delineation

DEM comes into play to determine the contributing areaUse “watershed” to categorize the landuse and soilEach landuse-soil combination has a specific impact on runoff chemistry Calculate NPS effects based on curve numbers

Turn on aerial

photos.

+

1. Click this button.

2. Click along a stream segment.

288 acre watershed

Selected characteristics available as tables for viewing or map layers.

Includes tools to download raster layers of land use, hydrologic soil, curve number, and shapefile of watershed boundary.

Program reformats characteristics of the watershed as needed for several models

Click “Online Digitizing” button to start edit

session.

Edit landuse category or apply a BMP to change curve number method.

Watershed Characteristics are prepared as inputs to several models, using original and edited as separate scenarios.

Results include tables and graphics of runoff volume and characteristics.

Behind the scenes

Math is too slow without prepared layersFlow direction, flowpath precalculated.Large rasters (landuse, soils) are tiled to speed access8 –digit watersheds chosen as tile size

Need DEM, Soil, Landuse, Water features, Highways,

Calculate Flow Direction, Accumulation, Flowpath

Flowpath as vector, for visualizing catchment area.

Region 5 EPA provided funds to extend LTHIA to whole region

For each 8-digit watershed an Arc Info script clips landuse, soils, DEM then burns-in streams, runs delineation to get flow direction, flow accumulation, flowpath, and does miscellaneous reformat and cleanup.

Issues with DEMs and Delineations

“Noise” in data, particularly IndianaWater in features vs DEM low spots Tiger 95 / Census 2000 water versus NHD“Water” in landuse vs in STATSGO Soil

Original USGS NED DEM

Original USGS NED DEM

Original USGS NED DEM and Hillshade

Major area of unreported “Glacial Grooves?”

Original USGS NED DEM and Hillshade

Add quadrangles and note exact fit.

Original USGS NED DEM

Note sharp edge along quad lines.

2003 Orthophotos and Hillshade

Flowpath of water will follow the “grooves.”

Without funding to repair the USGS NED DEM, our approach was to warn users that in some areas the watershed calculated from the DEM was not reliable.

Indiana Geological Survey revised NED DEM

Indiana Geological Survey revised NED DEM

“Noise” is gone!

Burning Issues…

DEMS can be so detailed that they cause problems.

In flat areas it is critical to “burn in” physical path to get correct logical flow

Area 1

Our Calculated Flow (black arrows) runs backwards along Calumet SAG channel.This impacts all the way to Area 1 in Illinois

We discovered that the IGS DEM has road embankments in it which are not being fully cut when the streams are burned-in. One acting like a dam on the flat SAG channel backs water up into Indiana from Illinois.

This area shows the problem, as lakes east of the Interstate flow wrong way, to the north.

The Interstate clearly forms a “dam” (dotted lines) in the DEM.Original model flow depicted.

Flow from the east side is blocked by the interstate and forced to go north to the lake. Aerials clearly show flow passing under the bridge going to west.

Stream layer was edited to burn a channel into the east lakes and the bridge was “pitted” to also cut the embankment. Flow shown is BEFORE repairs, and goes east.

After adding the hydraulic connection under the Interstate the lakes all drain to the west. Final flow depicted.

Thanks

Questions?