1. GIS-Water Resources Term Project
Using NEXRAD and ArcGIS to quantify precipitation from debris flow producing storms in the canyons of the eastern Uinta Mountains, Colorado and Utah
GIS-Water Resources Term Project
Isaac Larsen
Utah State University
Geology Department
2002

2. Talk Outline Introduction Project Goals Study Site Methods Results
Discussion
                          

3. Introduction
What are debris flows?

4. Introduction (continued)
Why Study Debris Flows?
Important Sediment Source
Create areas of fine-grained sediment storage
Are responsible for most of the vertical drop in the longitudinal profile of the Green River

5. Project Goals 4 storms produced 13 debris flows between 1997 and 2001
Precipitation from these storms is largely unknown
Use NEXRAD to characterize precipitation
Possibly determine intensity/duration thresholds
Compare NEXRAD rainfall estimates with RAWS
gage data as an accuracy test

6. Study Area

7. Methods 1) Delineated debris flow contributing areas using TauDEM
2)Prepared NEXRAD grid
Downloaded data from Colorado Basin River Forecasting Center
Used make_raindat_v5 (Ross Woods, NIWA, New Zealand) to extract data and create a grid of NEXRAD cells

8. Methods (continued) 3) Viewing and Analyzing NEXRAD data
Create a file for each storm that has cumulative precipitation at each NEXRAD cell
Use spatial analyst to spline the data to interpolate between points
Determined minimum and maximum rainfall totals for each contributing area
Determined duration and rainfall intensity each storm
Determined maximum hourly rainfall total for NEXRAD cell closest to each initiation site

9. Methods (continued)

10. Methods (continued)
4) Comparison of NEXRAD and RAWS rain gage precipitation estimates
Compared NEXRAD and rain gage total precipitation estimates for each storm
Compared precipitation vs. time plots with rain gage data and two nearest NEXRAD cells.

11. Results 1) NEXRAD Rainfall Totals and Intensities
Storm-total rainfall ranges from 0 to 39 mm for each contributing area
Rainfall intensity ranges between 0 and 0.6 mm/hour

12. Results (continued)
2)Maximum hourly precipitation at nearest NEXRAD cell
Values range between 0.39 mm and mm

13. Results (continued)
…but NEXRAD estimates do not compare well with rain gage estimates
a) In all cases NEXRAD precipitation estimates are lower than rain gage estimates
b) The difference in values ranges between 25 and 100%

14. Results (continued)
Analysis of precipitation vs. time plots show that NEXRAD only detects precipitation some of the time that gages do

15. Discussion Precipitation estimates can be made from NEXRAD data
But they do not seem to be very reliable
Others have reported that NEXRAD underestimates precipitation two-thirds of the time (Mizzell, 1999)
Able to characterize storms better than without NEXRAD and see overall precipitation patterns

16. Questions?