GIS-Water Resources Term Project Isaac Larsen Utah State University Geology Department 2002

1. 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 36.51 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?