FLOOD05944

• County: Saguache
• Basin: Rio Grande
• Title: Radar - Rainfall Reconstruction of the Saguache Creek Flash Flood of July 25, 1999
• Date: 6/1/2001
• Prepared For: Saguache County
• Floodplain - Doc Type: Flood Documentation Report

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6.0 Comparison of Paleo-Hydrologic Derived Storm
and Radar Rainfalls

Bob Jarrett of the USGS performed a preliminary estimate of the run-off
derived rainfall based on a field survey of the flood damage fields in the
floodplains. Figure 12 shows the Jarrett isohyetal pattern developed by
"backing into" the rainfall by estimating the runoff for the Saguache flash flood.
Jarrett used time honored run-off estimation techniques based on observed high-
water marks, flood debris and floodplain scarring caused by the runoff.

After gathering this evidence, he calculated the area and volumes affected
by the flood, noted high water marks and estimated runoff. From the runoff
values he estimated the precipitation necessary to produce the evidence he
collected and from multiple locations he creates the isohyetal pattern observed.
This simple description does not do justice to the techniques and painstaking
labor of science and love he applies.

Figure 13 shows the Jarrett rainfall pattern overlain by the radar-derived rainfall
pattern in one-inch increments. Note the following similarities and differences in
the two patterns:

. First, the radar-derived pattern covers roughly twice the area of the runoff
derived precipitation pattern. However, the radar-derived 4-inch isohyet
covers roughly the same area as the entire run-off derived pattern.

. The peak runoff derived rainfall is over 6 inches while the radar-derived
value is just over 5 inches. Strangely, the run-off derived precipitation
area covered by 5 inches or more of rain is roughly twice the size of the
radar derived 5-inch area and anchored further east in the region of
tightest elevation gradient.

. Note that both patterns show a roughly east-west axis of the heaviest
rainfall and the southeastward extending lobes of heavier rainfall.

The possible reasons for these discrepancies will not be discussed in this paper
but it is heartening to see the many similarities in the two patterns. Additional
patterns will be compared and pOSSibly assist in providing a key to using the
runoff derived technique effectively in paleo-hydrologic studies for eras in which
no radar data exists.

7.0 Conclusions

A flash flood occurred over the higher terrain roughly 5-10 miles northwest of
the small town of Saguache, Colorado during the afternoon of July 25, 1999.
The meteorological causes of the flash flood were related to the passage of a
monsoon weather disturbance identified on satellite and associated with a very
moist area of mid-level air from 10,000 to 20,000 feet.

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