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October 4, 2013

Mojave Desert rainfall: Your guess is as good as mine

Rainfall totals for the flash flood event that hit Fort Irwin, Aug. 25.

Rainfall varied greatly across Fort Irwin landscape during Aug. 25 flash flood event

The Mojave Desert is the driest desert in North America, and is characterized by low and unpredictable rainfall. On Aug. 25, parts of Fort Irwin experienced severe flooding resulting from a monsoonal storm that delivered several inches of rain in a very short duration. However, this rainfall varied greatly in quantity across the Fort Irwin landscape. While some areas of post received no rain at all that day, others experienced almost a year’s worth of precipitation in just a few hours.

When rain falls faster than it can infiltrate into the soil it results in overland flow. Ask a whitewater river rafter and they’ll tell you water wants to be flat, hence the relentless flow from areas of higher to lower elevation. In the desert, this flow downhill accumulates water into channels or washes that spill into the lowest valleys and fill what are typically dry playas. Unfortunately, water simply obeys gravity, so the path downhill sometimes brings it rushing through otherwise dry human habitation zones. After a major rainfall the sheer volume can be astonishing, capable of carrying vehicles, fences, and other property great distances, as was witnessed that August afternoon.

The Directorate of Public Works Environmental Division manages a series of rain gauges and weather stations across Fort Irwin. Data from those gauges on Aug. 25 show a vast disparity in rainfall. Several gauges in the Western Expansion Area deployed to monitor conditions in critical habitat for the federally threatened desert tortoise and federally endangered Lane Mountain milkvetch recorded less than 0.5 inches of rain over the 24-hour period. Just a few kilometers (approximately 2.5 miles) east in the NASA Goldstone conservation area there was more than 1.5 inches of rain, while in garrison the Integrated Training Area Management gauge recorded 2.45 inches of rain in just a few hours. Likewise, a gauge near Silver Lakes Mine in the Eastern Expansion Area recorded and incredible 3.58 inches of rain, but only 15 km (nine miles) to the east near Salt Creek Playa only 0.16 inches fell. By comparison, the long-term (1948-2012) average yearly rainfall for nearby Daggett, Calif., is only 3.8 inches (source: Western Regional Climate Center); so, nearly a year’s worth of rain essentially fell in one event.

The August rainfall event serves as an important reminder that just because rain is not falling in an area does not mean it won’t experience flooding; although Salt Creek Playa at the far eastern boundary of Fort Irwin received less than 0.2 inches of rain on that day, a large portion of the trail to that gauge completely washed out from the 3.58 inches of rain that rushed downhill from the deluge that hit Silver Lakes Mine. Even distant rainfall in the desert should be taken as a sign to avoid desert washes and low-lying areas. Additionally, remember it is never safe to cross moving water after a heavy rain; this muddy water can contain unseen debris, unknowingly undercut or erode road crossings, and may have the power to push vehicles off the road.

For interested parties, archival data from rain gauges and weather station sensors can be obtained by calling Clarence Everly (380-3740) or Dave Housman (380-6235) at the Directorate of Public Works Environmental Division.




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