Bloom Reports from the Anza-Borrego Desert: Rainfall in 2012-2013

Rainfall is the most important determinant of blooms. Summer monsoonal rainfall germinates summer annuals and stimulates some perennial plants to bloom in the late summer and fall. Fall / winter Pacific storm rainfall germinates the late winter / spring annuals, and causes the usual late winter / spring bloom in the desert. See Annual Germination, Growth and Blooms for more information.

Summer monsoonal rainfall is from thunderstorms, is usually extremely spotty in coverage, and does not occur every year in sufficient amounts to stimulate blooms.

Fall / winter rainfall is usually highest on the mountain slopes, especially on the west edge of the Borrego Desert, and falls off dramatically with lower elevation to the east. This occurs whenever our rainfall is mostly orographic, and the storm winds are from the west. However, when rainfall is from convection, or if the storm winds are from the east, the deserts can at times get more rainfall than the coast. (See Precipitation types.)

In addition to desert stations, we've also given the rainfall from Tom's house in Fallbrook, on the coastal side at 680 feet elevation, to show the large difference in rainfall between the wet side of the mountains and the dry side.

Table 1 gives the storm totals, in inches, as of the last day of each storm. The storm totals were taken from the Weather Service Rainfall Storm Summary, except for Fallbrook, which is the amount recorded at my house. Occasionally other stations are missing in that report; if so, totals are taken from the Rainfall Summary Map.

If a station didn't appear in the summary, or we couldn't find it elsewhere on line, we usually assumed the rainfall total was zero. Although this assumption is probably usually correct, it is not necessarily always valid since missing data plague all rain reports. In a few cases, when it was clear that some rainfall must have been received at those missing stations, we've guesstimated the rainfall.

Note that the total rainfall at the bottom of the table is since 1 September, since rain that falls earlier doesn't germinate the desert annuals (see General Requirements for Annual Germination). This rainfall total may be different from the rainfall reported by the Anza-Borrego Desert State Park Visitor Center using the normal California rainfall year that begins on 1 July. Also, the total rainfall sometimes contains rainfall in Fallbrook not reported in the table if there was no major rainfall in the desert.

Table 1. Major Rainfall Events Since 1 September 2012

End DateFallbrookRanchitaSan FelipeAgua CalienteBorrego SpringsOcotillo Wells
Total All Rain5.893.022.811.191.930.86

The rainfall total for Borrego Springs was erroneously reported as 1.27" in the NWS Preliminary Rainfall Storm Summary for 15 December 2012. Kate's rain gauge, located very close to that NWS rain gauge, only recorded the 0.32" that is in line with the other rainfall gauges in the desert.

The total for Borrego Springs wasn't even reported in the 26 January 2013 summary since the gauge is obviously malfunctioning, reporting 1.52" from that rainstorm compared to 0.70" in Borrego Palm Canyon and 0.39" in Ocotillo Wells. Kate measured 0.70" at her Borrego Springs house.

On 2/21/13, the NWS rain gauges this time under-reported the rain in Borrego Springs significantly. Kate's gauge recorded 0.52", whereas the NWS gauges recorded only 0.12".

Borrego Springs received two significant monsoonal rainfall events, 0.45" on 21-22 August 2012, and 0.69" on 30 August 2012. Other areas in the desert received rain on different days. For example, a 15 foot wall of water swept through Fish Creek on 31 July 2012 from a very large rainfall event there, carrying a hiker's car 1.75 miles downstream from the Wind Caves Trailhead area. We visited Fish Creek on 10 October 2012 and saw clear evidence of flash flooding 10-15 feet above the wash bottom all the way from the open area at the Wind Caves Trailhead to about 1.7 miles downstream. We were very surprised that the water could be that high in the large open area above the narrows of Fish Creek at Split Mountain.

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Updated 22 February 2013