Bloom Reports from the Anza-Borrego Desert: Rainfall in 2011-2012

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 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 13 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 13 September 2011

End DateFallbrookRanchitaSan FelipeAgua CalienteBorrego SpringsOcotillo Wells
9/13/20110.00   0.240.28
10/5/20110.83   0.000.00
11/4/20110.510.510.240.110.030.03
11/6/20110.390.09 0.040.000.00
11/12/20110.601.290.950.410.390.18
11/20/20110.780.310.080.050.000.05
12/12/20110.990.380.440.560.250.15
1/16/20120.280.000.000.000.000.00
1/21/20120.470.000.000.000.000.00
1/23/20120.480.400.08?0.08?
       
Total All Rain
to 2/13/12
5.332.981.791.170.990.69

As of 11/16/11, the rainfall total for the 2011-2012 year reported at the Visitor Center, measured at the nearby Colorado District Headquarters, was 1.31 inches (data from Mary Jo Churchwell), compared to the Borrego Springs NWS total of 0.66 inches. The extra amount at the Visitor Center either included some monsoonal rainfall before 9/13/11, or represented heavier rainfall at that location on 9/13/11 and/or 11/12/11.


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Updated 13 February 2012