Rainy season has arrived in Portal, AZ. For those who are unfamiliar with the area, the majority of the year’s precipitation in the Chihuahuan Desert comes in July-August, also referred to as monsoon season. Instead of the steady, prolonged rains that the word monsoon usually calls to mind, the Arizona version consists of relatively short but intense storms interspersed with periods of blue sky. These storms also tend to be highly localized: you may watch the rain coming at you down the valley all morning, only to have it skirt around you to the west and miss you completely. See http://www.wrh.noaa.gov/twc/monsoon/monsoon_NA.php for more information on the North American Monsoon.
So far this year the Portal experimental site seems to be sustaining more hits than misses, and it’s shaping up to be a fairly “good” rainy season. To see how the current season is measuring up to recent years, I’ve plotted total monthly precipitation through time in the figure below, with monsoon seasons highlighted as grey bands. July 2014 was the 5th rainiest month in the past 14 years, exceeded only by the rainy seasons of 2004, 2006, 2008, and 2011. And we still have August to look forward to!
So what will this mean for our study? One may predict that a pulse of precipitation in a moisture-poor ecosystem should cause an increase in plant growth. Indeed, we have found evidence that the rainy season is correlated to a peak in vegetation, and that more precipitation generally means higher peaks. The following plot shows this (approximately) linear relationship between summer precipitation and summer vegetation. The astute observer may notice that 2002 and 2007 managed to reach high levels of plant activity with low levels of precipitation. I would first like to note that the site’s weather station was not operational during 2002 and so precipitation during this time was estimated using data from a nearby weather station which, due to the patchiness of storms mentioned above, may not be a valid estimation. For 2007 I have no such excuse; either there are anomalies in the vegetation dataset I’m using (which is satellite-derived Normalized Difference Vegetation Index), or the rain-plant relationship is somewhat more complicated than I’ve assumed. For the present season, if it keeps raining at the same rate we’ve seen so far, we will get about 180mm in total, putting my prediction of vegetation index at 0.42-0.43. We’ll see how this pans out in another month or two.
The response of the rodent community to rainy season is much more difficult to observe than the plant response. The plant activity inspired by rainy season provides a surplus of food to the rodents, which encourages reproduction and should result in an increase in rodent abundance. However, current research suggests that short-term weather events have little to do with rodent population dynamics, that models must incorporate weather patterns from the past year or more to detect subsequent changes in rodent abundance. For example, numbers are not likely to increase after a single “good” summer monsoon, but they may if the site experiences a wet winter followed by a wet summer or two wet summers in a row.
I want to conclude by thanking local contractor Bob Walton for the beautiful new roof on our ramada. Just in time to keep our heads dry during this past sampling weekend!