"Rain is grace; rain is the sky descending to the earth; without rain, there would be no life." — John Updike 

“The desert smells like rain” — Gary Paul Nabhan

Rain! Desert rain is fickle at best. Summer rains come up from the south and form patches of rain here and dry as a bone there, just a few hundred meters away.

Sometimes you can see the fingerprint of those patchy rains as you drive down the road or highway; “green” healthy-looking creosote extending for maybe a few kilometers and then, almost like a line in the sand, the green creosote give way to olive-brown thirsty plants. Winter rains come from the north and reach us, or not, due to the whim (strength) and pattern of the high elevation jet streams. A winter storm of average strength or less will have nearly all its moisture wrung from it as the water-laden clouds push, or try to push, over the cold air of the mountain ridges. Cold air cannot hold on to water as well as warm air does, and so rain falls onto the windward slopes of the mountains leaving precious little or none for desert dwellers on the leeward side of the mountains.

If it were any different it would not be a desert. Deserts are generally defined as any landscape that, on average, receives 25 cm (10 inches) or less per year. Portions of the arctic, because of that cold air, receive less than 25 cm, and so are technically deserts. Perhaps a better definition of a desert should include evaporation rates, giving us a better idea of just how much water we can use. The average rainfall in the eastern Coachella Valley is about 8 cm (3 inches), whereas further west, near Palm Springs it is closer to 13 cm (about 5 inches). However, the average annual evaporation rate is just about 190 cm (75 inches – or over six feet of water!). Unless desert life can hold on to whatever precious water it can find, it will be no more alive than a saltine cracker. Yet life here abounds. The question is how, and the answers are many.

We get about 70% of our rain in the winter, with more than 50% falling in just three months, December, January, and February. Less than 10% of our rain falls between April and June, leaving the chance of any additional rain evenly distributed between July and November. Further east in the heart of the Sonoran Desert, the rain is divided evenly between winter in summer. Still further east and south in the Chihuahua Desert almost all their rain falls in the summer. All are deserts, but winter rain, falling during cooler temperatures, has a better chance of being absorbed into the ground and then being accessible to plants’ roots. In the Chihuahua and Sonoran Deserts, the summer monsoons increase humidity and so temporarily reduce evaporation, giving their rain a chance to enter the food web as well.

It is raining while I am writing this, but so far, the rain total for December and January has been just 2 mm (less than 1/10^th of an inch) in the east Coachella Valley and 7 mm (close to 1/3 inch) further west. Imagine a sand verbena (Abronia gracilis) seed. Unless it grows and reproduces it is as good as dead, but should it take a chance that enough rain will be forthcoming and germinate now, or wait for more rain, or wait for a better chance next year? Many thousands of years of trial and error has honed the “decision process” for seeds of annual plants’ to maximize their chances of success. Those that do not choose wisely wither in the sun, do not flower, do not go to seed, and their genetic line hits a literal dead end.  Those that are too conservative, waiting years until optimal conditions give them a “sure thing," also take the risk of rodents or insects finding and eating those seeds before they have geminated, and so also could hit a genetic dead end. The go-no go (germinate or wait) criteria varies among species and varies among individuals within species. Every year the rain patterns are different and every year the annual wildflowers are a different mix of species, or sometime there are none. Even in good years, most seeds probably do not germinate, ensuring that the species will survive even if individuals do not. Desert annual seeds can remain viable in the soil for decades.

Back in 1993 there was a similar lack of rain going into February. We had resigned ourselves to yet another year of little or no wildflowers. Then in late February the jet stream suddenly shifted and brought multiple storms and a week or more of heavy rain to the desert. By early March, the desert floor was green and then there was one of the most spectacular wildflower displays anyone could remember. We called it the “miracle March." Every dry year since then I have held on to the hope that another miracle would be forthcoming. Maybe this year. 

One of the advantages of “late blooms” has been that the non-native and invasive Sahara mustard (Brassica tournefortii) does not consider those late rains a chance worth taking. The miracle March was free of any non-native invasive species. Only natives thought it was worth the risk. In 2005 it was one of the wettest winters in recent memory, but the rains started early in mid-November. Perfect for the mustard. With the heavy rains all the annual wildflower species thought it was an easy choice and began germinating. They germinated beneath the already growing mustard, and never saw the sun, their pollinators could never reach their flowers below the dense mustard canopy. An entire cohort of germinating native wildflowers failed. Every germinating individual was a genetic dead end, even though every clue told them this was going to be the year of years, when they could replenish their seed banks and ensure their species’ survival for decades to come. Except it wasn’t. Their evolutionary history had not prepared them for Sahara mustard. Fortunately, not all the native wildflower seeds germinated that year, and while individuals failed in their evolutionary imperative to reproduce, enough seeds held back allowing species to survive. Nature always hedges her bets. 

As climate change intensifies, we know it will be warmer. The effects on rainfall are much less clear. It will probably mean more severe droughts; the warmer temperatures will increase evaporation rates even more, leaving less water to quench the thirst of plants (and their seeds) and so less water moving through the food webs. When it does rain it will likely change the timing and intensity of those rains. This is one of the biggest unknowns in our ability to predict climate change’s impact on biodiversity (including our species). Life adapted to living in deserts has evolved with heat and droughts, but if the conditions exceed what that life has evolved with, will they be able to continue to cope and survive? Our team of community scientists are out every year measuring how plants and lizards are responding to these changing conditions. So far, the signal has been that in the drier east valley many (but not all) perennial plants are not sustaining populations at lower elevations but are doing ok at cooler-wetter higher elevations. In the wetter west valley, those same shrubs are doing well all the way down to the bottom of the mountain slopes. Perennial plants do not have the option to weather harsh conditions as seeds, they must endure what nature deals them all year round. As such they make good indicators of how climate change will take a toll on biodiversity and where. So far, creosote bush does not seem to care about climate change.

Water means life, and in desert that axiom is particularly stark. Go out and celebrate the rain, however little or much, and always hold out hope for miracles.

Go outside, tip your hat to a lizard (and a cactus), and be safe.