Desert Notes: Desert Dry

Flow Canyon in fall. Photo by Robin Patten

Robin Patten

Somewhere north of Bluff, a small side canyon cuts into the cliffs, seeps and springs feeding a small stream that flows down from the canyon’s head. When I first arrived in this region, that shady narrow ravine provided solace, its moisture and coolness a balm to the unaccustomed dry heat of this place. I dubbed it Flow Canyon, and it continues to be a favorite site, where I go to sit for a time on a particular rock, listening to the sweet sound of water in the desert. If I’m in the area and a week passes without time at that sit spot, I feel I’ve missed something, for the rhythm of Flow Canyon’s stream has been a teacher in the ways of water.

Flow Canyon seems to run year-round, water pattering down from its steep shrubby slopes even in the driest of times, dripping over ledges, pooling as it gets to the flatter reaches. The moisture sustains a thicket of diverse plants, including single leaf ash, oak, sumac, and three thriving cottonwood trees. As the seasons turn, I’ve watched the small stream lengthen as the days shorten, then shorten as the days lengthen, in between icing up in dramatic patterns during the winter chill.

My first autumn in Bluff, as summer slipped away, I watched with curiosity as the stream’s length gradually increased, for it was a dryer year than normal, without precipitation to replenish the sources that fed the spring. As the days shortened and temperatures softened, Flow Canyon’s stream stretched out, its water working its way down the once-dry bed, covering stones, submerging green plants, turning sand dark and clay into mud. Dry day followed dry day, yet still more water flowed. When temperatures fell below freezing after dark, I speculated perhaps water froze at night, to melt in a rush that pushed the stream further down canyon – not such a plausible idea.

I found the answer to the stream’s autumnal expansion in the thick Flora of the Four Corners Region,[1] in a table of temperature, precipitation and potential evapotranspiration (PET), the last a major factor in the desert climate. PET is how much water would evaporate and transpire away if there was an unlimited supply of water – which is not the case in the desert. Because desert water is limited, actual evapotranspiration is less than PET.

Of course the desert is dry; that is its defining characteristic, and often we focus on the lack of rain when thinking of the desert environment. Yet minimal precipitation is only one element of DRY.  Evapotranspiration is another driving force in desert aridity – the unseen miniscule bits of moisture that move upward, slipping away back into the sky, counteracting any water that falls from above. Part of that lost moisture is simply evaporation off bare soil and canopy-intercepted precipitation, the rain or snow that lands on plants and evaporates away. The other part of the equation is transpiration from plants, the water that flows through a plant from root to leaf, then moves out of leaves into the air. Significantly, plants transpire a vast majority of the water they pull out of the soil. Evapotranspiration increases as temperatures go up, humidity goes down, and winds rise, so water moving into the atmosphere is greater on hot, dry, windy days – a bit like the weather can be in Bluff.

The critical fact for the desert is that most of the year, evapotranspiration exceeds precipitation. Heil’s table shows that in 2004, over 76 years of measurement, Bluff averaged 19.6 cm of annual precipitation, and 75.2 cm of PET, meaning more water potentially escaped into the sky than fell to the earth during all those years, mostly during the hot summer months. During times of high evapotranspiration, desert streams hardly stand a chance, the amount of water leaving the system more than any input, with water evaporating off the stream itself, even as plants actively tap the groundwater to use some and send more into the atmosphere.

Streams like the one in Flow Canyon are labeled “losing reaches”, water simply disappearing at varying distances along their run. As winter gave way to spring, I watched this disappearing act, the stream I sit by losing its length, pulling back into the high reaches of the canyon, until there was only a small rivulet reaching my sitting rock. By end of summer, only a pattering of drops sings the song of Flow Canyon’s water. That is the impact of evapotranspiration mixed with minimal precipitation, driving trickle to flow and back to trickle, following the cycle of the season.

Of course, precipitation is critical for streamflow as well. Last spring, a few nights of showers sent Flow Canyon’s water to the end of its short drainage, a lovely run of moisture.

But as the southwest enters another year of drought, I begin to question how much longer Flow Canyon will run. Will there be a day when the stream does not flow and the cottonwoods wither and years from now another wanderer will enter that canyon and wonder at the old stumps, imagining what it was like when there was enough water for the trees to thrive?

Yet for now, I can sit with the stream and its song, and for just a moment set aside thoughts of drought and global crises. And sometimes I imagine all that moisture lifting upward, dissipating into the air – and think perhaps that unseen water will come back to Bluff, riding air currents eastward, lifting against the slopes of the San Juan Mountains, where the cooling temperatures condense the moisture and create rainfall. There, the water descends back to the earth, feeding the San Juan River, returning to Bluff in its liquid form as it flows past town.

[1] Spence, J. (2013). Climate. In K. D. Heil, S. O’Kane, L. M. Reeves, & A. Clifford (Eds.), Flora of the four corners region : vascular plants of the San Juan River drainage, Arizona, Colorado, New Mexico, and Utah (pp. 1098 p.). Missouri Botanical Garden.