COMMENTARY:
In the last days of September, I watched parts of University Avenue flood so badly that every passing car sent a wave of water onto the sidewalk. Umbrellas, a rare sight in our desert city, suddenly seemed to be everywhere. Rain like this is unusual so late in the season, and its toll went far beyond wet sidewalks. The most visible sign was the closure of La Llorona Park, after the Rio Grande swelled to dangerous levels. Farther up the valley, the damage was even greater: Hatch and Rincon were evacuated, and pecan orchards near Rincon sat under water.
How does a region stuck in a 20-year megadrought flood this easily? The answer lies in the ground beneath our feet.
Recent shifts in our weather patterns played a major role. From Sept. 22 to 24, monsoon moisture combined with an upper-level trough to drop 1 to 3 inches of rain on Las Cruces. Before the soil had a chance to dry out, moisture from Hurricane Polo, joined by the remnants of Odalys, merged with a low-pressure system from California and unleashed another round of downpours. National Weather Service meteorologist Brian Guyer warned that because of the earlier storms across New Mexico, "the soils are saturated, and so it's going to be easy to flood".
That warning points to something many people miss. When rain hits the ground, the soil soaks up what it can until it reaches its maximum capacity. Beyond that point, the ground simply sheds what it can't hold, sending water racing across the surface into streets, arroyos and the Rio Grande. So whether a storm becomes a flood depends heavily on how wet or dry the soil is before the rain starts. Hydrologists who studied floods across the U.S. found that extreme rain led to extreme flooding 62% of the time when the ground was already wet, but only 13% of the time when it was dry.
Those numbers might suggest that dry soil protects us from flooding, but in the desert the opposite is often true. Much of Doña Ana County is sandy loam sitting on top of hard caliche and after years of drought, a lot of that ground is bare and crusted, so it sheds water almost like pavement.
Geomorphologists have long shown that when heavy rain hits that kind of surface, the water races into arroyos and carries topsoil with it. Through my research on soil erosion in southern New Mexico's watersheds at NMSU, I have seen how this becomes a vicious cycle: every storm that strips away topsoil and cuts deeper into arroyos leaves the land even less able to absorb the next one.
Our soils, in other words, fail at both extremes: too hard to absorb rain during drought, and too full to take any more after back-to-back storms. Climate change is pushing us further toward both. In its 2021, the Intergovernmental Panel on Climate Change reported that heavy rainfall becomes about 7% more intense for every 1°C of warming, because warmer air can hold more moisture. The 2023 Fifth National Climate Assessment echoes this finding for the Southwest, but adds a troubling twist. Even as the storms that do arrive grow more intense and contribute to more flooding, rising temperatures are drying out our soils and shrinking our rivers and groundwater. We are left with less of the water we can use, and more of the water we can't.
To be fair, climate change did not single-handedly cause September's flooding. The atmosphere dealt us a rough hand: a hurricane's leftover moisture arrived just as a passing low was there to wring it out over ground that was already soaked. Storms like this are not new to our region; remnants of Pacific hurricanes can supply up to 30% of the Southwest's annual rainfall in some places. But climate change is stacking the deck. A warmer atmosphere means these storms carry more water, while our soils, whether baked hard by drought or already saturated, are less able to absorb it..
Is the rain good for farmers? After three dry years the moisture helps, but flooded orchards and lost topsoil are a steep price. Too often, our water managers, city and county planners, and farmers prepare for drought or for floods, but rarely for both at once. Southern New Mexico needs to plan for both, starting with restoring vegetation and soil in our most vulnerable watersheds, so the land can hold water when the rain does come..
Albert Opare Agyei is a graduate research and teaching assistant at the Department of Geography and Environmental at New Mexico State University, where he studies soil erosion and watershed restoration in southern New Mexico.