Following Drought and Wildfires, Monsoon Rains Flood the Southwest

Copious amounts of monsoon moisture have been streaming into the Southwest U.S. over the past two weeks, increasing flood threat on a land primed for enhanced rainfall runoff by two precursors — drought and wildfires. The sudden regime change from dry to wet may bring flash floods to parched land that has been cracking under a sizzling sun.

Published on:

June 30, 2022

Dr. Hal Needham stands in front of Lake Mead at the Hoover Dam where water levels are visibly low. This is shown by a water ring around the base of the lake. He wears a black scarf and jacket with a navy-blue ski cap to keep warm.

Mega-Drought Expands and Intensifies in the West

Lake Mead’s bathtub-like ring, Hoover Dam, Dr. Hal Needham — A selfie pic with Lake Mead’s bathtub-like ring in the background. Photo taken at the Hoover Dam, January 19, 2021. Photo: Dr. Hal Needham.

The drought in the Southwest has become a multi-decadal phenomenon, classified by some scientists as a megadrought. Williams et al. (2020) conclude that the drought in this region is the worst in more than five centuries, as they found the 19-year period 2000 to 2018 the driest since the late 1500s in this region.

Elephant Butte Reservoir, New Mexico, fell to just 7% capacity when this photo was taken on January 23, 2021. Photo: Dr. Hal Needham.

Last summer, we watched as the water level of Lake Mead, the largest reservoir in the U.S., fell to its lowest level on record. The water level by late July fell to 1067.65 feet, the lowest level since April 1937, when the lake was still filling up behind Hoover Dam.

I visited the Lake Mead for the first time last year and was astonished by the pronounced bathtub-like ring marking former water levels on the reservoir walls. Those rings on Lake Mead’s reservoir walls continue to grow, as the Lake has fallen another 23 feet since the record was set last July, dropping to 1044.06 feet by June 21, 2022.

When I visited Elephant Butte Reservoir, New Mexico, on the same trip, I was surprised to find a mostly dry lakebed in many locations, as the reservoir had fallen to just 7% capacity. That reservoir was more spread out; although I did not see rings on the reservoir wall where I visited, I did see vast stretches of dry sediment where water obviously once flowed.

The drought continues to grow spatially-as of June 16, 2022, every western state except Washington contained at least some land classified as “Extreme Drought” by the U.S. Drought Monitor. From another perspective, on the same date, 44% of land in the West was classified as “Extreme Drought”, or worse.

The U.S. Drought Monitor classifies 44% of land in the West as “Extreme Drought” (medium red) or worse, as of June 16, 2022.

‍Drought Leads to Fires

With the backdrop of this strengthening drought, the 2021 wildfire season produced an exceptional number of blazes, particularly in the western states. Through June 22, 2021, 29,149 fires were reported to the National Interagency Fire Center, the greatest number of fires in that same period for statistics going back to 2012.

Among high-profile fires from the 2021 season, the Telegraph Fire, south of Superior, Arizona, stands out as one of the fires that inflicted substantial impacts. The fire cut transportation routes, closed several airports and burned 52 buildings. This was also one of the largest wildfires on record in Arizona, burning more than 180,000 acres.

The 2022 fire season has already outpaced 2021, as more than 31,000 fires have been reported through June 22, the first time that the 30,000-fire threshold was surpassed by this date. These fires have burned more than 3.2 million acres, the first time that 3 million acres was surpassed in 10 years of statistics.

This season’s Calf Canyon - Hermits Peak Fire made headlines when it became the largest fire on record in the state of New Mexico. The fire has burned more than 341,000 acres by June 21, and is visible from space in the NASA satellite image below.

Calf Canyon – Hermits Peak Fire in New Mexico is visible in this satellite image. — A smoke plume from the Calf Canyon – Hermits Peak Fire in New Mexico is visible in this satellite image posted to a NASA website on May 10, 2022.

Monsoon Rains Falling on Fire Scars Produce Enhanced Flooding

The National Weather Service designates the North American Monsoon season as the period running from June 15^th through September 30^th in the Southwestern U.S. During this period, winds generally shift from easterly to southerly, drawing in moisture from the Pacific Ocean, Gulf of California, and Gulf of Mexico. The change in wind direction increases atmospheric moisture and leads to a season of enhanced rainfall that can account for much of the average annual rainfall for the region.

The steep topography of this region increases flood risk for several reasons. Mountainsides enhance rainfall totals, as rising air traveling up mountain slopes increases total precipitation in a process known as orographic rainfall enhancement. The heavy downpours run off quicker from steep slopes than they would on flat land, enhancing flood potential even more in mountainous terrain.

Fortunately, many mountainsides in the Southwest U.S. are forested. The trees, shrubs, and grasses serve to slow the runoff of heavy rainfall as the vegetation breaks the fall of rain drops, redirecting these drops down tree branches and leaves, and prolonging the time it takes for rain drops to reach the soil. This spreads out the duration of rainwater entering the ground, effectively slowing the flood process. This slowing effect is most pronounced in thicker forests with dense canopies.

Dense forest cover in the San Francisco Mountains of Northern Arizona, north of Flagstaff. The wooded mountainside reduces rainfall runoff and erosion during summer monsoon season. Photo: Dr. Hal Needham.

Vegetation also absorbs rainwater through roots, as grasses, shrubs, and trees soak up water in the soil. These roots help hold the soil in place when heavy rain falls. This explains why vegetated mountainsides generally observe less severe erosion than barren slopes.

Without the presence of thick vegetation, like where a raging wildfire leaves a burn scar, heavy rain falls directly on soil, quickly saturating the ground and causing enhanced surface runoff. Also, the force of each raindrop is greater when falling directly on the ground, enabling it to erode soil more efficiently.

For these reasons, fire-scarred landscapes enhance heavy rainfall runoff. Locations downhill and downstream from such fire scars, which often resemble barren moonscapes, often observe rapid water level rises soon after heavy rain falls on the burned area.

Wildfires often leave behind a charred landscape that many people say looks more like a “moonscape”. With an absence of vegetation, such landscapes observe enhanced rainfall runoff, particularly in mountainous regions. Photo: Christine, Texas, fire zone in March, 2022, by Dr. Hal Needham.

According to Fernando Moreu, Assistant Professor in the Department of Civil, Construction and Environmental Engineering at the University of New Mexico, burn scars can not only increase the amount of runoff down a mountainside, but make the runoff pattern less predictable. He clarified, “We use models and our history to predict flooding, but burned areas make this less predictable. We know what flooded before but relying on the past does not work after a large fire, because the runoff patterns have changed.”

A particular challenge for predicting this year’s monsoon flooding in such areas is the short window of time the environment had to respond between when the fires struck, in April and May, and the onset of monsoon season, in June. Moreau reflected, “This year’s monsoon is coming just one to two months after large fires in some locations. We still have many questions about how the fire scars will impact runoff. For example, how much water will a burned area retain? We need field verification to better understand this.”

This makes sense because every fire is unique, and burn patterns are often erratic. A generalized model cannot adjust runoff predictions without accurate inputs of many variables, like vegetation and soil type, slope angle and aspect (direction of slope). Even if a model accurately inputs those data, assumptions must be made about water retention and runoff rates, until scientists can get in the field and collect data during storms.

Map of the Calf Canyon and Hermits Peak Fires show irregular burn patterns on the landscape. Such patterns create increased uncertainty for flood runoff models.

Darde De Roulhac, Chief Engineer with Gila County Flood Control District, added observations he witnessed during monsoon floods in 2021, coming off the burn scar from the Telegraph, Arizona, fire. He shared, “The landscape of the Telegraph fire was nearly completely burned, looking like a moonscape. When monsoon rains arrived, raindrops landing directly on this barren surface were able to loosen up the soil, enhancing erosion and increasing the total volume of fluid (from water and sediment) flowing downstream.” He also referenced the fact that more gullies can form near these burned areas, and commented, “Once you erode more gullies upland it will decrease the travel time for water to runoff and increase the flows.”

This setup explains the enhanced flooding observed in central Arizona during the 2021 monsoon season, which inundated towns like Globe and Miami. According to the National Weather Service, between 1.5 and 3.0 inches of rain fell in two hours south of Globe, near the Telegraph Burn Scar. This storm generated substantial flooding in nearby canyons and washes, extending to Globe. More than 50 homes were damaged in the region, according to the NWS report, referencing a statistic from Gila County Emergency Management.

Flooding at US Route 60 in Globe, Arizona, during the 2021 Monsoon Season. Photo: Twitter account @ArizonaDOT.

A Land of Two Extremes

When we visit coastal cities like New York, Miami and New Orleans, the threat of flooding in the built environment is evident, as these cities sit on the edge of ever-present saltwater. Even inland cities, like St Louis and Cincinnati, are situated on large rivers, making the concept of riverine flooding plausible. We see the cityscape in the reflection of flowing water, enabling our mind to imagine enhanced flood risk when those waters rise.

In the Southwest U.S., however, the landscape looks very different. Numerous mountain ranges separate this region from the nearest sea, months may pass without a drop of rain, and the rivers that do exist often dry up for much of the year. These rivers leave dry riverbeds, often strewn with large boulders, evidence of violent flooding in the past. These channels on the landscape are often called arroyos, in other places they are called washes. They are usually dry but following heavy rain they can be swollen with a raging torrent. Scientists refer to such water features as ephemeral, meaning they only contain water after a heavy rain.

The Hassayampa riverbed was completely dry when this photo was taken in January 2021. A nearby sign told of a fatal flood that killed between 30 and 70 people in the year 1890. Photo: Dr. Hal Needham.

I came across such an extreme landscape one evening last year, when I stumbled across the town of Wickenburg, Arizona. Several bridges in this town cross a dry riverbed, where banners for the Hassayampa River Walk promote a vibrant urban space to residents and visitors. The bed of the Hassayampa River was bone dry when I visited, and I was there in January, when temperatures, and sun angles, were among the lowest of the year.

However, a historic sign near the river told another story. It recounted how the Walnut Grove Dam upstream from Wickenburg failed in 1890, killing between 30 and 70 people, and destroying crops in the region. It also mentioned other hardships that the town has endured. Some of them were weather-related, like drought, while others related to mining booms and busts, and “Indian Wars”.

A sign promotes the Hassayampa River Walk in Wickenburg, Arizona, during January 2021. Photo: Dr. Hal Needham.

If the landscape of the Southwest tells the story of extreme weather, painted by long dry stretches interrupted with severe floods, it also tells the story of unpredictable flood patterns, which are best seen from the air. These patterns develop from the unique geography of the region. Rainfall is generally maximized in the mountains, where runoff scours out deep canyons. Where these mountains end and an open landscape begins, the flood water spreads out over the valley floor, in a geographic feature known as an alluvial fan.

Flood patterns associated with alluvial fans can be difficult to predict, because of the flood footprint is inconsistent from one storm to the next. These flood patterns, “Can take a different path each time,” according to J.D. Padilla, Executive Director of the New Mexico Floodplain Managers Association. This makes sense when we consider that canyons and pronounced riverbanks confine flood water to more predictable flow patterns-when the flood water loses these guides and is free to spread out on the desert floor, the pattern will be less predictable.

Mitigation Efforts to Reduce Flood Impacts in the Southwest

Short- and long-term mitigation efforts reduce flood impacts in the region. Short-term strategies are often used in response to rapidly changing conditions related to increased flood threat on a scale of days to weeks. Long-term efforts include infrastructure improvements, buyouts/relocation of homes, and funding of collaborative projects that involve diverse stakeholders dialoguing and taking action over a timescale of years.

In light of the increasing flood threat following the Telegraph Fire in 2021, Gila County Emergency Management, the Natural Resources Conservation Service, and a consulting firm named J.E. Fuller, coordinated efforts to remove debris that could obstruct the flow of water in key river channels. Such efforts were rushed to completion before the monsoon season set in, requiring mapping and spatial analysis to identify target areas, then coordinating field crews to remove the debris.

Flood debris associated with burn scars. Photo: Phoebe Suina.

As the 2022 monsoon season begins, communities in this region still face elevated flood risk. The threat of increased flooding downstream and downhill from the Telegraph Burn Scar will remain elevated for several more years, until substantial regrowth has taken place, according to Darde de Roulhac. In the face of such flood threats, sandbag distribution is a short-term mitigation effort that can protect individual homes and businesses. This is an effective short-term strategy used throughout the region; for example, Coconino County Flood Control District has organized several sand bag stations near the Pipeline Fire Area in Arizona.

Riverbank stabilization projects are an example of a longer-term mitigation strategy that reduces flood loss in communities. The first step in this process involves identifying key locations that are important to the community, but at risk from flooding or erosion. Sites like hospitals, utilities and key transportation corridors that connect populated areas are prioritized for protection.

Once such hotspots are identified, a popular strategy for riverbank stabilization includes the introduction of soil cement, also known as Cement Stabilized Alluvium (CSA), into the riverbank. According to Darde De Roulhac, this process involves excavating the riverbank, and then laying down layers of CSA, which serve to stabilize the bank.

Another strategy to mitigate long-term flood impacts involves removing residents from areas that face repetitive floods. J.D. Padilla reflected that in his region of New Mexico, he does not see many elevated buildings, like one would expect to see along coastal areas with hurricane risk. He has seen the Federal government buy out homeowners in repetitive flood areas, and then turn to the local government to regulate that nobody can live in that location again.

As seen in other flood-prone areas, ongoing community education and engagement is a successful long-term strategy to mitigate flood losses in the Southwest. Gila County Public Works has scheduled two public meetings, on June 29 and 30, 2022, in Globe, Arizona, to inform residents about the County’s Telegraph Fire Flood Mitigation Projects.

In New Mexico, collaboration between the University of New Mexico, National Science Foundation, and a consulting firm called High Water Mark, led to constructive dialogue during a workshop on May 6, 2022. Research from this collaboration will improve rainfall runoff models downstream from burn scars, which will provide more accurate flood forecasts in the future. Through this collaboration, student interns are building and deploying sensors that function as “early warning systems” that help downstream communities respond to flood events and reduce the destructive impact of flood events.

Communities should take precaution to prepare for monsoon rainfall flooding in the region during late June and early July. — Photo: Phoebe Suina

Residents in the Southwest should check with their local floodplain managers to find out about local meetings that will update citizens about the status of flood control projects, seasonal monsoon rainfall forecasts and other important items that will help them prepare for flood season.

Outlook for Monsoon Flood Potential This Season

The National Weather Service designates the North American Monsoon season as the period running from June 15^th through September 30^th in the Southwestern U.S. According to the National Weather Service – Las Vegas, monsoon forecasting for five to seven days generally predicts if weather patterns will favor monsoon moisture in the area. For shorter timeframes, of less than four days, forecasts can begin to pinpoint areas that may see heavier thunderstorms.

Beyond seven days, people may consult NOAA’s mid-term climate prediction forecasts that provide temperature and precipitation outlooks for 6-to-10 days and 8-to-14 days. These forecasts should be considered in a general sense, and users should expect shifts in the forecast.

This season, communities should take precaution now to prepare for monsoon rainfall flooding in the region during late June and early July. NOAA’s 8-to-14-day outlook, issued on June 22, and covering the forecast period June 30 – July 6, forecasts above normal precipitation in the region, as moisture is forecast to stream into the region.

NOAA’s 8-to-14 day outlook forecasts above normal precipitation in the Southwestern U.S., as monsoon moisture is forecast to stream into the region.

Longer-term monsoonal rainfall forecasting has often been unreliable but is expected to improve over the upcoming years. Recent research has found a correlation between moisture forecasts from the European Center for Medium-Range Weather Forecasts and monsoonal rainfall, which should improve longer-term seasonal monsoonal rainfall forecasts. Such advances will be useful for decision makers who seek to mitigate flood risk in the Southwestern U.S.

Regardless of the mid- and long-term monsoon flood forecasts, residents of the Southwest should prepare for flood season by reviewing their flood insurance policies and taking photographs of new purchases or home improvements from the past year. They should also have important documents stored in a zip lock bag or water-tight container, and have a “go bag” containing food, water, clothing, and medication, in case they need to evacuate at a moment’s notice.

Come Join Me On The Road!

GeoTrek loves to travel to the center of all the action! And we are doing exactly that with this topic of monsoonal flooding in the Southwest U.S.

Join me, as I travel to New Mexico and Arizona from Friday, June 24^th to Friday, July 1^st, 2022. I will investigate monsoon floods, document wildfire burn scars, interview scientists and interact with impacted residents.

Along the way, I’ll probably car camp in the middle of nowhere for a night or two, while enjoying canned food and a swig of water from my one-gallon jug.

The Holy Grail of this trip? I REALLY want to see a wall of water push through the desert, at the inception of a flash flood. From a safe vantage point, of course.

You can follow me on Facebook at GeoTrek the Community for travel updates. You can also follow along on Twitter, Instagram and TikTok. I look forward to traveling together!

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