California’s River in the Sky
An Atmospheric River was how she termed the heavy rainfall, as a friend talked about the risk to the nearby Oroville dam. The downpour threatened 190,000 people living downstream in 2017. The rain caused damage to the dam’s main spillway, so the regular release of water was stopped. The lake level quickly rose to overflowing. As water flowed over the emergency spillway, erosion threatened to undermine and collapse the concrete. No collapse occurred, but there was a fear that a 30-foot wall of water would soon be headed down the Feather River.
February 2017 feels like a lifetime ago. These memories are from a time before Covid upended our lives. On the personal side, these memories are before SymSoil filed its first patent application. As I read The Coming California Megastorm, by Raymond Zhong, stories from 2017 returned.
There is irony in remembering excess rain in a state focused on drought. As a California resident, I believe the last 150 years have been an anomaly, a period with more precipitation in the golden state than the norm over the last 2,000 years.
A calamity caused by heavy rain is a real risk I had forgotten. If you missed the NYTimes article, here are the high points:
- Higher global temperatures are putting more humidity into the air and generating stronger winds.
- There is now a reasonable probability that in some years, moisture-laden air will come towards California, leading to a 30-day atmospheric river drenching the entire state.
- New research suggests that this could be hundreds of miles wide, 1,200 miles long and drop the equivalent of 26 Mississippi Rivers onto California.
- The state’s infrastructure is unprepared for something as strong as the 2017 rains, which threatened major dams. Given changes in humidity and wind speed, the next megastorm could deliver as much as twice as much water as the 2017 storm.
- The average precipitation throughout the state would be 16”, but the coastal mountains could get as much as 40”.
- The estimated economic damage to California could be a high as $725 Billion, which is 5 times the cost of Katrina. Previously, the highest risk to California was a severe earthquake along the San Andreas fault, which would cause $250 Billion in damages.
My first thought was we need more trees!
I will admit, to a hammer, everything looks like a nail, and almost everything makes me think of trees and soil. But planting trees needs to be part of California’s solution to climate change for several reasons.
Trees Reduce Rain’s Kinetic Energy — Trees shield the ground from the full impact of the rain. The leaf canopy absorbs most of the kinetic energy of the rainfall. A slow motion video of a raindrop hitting bare soil looks like a metal ball slamming into a powdery surface. The energy of the raindrop’s impact is transferred to the soil particles directly under the drop, which then fly in all directions. This is the power that drives soil erosion.
Tree Roots Reduce Erosion — While the tree canopy is reducing the rain’s power, the tree’s roots are strengthening the cohesion of the soil. Different trees have different root patterns, some only directly under the tree, but others grow laterally and extend more broadly. Still other trees grow deep taproots as tall as the tree. A tree’s thickest roots are designed to stabilize the tree while storing water. From the thick roots, smaller branches grow, which stabilize the soil and feed the microbiome. They act like our circulatory system, bringing nutrients into the tree and sending food out to the microbial community.
Leaf Litter and Organic Matter Absorb Rain — Trees keep soil healthy so they can tap into the nutrients they need. Healthy soil contains plant material, roots, leaves and other debris in various states of decay by soil microbes. That organic material is both the glue that keeps the soil together and the key to water absorption by the ground. .
Without organic matter, even small amounts of rainfall cause problems. In California’s Coachella Valley, for example, the lack of vegetation on the mountains causes even ¼” of rain to create flash floods, as it rushes down into the desert valley. Flash floods happen in all deserts when there is rain. The lack of vegetation, for any reason (floods, fires, toxins, excess herbicides and other chemicals, salinity) will result in soil with little to no absorption capacity.
Each square foot under a healthy, full grown tree absorbs about 4 inches of rain.
The often cited statistic is 1% organic matter increases the land’s ability to absorb water by a 20,000 gallon acre. That translates into about half a gallon per square foot, or about .9” of rain.
Acres, inches, feet, gallons, percentages … Oy! … Let me walk you through the math:
- Each 1% organic matter increases the soil’s absorption by 20,000 gallons per acre.
- An acre is 43,560 square feet, so 1% organic matter absorbs 20,000 gallons, or a bit less than 1/2 gallon per square foot.
- Rainfall is described in inches. Half a gallon, put in one square foot container, is just under 1 inch tall.
- The area under mature, healthy trees typically has at least 5% organic matter. Thus, with 5% organic matter in soil under a healthy tree, the soil can absorb at least 4” of rain before it is saturated.
Trees initially grow organic matter directly underneath the canopy, and then it spreads beyond the canopy. California forests have 5% to 12% organic matter, so they can absorb as much as 6 gallons of water per square foot.
To summarize the math in plain English:
A single mature tree with a crown of leaves that is 30 feet wide, and healthy soil, will absorb about 1,800 gallons of rain water in a storm, or all the water from 4 inches of rainfall. If we included the roots outside the tree’s shadow, or the organic matter was higher than 5%, then the amount of water absorbed would be higher.
Soil Erosion and Mudslides — The reduction in trees, after a forest fire, often causes a second set of challenges when the rains come. Wildfires impact the watershed, where urban drinking water originates. Without trees, the rain can trigger mudslides. Large mudslides, full of toxic burnt material, are usually the primary challenge to these communities in the first winter after the fire, with soil remediation and replanting not even being possible until the second year. You might think the tree roots are still in the ground, but without the life in the roots and the soil, the cohesion of the soil is lost.
Instead, after a wildfire, streams and rivers fill with sediment and with toxins. Since two-thirds of urban drinking water originates in forest lands, the downstream impact of the lack of trees can hardly be overstated. During a rainstorm, the role of trees in filtering water, keeping the soil healthy and intact, by reducing the energy of rainfall during a storm and feeding the ecosystem is every bit as critical to humans as their role in shading us in the heat.
California needs more trees for the dual purposes of reducing the impact of heat and rain. They require healthy soil, and some nurturing until they are established, but trees provide over 25 ecosystem services (click to see the full list) which will help us with extreme weather events, including severe rains, drought and heat waves.
Why does SymSoil care? We focus on solutions to environmental issues, with a focus on soil biology. Trees and plants feed, and are fed by, the soil microbiome. Healthy soil influences water, carbon sequestration and human health. SymSoil holds a patent on the first scalable approach to manufacturing Soil Food Web products as an alternative to agrochemicals.
