Rewetting the Sponge
How to increase biological diversity, increase the removal of greenhouse gases from the atmosphere, and cool the environment.
The Sponge:
It takes a lot of solar energy to convert liquid water into water vapor. When that happens, solar energy used in this process does not get converted into forms that contribute to local heating. Hence, when water is either evaporated from a surface or transpired through plants (i.e., when plants take up liquid water from the soil and release water vapor into the atmosphere from their leaves), the environment remains cooler than in areas where this process is not occurring. Collectively, the two processes are called evapotranspiration, and effectively cool off our environments - similar to how sweating cools us off.
In areas with lots of precipitation and deep soils, the amount of evapotranspiration and associated cooling is only limited by the ability of plants to pump that water back to the atmosphere. In arid regions, the absence of available liquid water in soil becomes the limiting factor. But, what about our Front Range, a semiarid region where droughts are common, and irrigation is often essential for plants to grow? The answer is that both the abundance of plants and the size of the soil water storage bucket are controlling factors for the amount of evapotranspiration that plants can perform.
Figure 1: Soil acts as a critical water storage component, or a ‘sponge’, that is recharged by precipitation. Gravity forces water out, replenishing surface/ground waters, while further water is lost to evapotranspiration, as water evaporates from the surface or is pumped into the atmosphere by plants. To increase evapotranspiration and cooling, we must increase the total size and health of the sponge, as well as increase the abundance of plants.
Photo Credit: Tim Seastedt
We can think of this soil water storage bucket as a sponge (Figure 1). This sponge can be filled with water, but then gravity forces water to drain from it, leaving only a partially wet soil. A healthy soil that is a foot deep can hold about 0.4 feet of water against the force of gravity. Much of this water is what the plants will pump in order to grow. However, the problem is we have damaged the size and functioning of the sponge. We’ve increased surface water loss -- runoff -- such that less water enters the sponge, and less is retained. Two separate damaging forces are a work. First, we’ve created eroded landscapes and degraded soils, a process that decreases the size of the sponge and can make it less efficient to function as a sponge. Second, we’ve built impermeable structures such as houses and roads that prevent water from entering the soil, forcing the water to run off the surface. This water is sent to storm sewers rather than allowed to infiltrate and recharge the sponge. Soils have also been compressed and compacted such that water infiltration is further reduced. While these actions have occurred across all regions of the planet, their impacts in our region, a semi-arid landscape historically capable of using water for cooling, are arguably much more damaging than elsewhere. Arid lands will always be arid with or without a sponge, and high precipitation areas hardly need a sponge for plants to maximize production and cooling. But for areas with seasonally limited water such as ours, the size of the sponge and its recharging abilities is very important.
How do we fix this?
Two very doable and inexpensive restoration processes are applicable. In the uplands (areas above any flowing water except for snow and storm runoff), we restore riparian and wetland areas by adding structures that make the water slow down and move parallel to the flow, thereby spreading out the water (Figures 2 and 3). We also plant the appropriate vegetation, usually native trees and shrubs. These wet spots can create speed bumps for wildfire, since green, non-coniferous vegetation does not burn or burns very poorly and slowly.
Figure 2: Time-series of several rewetting structures constructed in one location on Colorado’s Western Slope. Two growing seasons after initial construction (Fall 2020 - Summer 2022) show reduced erosion, improved water infiltration and retention, improved soil health and increased abundance of vegetation.
Photo Credit: Cathy Tate
Figure 3: Three growing seasons after initial construction (Fall 2020 - Summer 2023) show further impacts - notably, during a heavy precipitation event, surface run-off was carrying away eroded soils from damaged areas uphill, but soil was trapped by the rewetting structures and the roots from increased grass production. This effectively rebuilt the soils in this location - increasing the size and health of the sponge, and allowing for increased evapotranspiration and cooling.
Photo Credit: Cathy Tate
A historical map of our streams in the Front Range and adjacent prairie would have shown dozens of beaver dams providing this service of spreading out and slowing down water flow, allowing for increased infiltration into the soil sponge. Now, with beaver populations and dams sparse, we must first plant ‘beaver food’ (cottonwoods and appropriate shrubs) in bottomlands before beaver populations can fully return and rebound sustainably. However, until that time, we can fill this role by creating beaver dam analogs (Figure 4). Building beaver dam analogs enables us to spread out water and slow the flow, allowing for the soil sponge to recharge its water storage bucket. By ‘rewetting the sponge’, we can increase the ability of plants to cool the landscape, increase biodiversity, and increase greenhouse gas removal!
Figure 4: An example of a beaver dam analog structure. Constructed using mostly on-site materials (grasses, stones, soils, etc.), beaver dam analogs are low-cost, easy to implement methods used to restore wet meadows.
Photo Credit: Tim Seastedt
In more upland sites, areas that have channels but only have surface runoff during snowmelt or heavy rainstorms, the process is a little more difficult, but not by much. The first action is to “stop the bleeding”, i.e., terminate unacceptable erosion caused by harmful land management legacies or impermeable surfaces. The low-tech way of doing this is to build stone structures in ways that keep plant roots from drying out in head-cuts (those erosional features in gullies caused by rapidly eroding sediment being transported away, cutting into the hillsides) that are in the process of forming. The idea of ‘rock mulch’ (where rocks protect plants, soil, and capture sediments) actually makes sense in many of our upland landscapes (Figure 5). Once protected from air, the plant roots that were previously vulnerable on head-cuts can now stabilize the soils and landscape, and the plants take over the role of preventing further erosion. Rock mulch can also force runoff waters to deposit their sediments on the landscape, thereby rebuilding soils and increasing the size of the sponge.
Figure 5: Example of a ‘rock rundown’ built on Colorado State Land Board property at Lowry Ranch in 2023. A head cut on the right side of the damaged area was sealed with rock so that plant roots no longer dry out. Vegetation growth and sediment capture will make this repair largely invisible in 1-3 years.
Photo Credit: Doug Swartz
The Front Range has been slow to adopt these actions, but their use west of the Continental Divide has been ongoing for 14 years and has helped to maintain populations of the endangered Gunnison Sage Grouse (because wetter meadows provide more food), and in general are enhancing the grazing and wildlife values of western landscapes. All landscapes – urban – agricultural – semi-natural – can benefit from these actions. ‘Rewetting the sponge’ involves low-input, low-cost actions that can rebuild soils and recharge soil water storage, increasing the ability of plants to perform evapotranspiration. By implementing these actions, we can increase biodiversity by restoring landscapes and habitat, while cooling off the landscape and intensifying greenhouse gas removal by increasing the abundance of plants! Recently,
Wildlands Restoration Volunteers (WRV: www.wlrv.org) rewetted some prairie owned by the Colorado Land Board east of Denver. This was the first project of this sort on the Front Range, and hopefully will be the first of many (Figure 5). You can get involved by visiting WRV’s website in the coming months for the list of 2024 projects.
