Embedded Files
Water
by
Paul Maslin
&
Gary Day
December 22, 2024
Aquifer -- a body of rock and/or sediment that holds groundwater.
Water is essential to life. All ecosystems, from aquatic to desert, require water and the nature of each ecosystem is more or less defined by water's relative abundance.
Water comes to an ecosystem by precipitation or direct condensation. Once there it may evaporate, run off over the surface, or soak into the ground. The groundwater may subsequently be evaporated from the surface, moved slowly downhill by gravity, or taken up by plant roots to be transpired by the leaves.
In a relatively dry climate, keeping water on the land is a priority. There are three basic ways:
Increase soil penetration
Slow runoff
Reduce transpiration
Increase soil penetration
Rain hitting a bare surface will tend to run off. Vegetation and duff catch it in mini dams and give it time to soak in. How far it soaks in is a function of soil compaction/porosity.
Plant roots are a primary source of soil porosity, pushing through the soil to leave channels after they decompose. If the vegetation is shallow rooted (annuals) the soil will only be loosened for the first few inches (less if cattle are trampling it) so the water will move downhill through (and over) that surface soil.
If the vegetation is deep-rooted (perennials) a much greater volume of soil will be involved and downhill transport will be slower. Burrowing animals also open channels through the soil, but since they primarily feed on plant roots, they tend to loosen the same layer as the plants do.
In California's grasslands and savannas, there has been a historic shift from predominantly perennial herbaceous plants to annuals from around the world. Consequently, these ecosystems are working on far less total available water than they evolved with. See The Blue Oak Enigma. An obvious solution is to shift the dominant vegetation back to native perennials.
Annual Plant Roots vs Perennial Plant Roots
https://www.usbg.gov/visit/exhibits/exposed-secret-life-roots
https://www.homegroundhabitats.org/content_topic/creating-habitat/building-healthy-soil/
Slowing runoff
Groundwater flow is about 100,000 times slower than surface flow. Water can flow over the surface further in a day than it would move through the ground in a year.
Small, ephemeral, streams can be manipulated to slow runoff and allow more to soak into the ground. (Avoid creating barriers to fish migration in perennial streams that support fish.) If water is forced to flow around obstacles such as rocks, brush, or plant stems, the velocity will decrease and the depth will increase, increasing time and surface area for more to soak into the ground. Some will continue through the ground and some will seep back into the stream as surface flow subsides (bank storage) keeping the stream flowing longer. Either way, water will be shed from the land more slowly. Obviously, the makeup of the channel is important. If the channel is incised into bedrock, only the slowing effect is important, but if the banks are permeable (weathered soil, alluvium, colluvium), large quantities can be stored in the ground.
An obvious way of slowing flow and increasing depth is by construction of a dam from available materials such as rocks or woody debris.
Micro BDAs
Beaver dam analogs (BDAs) have been acclaimed for restoring stream access to floodplains, raising groundwater, restoring wetlands, reducing downstream flooding, and increasing duration of stream flow.
The people who support BDAs emphasize dams but beavers affect streams in other significant ways, they dig canals lateral to the stream increasing percolation and water storage and move large amounts of organic material from land to water.
Beavers are unable to colonize ephemeral or seasonal streams, but humans can produce similar beneficial effects there by mimicking beaver activity on a small scale, creating micro BDAs ranging in size and complexity from simple leaf wads through brush dams to rock dams. Individually these small structures have a negligible effect, but cumulatively the nearly infinite sites for their construction make possible substantial landscape effects. Streams aggrade rather than degrade, reducing soil loss and downstream siltation while accumulating lenses of alluvium to store water and provide plant habitat.
Obviously the location of a micro dam will affect its individual impact. Constructed over impervious bedrock, a dam will store only the volume of its pool. (However, if there is a crack in the bedrock below the pool, the extra pressure head may force more water into a deep-lying aquifer.) Constructed over weathered soil, the pool will raise the surface level and thus pressure head of the stream, forcing water into the permeable soil thereby creating longer-term storage of more water than the pool will hold. The effect can be further augmented by creating small mimics of the lateral canals that beavers make. Constructed over alluvium or colluvium the increased pressure head can potentially force large quantities of water into the permeable substrate, creating a local seasonal or even perennial aquifer.
Where a micro dam raises a seasonal stream to access its floodplain, a seasonal wetland may be formed. See https://sites.google.com/view/wet-meadow/home.When soil is waterlogged, decomposition of organic matter is reduced. The longer the floodplain stays saturated, the more organic material will accumulate in the soil, increasing its capacity for water storage and its wetland character. If upstream micro dams store and gradually release enough water, the wetland may even become perennial.
In very tiny streams, woody debris naturally catches leaves to form minute dams, slowing flow and raising stream level. The process can easily be augmented with a leaf rake or blower. If the leaf wads get washed downstream, they may hang up on a larger obstacle where they will contribute to a larger micro dam or they may be deposited on a floodplain to contribute to the organic content of the soil. Raking leaves and debris into a tiny stream will create multiple micro dams with a generally positive effect (However, areas immediately upstream of a culvert should be avoided).
Aggradation of small streams creates an alluvial band colonized by hydrophytes (can be enhanced by deliberate introduction). The hydrophytes’ extensive root systems lock the alluvium in place while their stems further slow the flow of water and trap debris causing long-term conversion of stream channel to swale, a broad swath where water trickles slowly downhill through vegetation and soil. Best of all, a swale is self-maintaining; the hydrophytes spread to wherever they have water.
Beavers building a dam use available materials ( wood, rocks, mud) and the builder of BDAs should be equally opportunistic. However, the beaver remains on site, replacing parts that fail and the beaver mimic needs to return periodically to conduct similar maintenance. Micro dams built of rocks properly keyed to embedded boulders and having additional rocks in the spill area to dissipate energy and prevent undercutting will last indefinitely (but still need regular inspection in case a carelessly placed rock has moved).
The dams are initially built one rock high, so are known as one-rock dams, but, as the upstream pool fills with alluvium, a second layer can be added on the upstream side of the first and so on.
If the channel is incised into bedrock, only the slowing effect is important, but if the banks are permeable (weathered soil, alluvium, colluvium), large quantities of water can be stored in the ground.
Dams Made of Rock
Slowing Stream Flow
Principles of construction of small rock dams, also known as "trincheras" or “one rock dams”.
Anchor them to the banks by building against embedded rocks or placing very large rocks against the banks.
Build the skeleton of the dam of large rocks firmly in contact with one another allowing a lower area near the middle for a spillway.
Wedge appropriately shaped rocks into gaps from upstream.
Face the upstream surface with smaller rocks, then dribble gravel between these to partially seal the dam.
Place medium rocks just downstream of areas where water will spill to create a “splash apron” to keep the dam from being undercut.
If needed, use flat rocks to armor the banks above the anchor rocks to prevent erosion around the dam.
Dams Made of Woody Debris
While rocks are usually the best material for small dams (they don't rot, burn, or float away), in some sites they are simply unavailable. Dams of woody materials, especially in areas where they stay wet, can catch enough silt to gradually bury the wood and enable hydrophytes to replace it. Since wood tends to float, a major concern is keeping the water from going under the dam. Small branches, preferably leafy twigs , are placed at the bottom, then held in place by progressively larger materials with the heaviest on top. Logs should always be oriented lengthwise to the stream to avoid becoming bridges. Use pieces as long and heavy as convenient and, if available, include some rocks to help anchor the structure.
Longitudinal section of a woody debris dam
Newly constructed dam of woody debris.
After a significant rain event,
Dams of woody debris can vary in length; a long section of stream can even be packed, taking care to keep small materials on the bottom, with heavier things on top to pin them down. The water, flowing around the twigs and branches, wil deposit silt, gradually burying the wood to allow hydrophytes (plants that thrive in wet areas) to colonize, converting the stream to a swale.
This packed stream has captured about a foot of silt all the way along its length.
Note: The purpose of these dams is not to create pools, but to raise the level of the stream when it is flowing and thus the level of adjacent groundwater. For this the “pools” function just as well after they fill with sediment. An argument might be made that they deprive the main stream of alluvium and spawning gravel but this is temporary. After one or two rainstorms, the “pools” will be filled with sediment, alluvium will pass freely, and the tributary will resume its role of supplying spawning gravel to the main stream. Any negative effects on fisheries will be short term and overridden by the long-term increase in groundwater input during the dry season.
A swale on the BCCER recovering after a severe fire.
Swales
Swales
Aggradation of small streams creates an alluvial band colonized by hydrophytes (can be enhanced by deliberate introduction). The hydrophytes’ extensive root systems lock the alluvium in place while their stems further slow the flow of water and trap silt and debris causing long-term conversion of stream channel to swale, a broad swath where water trickles slowly downhill through vegetation and soil.
A swale is self-maintaining; the hydrophytes spread to wherever they have room and water.
Swales act like sponges, reducing downstream flooding and contributing to aquifer recharge as well as to carbon storage by stimulating tree growth and soil humus development.
Hydrophytes can be transplanted to jump-start swale development. Plants should be chosen from sites with a similar wet period.
Hydrophytes shown in this photo include Santa Barbara Sedge, Western Spice Bush, and California Giant Chain Fern.
A steep stream incised into bedrock like this does not lend itself to water storage. Explore upstream and downstream to find flatter areas with permeable substrate.
Although not ideal for water storage, bedrock scour areas are important since ground water surfacing there becomes available to animals for drinking, cooling, or reproducing.
Bank storage in stream reaches upstream of the bedrock pool will extend the time the pool contains water. This, of course, can be enhanced by building water-retention structures in those upstream reaches.
If bedrock pools are unavailable in a particular area, game guzzlers can be substituted. A perforated pipe buried in the bed of an ephemeral stream can be used to fill a 500 gal. water tank which can keep the guzzler filled via a float valve all courtesy of gravity flow.
Springs and Seeps
Groundwater will seep down through a permeable layer until it strikes an impermeable layer, then move laterally across the top of that layer, often following a buried ancient stream channel. Where it reaches the ground surface it emerges as a spring to supply an area of wetland. A pool maintained there provides an important water source for animals. Spring pools should be cleaned out periodically to keep them from filling with leaves or silt.
Sometimes a trickle of water will follow a crack in otherwise impermeable rock and emerge as a seep on the side of an exposed layer. A little diligent work with hammer and cold chisel can create a basin, perhaps only half a cup, to fill from the seep. Such a minute source of water can sustain a covey of quail through the summer.
Reduce transpiration
Transpiration is good as it allows for plants to grow, storing carbon and producing food and cover for animals. However, transpiration by unwanted plants produces excess fuel while losing water that could be better used. Because most California rain falls in winter, evergreen plants will take up some as soon as it falls whereas deciduous plants, being less active at that time, will allow more to soak in to replenish groundwater and aquifers. Deciduous trees also allow winter sunlight to reach the herbaceous layer, creating a mat of vegetation for soil protection, wildlife food and cover. Evergreen plants are not uniform in transpiration, broad-leaved evergreens, such as canyon oak, transpire water much more rapidly than pines. While deciduous trees, like big-leaf maple and valley oak, transpire water very rapidly, they are inactive during most of the wet season, allowing more water to enter deeper soil and aquifers.
A girdled canyon oak.
A simple way to select against unwanted trees is by girdling -- removal of a ring of phloem and cambium from the base of the tree to prevent translocation of carbohydrates to the roots.
Techniques vary depending on the size of the area. If the area to be treated is relatively small, a handlebar trimmer works well and allows precise selectivity. A trimmer with triangle metal blade can be used to cut small shrubs and efficiently girdle trees up to a foot in diameter. If a saw-blade is substituted, the trimmer can cut stems up to 4 inches (but not be useful for girdling). Whether cut or girdled, many species will stump-sprout necessitating repeat treatments, but treatment frequency declines as the plant roots run out of stored energy. For larger areas, techniques such as prescribed fire, mastication, or goat browsing, are more efficient. These techniques select mostly for size rather than species. While a goat will select for species, it will be from the goat’s perspective, not yours. Partial control can be attained by using the goats in winter when they are less likely to browse on deciduous species.
Differing topographies will require different treatments: A south-facing slope can be burned when other nearby aspects are too wet to carry fire. By contrast, when a north-facing slope will burn, other aspects would burn dangerously hot. Steep areas with lots of rocks would be better for goats than masticators.
Whatever treatment is used, remember you are in for the long game. Think in terms of maintenance, rather than restoration.
Since the purpose of slowing water loss from the ecosystem is to have more available locally, one might argue that it deprives downstream reaches of water. However, when simultaneous efforts are made to reduce transpiration, downstream water availability is likely to be increased.
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