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As some water agencies scramble to shore up their funding proposals for water storage, it seems timely to point out opportunities to expand existing reservoir capacities non-structurally, especially where watersheds encompassing reservoirs are dominated by nonnative annual grasslands, including as woodland understories.  That is, by restoring detention (infiltration and percolation) functions to these historically degraded watersheds we can increase their effective storage capacities without erecting new engineered structures, thereby significantly reducing costs, including those for environmental review.

How would we do this?  By restoring native (thus pre-adapted) woody and other perennial vegetation types to those uplands.  Those root systems, along with the soil ecosystems they coevolve with, will begin to increase detention storage capacity – the “sponge” – soon after establishment.  Thereafter, as the vegetation becomes self-sustaining, vadose zone detention storage will only increase over time, without the need for continuous human inputs.

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This may seem a non sequitur to many.  After all, aren’t we concerned about all that water lost to transpiration from the proposed trees shrubs and perennials?  Never mind that grass, specifically lawn grass, is the poster child for high transpiration estimates.  Never mind enormous water losses from surface reservoirs to evaporation.

Native California vegetation only became indigenous through adaptation and resilience to regional climate and weather patterns.  Generally and specifically, their transpiration patterns differ from that of lawn grass.

One adaptation of both native and imported plant species to California’s Mediterranean-type climate is an annual life cycle.  That strategy proved increasingly advantageous with the introduction of European life-ways to the state.

Mediterranean annual grasses, long adapted to those patterns, accompanied new settlers who expanded niches for them through sequential land disturbance patterns attending each new cultural or economic wave.  While they are growing these grasses’ transpiration patterns are similar to the high transpiration model of lawn grass, in that their primary defense against drought is to reproduce and die.

In contrast, native woody and perennial species have a panoply of adaptations to heat, cold, drought and other stressors that moderate transpirational losses.  But plants actually play a much bigger role in the hydrologic cycle than that typically attributed solely to transpiration.  See Plants in an Ecohydrology Context.

While what we currently know about the soil ecosystem is doubtless just the “tip of the iceberg”, we do know that a product of mycorrhizal relationships with (perennial) plant roots is an amazingly indestructible group of proteins referred to as glomalin, first documented by USDA researchers.  Glomalin is considered a keystone factor in stabilizing soil structure.  Soil structure is what enables the vadose zone to act as a “sponge”, detaining water as it slowly responds to gravity.  See Surface-Groundwater Systems in a Holistic Water Cycle .

By restoring native woody and other perennial vegetation in the watersheds above existing reservoirs, we can increase uplands detention storage.  Water detained there will: 1.) drain to bedrock aquifers feeding downstream alluvial aquifers over time, and/or 2.) slowly drain, through the vadose zone, to the reservoirs themselves.  Either way, it’s “water in the bank”.

OK, some of that detained water will be subject to 3.) transpiration, but such water is not “lost” as commonly conceived.  It doesn’t just go shooting off into the stratosphere.  Some of it condenses and sticks around quite close and some of it influences regional climate.  But it’s too soon in our discussions here to get into that heady topic.

Just throwing out watershed, more specifically catchment, restoration as a water storage strategy to consider for which cost/ benefit ratios should be attractive.

While watershed restoration remains in its infancy and some degree of experimentation will doubtless be required, lessons about restoring uplands may be learned from examples such as coastal sage scrub restoration efforts in southern California, precipitated by threatened and endangered species listings, like California Gnatcatcher, Cactus Wren and others.

Costs for planning, design and environmental review for watershed restoration projects should be far less than those for structural modifications to the same reservoirs.

Reservoir watersheds will require more active, adaptive management than less sensitive areas.  Fuel management will doubtless be required in most cases to avert wildfires.  This can be achieved through a combination of strategic planting and perhaps in many cases, prescription burning.  Such short and long-term considerations must be weighed in the overall equation.

Given that at least some California water agencies are currently striving to fund water storage projects, I’d be remiss if I didn’t put this out there now.  It is one, among several, applications of the Rainfall to Groundwater approach I wish to share with all who care about such issues.  I trust I’ve made the beginnings of understanding accessible through the Rainfall to Groundwater website – ?

I hope to make much more available through the proposed Rainfall to Groundwater Learn, Apply program.  You can help make that happen by sponsoring two proposed books, or pre-enrolling in four online courses/forums, as detailed on the Course Pre-Enroll & Sponsorship Order page.

Thank you in advance for helping bring Rainfall to Groundwater to fruition.