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8/2/2019 Rainwater Harvesting in the Chihuahuan Desert
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John Walton
Abubaker Alamailes
University of Texas at El Paso
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Challenge: How to maintain a lush green environmentin the Chihuahuan Desert?
Solution: Passive Rainwater harvesting
Why use passive rainwater harvesting rather than rain
barrels I see on the internet (active harvesting)? How does it work in theory? Will it work in El Paso?
How can I implement it in my yard, subdivision, andcity?
How much money does it cost? Or does it save money? Capital costs
Operational costs
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Development addsimpermeable surfaces: roads,roofs, sidewalks, driveways
Harvesting water from theseareas multiplies the availablemoisture above the climaticnorm
If only 20% of the lot hasplants, El Paso is as wet asAtlanta
If desert species are used,80% or more of typical ElPaso lawns can be tree andbrush covered with nowatering
0
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Atlanta Austin El Paso 5X El Paso
Inches/yrea
Annual Precipitation
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Water fromimpermeable surfaces(roofs, driveways,roads, parking lots) is
captured during stormevents
Water available is muchgreater than normal forthe climate
Captured water isstored for later use byplants
Capture area, waterstorage volume, andarea of plants supportedmust be carefully
matched Matching depends
upon climate and typeof plants supported
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native
plants
Impermeable areas concentrate water in vegetated areasConsider that if rainfall is increased by 5X, El Paso has a lotof water for watering trees.
capture area
capture area/plant area
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swale
Stormwater periodically diverted toshallow depressionswith native vegetation
swale
street
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Natures place to store water is in the soil
A year ago we had a wet winter followed by adry spring
Everything in the desert bloomed because thewinter precipitation was stored in the soil
This natural process can be enhanced to store
the moisture in the soil beneath the yard Native species are very drought resistant
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Active rainwater harvesting stores water in a tank;passive rainwater harvesting stores water in the soil natures way of storing water during dry periods
Most hydrological methods are designed for non-desert locations & dont work well here, the timeperiod between precipitation events in El Paso and thehot climate mean very large tanks are required foractive systems
The cost of active rainwater systems is dominated bythe cost of the storage tank
Passive systems always payback financially, activesystems generally do not in this climate
Passive systems simply enhance natural processes design with nature
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0
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100000
150000
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0 2000 4000 6000 8000 10000 12000
GallonsofWaterSto
red
Lawn Area, ft2
5,000 gallon water tankcosts $2,000
The soil can store theequivalent of 1-2 feet deep ofwater over the entire yard
Tanks store much less water
and are expensive In desert climate tanks are
only useful for wateringsmall flower or herb gardens
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soil
Mulch (usually rock)
Landscape cloth (screen)
Must block weeds and let water into soil, storage is in the soil. Storage capacityof soil can be enhanced by additives (fines, organic matter, hydrophilic materials).
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Shallow depressions are backfilled with gravel No open water for safety or vectors
Protects from evaporation during infiltration
French drains (gravel filled trenches) can beused to transport water from downspouts orother water sources
Soil must be covered with landscape cloth toprevent weed growth
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water fromparking lots
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A watershed is the area of land where all of thewater that is under it or drains off of it goesinto the same place. (EPA definition)
Think of a house and yard (or subdivision) as aseries of mini watersheds
Where does each portion of roof drain?
How can the water from roof, sidewalks,driveways, yards be stored whenever it rainsso that plants can live through droughts?
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Divide your yard into watersheds Think of where every portion of the
roof/sidewalk/driveway drains Make shallow rock filled depressions to receive the runoff
and allow it to infiltrate into the soil Use landscape cloth to prevent weed growth, water cannot
be stored if it is robbed Plant native vegetation with density related to capture area/
growth area Capture/growth area = 5 (limited watering in May/June to
15 (never water once plants are established) Plants will need watering for about a year, until roots are
established, about once every two weeks during growthperiods; this can be accomplished with a truck; after thatmaintenance costs drop
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El Paso
Native species (e.g., mesquite, desert willow,acacia)
10 years of historical temperature and rainfalldata
Capture area =
(roof+sidewalk+driveway+yard) in watershedof concern
Crown area = total crown area of plants inlooking from above
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Roof andcarport waterexit carportcorner
Cobbles allowsubsurfaceponding andinfiltration
into soil Soil stores
water betweenrains
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0
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0 10 20 30 40 50
Day
Soil Suction (centibars)
Water Content (VWC%)
rain rain
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Passive rainwater harvesting works in El Paso
Capture/green area ratio from 5-10
Saves money
Saves water
Provides a green, shaded lot, not xeriscapingwith a bunch of hot rocks
Active systems (most of what you will find oninternet) generally not appropriate for El Paso our climate is different
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