rainwater harvesting in the chihuahuan desert

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

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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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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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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

rainwater harvesting in the chihuahuan desert

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