John WaltonUniversity of Texas at El Paso

Challenge: How to maintain a lush green environment in the Chihuahuan Desert? Conditions harsher, rain more sporadic

Solution: Passive Rainwater harvesting – no rain barrels or tanks

Passive is cost effective, tanks are generally not

How does it work in theory? Will it work in El Paso?

How can I implement it in my yard, subdivision, and city?

Development replaces desert with impermeable surfaces: roads, roofs, sidewalks, driveways

Harvesting water from these areas multiplies the available moisture above the climatic norm

If only 20% of the lot has plants, El Paso is as wet as Atlanta

We can reduce storm runoff and save water while having lush green yards

A watershed is the area of land where all of the water that is under it or drains off of it goes into the same place. (EPA definition)

Think of a house and yard (or subdivision) as a series of mini watersheds

Where does each portion of roof drain? How can the water from roof, sidewalks,

driveways, yards be infiltrated into the soil whenever it rains?

nativeplants

Impermeable areas concentrate water in vegetated areasConsider that if rainfall is increased by 5X, El Paso has a lot of water for watering trees.

capture area

capture area/plant area

swale

a) Reduces floodingb) Lowers water usec) Green environment

swale

street

Nature’s place to store water is in the soil Two years ago we had a wet winter followed

by a dry spring Everything in the desert bloomed because

the winter 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, most just go dormant

• The soil can store the equivalent of 1-2 feet deep of water over the entire yard

• Tanks store much less water and are expensive

• In desert climate tanks are only useful for watering small flower or herb gardens

Active rainwater harvesting stores water in a tank; passive rainwater harvesting stores water in the soil – nature’s way of storing water during dry periods

Most hydrological methods are designed for non-desert locations & don’t work well here, the time period between precipitation events in El Paso and the hot climate mean very large tanks are required for active systems

The cost of active rainwater systems is dominated by the cost of the storage tank

Passive systems always payback financially, active systems generally do not in this climate

Passive systems simply enhance natural processes – design with nature

soil

Mulch (usually rock)

Landscape cloth (screen)

Must block weeds and let water into soil, storage is in the soil

Root depths > 5 meters (16.4 ft) (mesquite)

Root span > 12 m (39 ft) (mesquite) Volume > 565 m3 (20,000 cubic feet)

Soil moisture storage: 113 cubic meters, 4,000 cubic feet, 30,000 gallons

Soil is a much more cost effective storage location than “rain barrels”

Rain barrels are not practical in the desert except for small gardens

water from parking lots

Divide development 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 growth periods

Concepts are evolving We prefer filling depressions with sorted

rock (gravel) Avoids drowning risks Avoids vector risks

Depressions can be any shape, but think of unlined French Drains to spread water over all of soil area

Sized to store ~1 inch of runoff (change as desired)

Balance of flood control, water storage, cost

The Model House

Runoff Paths. Option 1. Option 2.

Locations of LID Practices and Flow Path.

Cross Sections of a Bioretention Cell and a Vegetated Swale.

French Drain

Impermeable Mulch

Native VegetationScientific Name Common Name Type Height

FtWidth

FtEvergreen

Or Deciduous

Water Requirements

Ceratoides Lanata Winterfat Shrub 3 2 Evergreen Low

Larrea Tridentata Creosote Bush Shrub 8 6 Evergreen LowKoberlinia Spinosa Crucifixion

ThornShrub 5 7 Evergreen Low

Atriplex Canescens Four Wing Saltbush

Shrub 6 8 Simi-Evergreen

Low

Leucophyllum Frutescens

Texas Sage/Ranger

Shrub 4-8 4-8 Evergreen Low

Acacia Berlandiera Guajillo Shrub 12 12 Deciduous Low

Prosopis Glandulosa Honey Mesquite Tree 30 30 Deciduous LowChiloposo Linearis Desert Willow Tree 25 20 Deciduous Low

Fraxinus greggii Gregg’s Ash Tree 15 8 Semi-Evergreen

Low

Quercus Arizonica Arizona White Oak

Tree 35 30 Evergreen Low

Class Exercise: Take the example house and lot. Divide the house/lot into mini-watersheds. Show where water will be infiltrated and where the native vegetation will be planted. Draw in the crown of the vegetation.

El Paso Native species (e.g., mesquite, desert

willow, acacia) 10 years of historical temperature and

rainfall data Capture area =

(roof+sidewalk+driveway+yard) in watershed of concern

Crown area = total crown area of plants in looking from above

Change in soil moisture storage = runoff in – evapotranspiration loss

Concept is to design system so we never reach wilting point

Alternatively can design so plants need watering once per year (or during extreme droughts)

A passive rainwater harvesting system is to be installed in conjunction with a parking lot. The system consists of a set of shallow depressions in a 10 foot wide swath in the middle of a 200 by 200 foot parking lot. The depressions are two feet deep and backfilled with size sorted gravel having a final porosity of 35%. The parking lot halves each slope into the depressions. For really large storms, the depressions will overflow into a storm sewer. For small storms the depressions will capture and infiltrate all the water. The soil below and surrounding the depressions has a field capacity of 0.3 and a permanent wilting point water content of 0.15. The depression is planted with Honey Mesquite trees that, when fully grown, will have a crown with a 30 foot width (i.e, will cover the depressions and extend 10 ft. out into the parking lot on each side). The Mesquite trees have a transpiration rate of 0.04 inch/day spread over the crown area. The Rational Coefficient for the parking lot is 0.90. The moisture and roots spread over a soil area 20 ft. wide by 10 ft deep.

Roof and carport water exit carport corner

Cobbles allow subsurface ponding and infiltration into soil

Soil stores water between rains

Soil Moisture Energy Volumetric water

content (VWC%) Soil suction

(cetibars)

TDR

Tensiometer

afaf

A1

A2

A3

Carport

N

French drain

A1

A2

A

NW roof

Passive rainwater harvesting works in El Paso Capture/green area ratio from 5-20 Saves money Saves water Provides a green, shaded lot, not xeriscaping

with a bunch of hot rocks Mixture of trees and shrubs is optimal Active systems generally not appropriate for

Southwest Passive systems work (have analyzed

Phoenix, Albuquerque, El Paso)