ADAPTING LOW IMPACT DEVELOPMENT TO THE CHIHUAHUAN DESERT

John WaltonUniversity of Texas at El Paso

Low Impact Development (LID)

Evolution of Hydraulic Engineering Development leads to greater runoff and shorter time

of concentration Phase I: don’t worry, be happy, new

development dumps on downstream land owners (e.g., most of 2006 flood damage)

Phase II: Retention ponds hold all water Phase III: Smaller detention ponds route water

by shaving peak discharge Phase LID: Bioretention and infiltration areas use

stormwater to lower stormwater runoff and make a greener environment

El Paso

El Paso

Note that setting duration to 24 hours leads to under design of facilities unless NRSCS synthetic hydrograph is used (it embeds shorter duration peaks inside 24 hour hydrograph)

LID Documents and Training Oriented to Different Climates

Rain gardens, rain barrels, green roofs don’t work in desert environment

Conditions harsher, rain more sporadic What does work? How can we use LID to maintain a lush

green environment in the Chihuahuan Desert?

How can we put it in local streets, subdivisions, and industrial areas to make El Paso a greener and less flood prone area?

Basic Concept

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 Atlanta Austin El Paso 5X El Paso

0

10

20

30

40

50

60

Annual Precipitation

Inches/y

rea

Watershed

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?

Capture Zone

nativeplants

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

capture area

capture area/plant area

General Scheme

Break development into a series of microwatersheds Where does every portion of roof, sidewalk,

road, driveway drain? Build bioretention areas, properly sized, in

each microwatershed Carefully balance flood control, water

storage, and plant evapotranspiration in each microwatershed

No sprinklers needed, little maintenance, more knowledge applied, less money

But it doesn’t rain very often, where do we store the moisture? 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

Soil Moisture Storage

Water storage = Vsoil * (field capacity – wilting point)

Soil stores more water than tanks at lower cost (free)

0 2000 4000 6000 8000 10000 120000

50000

100000

150000

200000

Lawn Area, ft2

Gallons o

f W

ate

r Sto

red

5,000 gallon water tankcosts $2,000

• 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

Why Passive?

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

Storage in Soil

Mulch (usually rock)

Landscape cloth (screen)

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

bioretention

Stormwater diverted to gravel filled trenches and depressions, moisture moves into soil where it is stored indefinitely, mulch and landscape cloth stop weeds and evaporation (sources of water loss)

Everything is Sized

Bioretention volume = depth*area*porosity Sized to hold size of storm desired Sized to hold enough water to transfer to soil

for plant growth during dry periods (we have a lot of them)

Soil moisture holding capacity Sufficient area and depth of soil to hold

moisture to support plant growth without external watering

Soil moisture holding capacity can be increased by adding diatomaceous earth, fines, and organic matter

Mesquite Root System

Plants can be located some distance from bioretention areas

Mesquite Roots

Soil Moisture Storage Beneath One Mesquite Tree

Root depths > 5 meters (16.4 ft) (mesquite) Root span > 12 m (39 ft) (mesquite) Volume > 565 m3 (20,000 cubic feet) Assume field capacity, 0.3, wilting point 0.1 Soil moisture storage:

113 cubic meters, 4,000 cubic feet, 30,000 gallons How much would a rain barrel that size cost? Rain barrels are not practical in the desert

except for small gardens

BUT YOU CAN’T GROW TREES IN EL PASO

WITHOUT WATERING!

Accidental Example Near UTEP

Second UTEP Example

water from parking lots

Accidental case 3

How is it done?

Divide development into watersheds Think of where every portion of the roof/sidewalk/driveway

drains Make shallow rock filled depressions – bioretention areas Match bioretention volume to desired retention (e.g, 2 inch

rain) Use landscape cloth to prevent weed growth, water cannot

be stored if it is robbed Use spreadsheet to estimate: plant density, groundwater

recharge, bioretention volume Plant native vegetation with density related to capture area/

growth area Plants will need watering for about a year, until roots are

established, about once every two weeks during growth periods

Distribute water to soil and have sufficient storage for flood control and plant growth

Impermeable area (roof, parking lot)

French drains/depressions/trenches filled with sorted gravel

LID Design

The Model House

LID Design Con. Mini-Watersheds Options.

Runoff Paths. Lot OnlySubdivision water neutral

LID Design: LID practices – Entire subdivision or just the lot can be hydraulically neutral (same pre and

post development)

Locations of LID Practices and Flow Path.

Passive landscape

Native VegetationScientific Name Common Name Type Height

FtWidth

FtEvergreen

Or Deciduous

Water Requirements

Ceratoides Lanata Winterfat Shrub 3 2 Evergreen LowLarrea Tridentata Creosote Bush Shrub 8 6 Evergreen Low

Koberlinia Spinosa Crucifixion Thorn

Shrub 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 LowProsopis 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

Simulations

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

willow, acacia) 30 years of historical temperature and

rainfall data Plant 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)

Walton Household Example

Roof and carport water exit carport corner

Cobbles allow subsurface ponding and infiltration into soil

Soil stores water between rains

Design Spreadsheet

0 5 10 15 20 25 300%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%Soil Moisture Storage Relative to Capacity)

Time of Simulation, year

Wat

er S

tora

ge S

tatu

s, %

ful

l)

Monitoring

0 5 10 15 20 25 30 35 40 450

10

20

30

40

50

60

70

Soil Suction (centibars)Water Content (VWC%)

Day

rain rain

Summary

Passive rainwater harvesting works in El Paso

Capture/green area ratio from 10-25% Saves money Saves water Reduces flooding Provides a green, shaded lot, not

xeriscaping with a bunch of hot rocks Active systems generally not appropriate

for Southwest