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?
native plants Impermeable areas concentrate water in vegetated areas Consider 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 flooding b)Lowers water use c)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
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
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
Native Vegetation Scientific NameCommon NameType Height Ft Width Ft Evergreen Or Deciduous Water Requirements Ceratoides LanataWinterfatShrub32EvergreenLow Larrea TridentataCreosote BushShrub86EvergreenLow Koberlinia Spinosa Crucifixion Thorn Shrub57EvergreenLow Atriplex Canescens Four Wing Saltbush Shrub68 Simi- Evergreen Low Leucophyllum Frutescens Texas Sage/Ranger Shrub4-8 EvergreenLow Acacia BerlandieraGuajilloShrub12 DeciduousLow Prosopis GlandulosaHoney MesquiteTree30 DeciduousLow Chiloposo LinearisDesert WillowTree2520DeciduousLow Fraxinus greggiiGregg’s AshTree158 Semi- Evergreen Low Quercus ArizonicaArizona White Oak Tree3530EvergreenLow
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
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)