Adapting low impact development to the Chihuahuan Desert

1. Adapting low impact development to the Chihuahuan Desert John Walton University of Texas at El Paso

10. Flood control, water savings, green environment • 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?

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

12. 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?

13. Capture Zone 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 native plants capture area/plant area

14. Stormwater periodically diverted to shallow depressions with native vegetation • Reduces flooding • Lowers water use • Green environment swale swale street

15. 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

16. Soil Moisture Storage

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

18. 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

19. Storage in Soil Mulch (usually rock) soil Landscape cloth (screen) Must block weeds and let water into soil, storage is in the soil

20. Mesquite Root System

21. Mesquite Roots

23. Soil Moisture Storage • Root depths > 5 meters (16.4 ft) (mesquite) • Root span > 12 m (39 ft) (mesquite) • Volume > 565 m3 (20,000 cubic feet)

24. Soil Moisture Storage • 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

25. Accidental Example Near UTEP

26. Second UTEP Example water from parking lots

27. Accidental case 3

28. How is it done? • 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

29. Depressions • 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

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

31. LID Design The Model House

32. LID Design Con. Mini-Watersheds Options. Runoff Paths. Option 1. Option 2.

33. LID Design: LID practices – Selection, and Placement. Locations of LID Practices and Flow Path.

34. LID Design Con. Cross Sections of a Bioretention Cell and a Vegetated Swale.

35. LID Design Con. French Drain Impermeable Mulch

36. Passive landscape • Native Vegetation

37. 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.

38. Simulations • 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

39. 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)

41. El Paso: Capture Area/Crown Area = 2Not Enough Water

42. El Paso: Capture Area/Crown Area = 5Water in Dry Periods (usually May-June)

43. El Paso: Capture Area/Crown Area = 10 Water in Extreme Drought 1/10 years

44. El Paso: Capture Area/Crown Area = 15Drought Proof

45. Class Exercise • 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.

46. Walton Household Example • Roof and carport water exit carport corner • Cobbles allow subsurface ponding and infiltration into soil • Soil stores water between rains

47. Field Method • Soil Moisture Energy • Volumetric water content (VWC%) • Soil suction (cetibars) TDR Tensiometer

48. Field Study. Con…

49. Monitoring

50. Summary • 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)