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

15. 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

16. El Paso

17. 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)

18. 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?

19. 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

20. 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?

21. Capture Zone Impermeable areas concentrate water in vegetated areas Consider that if rainfall is increased by 5X, El Paso has a lot of water capture area native plants capture area/plant area

22. 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

23. 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

24. Soil Moisture Storage Water storage = Vsoil * (field capacity – wilting point)

25. 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

26. 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

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

28. 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

29. Mesquite Root System Plants can be located some distance from bioretention areas

30. Mesquite Roots

32. 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

33. But you can’t grow trees in El Paso without watering!

34. Accidental Example Near UTEP

35. Second UTEP Example water from parking lots

36. Accidental case 3

37. 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

38. Distribute water to soil and have sufficient storage for flood control and plant growth French drains/depressions/trenches filled with sorted gravel Impermeable area (roof, parking lot)

39. LID Design The Model House

40. LID Design Con. Mini-Watersheds Options. Subdivision water neutral Runoff Paths. Lot Only

41. 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.

42. Passive landscape • Native Vegetation

43. 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

44. 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)

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

46. Design Spreadsheet

47. Monitoring

48. 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