Not a Drop to Spare-Rain Harvesting in West Texas

Special to the Reporter

Plant Water Requirements. The types and numbers of plants in your landscape, along with their growth stages and sizes, determine the amount of water your plants need to be healthy. Because rainfall varies throughout Texas, different plants have become adapted to conditions in different regions of the state. Plants native to your region are the best choices for your landscape because their water requirements are usually met by normal rainfall amounts.

Water Collection and Distribution System Rainwater collection and distribution systems can be incorporated into almost any existing site, although it is easier to incorporate them into new construction. Always contact your local water supplier prior to installing any system to ensure compliance with local requirements.

Simple Rainwater

Harvesting Systems A simple water harvesting system usually consists of a catchment, a distribution system and a landscape holding area, which can be a concave or planted area with a border or earthen berm to retain water for immediate use. Gravity moves the water from the catchment (for example, the roof) to a different location. Sometimes water is caught in small containers and stored for later use. Water dripping from the edge of a roof to a planted area or a diversion channel located directly below the drip edge is an example of a simple water harvesting system.

Catchments. A catchment is any area from which water can be collected, including roofs, paved areas and the soil surface. The best catchments have hard, smooth surfaces, such as concrete or metal roofing material. The amount of water harvested from a catchment depends on its size, surface texture, slope and rainfall received.

Distribution Systems Distribution systems channel water from catchments to landscape holding areas. Examples of distribution systems include gutters and downspouts, sloped sidewalks, hillsides, street and parking lot curb cutouts and channels, ditches and swales. If gravity does not cause water to flow though your distribution system, you may need to install a small pump, gates or diverters. You may need to line earthen distribution systems with an impermeable material such as plastic to keep water from soaking into non-target areas. Complex distribution systems, discussed later, also may include pipelines.

Landscape Holding Areas Concave depressions covered by grass or plants can store water for direct landscape use and reduction of flooding and erosion. Several such holding areas can be chained together through spillways. You can create holding areas by digging out depressions and keeping the resulting soil as a berm or by using berms, moats or soil terracing to make flat areas hold water. You should, however, be aware that digging may expose poorer quality subsoils unsuitable for landscape plants.

(continued from Page 1) Designing and Building a Simple Rainwater Harvesting

System

Step #1. Design the Collection and Distribution Systems.

Observe your landscape during a rain to identify its drainage pattern, and then use that pattern to decide how to move water from catchments to plants. Ideas for collection/ distributionsystemsinclude:

• Use your roof to collect rainwater, then extend downspouts, provide paths or use hoses to move the water to plants.

• Use existing sloped paving to reach plants.

• Design new sidewalks, terraces or driveways with a two percent (1/-l inch per foot) slope toward plants.

• Grade unpaved, bare soil to increase and direct runoff.

• Design landscape soil and plantings around foundations to slope away from buildings.

Step #2. Design Landscape Holding Areas.

Next, identify landscape depressions with potential for holding water or create new ones near plants. Tips for designing landscape holding areas include• Locate holding areas at least 10 feet away from buildings to avoid structural or pest problems.

• Build level berms or moats around plants to avoid

damaging roots, but do not mound soil at plant bases.

• To encourage large root systems, extend holding areas beyond the” drip line/’ the

outer perimeter of possible runoff from the catchment. (Plants with well-developed root systems are more drought tolerant.)

• Locate new plants at upper edges of concave holding areas to encourage large root systems and to protect them from flooding.

• Connect several holding areas with spillways or channels to distribute water throughout your site.

Step #3. Select Plants. To help your water harvesting project succeed, choose native or regionally adapted plants that can withstand both prolonged drought and prolonged inundation.

For example, excess water may collect at the bottom of a holding area due to soil saturation, so low-water-use, native riparian trees may be the best choice for large, deep basins. To determine the best plants for your region, consult a guide such

as Xeriscape-Landscape Water Conservation, publication B-1584 from the Texas A&M AgriLife Extension Service, a part of the Texas A&M University System (see http://agrilifebookstore.org). Other plant lists and resources are available at the Texas Master Gardeners’ Web site: http://aggie-horticulture. tamu.edu/mastergd.mg.html or http://aggie. horticulture. tamu. edu/extension/pu blications. h tm I.

To seed holding basins, select seed mixes containing native or adapted wildflowers, grasses and herbaceous plants. Perennial grasses will hold the soil and prevent erosion.

Compacting soil in landscape holding areas inhibits the movement of water through the soil. Loosen compacted soil by tilling. Add organic matter such as compost to soil that is too sandy to hold water. To reduce evaporation, apply a 1.5- to 2-inch layer of mulch after planting. Organic mulches may increase permeability of tight clay soils but may float if inundated.

Tips on how best to use water free-falling from roof downspouts include:

• Plant large, rigid plants where the water falls.

• Hang a large chain from the downspout to the ground to disperse and slow the water.

• Provide a storage basin to slow the falling water.

• Place rocks or other hard materials under the downspout to prevent erosion by breaking the fall of the water.

• Plant large, rigid plants where the water falls.

• Hang a large chain from the downspout to the ground to disperse and slow the water.

• Provide a storage basin to slow the falling water.

• Place rocks or other hard materials under the downspout to prevent erosion by breaking the fall of the water.

Rain falling on impermeable surfaces runs off immediately, potentially scouring bare soil and creating pockmarked roads. Because the roofs, roads and parking lots that cluster in urban areas are impermeable, even moderate rainfall there may produce large runoff. Water harvesting can prevent flooding and erosion and turn storm water problems into water supply assets by slowing runoff and allowing it to soak into the ground.

Methods to harvest urban rainwater include:

• Crescent-shaped berms around plants on slopes.

• Gabions (piles of large rocks encased in wire mesh).

• French drains (holes or trenches filled with gravel).

• Permeable paving materials, such as gravel, crushed stone and open or permeable paving blocks, on steep slopes.

• Terrace grading (stair-step-like shelves) of slopes.

Designing and Building Complex Rainwater Harvesting Systems

Steps involved in designing a complex water harvesting system include site analysis, calculations, system design, construction and field testing. For large projects or those with several catchments and planting areas, divide the project site into subdrainage areas and repeat these steps for each.

Step #1. Site Analysis

• Draw the site to scale, using arrows to plot existing drainage patterns observed during a rain and showing high and low areas on your sketch.

• Identify possible catchments, such as pavements, roof surfaces and bare earth.

• Identify areas requiring irrigation and sites near them where storage could be located (either above ground or underground).

• Think about ways to move water from catchments to holding areas or storage containers. (Use gravity wherever possible.)

• Think about ways to move water through the site from one landscaped area to another.

Step #2. Calculations To design a complex water harvesting system, you must first calculate monthly “Supply” (rainfall harvest potential) and monthly “Demand” (plant water requirements), then calculate monthly “Storage/ Supplemental Municipal Water Requirement.”

Calculate Supply. The following equation calculates the amount of water (in gallons) that can be harvested from a catchment.

• Multiply rainfall in inches (see Appendix I) by 0.623 to convert inches to gallons per square foot.

• Multiply the result by the area of catchment in square feet (ft2 ). (For example, a 10 foot x 20 foot roof is 200 fo. For a sloped roof, measure the area covered by the entire roof, usually the length times the width of the building.)

• Multiply this result by the “Runoff Coefficient” (see Appendix II) to obtain the available supply. (The runoff coefficient is the percentage of total rainfall that can be harvested from a particular surface. The higher the runoff coefficient, the less absorbent the surface.)

Example 1: Calculating

Supply

Susana wants to build a rainwater harvesting system for her home in San Antonio, Texas. From Appendix I, she enters the rainfall for each month on the Supply Worksheet (see sample on next page). Then she multiplies the inches of rainfall by 0.623 to convert inches to gallons per square foot.

Susana has an ‘’L’’-shaped house with asphalt shingle roofing that she plans to use as her primary catchment area. To simplify measurements, she divides the house into two rectangular sections, A and B. The eave-to-eave measurements for section A are 45 ft. x 25 ft., and for section Bare 20 ft. x 25 ft.:

Susana has 1,625 square feet of catchment area. She enters this value in Column C of the Supply Worksheet. She then multiplies the gallons per square foot in Column B by the square footage in Column C to determine the total gallons of rainfall received each month by her catchment. Since the asphalt shingle roof won’t shed all of this rainfall, Susana finds the appropriate runoff coefficient ( 0.9) in Appendix II and enters it in Column E.

Finally, Susana multiplies Column D by Column E to obtain the net harvestable rainfall for the month. Calculating Demand: Susana has a small lawn with a sprinkler system and a 1,200-square-foot densely planted area of high-wateruse trees, shrubs and flowers. To avoid the expense of installing an electric pump, Susana wants to operate her rainwater project by gravity flow. But because the sprinkler system cannot be operated by gravity flow, she decides to use her rainwater system to irrigate only her flowers, trees and shrubs.

Once Susana has calculated supply and demand for each month1 she can determine her system’s maximum storage needs. Although containers of any size will reduce Susana’s dependence on municipal water1 to take full advantage of available rainfall she should build enough storage to meet total irrigation water needs

Calculate Maximum Storage/Supplemental Water Use. Once you have calculated how much rainfall you can potentially harvest and how much irrigation water you need1 use a “checkbook” method to determine monthly harvested water balance and amount of supplemental water (municipal or from another source) needed to meet any shortfalls.

To keep things simple1 calculations are performed on a monthly basis1 although the amount of available water changes daily.

“Cumulative Storage” refers to available water. To determine the current month’s cumulative storage1 add the previous month’s cumulative storage to the current month’s yield, then subtract the current month’s demand from that total. If the remainder is positive, place it in the Cumulative Storage column for the current month. If the remainder is negative ( that is, if irrigation demand is greater than stored water supply), place it in the Supplemental Water Use column to indicate the amount of supplemental water needed for that month.

Balancing Supply and Demand. In this scenario1 during the summer Susana’s landscape demand always requires a supplemental water supply. But during the winter months1 rainwater supply exceeds demand due to low evapotranspiration rates1 so water can be saved for spring and early summer water deficit periods.

Every site generates unique supplies and demands. For some sites1 rainwater harvesting systems always provide enough water to meet irrigation demands1 while for others1 harvesting can only partially satisfy such demands. Remember that supply fluctuates from year to year1 depending on the weather (when and how much it rains). Demand can increase with warmer-than-normal weather1 as the landscape ages and plants grow larger1 and while new plants get established.

If supplies of harvested water do not meet irrigation demands1 balance your water harvesting checkbook either by increasing supply or by reducing demand.

To increase supply: • Increase your catchment’s area or runoff coefficient.

• Use another source of water1 such as your municipal supply.

To reduce demand:

• Reduce landscaped area.

• Reduce plant density.

• Replace high-wateruse plants with lower-wateruse plants.

• Use mulch to reduce surface evaporation.

Step #3. Final design and construction.

Catchments and Landscaping. Use your site analysis and your supply and demand calculations to determine size and location for catchment areas. Use gutters and downspouts to carry water from a roof catchment to your storage areas. ( Consult Appendix VI for tips on selecting and installing gutters and downspouts.) Design or retrofit roofs or shade structures to maximize your catchment area. If you cannot provide a catchment large enough to meet maximum landscape water requirements:

• For existing landscapes1 reduce plant water demand either by lowering plant density or by selecting lower-water-use plants.

• For new landscapes1 select types and numbers of plants that can be supported by the water harvested from your existing catchment.

To use harvested water most efficiently1 group together plants with similar water needs. And remember that new plantings1 even of native plants1 need increased amounts of irrigation during their establishment period, which can range from 1 to 3 years. (Use supply and demand calculations to determine the amount of water needed for new plantings.)

Storage Containers and Distribution. Use storage containers large enough to hold your calculated supply. Place containers close to plants and, to take advantage of gravity flow, higher than planted areas. Use pipes, hoses, channels and drip systems to distribute water. For drip systems and those without gravity flow, use a small pump to move water through the lines. Select drip irrigation filters with 200mesh screens and clean them regularly.

Step #4. Field testing.

Once you’ve built your water harvesting system, “field test” it during rains. Determine whether water is moving where you want it to go or whether some of it is being lost. Determine if holding areas adequately contain water. Make changes to your system as required.

Developing a water harvesting system is actually an ongoing process to be improved and expanded over time. For example, you may discover additional areas where water can be harvested or channeled. Inspect your water harvesting system before each rainy season (and, ideally, after every rainfall) to keep the system operating optimally.

Use this maintenance checklist to keep your system in top condition:

• Keep debris out of holding areas.

• Control and prevent erosion; block erosion trails.

• Clean and repair channels.

• Clean and repair dikes, berms and moats.

• Keep debris out of gutters and downspouts.

• Flush debris from storage container bottoms.

• Clean and maintain filters, especially those on drip irrigation systems.

• Expand watering basins as plants grow.

Once your system is operating, monitor landscape water use to find out the amount of water saved. For new water harvesting basins in existing landscapes, compare previous years’ (preharvesting) water bills with postharvesting figures. When new plants are added to a water harvesting area, water savings begin as soon as they are planted and continue for as long as you irrigate with harvested rainwater. Appendix VI

Guidelines for Gutter and Downspouts

Gutters and downspouts for distributing rainwater should be the correct size, durable, attractive and well-suited to buildings on which they are used. Because specifications depend on gutter type and special considerations such as snow load or roof construction, you should consult a company specializing in gutter design and installation for product advice.

General guidelines for selecting and installing gutters include:

• Select gutters at least 5 inches wide.

• Select gutters made from galvanized steel (29 gauge minimum) or aluminum (.025 inch minimum).

• To enhance flow, slope sectional gutters 1/16 inch per 1 foot of gutter; slope seamless gutters 1/16 inch per 10 feet.

• Use expansion joints at connections for straight runs exceeding 40 feet.

• Keep the front of the gutter 1/2 inch lower than the back.

• Provide gutter hangers at least every 3 feet (every foot in areas of heavy snow load).

• Select elbows with 45, 60, 75 or 90 degree angles, as needed.

General guidelines for selecting and installing downspouts include:

• Space downspouts from 20 to 50 feet apart.

• Provide 1 square inch of downspout area for every 100 square feet of roof area. (A 2-inch-by-3-inch downspout will accommodate 600 to 700 square feet; a 3inch-by-4-inch downspout will accommodate up to 1,200 square feet.)

• Do not exceed 45-degree angle bends.

• Select downspout configuration (square, round or corrugated round) depending on your needs.

• Use 4-inch-diameter pipes to convey water to storage containers or filters.

Any questions? Please contact:

Dale Adams, Manager, Wes-Tex Groundwater Conservation District, 100 E. 3rd street, STE 305b, Sweetwater, Texas, 79556. Office #325-236-6033