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Water Requirements

Irrigation water requirement is the quantity of water which needs to be applied with the irrigation system. The irrigation water requirement considers a plant’s evapotranspiration (ET), irrigation losses, rain, and leaching requirements. Typically only about 65 percent of the total irrigation water delivered is available for plant growth, the balance is lost to leaching or held too tightly by soil particles to be absorbed by plant roots. The portion of the precipitation available to plants depends on the timing and amount.

Evapotranspiration (ET)

Weather and Microclimate Evapotranspiration is the combination of evaporation of water from the soil and transpiration from the plants. Evapotranspiration is necessary for plant growth (photosysthensis); it maintains a healthy plant temperature and provides for the transportation of nutrients to and through the plant. Evapotranspiration requires energy. Energy comes from radiation and advection. Radiation usually comes from direct sunlight, and advection comes from heated air surrounding the plant. In addition to the energy required for evaporation, the air above the plant needs to be able to accept more water. The drier (lower humidity) and hotter the air, the more water the air can hold. If energy is being added to the plant and no evaporation is taking place, the temperature of the plant will increase.

ET can be estimated by mathematics equations that use weather data (temperature solar radiation, humidity, rain and wind) to determine available energy and humidity to evaporate water. A potential ET can be adjusted to a particular plant type, plant growth stage, plant population, vigor, and stress.

The location of a plant in a landscape affects its ET rate because of differences in available energy for evaporation. The following lists show locations that increase and decrease plant ET.

Increases available energy
  • — south or west exposures
  • — reflected sunlight from surfaces
  • — non-vegetative surroundings parking lots, streets
  • — proximity to desert
  • — exposure to dry hot winds or wind channeling

Decreases available energy
  • — north or east exposures
  • — shade by building
  • — shade by other plants
  • — sheltered from the wind
  • — locations in the center of largeirrigated or wet areas

Plant Water Use

Plant species have different rates of ET based upon the characteristics of the plants and available soil water. Plant stomates are the evaporation surfaces on the leaves. Plant leaves control transpiration by stomatal closure. Leaves that reflect more of the sun's radiation (gray or silver) usually transpire water at a lower rate than green leaves. Plants that can tolerate higher leaf temperatures evaporate water at a lower rate.

Low water use plant characteristics
  • — Low fertilizer requirements
  • — Slow growing plants
  • — Small or narrow leaves or leaves that roll-up during high temperatures
  • — Leaf modifications (color, hairy, waxy, sunken or reduced number of stomates)
  • — Small plant size

Drought tolerant plants are not necessarily low water use plants and vice versa. For example, mesquite trees are drought tolerant, but are high water users when water is available. Drought tolerant plants go dormant or near dormant when soil water is unavailable and then become active when water is available. Some low water use plants are not drought tolerant. Many plants not normally considered low water use species become water thrifty for survival when soil moisture is limited. Some plants considered low water use species will use water at a high rate if water is available and revert to low water use when not available. Low water use plants don’t conserve water if they are irrigated as high water use plants.

Available soil water also affects the rate of transpiration from plants. If soil moisture is limited, then transpiration and plant growth decrease. If soil water is abundant and not limiting plant growth (this does not mean that it is being over-irrigated), turfgrasses and many woody plants will maximize their water use and maximize their growth. This may or may not be desirable. For plants that need to survive and reproduce from season to season, this allows them to optimize a limited amount of natural rainfall. For landscape plants under constant abundant irrigation, this promotes succulence. Succulent growth does not withstand traffic or wear (turf); is more susceptible to disease and mechanical damage (wind); promotes uncontrolled growth; encourages high water use; decreases drought tolerance; and decreases tolerance to heat and cold. Research has established that cool season turfgrasses like perrenial ryegrass can survive at 80 percent of its maximum water use and not reduce turf quality (80 percent of actual Evapotranspiration (ET) rate). Bermudagrasses (not overseeded) can preform well at 60 percent of its actual ET. However, caution should be exercised when trying this over extended periods because of the accumulation of soil salts that will occur over time when irrigating with water containing these salts. This same reduction in applied water on ornamental plants and its effects has not been examined. Generally speaking, the higher the aesthetic expectations are from a landscape and the amount of "abuse" expected, more water is required to meet these requirements.

Other factors that affect plant water use are soil fertility, turf mowing height and frequency. Fertilizer applications that stimulate growth increase plant water use in turf, ornamentals, fruits and vegetables. Pruning of landscape plants also promotes growth that results in greater water use. High, frequent mowing of turfgrass increases water use by providing more leaf surface for transpiration. However, this type of mowing also increases rooting depth , thus making turfgrass more drought tolerant.

Evaporation from open water surfaces is about 1.25 to 1.5 times more than from a well irrigated turf area. There are no leaves and stomates to limit evaporation from an open water surface. Natural and manmade lakes can also lose water through leakage and deep percolation.

Tips for minimizing plant water use:
  1. Select native and low water use non-native plants whenever possible.
  2. Select smaller plants over larger plants whenever possible.
  3. Use as much hardscape or surface mulched areas as possible.
  4. Reduce fertilizer use to the lowest level possible while maintaining acceptable plant health and aesthetics.
  5. Use surface mulches around plants and in bare soil areas.
  6. Avoiding excessive irrigations.
  7. Water trees, shrubs, ground covers, and herbaceous plants to their potential rooting depth.
  8. Zone irrigation systems, separating plant materials by water use, exposure, topography and soil type.
  9. Increase mowing height of lawns to allow plants to develop deeper root systems.
  10. Keep the lawn mower blade sharp. Sharp mower blades make cleaner cuts that cause less water loss than cuts from dull mower blades.
  11. Control all weeds. Weeds use water that would otherwise be available for desirable plants.
  12. Cull plants that are growing poorly. Don't waste water caring for marginal or undesirable plants.
  13. Apply wetting agents to hydrophobic (water repelling) soils.
  14. Match nozzle and emitter to deliver the same gallonage output.
  15. Keep sprinkler heads and drip emitters clean to ensure uniform water distribution.
Irrigation losses

Irrigation losses consist of spray drift and evaporation losses, deep percolation due to over-irrigation, uniformity losses, required salt leaching and runoff. Spray and drift losses range from 10 to 30 percent. The losses depend on time of application, wind, sprinkler type and sprinkler water pressure. High pressures break up the water into small drops that creates more evaporation and drift. Deep percolation due to irrigation non -uniformity and overestimates of plants' needs range from 10 to 35 percent. Uniformity losses include uniformity of application, as well as uniformity of soil infiltration. Once an irrigation system is installed, it will have a characteristic uniformity of application (how evenly water is applied to the site), which is dependent on:

  • how well the irrigation components were engineered.
  • how the system was designed.
  • how the system was installed.
  • how the system was maintained.

Irrigation components from major manufacturers will give reasonable uniformities when the correct nozzles are chosen, water pressure is regulated with an appropriate design and maintenance schedule.

It would be advisable that a new system should be designed or approved by a certified irrigation designer. After installation, the system should be audited by a certified auditor or someone trained to perform audits, with a minimum acceptable, uniformity established prior to the installation and agreed upon by the client, designer and contractor. Once a system has been installed with a guaranteed uniformity and audited, the maintenance contractor must be aware that any changes in head spacing, number of heads, nozzle sizes, head manufacturer or model, pipe sizing and operating pressure will decrease uniformity and increase water use. The maintenance contractor (when applicable) should be responsible for regular replacement of worn nozzles and emitters, checking heads and emitters for proper operation, clearing and cleaning heads and emitters of blockage and debris, replacement or maintenance of non-operating heads and emitters, fixing leaks and breaks, regular maintenance of irrigation components and monthly irrigation scheduling.

Just because a system applies water uniformly does not mean plants receive water uniformly. Problems associated with systems applying water more rapidly than soils can absorb; 2) slopes and landscape mounds; and 3) compacted soils all decrease the time available for infiltration to take place, leading to runoff and puddling in low spots. This requires the application of more total water than is needed to cure dry areas.

Correcting irrigation losses from spray drift, wind and evaporation:
  1. Use low volume drip or micro-spray irrigation.
  2. Use a pressure regulator on the system if pressure is too high or booster pumps if too low.
  3. Use low angle nozzles in windy locations.
  4. Irrigate during early morning hours when winds and evaporation are typically lower.
  5. Design spacing of heads to compensate for windy locations.
  6. Select heads and nozzles that provide a predominance large water droplet sizes rather than fine sprays.
Correcting losses from deep percolation:
  1. Calculate irrigation run times to wet the observed root zone, no more.
  2. Zone irrigation systems to types of plant material and their characteristic rooting depths
  3. Know your water quality so the proper leachingfraction can be included in an irrigation.
Correcting uniformity losses include:
  1. Correct all obvious irrigation distribution problems, e.g., sunken heads blocked heads, non-rotating or plugged heads, tilted heads, replace worn or improperly sized nozzles and spray angles replace substituted heads for design-specified heads, check "As-Built" irrigation design for field compliance, and review any designs done by a non-certified designer with a certified designer.
  2. Do a complete irrigation audit, recording catch can values corresponding to sprinkler head locations.
Correcting runoff losses:
  1. Aerate slopes and compacted soils.
  2. Construct reservoirs around landscape plants irrigated with bubblers.
  3. Select drip emitters with a lower G.P.H. (gallons per hour) output.
  4. Schedule irrigations with several stop/start cycles to increase infiltration time.
  5. Redesign slopes and mounds to eliminate turf and concentrate turf on flat areas.
  6. Place a landscape "buffer area" designed with drip irrigation between turf and parking lots, sidewalks, driveways and hardscapes.
  7. Not placing overhead irrigation on median strips or planter areas.

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