Simplified Landscape Irrigation Demand Estimation Roger Kjelgren and Larry Rupp Center for Water Efficient Landscaping Utah State University A New Method of Estimating Landscape Water Demand

Why do we use water in landscapes? “I was nearly twelve before I saw either a bathtub or a water closet; and when I walked past my first lawn,… I stooped down and touched its cool nap in awe and unbelief. I think I held my breath - I had not known that people anywhere lived with such grace.” Wallace Stegner

Why should you understand landscape water demand? 1.Water allocation o Water agencies increasingly are allocating fixed amounts to end user o Landscape designs need to estimate expected demand within water allocation; agencies can track through billing data analysis 2.Landscape architect/contractor/maintenance o Guidelines for irrigation system design o Aid in irrigation scheduling – when and how much? o Drought o Minimum water for survival o Especially important for woody plants

Defining Landscape Water DemandDefining Landscape Water Demand Irrigation is used to bridge the difference between water demand and water availability

How is water demand typically quantified? Inches of water lost to evapotranspiration (ET) Amounts to about inch/day

Evapo-transpiration Rate: EToEvapo-transpiration Rate: ETo Calculated water lost from hypothetical 12 cm/4 inch high cool season, clipped/mowed turf<=sun, heat, wind, humidity Range 0 – 6 mm/day (0-0.25”/day) Plant Transpiration Soil Evaporation + ET=

ETo is based on field research with agricultural crops Uniform crop surface similar to turfgrass surface

Estimating Water Demand Based on ETo Determine the ETo (historical or real time) Determine the percentage of ETo that provides optimum yield for a specific crop (Plant Factor-Kp) Determine ETa (actual) by multiplying ETo by Kp Replace ET by irrigation as needed based on soil type, etc. Plant Water Use (Eta) = ETo X Kp

ETo-Kp Approach: Turf OKETo-Kp Approach: Turf OK

ETo Approach: Problems Assumes continuous uniform surface, water use 2-D; urban landscapes often non-uniform, non-continuous, water use 3-D Assumes fixed Kp to avoid water stress for optimum yield; landscapes do not have economic yield-how much is enough? Assumes small # food/fiber species; >1,000 landscape species differing in water use traits

13 ETo Background: Problems; #2ETo Background: Problems; #2 In landscapes, $ value water input ≠ ≈ value of landscape Instead, minimum water demand for acceptable appearance Water applied Economic Yield

Prescriptive Landscape Water Demand Estimation: ETo-Kp California-Water Use Coefficients of Landscape Species (WUCOLS) Landscape Water Use = ETo X Kp X Density Factor X Microclimate Factor Species factor=3 water demand categorizations based on committee decision EPA Water Sense ETo x Kp, but Kp variable based on plant type, size Too comple: c omplicated, difficult, false precision

The very complexity of excellent landscapes makes it difficult to prescribe a water management plan A descriptive approach is more practical

Law complexity- simplicity: Complexity, precision Precision, simplifying Simplifying, knowledge Law supply-demand: Water supply, Knowledge Law complexity- simplicity: Complexity, precision Precision, simplifying Simplifying, knowledge Law supply-demand: Water supply, Knowledge Prescriptive=complex Prescriptive=complex Descriptive=simplifying with knowledge Descriptive=simplifying with knowledge

Descriptive Approach: Simplified Landscape Irrigation Demand Irrigation= SLIDE Rules Kp = minimum water demand =>acceptable appearance may include water stress Assume all species Kp = 50% of ETo unless evidence otherwise Number of transpiring leaves more important than Kp

Minimum water demand; many landscape species can be water stressed but look OK

SLIDE RulesSLIDE Rules If water stress OK, how much water needed to avoid not OK Assume Kp = replace 50% of ETo will work for almost all landscape plants When you know a species can tolerate less, irrigate at lower rate; greatest risk with trees that are greatest investment

Minimum % of ETo required for trees varies with climate and species, but 50% safely overlaps most non turf plants

Reference evapotranspirationReference evapotranspiration ETo for Salt Lake City: water to MonthMayJuneJulyAug. Sept. Ave Monthly ETo, in Rainfall, inches Net monthly water demand turf, 80% ETo minus rain Net monthly water demand trees 50% ETo minus

Reference ET and Leaf AreaReference ET and Leaf Area More leaves = more water loss Larger trees or increased numbers of trees takes more water Desert is characterized by large distances between plants

Native Species: Evidence of Water UseNative Species: Evidence of Water Use 23

Native Species: Response to MicroclimateNative Species: Response to Microclimate 24 Eriogonm corymbosum Medium/small leaves, evergreen More broadleaf species Small, thick, blue leaves Very small to no leaves Aspen Oak Buffaloberry Mormon tea

USU Botanical Center Landscape Lysimeter Study

Simplified Landscape Irrigation Demand Estimation: SLIDE Rules Minimum demand is based on acceptable appearance, therefore a mild deficit is okay Basic assumption is woody plants have a coefficient of 0.5 unless there is concrete evidence for a higher or lower value Actual leaf area is best indicator of total water use Total Plant Water Use = ETo X Kp X Leaf Area

27 Transpiring leaf area ≈ crown diameter 2 Crown diameter 2 may underestimate true leaf area

How do you estimate leaf area?How do you estimate leaf area? 6’ A tree with a 6 ft crown diameter has roughly 36 ft 2 of leaf area Simplifying assumption: crown diameter in ft 2 ≈ gallons needed to apply 2 inches of water

Simplest SLIDE Approach for Drip Irrigated Non Turf Plants How much: apply 2 inches water every irrigation = crown diameter 2 How often: seasonal ETo (after rain stops) x 0.5 (Kp) ÷ 2 inches/irrigation

What is the risk of plant damage if estimate to high or low ? Drought can also harm or kill landscape plants

Reducing risk? KnowledgeReducing risk? Knowledge Understand plant types and adaptations Use good plant selection for situation Be aware of soil conditions Be observant Be flexible

Summary SLIDE RulesSummary SLIDE Rules Non turf urban landscapes are complex; simplify water demand Knowing volume of transpiration (from leaf area) more important than rate of water use Assume 0.5 Kp as rate of water use (% ETo) for all plants unless evidence otherwise Irrigation amount: same each irrigation Drip irrigation, 2” water ≈ gallons needed crown diameter 2 Irrigation frequency ≈ ETo x 0.5 ÷ inches per irrigation

Questions?