Irrigation Scheduling

General Approaches Maintain soil moisture within desired limits Maintain soil moisture within desired limits – direct measurement – moisture accounting Use plant status indicators to trigger irrigation Use plant status indicators to trigger irrigation – wilting, leaf rolling, leaf color – canopy-air temperature difference Irrigate according to calendar or fixed schedule Irrigate according to calendar or fixed schedule – Irrigation district delivery schedule – Watching the neighbors

Yield/Appearance vs. ET c

Deficit Irrigation operate here for full irrigation operate here for deficit irrigation (reduced water use & reduced yield)

Growth/Yield vs. fr (Figure 6.2)

Water Availability Thresholds fr min = minimum fraction of available water remaining before plant stress occurs fr min = minimum fraction of available water remaining before plant stress occurs fd max = maximum allowable fraction of available water depletion before plant stress occurs fd max = maximum allowable fraction of available water depletion before plant stress occurs fd max = 1 - fr min fd max = 1 - fr min fd max depends on species, genotype, weather fd max depends on species, genotype, weather Rule-of-Thumb:fd max = 0.50 (use this rule with caution) Rule-of-Thumb:fd max = 0.50 (use this rule with caution)

fd max by Crop and Maximum Crop ET Rate

Possible Irrigation Scheduling Management Objectives Maximum yield/biomass productionMaximum yield/biomass production Maximum economic returnMaximum economic return Functional value of plants (e.g., athletic fields)Functional value of plants (e.g., athletic fields) Aesthetic value of plants (e.g., landscapes)Aesthetic value of plants (e.g., landscapes) Keeping plants aliveKeeping plants alive

-Moisture Accounting- Soil Water Reservoir TAW = (R d ) (AWC) (from Chapter 2) TAW = (R d ) (AWC) (from Chapter 2) – TAW = total available water, (in. or mm) AD = fd max (TAW) AD = fd max (TAW) – AD = management allowed deficit, (in. or mm) MB = fr min (TAW) MB = fr min (TAW) – MB = minimum balance (min. allowable available soil water),(in. or mm) TAW = AD + MB

Plant Root Zones Depth used for scheduling vs. maximum depth where roots are found Depth used for scheduling vs. maximum depth where roots are found Influenced by soil characteristics Influenced by soil characteristics –Soil texture –Hardpan –Bedrock Perennial vs. annual plants Perennial vs. annual plants

Turf & Shrub Rooting Depths

Turfgrass Root Depth Potential Rooting Depth for Turfgrass Species (Dr. L. Wu, U. C. Davis, 1985) Shallow (1-8 inches) Medium (8-18 inches Deep (18-60 inches) Poa annua Kentucky bluegrass Zoysiagrass Creeping bentgrass Red fescue Bermudagrass Colonial bentgrass Ryegrass Tall fescue St. Augustinegrass

Root Development of Annual Plants Assumes linear increase in rooting depth from germination to maximum depth

Rule-of-Thumb Divide the crop root depth into quartersDivide the crop root depth into quarters Upper ¼ provides 40% of water uptakeUpper ¼ provides 40% of water uptake 2 nd ¼ provides 30% of water uptake2 nd ¼ provides 30% of water uptake 3 rd ¼ provides 20% of water uptake3 rd ¼ provides 20% of water uptake Lowest ¼ provides only 10% of water uptakeLowest ¼ provides only 10% of water uptake Applies only when most of root zone irrigated to field capacityApplies only when most of root zone irrigated to field capacity Dictated by distribution of root massDictated by distribution of root mass

Maximum vs. Effective Rooting Depth

Irrigation Timing Maximum irrigation interval, (days)Maximum irrigation interval, (days) Actual irrigation interval, (days)Actual irrigation interval, (days) d e = effective depth of irrigation, (in. or mm)

Latest Date LD = maximum number of days before irrigation should occur LD = maximum number of days before irrigation should occur ET c (forecast) can be based on long-term averages or last few days ET c (forecast) can be based on long-term averages or last few days

Latest Date

Earliest Date ED = minimum number of days before irrigation should occur ED = minimum number of days before irrigation should occur d ep = planned effective depth of water d ep = planned effective depth of water r a = rainfall allowance (allow room in the profile beyond d ep ) r a = rainfall allowance (allow room in the profile beyond d ep )

Earliest Date

Components of Crop Root Zone Water Balance

Soil Water Budget Calculations SWD i = SWD i-1 + ET c i-1 - d e i-1 - P e i-1 - U f i-1 Subscripts: i = todayi-1 = yesterday (all quantities below in consistent depth units: inches, mm, etc.) SWD= soil water deficit ET c = crop evapotranspiration d e = effective irrigation P e = effective precipitation U f = upward flow of water from a shallow water table

LD

Other Irrigation Scheduling Methods Soil Water MeasurementSoil Water Measurement –Determine SWD by measuring: fr or fd (feel and appearance of soil)fr or fd (feel and appearance of soil)  m (gravimetric sampling)  m (gravimetric sampling)  v (neutron scattering)  v (neutron scattering)  p (potential: w/ tensiometers or resistance blocks (must convert  p to water content)  p (potential: w/ tensiometers or resistance blocks (must convert  p to water content) –Need measurements at several locations –Need measurements throughout root zone depth –Difficult to predict Latest Date –Doesn’t indicate how much water to apply

Other Irrigation Scheduling Methods Plant Status IndicatorsPlant Status Indicators –Leaf water potential (energy status of leaf water) Use pressure chamber or thermocouple psychrometerUse pressure chamber or thermocouple psychrometer Measured at mid-day; many samples neededMeasured at mid-day; many samples needed –Foliage/Air temperature difference Well-watered plants cooler than airWell-watered plants cooler than air Use infrared thermometerUse infrared thermometer –Leaf appearance Color, wilting, etc.Color, wilting, etc. Indicators show up too lateIndicators show up too late –Irrigate at critical growth stages (e.g.: flowering)