AZMET EVAPOTRANSPIRATION ESTIMATES: A TOOL FOR IMPROVING WATER MANAGEMENT OF TURFGRASS Paul W. Brown Extension Biometeorologist INTRODUCTION Proper irrigation management is essential to the successful production of turfgrass in Arizona. The Arizona Meteorological Network (AZMET) presently provides weather informationto assist irrigation management throughout southern and central Arizona including the Phoenix and Tucson metropolitan areas. AZMET information can be particularly useful when determining how much water has been used by a turfgrass surface since the previous irrigation. This document discusses the proper procedures for using AZMET information for estimating turf water use. EVAPOTRANSPIRATION Evapotranspiration (ET) is the loss of water from a vegetative surface through the combined processes of plant transpiration and soil evaporation (ET is equivalent to and frequently referred to as consumptive use). Both environmental and biological factors affect ET. Important environmental factors include solar radiation, temperature, atmospheric dryness (vapor pressure deficit), wind and soil moisture. Biological factors affecting ET include type of vegetation, foliage geometry and foliage density. Several methods have been developed to estimate crop ET. Most methods use weather data to provide an estimate of reference (or potential) evapotranspiration (ETo), often convert the ETo to "actual" ET using a multiplicative factor known as a crop coefficient (Kc): ET = Kc x ETo The Arizona Meteorological Network provides daily estimates of ETo for all locations served by AZMET's remote weather station network. AZMET ETo values are determined using a weather-based model known as the Penman Equation. Weather parameters utilized in the Penman calculation include solar radiation, temperature, humidity and wind speed. CROP COEFFICIENTS Reference crop evapotranspiration (ETo) is an estimate of the water used by a well-watered, full-cover grass surface, 8-15 cm in height (the reference crop). As previously mentioned, a correction factor or crop coefficient (Kc) is required to convert ETo to ET for a specific crop. Crop coefficients for turfgrass depend on the type of grass (warm or cool season), cutting height and desired turf quality. The following Kc recommendations are based on results of research conducted at the University of Arizona: Type of Grass Cutting Height Quality Kc Warm Season 2 - 2.5 cm High (Golf Course) 0.8 Warm Season 4 - 5 cm Acceptable (Park) 0.65 Cool Season 2 - 2.5 cm High (Golf Course) 0.95 ESTIMATING TURF WATER USE USING ETo AND CROP COEFFICIENTS The following example shows the procedure for using AZMET ETo to estimate daily water use of turf. Remember Turf ET = Kc x ETo. Step 1. Obtain the AZMET ETo estimate from an AZMET Daily Weather Summary. From Figure 1, the ETo estimate is 0.33" Step 2. Select the Kc value appropriate late for your turf. Assume a high quality warm season turf: Kc = 0.8. Step 3. Determine turf ET value. ET = Kc x ETo = 0.8 x 0.33 = 0.26" PRECAUTIONS Use AZMET ET estimates with appropriate caution. We suggest using the Kc's provided with this document, then adjusting the coefficients up or down for your particular situation. Adjustments should be made on the basis of turf quality. Increase the value of the coefficients if unacceptable turf quality results from their use. By the same token, you may wish to reduce the coefficients if resulting turf quality is higher than deemed necessary. AZMET ET estimates provide a convenient means of determining turf water use, and thereby, provide a method of calculating how much water to apply with each irrigation. Remember, irrigation system efficiency and water quality must be taken into consideration when determining the final amount of water to apply. Use of inefficient systems and/or irrigation water with a high salt content will require applications of water over and above that lost by ET. NOTE ! : ETo is a Reference Evapotranspiration. --------- This is NOT equal to a Pan Evaporation (ie: open body of water) value. To get an approximate Pan Evaporation value from the ETo value use this 'rule-of-thumb' conversion: divide the ETo value by a conversion constant. In winter or cooler times of the year, divide by 0.7. During summer or warmer periods, divide by 0.6. || ETo Winter || ETo Summer ------- = Approx. Pan Evap. || ------- = Approx. Pan Evap. 0.7 || 0.6 || ETo units can be in 'English' (inches) or Metric (millimeters). Many factors can affect the rate of evaporation from an open body of water; depth of water, area of the water, temperature of water, topography and vegetation surrounding the body of water, etc. The rate of evaporation from a irrigated field (drip,flood, sprinkler) can be affected by the same factors as an open body of water (above). The soil type, texture, color and porosity will also influence evaporation. AZMET DAILY WEATHER SUMMARY: PHOENIX-GREENWAY SEP 1 1987 MAX. MIN. MEAN TOTAL UNITS TEMPERATURE 103.2 81.5 91.4 DegF RELATIVE HUMIDITY 35.6 11.1 20.9 % VAPOR PRESS DEF. 4.1 KPas SOLAR RADIATION 592.9 Langleys PRECIPITATION 0.00 Inches SOIL TEMP. 2 IN 103.9 81.4 90.5 DegF SOIL TEMP. 4 IN 102.9 85.2 92.8 DegF WIND SPEED 16.7 5.3 MPH WIND VECTOR MAG. 3.1 MPH WIND VECTOR DIR. 110 Degrees ---------- REF. EVAPOTRANSPIRATION | 0.33 | Inches ETo ---------- HEAT UNITS 86/55F 86/50F 86/45F DAY CUM DAY CUM DAY CUM INTEGRATED 30.4 1000 35.4 40.4 SINE CURVE 30.2 982 35.2 1157 40.2 1332 DAY : Daily Heat Unit Total CUM : Heat Units Accumulated Since Installation Figure 1. AZMET Daily Weather Summary for Phoenix-Greenway. The boxed area encloses the daily ETo value. ESTIMATING REFERENCE EVAPOTRANSPIRATION WITH HOURLY DATA ============================================================================ Real-time (daily) reference evapotranspiration (ETo) is estimated using hourly weather data from the AZMET weather stations using a modified version of Penman's equation (Pruitt and Doorenbos, 1977). The advantages of using hourly data to estimate real-time ETo have been recognized by many researchers including: Pruitt and Doorenbos (1977), Slatyer (1971), Tanner and Pelton (1960), and van Bavel (1966). Penman himself recognized the limitations of using daily data to estimate real-time ET and recommended that his equation using daily data be used to estimate average ET over periods of 10 days or longer (Penman, 1948). The calculation of ETo from hourly data using the Pruitt and Doorenbos modification of Penman's equation is discussed here. Hourly estimates of reference evapotranspiration (RET) are calculated as: (1) RET = W(Rn)+(1-W)(VPD)(FU2) where Rn is the net radiation in equivalent mm of evaporation, W is a dimensionless weighing function, VPD is the vapor pressure deficit in kilopascals (KPa), and FU2 is an empirical wind function in mm of evaporation per KPa. Daily, real-time estimates of ETo are calculated as the sum of the 24 hourly estimates of RET. Equation (1) does not include a contribution of soil heat flux to evapotranspiration. The effect of soil heat flux on ETo is assumed to be small and the heat flux nearly adds to 0 (zero) over a day, so it is neglected in the calculation of RET. Measurements are taken on all parameters once each minute and hourly averages of the 60 one minute samples were used to calculate hourly RET. Net radiation (R) is measured with Fritchen net radiometers located 1 meter over irrigated grass and the data are recorded as hourly averages in W/M*M. Net radiation is then converted to net radiation in mm of evaporation (Rn) using Eq. 2: (2) Rn=R/(694.5(1-0.000946T)) where T is the hourly average air temperature in degrees celsius. The weighing function (W) is determined from each hourly average air temperature (Ta) in kelvins. The average air temperature (T) in degrees celsius is converted to absolute temperature (Kelvin) as: Ta=T+273.16. W is calculated as: (3) W=DEL/(DEL+GAM) where DEL is the slope of the saturation vapor pressure curve at the air temperature and GAM is the psychrometric constant. DEL is calculated as: (4) DEL = {6790.5-5.02808(Ta)+4916.8 x (Ta^[2]) x 10^[-0.0304(Ta)] + 174209 x 10^[-1302.88/Ta]} x {Es/Ta^[2]} GAM is calculated as: (5) GAM = 0.000646(1+0.000946(Ta-273.16))P where P is approximately 101.3 kilopascals (KPa) at sea level. Vapor pressure deficit is calculated as: (6) VPD = Es-E where Es is the saturation vapor pressure at the hourly average air temperature and E is the vapor pressure in kilopascals. The saturation vapor pressure (Es) in KPa at Ta is calculated using the Goff-Gratch equation: (7) Es = 10^[X] where X = -7.90298((373.16/Ta)-1)+5.02808 Log(373.16/Ta) -1.3816(10^[7])(10^[11.344(1-Ta/373.16]-1) +8.1328(10^[-3])(10^[-3.49149((373.16/Ta)-1]-1) + 2.0057 Relative humidity is measured once each minute by the weather stations and is converted to vapor pressure in KPa using an equation after Lowe (1976). Hourly averages of the 1 minute vapor pressure readings (E) are retrieved for the RET calculation. The wind function (FU2) consists of two empirical equations, developed by W.O. Pruitt using the Davis Lysimeters planted to cool-season grass. The results were reported by Pruitt and Doorenbos (1977). When the net radiation is less than or equal to 0 (zero), the equation for FU2 is: (8) FU2 = 0.125+0.0439(U2) where U2 is the average hourly wind speed measured at 2 meters height in m/s. When the net radiation is greater than 0, the equation is: (9) FU2 = 0.030+0.0576(U2) References 1. Penman, H.L. 1948. Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. London, A193:120-146. 2. Pruitt, W.O., and J. Doorenbos. 1977. Empirical calibration a requisite for evapotranspiration formulae based on daily or longer mean climatic data. Proceedings of the International Round Table Conference on "Evapotranspiration", Budapest, Hungary. 20 pages. 3. Slatyer, R.O., and I.C. McIlroy. 1961. Practical microclimatology, CSIRO, Melbourne (UNESCO). 4. Tanner, C.B., and W.L. Pelton. 1960. Potential evapotranspiration estimates by the approximate energy balance method of Penman. J. Geophys. Res., 65:3391-3413. 5. van Bavel, C.H.M. 1966. Potential evapotranspiration: The combination concept and its experimental verification. Water Resources Res., 2:455-467.