ET Methods Climate/Weather Data Crop Characteristics · ASCE Standardized Reference...

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ET Methods Climate/Weather Data

Crop Characteristics Evapotranspiration Workshop

March 21, 2012 Ft Collins CO

Thomas W. Ley, PhD, PE

StateCU—Crop ET Methods ó Monthly Time Step: ó Original Blaney-Criddle ó Modified Blaney-Criddle (SCS TR-21) ó Pochop (for bluegrass only)

ó Daily Time Step: ó ASCE Standardized Reference ET ó Penman-Monteith ó Modified Hargreaves

March 21 2012 2 ET Workshop

Modified Blaney-Criddle (SCS TR-21) Method

u = kc x kt x (t x p) / 100

u = monthly consumptive use (inches) kt = climatic coefficient = 0.173 * t - 0.314 kc = crop growth stage coefficient t = mean monthly air temperature (F) P = monthly percentage of annual daylight

Modified Blaney-Criddle (SCS TR-21) Method—Pros

ó Long history in determining historical consumptive use of water ó Generally accepted by Colorado water courts ó Recognized by water court judges and water

attorneys ó Easy to use ó Minimal data requirement--mean monthly

temperature ó Many NOAA Coop stations in Colorado with

lengthy records of max temp/min temp/precip

Modified Blaney-Criddle (SCS TR-21) Method—Cons

ó ASCE Manual 70 (1990) evaluation of 19 ET methods against carefully screened lysimeter data from 11 worldwide locations representing a range of climatic (arid to humid) conditions ó SCS Blaney-Criddle (TR-21) ranked 15th at

both arid (average 16% underestimation) and humid sites (average 20% overestimation) on peak monthly and seasonal basis ó Crop factors are method specific-not usable

with other methods ó Accurate results requires local calibration

ASCE Standardized Reference Evapotranspiration Equation

ó Based on full form Penman-Monteith Eqn. ó ET for hypothetical standardized reference crop

ETos :12 cm tall grass OR ETrs :50 cm tall alfalfa

ó Reference ET is converted to crop ET using crop coefficients

ó Simplified and standardized computations for variables

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ASCE Standardized Reference ET Equation—Pros

ó ASCE Task Committee evaluated the predictive accuracy of 13 reference equations (including the standardized equation) at 49 sites across the US ó Standard of comparison was the (ASCE full form)

Penman-Monteith equation ó ASCE Standardized Ref ET Equation performed

well on hourly and daily basis ó simplifications and standardized computations

included in the ASCE standardized equation considered acceptable

ó Detailed daily weather data requirements ó air temperature, relative humidity, solar

radiation and wind speed at 2 m height

ó Appropriately sited weather stations ó over and downwind of vegetation that

approximates a well-watered, clipped green grass surface in an open, irrigated setting à “reference” site

ó ideally 100 m of green, well-watered fetch in all directions

ó Shorter period of record— ~20-25 years

ASCE Standardized Reference ET Equation—Cons

Air temperature

Relative humidity

Wind speed at 2-m height

Rainfall

Solar radiation

Holly02 CoAgMet Station

Wind direction

Weather Data Requirements

Climate/Weather Data ó Long term climate data: ó NOAA Longmont 2ESE (Lat 40.15N, El 4950 ft) ó Mean monthly temp and precip: 1950-2004 ó Generally very complete: regression used to fill a

few missing values ó Daily weather data (provided courtesy of Northern

Colorado Water Conservancy District, February 2012): ó NCWCD Longmont South (103) electronic weather

station (Lat 40.07N, El 4984 ft) ó Daily weather parameters: 1994-2011 ó Very complete, high quality data set

James Ditch service area and relative locations of NOAA Longmont 2ESE long term climate station and NCWCD Longmont South (103) electronic weather station

Weather Data Quality Assessment ó Visually screen data plotted versus time ó Check for “reasonableness” ó Consider environmental impacts at measurement site

ó knowledge of station siting and environment ó station location data and photos

ó Compare measured data with physically known ranges or constraints

ó Detailed QA/QC procedures available (e.g. EWRI, 2005; Allen, 1996)

ó Typical QC procedures may not identify sensor measurement bias

ó Poor data à poor results

NCWCD Longmont South (103) Weather

Station

Many daily max RH values > 103-105%

After correction

Clear sky solar radiation (App. D, ASCE-EWRI; 2005):

Sun angle Atmospheric thickness Atmospheric water vapor Extra-terrestrial radiation (dependent on day of the year and latitude)

Highest measured values (clear days) should “bump” up against clear sky envelope

Visually inspect for consistently low readings (i.e., anemometer threshold values) è indicates “frozen” bearings, or, increased starting threshold due to dirty or fouled bearings

Crop Characteristics ó Growing season dates ó Frost dates ó Typical cropping dates ó Length of season ó Planting, harvest dates

ó Days from planting to full effective cover

ó SCS TR-21 monthly crop factors ó Crop coefficients for ASCE Standardized

Reference ET equation ó Calibrated SCS TR-21 crop factors

Crop Characteristics

Typical Planting Date or Spring Greenup

Days to Full Cover

Length of Season (days)

Typical Harvest Date

Alfalfa Apr 1 --- 75 45 45

Jun 15, Jul 31, Sep 15

Grass/Pasture Mar 20-Apr 1 --- --- Hard Fall Freeze

Corn Grain May 5-10 72 135-140 Sep 17-22

Dry Beans May 20-25 55 105-115 Aug 30-Sep 10

Spring Grains Apr 7-12 70 125-135 Aug 10-20

Sugar Beet Apr 28-May 5 87 180-185 Oct 10-15

Crop Coefficients for ASCE Std Reference ET Method

ó Guidance in ASCE (2005) report on the standardized reference ET equation—use coefficients published in ASCE Manual 70 based on 1982 Kimberly Penman method ó ASCE Std Reference ET method (for tall crops)

is based on a hypothetical 0.50 m vegetative surface (similar to alfalfa) for all times of year ó ASCE Std Reference ET method predicts

higher than 1982 Kimberly Penman in spring and fall, lower during peak of summer

Monthly ratios of 1982 Kimberly Penman to ASCE Standardized Penman-Monteith alfalfa reference ET (adapted from Allen, 2001 unpublished paper).

Vineland 1994-2000

Avondale 1994-2000

Rocky Ford 1993-2000 Mar

Crop Coefficients for ASCE Standardized Reference ET Method ó Converted coefficients have been developed

by Allen and Wright (2002) ó Published in: ó Table 8.7, Chapter 8: Irrigation Water

Requirements, Design and Operation of Farm Irrigation Systems, 2nd ed. ASABE, 2007 ó And, in forthcoming 2nd edition of ASCE Manual

70, Appendix E

Calibration of SCS TR-21 Crop Factors—Why?

óMost historical consumptive use analyses based on long evaluation period: 50+ years ó Long periods of record available at many

NOAA Coop stations ó Limited period of record at electronic weather

stations (20-25 years or less) ó Local calibration/verification is strongly

recommended to obtain more accurate historical crop ET estimates

ó Compute calibration coefficients for some specific time interval during growing season (e.g., monthly)

Calibration Measured Crop ET coefficient, Kcal = Crop ET by method being

calibrated

ó In the absence of measured ET (i.e., lysimeter), use crop ET based on ASCE Std Ref-ET Equation

Calibration of SCS TR-21 Crop Factors—Approach

ó Compute average calibration coefficient values for overlapping period of record ó Monthly time step

óMinimum 5-7 years of overlapping record

SPDSS Regional Calibrated Crop Factors (SPDSS Memo Task 59.1)

ó Upper and lower plains regions ó Alfalfa, Corn Grain, Dry Beans, Grass/Pasture,

Spring Grains, Sugar Beets ó Period: 1993-2003 ó Crop ET for both SCS TR-21 and ASCE Std Ref ET

computed using CoAgMet station data ó Original ASCE Manual 70 crop coefficients used with

ASCE Std Ref ET

ó Upper plains region crop factors ó Average of Ft Collins (FTC01), Ft Lupton (FTL01),

and Greeley (GLY03) coefficients

ó Extent of areal representation ? óWeather station pairing ó Compute calibration coefficients by pairing each

NOAA station of interest with electronic weather station ó Site bias at the NOAA site may not be properly

captured

Calibration: Precautions/Limitations

Coefficients for one EWS-NOAA station pair generally not applicable at other NOAA stations when conditions at the NOAA sites are dissimilar

Rocky Ford CoAgMet Rocky Ford NOAA

Las Animas NOAA La Junta NOAA

Site Specific Calibrated Crop Factors ó Longmont area: James Ditch Case Study ó Alfalfa, Corn Grain, Dry Beans, Grass/Pasture,

Spring Grains, Sugar Beets ó Period: 1994-2004 ó Crop ET by SCS TR-21 ó using Longmont 2ESE data

ó Crop ET by ASCE Std Ref ET ó using NCWCD Longmont South (103) station data ó converted crop coefficients for use with ASCE Std

Ref ET

1 31 61 91 121 151 181 211 241 271 301 331 361

Day of the Year

Alfalfa

Original TR21 Crop Factor

SPDSS Upper Plains Region Calibrated Crop Factor

Site specific (Longmont 103-Longmont 2ESE) Calib Crop Factor (w/cuttings)

Site specific (Longmont 103-Longmont 2ESE Calib Crop Factor (seasonal average)

1 31 61 91 121 151 181 211 241 271 301 331 361

Day of the Year

Grass/Pasture

Site specific (Longmont 103-Longmont 2ESE) Calibrated Crop Factor

0 10 20 30 40 50 60 70 80 90 100

Percent of Growing Season

Corn Grain

0 10 20 30 40 50 60 70 80 90 100

Dry Beans

0 10 20 30 40 50 60 70 80 90 100

Spring Grain

0 10 20 30 40 50 60 70 80 90 100

Sugar Beets

Why the Differences? a little bit of apples and oranges

ó SPDSS Upper Plains Region calibrated crop factors ó Ft Lupton EWS data ó solar data low, RH data

high è lower ETr ó EWS temperature data

used for both the ASCE Std Reference ET and the SCS TR-21 crop ET estimates

ó Used crop coefficients from ASCE Manual 70 developed using the 1982 Kimberly Penman

ó James Ditch case study calibrated crop factors ó High quality EWS weather

data ó Calibration was a specific

NOAA station to a specific EWS station

ó Used converted crop coefficients for ASCE Standardized Reference ET Method

ET Methods Climate/Weather Data Crop Characteristics · ASCE Standardized Reference...

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