Calibration for Historical Crop ET Estimates – A Process Ivan A. Walter, P.E. Ivan’s Engineering, Inc. EVAPOTRANSPIRATION WORKSHOP 3/12/20101Ivan A. Walter, P.E.

Kc ETstzd vs SCS BC n Kc ET stzd is more accurate than SCS BC – Kc -- Generate from measurement at known sites – Kc -- Transferrable – ET stzd – ET professional & researchers have accepted – SCS BC – Crop Coefficients derived where??? – SCS BC – Requires local calibration 3/12/20102Ivan A. Walter, P.E.

Why Isn’t Kc ET stzd Used n Answers to “Why not?” – SCS BC is well accepted in Colo. Water Court. – Do not have the data for ET stzd – Do not have the Kc’s 3/12/20103Ivan A. Walter, P.E.

What is Desirable Result n Just some number for ETc n Accurate daily Temp, Rs, U, & Rh n Accurate daily ET rs & ET os n Accurate monthly ET rs & ET os n Excel files & Programs for analysis 3/12/20104Ivan A. Walter, P.E.

Criteria (ASCE Task Committee) n Understandable n Defensible n Simplification without loss of accuracy n Based on & Traceable to Experimental Data 3/12/20105Ivan A. Walter, P.E.

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Raw Data >>>>>> ETc n Download data from Web site n QC the data – Select good data n Calc ET rs, ET os, and ET Hrgv n Calibration Calc’s with good data n COEF Hgrv x ET Hrgv = ET rs n COEF1 Hgrv x ET Hrgv = ET os 3/12/20109Ivan A. Walter, P.E.

Begin Here Standardized Reference ET Equation: 3/12/201010Ivan A. Walter, P.E.

Begin Here w/ Data from Web 3/12/201011Ivan A. Walter, P.E.

Calculate & Analyze & Compare Standardized Reference ET Equation: 3/12/201012Ivan A. Walter, P.E Hargreaves Equation:

Final Product n Standardized Reference ET values – Short Crop: ETos – Tall Crop: ETrs – Daily values 3/12/201013Ivan A. Walter, P.E.

Step 1: XXX COAG.xls n Download data from COAGMet site n QC the data – Identify the Good Data – Fix small gaps & errors – Throw out everything else 3/12/201014Ivan A. Walter, P.E.

Step 2: ColCrbt2.exe -- Application n Input data from XXX COAG.xls – Text file XXXEtrData.prn n Calculates Reference ET (daily) – ETrs – ASCE PM Etr, 1982 Kimberly Penman ETr – ETos – ASCE PM ETo, Hargreaves ETo 3/12/201015Ivan A. Walter, P.E.

Step 3: Colcrbt2.exe -- Results n Linear Regression Analysis (daily) n Zero intercept n Monthly Ratio: ET rs to ET Hgrv n Monthly Ratio: ET os to ET Hgrv n Monthly Ratio: ET rs to ET os n Regression Coef & SEE 3/12/201016Ivan A. Walter, P.E.

Step 3 Results: Ratio ET rs : ET Hrgv 3/12/2010Ivan A. Walter, P.E.17 MAYJUNJULAUGSEP Fort Morgan(Tdpt Adjusted) Fort Collins Center Avondale Rocky Ford Yuma Fruita Yellow Jacket

Center: Ratio ET rs : ET Hrgv 3/12/2010Ivan A. Walter, P.E.18 MAYJUNJULAUGSEP All

Step 3 Results: Ratio ET os : ET Hrgv 3/12/2010Ivan A. Walter, P.E.19 MAYJUNJULAUGSEP Fort Morgan(Tdpt Adjusted) Fort Collins Center Avondale Rocky Ford Yuma Fruita Yellow Jacket

Step 4: ETRETOH 2.exe n Calculate ET rs & ET os using ET Hgrv n Input File – DataT&P.prn from XXX COAG.xls n Latitude, Tmax, Tmin, Tavg – COEFHRGV.dat n Monthly Coefficient all Station (15 max.) 3/12/201020Ivan A. Walter, P.E.

Step 4: COEFHGRV.dat 3/12/201021Ivan A. Walter, P.E.

Step 4: ETRETOH 2.exe n Calculation of ET rs & ET os using 1985 Hargreaves n ET rs = COEF x ET Hgrv (Daily) n ET os = COEF1 x ET Hgrv (Daily) 3/12/201022Ivan A. Walter, P.E.

Step 4: Final Results n Year, DOY, Month, Day, ETrs,ETos, Precip. n 3/12/201023Ivan A. Walter, P.E.

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Process -- Historical ETref Estimates n CoAgMet Data n XXX COAG.xls -- QC for Good data n Colcrbt2.exe -- Cal’c Monthly Hgrv Factors n ETRETOH 2.exe - Calculate ET rs & ET os 3/12/201032Ivan A. Walter, P.E.

Meadow Grass ET n Review of Methods n Recommendation for Calculation ETc 3/12/2010Ivan A. Walter, P.E.33

Reported Procedures n SCS –BC Pasture Grass – 10% / 1,000 m. change in elevation – ASCE Manual 70 does not recommend n Pochop Method -- ASAE 1984 – Limited to Alfalfa (9%) & Turf (6%) – Adjusted kt --- thus not true SCS BC – Adjust from elevation Coefficient derived – Wyoming 3/12/201034Ivan A. Walter, P.E.

Reported Procedures n Denver Water Board, BC “k” coef. – South Park, Elev. 8,900 ft msl – Standard BC not modified BC – Local calibration – ET calculated as: n Lysimeter water use + Precipitation (100%) – ET irr wtr = ETc - Precipitation 3/12/201035Ivan A. Walter, P.E.

Recommend Transferrable Kc n Smith’s Gunnison basin – Hargreaves Monthly Coefficients – Letter Describing included in Materials n Maly & Walter – Kc’s Meadow Grass – Memo Describing process included in Materials 3/12/201036Ivan A. Walter, P.E.

Hargreaves Kc’s for Meadow Grass n Multiple Sites -- 5 years of data n May=1.16 n June=1.28 n July=1.04 n August=0.86 n September =0.88 3/12/201037Ivan A. Walter, P.E.

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Software & Documents Provided n XXX COAG.xls -- QC data n Colcrbt2.exe -- Calc’ Monthly Coef. n ETRETOH 2.exe -- Calc’ ET rs & ET os n Example Input and Output files n Memo: Coefficients for Meadow Grass n Smith Letter: Hargreaves Kc M.Grass n Daily Ra calculation Application 3/12/201039Ivan A. Walter, P.E.

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