Upload
others
View
7
Download
0
Embed Size (px)
Citation preview
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
hours
March 21 2012 3 ET Workshop
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
March 21 2012 4 ET Workshop
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
March 21 2012 5 ET Workshop
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
March 21 2012 ET Workshop 6
)uC1(
)ee(u273T
C)GR(408.0ET
2d
as2n
n
sz +g+D
-+
g+-D=
ASCE Standardized Reference ET Equation—Pros
March 21 2012 ET Workshop 7
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
March 21 2012 ET Workshop 8
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
March 12 2010 ET Workshop 9
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
March 21 2012 ET Workshop 10
James Ditch service area and relative locations of NOAA Longmont 2ESE long term climate station and NCWCD Longmont South (103) electronic weather station
March 21 2012 ET Workshop 11
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
March 21 2012 ET Workshop 12
March 21 2012 ET Workshop 13
March 21 2012 ET Workshop 14
NCWCD Longmont South (103) Weather
Station
March 21 2012 ET Workshop 15
Many daily max RH values > 103-105%
After correction
March 21 2012 ET Workshop 16
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
March 21 2012 ET Workshop 17
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
March 21 2012 ET Workshop 18
Crop Characteristics
March 21 2012 ET Workshop 19
Crop
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
March 21 2012 ET Workshop 20
March 21 2012 ET Workshop 21
Monthly ratios of 1982 Kimberly Penman to ASCE Standardized Penman-Monteith alfalfa reference ET (adapted from Allen, 2001 unpublished paper).
Month
Vineland 1994-2000
Avondale 1994-2000
Rocky Ford 1993-2000 Mar
0.77
0.73
0.76
Apr
0.85
0.81
0.85
May
0.92
0.90
0.94
Jun
1.01
0.98
1.00
Jul
1.05
1.00
1.03
Aug
1.05
0.98
1.01
Sep
0.95
0.90
0.95
Oct
0.84
0.81
0.85
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
March 21 2012 ET Workshop 22
March 21 2012 23 ET Workshop
March 21 2012 ET Workshop 24
March 21 2012 25 ET Workshop
March 21 2012 26 ET Workshop
March 21 2012 27 ET Workshop
March 21 2012 28 ET Workshop
March 21 2012 29 ET Workshop
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
March 21 2012 ET Workshop 30
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
March 21 2012 ET Workshop 31
Calibration of SCS TR-21 Crop Factors—Approach
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
March 21 2012 ET Workshop 32
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
March 21 2012 ET Workshop 33
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
March 21 2012 ET Workshop 34
Calibration: Precautions/Limitations
March 21 2012 ET Workshop 35
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
March 21 2012 ET Workshop 36
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
March 21 2012 ET Workshop 37
March 21 2012 38 ET Workshop
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
1.80
1.90
2.00
1 31 61 91 121 151 181 211 241 271 301 331 361
Crop
Fac
tor
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)
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
1.80
1.90
2.00
1 31 61 91 121 151 181 211 241 271 301 331 361
Crop
Fac
tor
Day of the Year
Grass/Pasture
Original TR21 Crop Factor
SPDSS Upper Plains Region Calibrated Crop Factor
Site specific (Longmont 103-Longmont 2ESE) Calibrated Crop Factor
March 21 2012 39 ET Workshop
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
0 10 20 30 40 50 60 70 80 90 100
Crop
Fac
tor
Percent of Growing Season
Corn Grain
Original TR21 Crop Factor
SPDSS Upper Plains Region Calibrated Crop Factor
Site specific (Longmont 103-Longmont 2ESE) Calibrated Crop Factor
March 21 2012 40 ET Workshop
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
0 10 20 30 40 50 60 70 80 90 100
Crop
Fac
tor
Percent of Growing Season
Dry Beans
Original TR21 Crop Factor
SPDSS Upper Plains Region Calibrated Crop Factor
Site specific (Longmont 103-Longmont 2ESE) Calibrated Crop Factor
March 21 2012 41 ET Workshop
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
0 10 20 30 40 50 60 70 80 90 100
Crop
Fac
tor
Percent of Growing Season
Spring Grain
Original TR21 Crop Factor
SPDSS Upper Plains Region Calibrated Crop Factor
Site specific (Longmont 103-Longmont 2ESE) Calibrated Crop Factor
March 21 2012 42 ET Workshop
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
1.60
0 10 20 30 40 50 60 70 80 90 100
Crop
Fac
tor
Percent of Growing Season
Sugar Beets
Original TR21 Crop Factor
SPDSS Upper Plains Region Calibrated Crop Factor
Site specific (Longmont 103-Longmont 2ESE) Calibrated Crop Factor
March 21 2012 43 ET Workshop
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
March 21 2012 ET Workshop 44