Google Earth Engine ASCE Standardized Reference Evapotranspiration Functions
Project description
Google Earth Engine (GEE) functions for computing daily and hourly reference ET.
Usage
Daily
The following demonstrates how to compute a single daily ETr value using weather data for 2015-07-01 from the Fallon, NV AgriMet station. The necessary unit conversions are shown on the input values. The raw input data is available here.
import math
import ee
import geerefet
# Unit conversions
tmin_c = (66.65 - 32) * (5.0 / 9) # F -> C
tmax_c = (102.80 - 32) * (5.0 / 9) # F -> C
tdew_c = (57.26 - 32) * (5.0 / 9) # F -> C
ea = 0.6108 * math.exp(17.27 * tdew_c / (tdew_c + 237.3)) # kPa
rs = (674.07 * 0.041868) # Langleys -> MJ m-2 d-1
uz = 4.80 * 0.44704 # mpg -> m s-1
lat = 39.4575 # degrees
etr = geerefet.Daily(
tmin=tmin_c, tmax=tmax_c, ea=ea, rs=rs, uz=uz, zw=3, elev=1208.5,
lat=lat, doy=182).etr().getInfo()
print('ETr: {:.2f} mm'.format(float(etr)))
Hourly
The following demonstrates how to compute a single hourly ETr value using weather data for 18:00 UTC (11:00 AM PDT) on 2015-07-01 from the Fallon, NV AgriMet station. The necessary unit conversions are shown on the input values. The raw input data is available here
import math
import ee
import geerefet
# Unit conversions
tmean_c = (91.80 - 32) * (5.0 / 9) # F -> C
ea = 1.20 # kPa
rs = (61.16 * 0.041868) # Langleys -> MJ m-2 h-1
uz = 3.33 * 0.44704 # mph -> m s-1
lat = 39.4575 # degrees
lon = -118.77388 # degrees
etr = geerefet.Hourly(
tmean=tmean_c, ea=ea, rs=rs, uz=uz, zw=3, elev=1208.5,
lat=lat, lon=lon_radians, doy=182, time=18).etr().getInfo()
print('ETr: {:.2f} mm'.format(float(etr)))
GRIDMET
A helper function for computing daily ETo and ETr for GRIDMET images is available.
import ee
import geerefet
gridmet_img = ee.Image(ee.ImageCollection('IDAHO_EPSCOR/GRIDMET').first())
etr = geerefet.Daily.gridmet(gridmet_img).etr().getInfo()
print('ETr: {:.2f} mm'.format(float(etr)))
Input Parameters
Required Parameters (hourly & daily)
Variable |
Type |
Description [units] |
---|---|---|
ea |
ee.Image |
Actual vapor pressure [kPa] |
rs |
ee.Image |
Incoming shortwave solar radiation [MJ m-2 day-1] |
uz |
ee.Image |
Wind speed [m s-1] |
zw |
ee.Number |
Wind speed height [m] |
elev |
ee.Image, ee.Number |
Elevation [m] |
lat |
ee.Image, ee.Number |
Latitude [degrees] |
doy |
ee.Image, ee.Number |
Day of year |
Required Daily Parameters
Variable |
Type |
Description [units] |
---|---|---|
tmin |
ee.Image |
Minimum daily temperature [C] |
tmax |
ee.Image |
Maximum daily temperature [C] |
Required Hourly Parameters
Variable |
Type |
Description [units] |
---|---|---|
tmean |
ee.Image |
Average hourly temperature [C] |
lon |
ee.Image, ee.Number |
Longitude [degrees] |
time |
ee.Number |
UTC hour at start of time period |
Optional Parameters
Variable |
Type |
Description [units] |
---|---|---|
method |
str |
Calculation method
|
rso_type |
str |
Override default clear sky solar radiation (Rso) calculation
Defaults to None if not set
|
rso |
ee.Image, ee.Number |
Clear sky solar radiation [MJ m-2 day-1]
|
Issues
Currently the user must handle all of the file I/O and unit conversions.
Cloudiness Fraction (hourly)
The cloudiness fraction (fcd) is computed as the ratio of the measured solar radiation (Rs) to the theoretical clear sky solar radiation (Rso). This ratio cannot be computed directly at night since Rso is 0. ASCE-EWRI 2005 suggests computing a representative nighttime fcd based on the fcd at sunset and/or sunrise.
In the RefET module fcd is hard coded to 1 for all time steps with very low sun angles since the hourly reference ET is computed independently for each time step.
ASCE vs. RefET
TODO: Discuss the differences between “asce” and “refet” methods.
Installation
To install the RefET-GEE python module:
pip install geerefet
Validation
Please see the validation document for additional details on the source of the test values and the comparison of the functions to the Ref-ET software.
Dependencies
Modules needed to run the test suite:
References
ASCE-EWRI Standardized Reference Evapotranspiration Equation (2005)
Project details
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