geerefet 0.1.6

Google Earth Engine ASCE Standardized Reference Evapotranspiration Functions

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_radians = (39.4575 * math.pi / 180)                    # degrees -> radians

etr = geerefet.Daily(
    tmin=tmin_c, tmax=tmax_c, ea=ea, rs=rs, uz=uz, zw=3, elev=1208.5,
    lat=lat_radians, 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_radians = (39.4575 * math.pi / 180)      # degrees -> radians
lon_radians = (-118.77388 * math.pi / 180)   # degrees -> radians

etr = geerefet.Hourly(
    tmean=tmean_c, ea=ea, rs=rs, uz=uz, zw=3, elev=1208.5,
    lat=lat_radians, 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]
zw	ee.Number	Wind speed height [m]
elev	ee.Image, ee.Number	Elevation [m]
lat	ee.Image, ee.Number	Latitude [radians]
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 ‘asce’ – Calculations will follow ASCE-EWRI 2005 (default) ‘refet’ – Calculations will follow RefET software
rso_type	str	Override default clear sky solar radiation (Rso) calculation Defaults to None if not set ‘full’ – Full clear sky solar formulation (default) ‘simple’ – Simplified clear sky solar formulation (Eq. 19) ‘array’ – Read Rso values from “rso” function parameter
rso	ee.Image, ee.Number	Clear sky solar radiation [MJ m-2 day-1] Only needed if rso_type is ‘array’ Defaults to None if not set

Issues

Currently the user must handle all of the file I/O and unit conversions.

The latitude/longitude units are in degrees, not radians (this is different than the python RefET modules).

Cloudiness Fraction (hourly)

The hourly reference ET calculation is currently performed independently for each time step. The cloudiness fraction (fcd) for very low sun angles (i.e. at night) is hard coded to 1 for very low sun angles instead of being derived from the .

ASCE vs. RefET

TODO Discuss 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

• earthengine-api

Modules needed to run the test suite:

• pandas

• pytest

• pytz

References

ASCE-EWRI Standardized Reference Evapotranspiration Equation (2005)

• Report

• Appendix