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Google Earth Engine ASCE Standardized Reference Evapotranspiration Functions

Project description

Latest version on PyPI Build status

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)))

NLDAS

Helper functions for computing daily/hourly ETo/ETr for NLDAS images are available.

For the daily function, the NLDAS collection must be filtered to a single 24 hour period.

import ee
import geerefet

nldas_coll = ee.ImageCollection('NASA/NLDAS/FORA0125_H002')\
    .filterDate('2017-07-01', '2017-07-02)
etr = geerefet.Daily.nldas(nldas_coll).etr().getInfo()

print('ETr: {:.2f} mm'.format(float(etr)))
import ee
import geerefet

nldas_img = ee.Image(ee.ImageCollection('NASA/NLDAS/FORA0125_H002').first())
etr = geerefet.Hourly.nldas(nldas_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
  • ‘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.

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)

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