Google Earth Engine based ASCE Standardized Reference Evapotranspiration Functions

## Project description

This repository provides Google Earth Engine Python API based implementation of the ASCE Standardized Reference Evapotranspiration Equations (ASCE2005) 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 openet.refetgee

# 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 = openet.refetgee.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 openet.refetgee

# 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 = openet.refetgee.Hourly(
tmean=tmean_c, ea=ea, rs=rs, uz=uz, zw=3, elev=1208.5,
lat=lat, lon=lon, 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 openet.refetgee

source_img = ee.Image(ee.ImageCollection('IDAHO_EPSCOR/GRIDMET').first())
etr = openet.refetgee.Daily.gridmet(source_img).etr\
.reduceRegion(reducer=ee.Reducer.first(),
geometry=ee.Geometry.Point(-118.77388, 39.4575),
scale=1000)\
.getInfo()

print('ETr: {:.2f} mm'.format(float(etr['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 openet.refetgee

source_coll = ee.ImageCollection('NASA/NLDAS/FORA0125_H002')\
.filterDate('2015-07-01', '2015-07-02')
etr = openet.refetgee.Daily.nldas(source_coll).etr\
.reduceRegion(reducer=ee.Reducer.first(),
geometry=ee.Geometry.Point(-118.77388, 39.4575),
scale=1000)\
.getInfo()

print('ETr: {:.2f} mm'.format(float(etr['etr'])))
import ee
import openet.refetgee

source_img = ee.Image('NASA/NLDAS/FORA0125_H002/A20150701_2000')
etr = openet.refetgee.Hourly.nldas(source_img).etr\
.reduceRegion(reducer=ee.Reducer.first(),
geometry=ee.Geometry.Point(-118.77388, 39.4575),
scale=1000)\
.getInfo()

print('ETr: {:.2f} mm'.format(float(etr['etr'])))

### CFSv2

A helper function for computing daily ETo and ETr for CFSv2 images is available.

For the daily function, the CFSv2 collection must be filtered to a single 24 hour period.

import ee
import openet.refetgee

source_coll = ee.ImageCollection('NOAA/CFSV2/FOR6H')\
.filterDate('2015-07-01', '2015-07-02')
etr = openet.refetgee.Daily.cfsv2(source_coll).etr\
.reduceRegion(reducer=ee.Reducer.first(),
geometry=ee.Geometry.Point(-118.77388, 39.4575),
scale=1000)\
.getInfo()

print('ETr: {:.2f} mm'.format(float(etr['etr'])))

### RTMA

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

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

import ee
import openet.refetgee

source_coll = ee.ImageCollection('NOAA/NWS/RTMA')\
.filterDate('2015-07-01', '2015-07-02')
etr = openet.refetgee.Daily.rtma(source_coll).etr\
.reduceRegion(reducer=ee.Reducer.first(),
geometry=ee.Geometry.Point(-118.77388, 39.4575),
scale=1000)\
.getInfo()

print('ETr: {:.2f} mm'.format(float(etr['etr'])))
import ee
import openet.refetgee

source_img = ee.Image('NOAA/NWS/RTMA/2015070120')
etr = openet.refetgee.Hourly.nldas(source_img).etr\
.reduceRegion(reducer=ee.Reducer.first(),
geometry=ee.Geometry.Point(-118.77388, 39.4575),
scale=1000)\
.getInfo()

print('ETr: {:.2f} mm'.format(float(etr['etr'])))

### ERA5-Land

Helper functions for computing daily/hourly ETo/ETr for ERA5-Land images are available.

For the daily function, the ERA5-Land collection must be filtered to a single 24 hour period.

import ee
import openet.refetgee

source_coll = ee.ImageCollection('ECMWF/ERA5_LAND/HOURLY')\
.filterDate('2015-07-01', '2015-07-02')
etr = openet.refetgee.Daily.era5_land(source_coll).etr\
.reduceRegion(reducer=ee.Reducer.first(),
geometry=ee.Geometry.Point(-118.77388, 39.4575),
scale=1000)\
.getInfo()

print('ETr: {:.2f} mm'.format(float(etr['etr'])))
import ee
import openet.refetgee

source_img = ee.Image('ECMWF/ERA5_LAND/HOURLY/20150701T20')
etr = openet.refetgee.Hourly.era5_land(source_img).etr\
.reduceRegion(reducer=ee.Reducer.first(),
geometry=ee.Geometry.Point(-118.77388, 39.4575),
scale=1000)\
.getInfo()

print('ETr: {:.2f} mm'.format(float(etr['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.

## Calculation Method - ASCE vs. RefET

The main difference between the two “methods” is that the “asce” method attempts to follow the equations in ASCE2005, whereas the “refet” method attempts to follow the calculations of the RefET Software as closely as possible. The difference in output between these methods is generally negligible (if not identical for realistic numbers of significant digits). Note that the default is set to “asce” to best match the calculations a user would expect to have happen. The “refet” method was added in order to help validate this code to the RefET Software.

## Installation

The OpenET RefET GEE python module can be installed via pip:

pip install openet-refet-gee

## OpenET Namespace Package

Each OpenET model is stored in the “openet” folder (namespace). The model can then be imported as a “dot” submodule of the main openet module.

import openet.refetgee as refetgee

## 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

[ASCE2005]
ASCE-EWRI (2005). The ASCE standardized reference evapotranspiration equation.