ASCE Standardized Reference Evapotranspiration (ET)

NumPy functions for computing daily and hourly reference ET following the ASCE Standardized Reference Evapotranspiration Equations (ASCE2005).

Usage

Daily Example

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 refet

# The actual vapor pressure could be computed from the dew point temperature below
#   or the tdew can be passed directly to the function
# Convert the dew point temperature to Celsius
# tdew = units._f2c(49.84)
# ea = 0.6108 * math.exp(17.27 * tdew / (tdew + 237.3))
# ea = refet.calcs._sat_vapor_pressure(tdew)

etr = refet.Daily(
    tmin=66.65, tmax=102.80, tdew=49.84, rs=674.07, uz=4.80,
    zw=3, elev=1208.5, lat=39.4575, doy=182, method='asce',
    input_units={'tmin': 'F', 'tmax': 'F', 'tdew': 'F', 'rs': 'Langleys',
                 'uz': 'mph', 'lat': 'deg'}
    ).etr()

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

Hourly Example

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 refet

etr = refet.Hourly(
    tmean=91.80, ea=1.20 , rs=61.16, uz=3.33, zw=3, elev=1208.5,
    lat=39.4575, lon=-118.77388, doy=182, time=18, method='asce',
    input_units={'tmean': 'F', 'rs': 'Langleys', 'uz': 'mph', 'lat': 'deg'}
    ).etr()

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

Input Parameters

Required Parameters (hourly & daily)

Variable	Type	Description [default units]
uz	ndarray	Wind speed [m s-1]
zw	float	Wind speed height [m]
elev	ndarray	Elevation [m]
lat	ndarray	Latitude [degrees]
doy	ndarray	Day of year

Required Ea Parameters (hourly & daily)

Either the "ea" or "tdew" parameter must be set

Variable	Type	Description [default units]
ea	ndarray	Actual vapor pressure [kPa]
tdew	ndarray	Dew point temperature [C]

Required Daily Parameters

Variable	Type	Description [default units]
rs	ndarray	Incoming shortwave solar radiation [MJ m-2 d-1]
tmin	ndarray	Minimum daily temperature [C]
tmax	ndarray	Maximum daily temperature [C]

Required Hourly Parameters

Variable	Type	Description [default units]
rs	ndarray	Incoming shortwave solar radiation [MJ m-2 h-1]
tmean	ndarray	Average hourly temperature [C]
lon	ndarray	Longitude [degrees]
time	ndarray	UTC hour at start of time period

Optional Parameters

Variable	Type	Description [default 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 'simple' -- Simplified clear sky solar formulation 'array' -- Read Rso values from "rso" function parameter
rso	array_like	Clear sky solar radiation [MJ m-2 d-1 or MJ m-2 h-1] Only used if rso_type == 'array' Defaults to None if not set
input_units	dict	Override default input unit types Input values will be converted to default unit types

Installation

The RefET python module can be installed with conda or pip:

conda install refet

Issues

The functions have not been tested for inputs with different shapes/sizes and the broadcasting may not work correctly.

The user must handle the following:

• File I/O
• QA/QC of the input data
• Filling missing or bad data

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. ASCE2005 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.

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

• numpy

Modules needed to run the test suite:

• pandas
• pytest
• pytz

References

[ASCE2005]

ASCE-EWRI (2005). The ASCE standardized reference evapotranspiration equation. https://ascelibrary.org/doi/book/10.1061/9780784408056