refet 0.3.0

ASCE Standardized Reference Evapotranspiration Functions

NumPy functions for computing daily and hourly reference ET.

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

# 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 = refet.Daily(
    tmin=tmin_c, tmax=tmax_c, ea=ea, rs=rs, uz=uz, zw=3, elev=1208.5,
    lat=lat_radians, doy=182, method='asce').etr()

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

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 math
import refet

# 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 = refet.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,
    method='asce').etr()

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

Input Parameters

Required Parameters (hourly & daily)

Variable	Type	Description [units]
ea	ndarray	Actual vapor pressure [kPa]
rs	ndarray	Incoming shortwave solar radiation [MJ m-2 day-1]
uz	ndarray	Wind speed [m/s]
zw	float	Wind speed height [m]
elev	ndarray	Elevation [m]
lat	ndarray	Latitude [radians]
doy	ndarray	Day of year

Required Daily Parameters

Variable	Type	Description [units]
tmin	ndarray	Minimum daily temperature [C]
tmax	ndarray	Maximum daily temperature [C]

Required Hourly Parameters

Variable	Type	Description [units]
tmean	ndarray	Average hourly temperature [C]
lon	ndarray	Longitude [radians]
time	ndarray	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 ‘simple’ – Simplified clear sky solar formulation ‘array’ – Read Rso values from “rso” function parameter
rso	array_like	Clear sky solar radiation [MJ m-2 day-1] Only used if rso_type == ‘array’ Defaults to None if not set

Issues

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

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

The user must handle all QA/QC of the input data and no missing data will be filled.

Cloudiness Fraction (hourly)

The hourly reference ET calculation is currently performed independently for each time step which causes the cloudiness fraction (fcd) calculation for very low sun angles to be incorrect.

ASCE vs. RefET

TODO Discuss differences between “asce” and “refet” methods.

Installation

To install the RefET python module:

pip install refet

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

ASCE-EWRI Standardized Reference Evapotranspiration Equation (2005)

• Report

• Appendix