ASCE Standardized Reference Evapotranspiration Functions

## Project description

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

Modules needed to run the test suite:

## References

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

## Project details

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