Model Description

The ET-Demands package

RefET

Reference evapotranspiration is calculated according to the ASCE Standardized Reference Evapotranspiration Equation (ASCE-EWRI, 2005).

Gap Filling and QA/QC

Missing Data

Missing values of maximum air temperature (T_max), minimum air temperature (T_min), and mean wind speed (u_x), up to six timesteps, are first filled through linear interpolation. Additional missing values not handled by the linear interpolation are filled using mean monthly values. Missing values of precipitation (P_r), snow, (S_n), and snow depth (S_d) are set to 0.

Maximum and Minimum Air Temperature

Maximum air temperature (T_max) values greater than 120°F are set to 120°F. Minimum air temperature (T_min) values greater than 90°F are set to 90°F. Maximum air temperature is checked against minimum air temperature at every time step. If minimum air temperature is greater than maximum air temperature, maximum air temperature is set to minimum air temperature.

ASCE Standardized Reference Evapotranspiration Equation

Daily Reference Evapotranspiration

\[ET_{sz} =\frac{0.408 \Delta (R_n-G) + \gamma \frac{C_n}{T_{mean} + 273}u_2 (e_s-e_a)}{\Delta + \gamma(1+C_d u_2)}\]

where:

ET_sz = standardized reference crop evapotranspiration for short ET_os or tall ET_rs surfaces [mm d^-1 for daily time steps or mm h^-1 for hourly time steps]

R_n = calculated net radiation at the crop surface [MM m^-2 d^-1 for daily time steps or MM m^-2 h^-1 for hourly time steps]

G = soil heat flux density at the soil surface [MM m^-2 d^-1 for daily time steps or MM m^-2 h^-1 for hourly time steps]

T_mean = mean daily or hourly air temperature at 1.5 to 2.5-m height [°C]

u₂ = mean daily or hourly wind speed at 2-m height [m s^-1]

e_s = saturation vapor pressure at 1.5 to 2.5-m height [kPa]

e_a = mean actual vapor pressure at 1.5 to 2.5-m height [kPa]

Δ = slope of the saturation vapor pressure-temperature curve [kPa °C^-1]

γ = psychrometric constant [kPa °C^-1]

C_n = numerator constant that changes with reference type and calculation time step [K mm s³ Mg_-1 d^-1 for daily time steps or K mm s³ Mg_-1 h^-1 for hourly time steps]

C_d = denominator constant that changes with reference type and calculation time step [s m^-1]

For a grass reference surface (ET_o),

C_n = 900

C_d = 0.34

For an alfalfa reference surface (ET_r),

C_n = 1600

C_d = 0.38

As soil heat flux density is positive when the soil is warming and negative when the soil is cooling, over a day period it is relatively small compared to daily R_n. For daily calculations it is ignored,

G = 0

Hourly Reference Evapotranspiration

The equation for ET_sz is the same as daily, with

For a grass reference surface (ET_o),

C_n = 37.0

At night, when R_n < 0,

C_d = 0.96

G = 0.5

For an alfalfa reference surface (ET_r),

C_n = 66.0

At night, when R_n < 0,

C_d = 1.7

G = 0.2

# UNIT CONVERSION

Mean Air Temperature (T_mean)

ASCE-EWRI (2005) advises to use the mean of daily minimum and daily maximum temperature to calculate mean daily temperature as opposed to the mean of hourly temperatures.

\[T_{mean} = \frac{T_{max} + T_{min}}{2}\]

where:

T_mean = mean daily air temperature [°C]

T_max = maximum daily air temperature [°C]

T_min = minimum daily air temperature [°C]

Ultimately, the ET_sz equation requires actual vapor pressure (e_a). This can be calculated from dew point temperature (T_d), specific humidity (q), or relative humidity (RH). If needed, dew point temperature can be calculated from minimum air temperature (T_min) and mean monthly dew point depression values (K₀).

Dew Point Temperature

\[T_{d} = T_{min} - K_0\]

where:

T_d = mean hourly or daily dew point temperature [°C]

T_min = mean hourly or daily minimum daily air temperature [°C]

K₀ = mean monthly dew point depression [°C]

Actual Vapor Pressure (e_a) from Dew Point Temperature (T_d)

\[e_a = 0.6108 \cdot \exp{\frac{17.27 \cdot T_{d}}{T_{d} + 237.3}}\]

where:

e_a = actual vapor pressure [kPa]

T_d = mean hourly or daily dew point temperature [°C]

# CALCULATE ACTUAL VAPOR PRESSURE FROM RELATIVE HUMIDITY

Actual Vapor Pressure (e_a) from Relative Humidity (RH)

\[e_a = \frac{RH}{100} \cdot e_{s}\]

where:

e_a = actual vapor pressure [kPa]

RH = relative humidity [%]

e_s = saturation vapor pressure [kPa]

Actual Vapor Pressure (e_a) from Specific Humidity (q)

\[e_a = \frac{q \cdot P}{0.622 + 0.378 \cdot q}\]

where:

e_a = actual vapor pressure [kPa]

q = specific humidity [kg/kg]

P = mean atmospheric pressure at station elevation [kPa]

Atmospheric Pressure (P)

\[P = 101.3 \cdot \left(\frac{293.15 - 0.0065z}{ 293.15} \right)^{(9.80665 / (0.0065 \cdot 286.9)}\]

where:

P = mean atmospheric pressure at station elevation [kPa]

z = station elevation above mean sea level [m]

This equation differs slightly from ASCE 2005 as it reflects full precision per Dr. Allen (pers. comm.).

Psychrometric Constant (γ)

\[\gamma = .0 000665 \cdot P\]

where:

γ = psychrometric constant [kPa °C^-1]

P = mean atmospheric pressure at station elevation [kPa]

Slope of the Saturation Vapor Pressure-Temperature Curve (Δ)

\[\Delta = 4098 \cdot \frac{0.6108 \cdot \exp \left( \frac{17.27T_{mean}} {T_{mean} + 237.3} \right)}{\left(T_{mean} + 237.3\right)^2}\]

where:

Δ = slope of the saturation vapor pressure-temperature curve (kPa °C^-1]

T_mean = mean daily air temperature [°C]

Saturation Vapor Pressure (e_s)

\[e_s = 0.6108 \cdot \exp \left( \frac{17.27T_{mean}}{T_{mean} + 237.3} \right)\]

where:

e_s = saturation vapor pressure

Tetens (1930)

Vapor Pressure Deficit (VPD)

\[\textrm{VPD} = e_s - e_a\]

where:

VPD = vapor pressure deficit [kPa]

e_s = saturation vapor pressure [kPa]

e_a = actual vapor pressure [kPa]

Extraterrestrial Radiation (R_a)

The calculations for hourly and daily extraterrestrial radiation (R_a) differ slightly as the hourly calculations require hourly solar time angles (ω) in addition to the sunset hour angle (ω_s) while the daily calculations just require the sunset hour angle.

Hourly and daily calculations require solar declination (δ), sunset hour angle (ω_s), and inverse square of the earth-sun distance (d_r).

Solar Declination (δ)

\[\delta=23.45 \cdot \frac{\pi}{180} \cdot \sin\left(\frac{2\pi}{365} \cdot(\textrm{DOY} + 284)\right)\]

where:

δ = solar declination [radians]

DOY = day of year

Sunset Hour Angle (ω_s)

\[\omega_{s} = \arccos(-\tan(\textrm{lat}) \cdot \tan(\delta))\]

where:

ω_s = sunset hour angle [radians]

lat = Latitude [radians]

δ = solar declination [radians]

To calcuate the inverse quare of the earth-sun distance, the day-of-year fraction (DOY_frac) is needed

Day-of-Year Fraction (DOY_frac)

\[\textrm{DOY}_{\textrm{frac}} = \textrm{DOY} \cdot \left(\frac{2\pi}{365}\right)\]

where:

DOY_frac = day-of-year fraction

DOY = day-of-year

Inverse Square of the Earth-Sun Distance (d_r)

\[d_{r} = 1 + 0.033 \cos(\textrm{DOY}_{\textrm{frac}})\]

where:

d_r = inverse square of the earth-sun distance [d^-2]

ω_s = sunset hour angle [radians]

lat = Latitude [radians]

δ = solar declination [radians]

Daily Extraterrestrial Radiation

\[ \begin{align}\begin{aligned}\theta = \omega_{s} \cdot \sin(\textrm{lat}) \cdot \sin(\delta) + \cos(\textrm{lat})\cdot \cos(\delta) \cdot \sin(\omega_{s})\\ R_{a} = \frac{24}{\pi} \cdot (1367 \cdot 0.0036) \cdot d_{r} \cdot \theta\end{aligned}\end{align} \]

where:

ω_s = sunset hour angle [radians]

lat = Latitude [radians]

R_a = daily extraterrestrial radiation [MJ m^-2 d^-1]

δ = solar declination [radians]

d_r = inverse square of the earth-sun distance [d^-2]

Hourly Extraterrestrial Radiation

Hourly calculations also require the calculation hourly solar time angles (ω), which requires the calculation of solar time (t_s).

Seasonal Correction (sc)

\[ \begin{align}\begin{aligned}b = \frac{2\pi}{364} \cdot (\textrm{DOY} - 81)\\sc = 0.1645 \cdot \sin(2b) - 0.1255 \cdot \cos(b) - 0.0250 \sin(b)\end{aligned}\end{align} \]

where:

sc = seasonal correction [hours]

DOY = day-of-year

Solar Time (t:sub:`s`)

\[t_{s} = t + (\textrm{lon} \cdot \frac{24}{2\pi} + sc - 12)\]

where:

t_s = solar time (i.e. noon is 0) [hours]

lon = Longitude [radians]

t = UTC time at the midpoint of the period [hours]

sc = seasonal correction [hours]

Solar Time Angle (ω)

\[\omega = \frac{2\pi}{24} \cdot t_{s}\]

where:

ω = solar hour angle [radians]

t_s = solar time (i.e. noon is 0) [hours]

Hourly Extraterrestrial Radiation

\[ \begin{align}\begin{aligned}\omega_{1} = \omega - \frac{\pi}{24}\cdot t\\\omega_{2} = \omega + \frac{\pi}{24}\cdot t\end{aligned}\end{align} \]

Checks on ω₁ and ω₂

\[ \begin{align}\begin{aligned}\textrm{if } \omega_{1} < -\omega_{s} \textrm{ then } \omega_{1} = -\omega_{s}\\\textrm{if } \omega_{2} < -\omega_{s} \textrm{ then } \omega_{2} = -\omega_{s}\\\textrm{if } \omega_{1} > \omega_{s} \textrm{ then } \omega_{1} = \omega_{s}\\\textrm{if } \omega_{2} > \omega_{s} \textrm{ then } \omega_{2} = \omega_{s}\\\textrm{if } \omega_{1} > \omega_{2} \textrm{ then } \omega_{1} = \omega_{2}\end{aligned}\end{align} \]

\[\theta = (\omega_{2} - \omega_{1}) \cdot \sin(\textrm{lat}) \cdot \sin(\delta) + \cos(\textrm{lat}) \cdot \cos(\delta) \cdot \sin(\omega_{2} - \omega_{1})\]

\[R_{a} = \frac{24}{\pi} \cdot (1367 \cdot 0.0036) \cdot d_{r} \cdot \theta\]

where: ω₁ = solar time angle at the beginning of the period [radians]

ω₂ = solar time angle at the end of the period [radians]

ω = solar hour angle [radians]

t = UTC time at the midpoint of the period [hours]

ω_s = sunset hour angle [radians]

lat = Latitude [radians]

δ = solar declination [radians]

R_a = hourly extraterrestrial radiation [MJ m^-2 h^-1]

d_r = inverse square of the earth-sun distance [d^-2]

Clear-Sky Radiation (R_so)

Sin of the Angle of the Sun above the Horizon (sin:sub:`β24`)

\[ \begin{align}\begin{aligned}\sin_{\beta24} = \sin(0.85 + 0.3 \cdot \textrm{lat} \cdot \sin(\textrm{DOY}_{\textrm{frac}}) - 1.39)) - 0.42 \cdot \textrm{lat}^2\\ \sin_{\beta24} = \max(\sin_{\beta24}, 0.1)\end{aligned}\end{align} \]

where:

sin:sub:`β24`= sine of the angle of the sun above the horizon [radians]

lat = Latitude [radians]

DOY_frac = day-of-year fraction

Precipitable Water (w)

\[w = P \cdot 0.14 \cdot e_{a} + 2.1\]

where:

w = precipitable water [mm]

P = mean atmospheric pressure at station elevation [kPa]

e_a = actual vapor pressure [kPa]

Clearness Index for Direct Beam Radiation (k:sub:`b`)

\[k_{b} = 0.98 \cdot \exp{\left(\frac{-0.00146P}{sin_{\beta24} - 0.0075}\right)} - 0.075\left(\frac{w}{\sin_{\beta24}}\right)^{0.4}\]

where:

k_b = clearness index for direct beam radiation

P = mean atmospheric pressure at station elevation [kPa]

sin:sub:`β24`= sine of the angle of the sun above the horizon [radians]

w = precipitable water [mm]

Transmissivity Index for Diffuse Radiation (k:sub:`d`)

\[\begin{split}k_{d} = \min \begin{cases} -0.36 \cdot k_{b} + 0.35 \\ 0.82 \cdot k_{b} + 0.18 \end{cases}\end{split}\]

where:

k_d = transmissivity index for diffuse radiation

k_b = clearness index for direct beam radiation

Daily Clear-Sky Radiation

\[R_{so} = R_{a} \cdot (k_{b} + k_{d})\]

where:

R_so = daily clear-sky radiation [MJ m^-2 d^-1]

R_a = daily extraterrestrial radiation [MJ m^-2 d^-1]

k_b = clearness index for direct beam radiation

k_d = transmissivity index for diffuse radiation

Hourly Clear-Sky Radiation

Several calculations, including the sin of the angle of the sun above the horizon (sin_β) and the clearness index for direct beam radiation (k_b) change when calculating hourly clear-sky radiation.

Sin of the Angle of the Sun above the Horizon (sin:sub:`β`)

\[ \begin{align}\begin{aligned}\sin_{\beta} = \sin(\textrm{lat}) \cdot \sin(\delta)+\cos(\textrm{lat}) \cdot \cos(\delta) \cdot \cos(\omega)\\\begin{split}\sin_{\beta,c} = \max \begin{cases} \sin_{\beta} \\ 0.01 \end{cases}\end{split}\end{aligned}\end{align} \]

where:

sin:sub:`β`= sine of the angle of the sun above the horizon [radians]

sin_{β,c`= sin:sub:`β} limited to 0.01 so that k_b does not go undefined

lat = Latitude [radians]

δ = solar declination [radians]

ω = solar hour angle [radians]

Clearness Index for Direct Beam Radiation (k:sub:`b`)

\[ \begin{align}\begin{aligned}k_{t} = 1.0\\k_{b} = 0.98 \cdot \exp \left(\frac{-0.00146P}{k_{t} \cdot \sin_{\beta,c}}\right) - 0.075 \left(\frac{w}{\sin_{\beta,c}}\right)^{0.4}\end{aligned}\end{align} \]

where:

k_t = atmospheric turbidity coefficient

k_b = clearness index for direct beam radiation

P = mean atmospheric pressure at station elevation [kPa]

sin:sub:`β,c`= sine of the angle of the sun above the horizon, limited to 0.01 [radians]

w = precipitable water [mm]

Transmissivity Index for Diffuse Radiation (k:sub:`d`)

\[\begin{split}k_{d} = \min \begin{cases} -0.36 \cdot k_{b} + 0.35 \\ 0.82 \cdot k_{b} + 0.18 \end{cases}\end{split}\]

where:

k_d = transmissivity index for diffuse radiation

k_b = clearness index for direct beam radiation

Hourly Clear-Sky Radiation

\[R_{so} = R_{a} \cdot (k_{b} + k_{d})\]

where:

R_so = hourly clear-sky radiation [MJ m^-2 h^-1]

R_a = hourly extraterrestrial radiation [MJ m^-2 h^-1]

k_b = clearness index for direct beam radiation

k_d = transmissivity index for diffuse radiation

Cloudiness Fraction (fcd)

Daily Cloudiness Fraction

\[ \begin{align}\begin{aligned}\textrm{fcd} = 1.35 \cdot \frac{R_{s}}{R_{so}}-0.35\\0.3 < \frac{R_{s}}{R_{so}} \leq 1.0\end{aligned}\end{align} \]

where:

fcd = daily cloudiness fraction

R_s = measured solar radiation [MJ m^-2 d^-1]

R_so = clear sky solar radiation [MJ m^-2 d^-1]

R_s / R_so is limited to 0.3 < R_s / R_so ≤ 1.0

Hourly Cloudiness Fraction

At low sun angles (β), cloudiness fraction (fcd) is set to 1.

\[ \begin{align}\begin{aligned}\beta = \arcsin(\sin(\textrm{lat}) \cdot \sin(\delta) + \cos(\textrm{lat}) \cdot \cos(\delta) \cdot \cos(\omega))\\\textrm{fcd}[R_{so} > 0] = 1.35 \cdot \frac{R_{s}}{R_{so}}-0.35\\0.3 < \frac{R_{s}}{R_{so}} \leq 1.0\\\textrm{fcd}[\beta < 0.3] = 1\end{aligned}\end{align} \]

where:

β = angle of the sun above the horizon [radians]

lat = Latitude [radians]

δ = solar declination [radians]

ω = solar hour angle [radians]

fcd = hourly cloudiness fraction

R_s = measured solar radiation [MJ m^-2 h^-1]

R_so = clear sky solar radiation [MJ m^-2 h^-1]

Net Longwave Radiation (R_nl)

Daily Net Longwave Radiation

\[R_{nl} = 4.901\textrm{e-9} \cdot \textrm{fcd} \cdot (0.34 - 0.14 \cdot \sqrt{e_{a}} \cdot 0.5 ((T_{max} + 273.16)^4 + (T_{min} + 273.16)^4)\]

where:

R_nl = daily net longwave radiation [MJ m^-2 d^-1]

fcd = daily cloudiness fraction

e_a = actual vapor pressure [kPa]

T_max = maximum daily air temperature [°C]

T_min = minimum daily air temperature [°C]

Hourly Net Longwave Radiation

\[R_{nl} = 2.042\textrm{e-10} \cdot \textrm{fcd} \cdot (0.34 - 0.14 \cdot \sqrt{e_{a}} \cdot(T_{mean} + 273.16)^4\]

where:

R_nl = hourly net longwave radiation [MJ m^-2 h^-1]

fcd = daily cloudiness fraction

e_a = actual vapor pressure [kPa]

T_mean = mean hourly air temperature [°C]

Net Radiation (R_n)

Daily Net Radiation

\[R_{n} = 0.77 \cdot R_{s} - R_{nl}\]

where:

R_n = daily net radiation [MJ m^-2 d^-1]

R_nl = daily net longwave radiation [MJ m^-2 d^-1]

R_s = measured solar radiation [MJ m^-2 d^-1]

Hourly Net Radiation

\[R_{n} = 0.77 \cdot R_{s} - R_{nl}\]

where:

R_n = hourly net radiation [MJ m^-2 h^-1]

R_nl = hourly net longwave radiation [MJ m^-2 h^-1]

R_s = measured solar radiation [MJ m^-2 h^-1]

Windspeed Adjustment

The standardized reference crop evapotranspiration equation assumes a 2-m height windspeed. Windspeed measured at different heights can be approximated as

\[u_2 = u_z + \frac{4.87}{\ln\left(67.8 z_w - 5.42 \right)}\]

where:

u₂ = wind speed at 2 m above ground surface [m s^-1]

u_z = measured wind speed at z_w m above ground surface [m s^-1]

z_w = height of wind measurement about ground surface [m]

## CACLULATE MIN AND MAX MONTHLY MEAN TEMPERATURES

Thornton and Running Solar Radiation Estimate

If measured solar radiation (R_s) is not provided, it can be estimated using the approach described in Thorton and Running (1999). This approach requires three calibrated coefficients [LINK TO PAGE ON HOW TO DO THIS].

\[ \begin{align}\begin{aligned}T_{diff} = T_{max} - T_{min}\\T_{mon,diff} = T_{mon,max} - T_{mon,min}\\B_{TR} = TR_{b0} + TR_{b1} \cdot \exp{(TR_{b2} \cdot{T_{mon,diff}})\\R_{s} = R_{so} \cdot (1 - 0.9 \exp{(-B_{TR} \cdot T_{diff}^{1.5})})\end{aligned}\end{align} \]

where:

T_diff = temperature difference [°C]

T_max = maximum daily air temperature [°C]

T_min = minimum daily air temperature [°C]

T_mon,diff = mean monthly temperature difference [°C]

T_mon,max = mean monthly maximum air temperature [°C]

T_mon,min = mean monthly minimum air temperature [°C]

TR_b0 = Thornton and Running b₀ coefficient

TR_b1 = Thornton and Running b₁ coefficient

TR_b2 = Thornton and Running b₂ coefficient

B_TR = Thorton and Running parameter

R_s = calculated solar radiation [MJ m^-2 d^-1]

R_so = clear sky solar radiation [MJ m^-2 d^-1]

For arid stations, [REFERENCE FOR THESE COEFFICIENTS]

TR_b0 = 0.023

TR_b1 = 0.1

TR_b2 = 0.2

[DISCUSSION OF THESE PARAMETERS, AND HOW TO GET THEM]

Other Potential ET Estimates

The RefET module code can also calculate potential evapotranspiration using several different approaches. This provides a comparison with reference ET.

Latent Heat of Vaporization (λ)

The latent heat of vaporization is calculated from mean air temperature. This differs from ASCE-EWRI (2005) which advises to use a constant value of 2.45 MJ kg^-1 as it varies only slightly over the ranges of air temperature that occur in agricultural or hydrologic systems. The equation used is from XXX.

\[\lambda = 2500000 - 2360 \cdot T_{mean}\]

where:

λ = latent heat of vaporization [MJ kg^-1]

T_mean = mean daily air temperature [°C]

Penman

\[ET_o = W \cdot R_n + (1-W) \cdot f(ur) \cdot (e_a - e_d)\]

where:

ET_o = grass reference evapotranspiration [mm d^-1]

W = weighting factor (depends on temperature and altitude)

R_n = net radiation in equivalent evaporation [mm d^-1]

f(ur) = wind-related function

(e_a - e_d) = difference between saturation vapor pressure at mean air temperature and the mean actual vapor pressure of the air [hPa]

\[f(ur) = 0.27 (1+(ur_2 / 100))\]

where:

f(ur) = wind-related function

ur₂ = daily wind run at 2-m height [km d^-1]

(Penman, 1948).

Kimberly Penman 1982

Hargreaves-Samani

(Hargreaves and Samani, 1985).

Priestley-Taylor

`(Priestley and Taylor, 1972) <https://doi.org/10.1175/1520-0493(1972)100//<0081:OTAOSH//>2.3.CO;2>`_ .

Blaney-Criddle

[THIS CURRENTLY ISN’T SUPPORTED]

(Blaney and Criddle, 1950).

CropETPrep

CropET

The CropET module of the ET Demands model is the FAO-56 dual crop coefficient model (Allen et al., 1998) .

\[ET_{c} = (K_c K_{cb} + K_e)ET_o\]

ET_c = crop evapotranspiration

K_c = crop coefficient

K_cb = Basal crop coefficient

K_e = coefficient representing bare soil evaporation

ET_o = reference crop evapotranspiration from a grass reference surface

Aridity Rating

Allen and Brockway (1983) estimated consumptive irrigation requirements for crops in Idaho, and developed an aridity rating for each meteorological weather station used to adjust temperature data. The aridity rating ranges from 0 (fully irrigated) to 100 (arid) and reflects conditions affecting the aridity of the site. The aridity rating was based on station metadata information, questionnaires, and phone conversations, and includes conditions close to the station (within a 50m radius),the area around the station (within a 1600m radius in the upwind direction), and the region around the station (within a 48km radius in the upwind direction).

\[AR_{cum} = 0.4AR_{St} + 0.5AR_{Ar} + 0.1AR_{Reg}\]

Allen and Brockway (1983) used empirical data from Allen and Brockway (1982) to develop monthly aridity effect values (A_e). These values were used as adjustment factors for the temperature data based on the aridity rating. Stations with an aridity rating of 100 applied the adjustment factor directly, while stations with aridity ratings less than 100, weighted the adjustment factor by the aridity rating.

\[T_{adj} = \frac{AR_{cum}}{100} \cdot A_{e}\]

The empirical temperature data and aridity effect values used are show in the table below. These data are the average monthly departure of air temperatures over arid areas from air temperatures over irrigated areas in southern Idaho during 1981, and the aridity effect.

Month	Tmax	Tmin	Tmean	Ae
April	2.7	2.4	2.5	1.0
May	1.3	0.6	0.9	1.5
June	2.4	1.8	2.1	2.0
July	4.8	2.9	3.8	3.5
August	5.2	4.3	4.7	4.5
September	3.3	2.7	3.0	3.0
October	0.3	1.6	0.9	0.0

HOW WAS THE ARIDITY EFFECT DETERMINED. ARE THESE DATA GENERAL ENOUGH TO USE AT OTHER LOCATIONS IF AN ARIDITY RATING IS DEVELOPED? IF NOT, CAN WE GENERALIZE THE APPROACH TO DEVELOPING AN ARIDITY RATING, AND ASSOCIATED ARIDITY EFFECT ADJUSTMENTS? ALSO, THE ‘CropET’ MODULE HAS A WAY OF PULLING IN ARIDITY EFFECT VALUES, HOWEVER, THE ‘RefET’ MODULE DOES NOT. THIS MEANS THAT WHILE TEMPERATURES USED IN THE CropET MODULE ARE ADJUSTED, TEMPERATURES USED TO CALCUATE REFERENCE ET ARE NOT. IF WE WANT TO CONTINUE TO SUPPORT THE ARIDITY RATING, THIS SHOULD BE ADDRESSED. WOULD ALSO REQUIRE PASSING THE MODEL THE ARIDITY EFFECT ADJUSTMENT FACTORS.

AreaET

PostProcessing

References

Allen, R. G., & Brockway, C. E. (1982). Weather and Consumptive Use in the Bear River Basin, Idaho During 1982.

Allen, R. G., & Brockway, C. E. (1983). Estimating Consumptive Irrigation Requirements for Crops in Idaho.

Allen, R. G., Pereira, L. S., Smith, M., Raes, D., & Wright, J. L. (2005). FAO-56 Dual Crop Coefficient Method for Estimating Evaporation from Soil and Application Extensions. Journal of Irrigation and Drainage Engineering, 131(1), 2–13. https://doi.org/10.1061/(ASCE)0733-9437(2005)131:1(2)

Allen, R. G., & Robison, C. W. (2007). Evapotranspiration and Consumptive Irrigation Water Requirements for Idaho.

ASCE-EWRI. (2005). The ASCE Standardized Reference Evapotranspiration Equation.

Blaney, H. F., & Criddle, W. D. (1950). Determining Water Requirements in Irrigated Areas from Climatological and Irrigation Data. SCS-TP-96. Washington D.C.

Hargreaves, G. H., & A. Samani, Z. (1985). Reference Crop Evapotranspiration from Temperature. Applied Engineering in Agriculture, 1(2), 96–99. https://doi.org/https://doi.org/10.13031/2013.26773

Penman, H. L. (1948). Natural Evaporation from Open Water, Bare Soil and Grass. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 193(1032), 120–145. https://doi.org/10.1098/rspa.1948.0037

Priestley, C. H. B., & Taylor, R. J. (1972). On the Assessment of Surface Heat Flux and Evaporation Using Large-Scale Parameters. Monthly Weather Review, 100(2), 81–92. https://doi.org/10.1175/1520-0493(1972)100<0081:OTAOSH>2.3.CO;2

Thornton, P. E., & Running, S. W. (1999). An improved algorithm for estimating incident daily solar radiation from measurements of temperature, humidity, and precipitation. Agricultural and Forest Meteorology, 93, 211–228. https://doi.org/10.1016/S0168-1923(98)00126-9