a |
coefficient of psychrometer [°C |

a |
fraction of extraterrestrial radiation reaching the earth on an overcast day [-] |

a |
fraction of extraterrestrial radiation reaching the earth on a clear day [-] |

c |
specific heat [MJ kg |

c |
soil heat capacity [MJ m |

CR |
capillary rise [mm day |

D |
cumulative depth of evaporation (depletion) from the soil surface layer [mm] |

D |
cumulative depth of evapotranspiration (depletion) from the root zone [mm] |

d |
zero plane displacement height [m] |

d |
inverse relative distance Earth-Sun [-] |

DP |
deep percolation [mm] |

Dp |
deep percolation from the evaporation layer [mm] |

E |
evaporation [mm day |

E |
pan evaporation [mm day |

e°(T) |
saturation vapour pressure at air temperature T [kPa] |

e |
saturation vapour pressure for a given time period [kPa] |

e |
actual vapour pressure [kPa] |

e |
saturation vapour pressure deficit |

EC |
electrical conductivity of the saturation extract of the soil [dS m |

EC |
electrical conductivity of the saturation extract of the soil above which yield begins to decrease [dS m |

ET |
evapotranspiration [mm day |

ET |
reference crop evapotranspiration [mm day |

ET |
crop evapotranspiration under standard conditions [mm day |

ET |
crop evapotranspiration under non-standard conditions [mm day |

exp[x] |
2.7183 (base of natural logarithm) raised to the power x |

F |
resistance correction factor [-] |

f |
fraction of soil surface covered by vegetation (as observed from overhead) [-] |

f |
effective fraction of soil surface covered by vegetation [-] |

1 - f |
exposed soil fraction [-] |

f |
fraction of soil surface wetted by rain or irrigation [-] |

f |
fraction of soil that is both exposed and wetted (from which most evaporation occurs) [-] |

G |
soil heat flux [MJ m |

G |
soil heat flux for day and ten-day periods [MJ m |

G |
soil heat flux for hourly or shorter periods [MJ m |

G |
soil heat flux for monthly periods [MJ m |

G |
solar constant [0.0820 MJ m |

H |
sensible heat [MJ m |

HWR |
height to width ratio |

h |
crop height [m] |

I |
irrigation depth [mm] |

I |
irrigation depth for that part of the surface wetted [mm] |

J |
number of day in the year [-] |

K |
crop coefficient [-] |

K |
crop coefficient during the initial growth stage [-] |

K |
crop coefficient during the mid-season growth stage [-] |

K |
crop coefficient at end of the late season growth stage [-] |

K |
maximum value of crop coefficient (following rain or irrigation) [-] |

K |
minimum value of crop coefficient (dry soil with no ground cover) [-] |

K |
basal crop coefficient [-] |

K |
basal crop coefficient during mid-season (at peak plant size or height) for vegetation with full ground cover of LAI > 3 [-] |

K |
basal crop coefficient during the initial growth stage [-] |

K |
basal crop coefficient during the mid-season growth stage [-] |

K |
basal crop coefficient at end of the late season growth stage [-] |

K |
soil evaporation coefficient [-] |

K |
pan coefficient [-] |

K |
soil evaporation reduction coefficient [-] |

K |
water stress coefficient [-] |

K |
yield response factor [-] |

k |
von Karman's constant [0.41] [-] |

k |
adjustment coefficient for the Hargreaves' radiation formula [°C-0.5] |

L |
length of initial growth stage [day] |

L |
length of crop development growth stage [day] |

L |
length of mid-season growth stage [day] |

L |
length of late season growth stage [day] |

L |
longitude of centre of local time zone [degrees west of Greenwich] |

L |
longitude [degrees west of Greenwich] |

LAI |
leaf area index [m |

LAI |
active (sunlit) leaf area index.[-] |

N |
maximum possible sunshine duration in a day, daylight hours [hour] |

n |
actual duration of sunshine in a day [hour] |

n/N |
relative sunshine duration [-] |

P |
rainfall [mm], atmospheric pressure [kPa] |

p |
evapotranspiration depletion factor [-] |

R |
specific gas constant [0.287 kJ kg |

R |
extraterrestrial radiation [MJ m |

R |
longwave radiation [MJ m |

R |
net radiation [MJ m |

R |
net longwave radiation [MJ m |

R |
net solar or shortwave radiation [MJ m |

R |
solar or shortwave radiation [MJ m |

R |
clear-sky solar or clear-sky shortwave radiation [MJ m |

r |
aerodynamic resistance [s m |

r |
bulk stomatal resistance of well-illuminated leaf [s m |

r |
(bulk) surface or canopy resistance [s m |

R |
relative solar or relative shortwave radiation [-] |

RAW |
readily available soil water of the root zone [mm] |

REW |
readily evaporable water (i.e., maximum depth of water that can be evaporated from the soil surface layer without restriction during stage 1) [mm] |

RH |
relative humidity [%] |

RH |
average hourly relative humidity |

RH |
daily maximum relative humidity [%] |

RH |
daily mean relative humidity [%] |

RH |
daily minimum relative humidity [%] |

RO |
surface runoff [mm] |

S |
seasonal correction factor for solar time [hour] |

SF |
subsurface flow [mm] |

T |
air temperature [°C] |

T |
air temperature [K] |

T |
virtual air temperature [K] |

T |
dewpoint temperature [°C] |

T |
temperature of dry bulb [°C] |

T |
daily maximum air temperature [°C] |

T |
daily maximum air temperature [K] |

T |
daily mean air temperature [°C] |

T |
daily minimum air temperature [°C] |

T |
daily minimum air temperature [K] |

T |
temperature of wet bulb [°C] |

TAW |
total available soil water of the root zone [mm] |

TEW |
total evaporable water (i.e., maximum depth of water that can be evaporated from the soil surface layer) [mm] |

t |
time [hour] |

u |
wind speed at 2 m above ground surface [m s |

u |
wind speed at z m above ground surface [m s |

W |
soil water content [mm] |

Y |
actual yield of the crop [kg ha |

Y |
maximum (expected) yield of the crop in absence of environment or water stresses [kg ha |

Z |
depth of surface soil layer subjected to drying by evaporation [m] |

Z |
rooting depth [m] |

z |
elevation, height above sea level [m] |

z |
height of humidity measurements [m] |

z |
height of wind measurements [m] |

z |
roughness length governing momentum transfer [m] |

z |
roughness length governing heat and vapour transfer [m] |

a |
albedo [-] |

g |
psychrometric constant [kPa °C |

g |
psychrometric constant of instrument [kPa °C |

D |
slope of saturation vapour pressure curve [kPa °C |

D SW |
variation in soil water content [mm] |

D t |
length of time interval [day] |

D z |
effective soil depth [m] |

d |
solar declination [rad] |

e |
ratio molecular weight of water vapour/dry air (= 0.622) |

h |
mean angle of the sun above the horizon |

q |
soil water content [m |

q |
soil water content at field capacity [m |

q |
threshold soil water content below which transpiration is reduced due to water stress [m |

q |
soil water content at wilting point [m |

l |
latent heat of vaporization [MJ kg |

l ET |
latent heat flux [MJ m |

r a |
mean air density [kg m |

r |
density of water [kg m |

s |
Stefan-Boltzmann constant [4.903 10 |

j |
latitude [rad] |

w |
solar time angle at midpoint of hourly or shorter period [rad] |

w |
solar time angle at beginning of hourly or shorter period [rad] |

w |
solar time angle at end of hourly or shorter period [rad] |

w |
sunset hour angle [rad] |