List of principal symbols and acronyms

apsy	coefficient of psychrometer [°C-1]
as	fraction of extraterrestrial radiation reaching the earth on an overcast day [-]
as+bs	fraction of extraterrestrial radiation reaching the earth on a clear day [-]
cp	specific heat [MJ kg-1 °C-1]
cs	soil heat capacity [MJ m-3 °C-1]
CR	capillary rise [mm day-1]
De	cumulative depth of evaporation (depletion) from the soil surface layer [mm]
Dr	cumulative depth of evapotranspiration (depletion) from the root zone [mm]
d	zero plane displacement height [m]
dr	inverse relative distance Earth-Sun [-]
DP	deep percolation [mm]
Dpe	deep percolation from the evaporation layer [mm]
E	evaporation [mm day-1]
Epan	pan evaporation [mm day-1]
e°(T)	saturation vapour pressure at air temperature T [kPa]
es	saturation vapour pressure for a given time period [kPa]
ea	actual vapour pressure [kPa]
es - ea	saturation vapour pressure deficit
ECe	electrical conductivity of the saturation extract of the soil [dS m-1]
ECe, threshold	electrical conductivity of the saturation extract of the soil above which yield begins to decrease [dS m-1]
ET	evapotranspiration [mm day-1]
ETo	reference crop evapotranspiration [mm day-1]
ETc	crop evapotranspiration under standard conditions [mm day-1]
ETc adj	crop evapotranspiration under non-standard conditions [mm day-1]
exp[x]	2.7183 (base of natural logarithm) raised to the power x
Fr	resistance correction factor [-]
fc	fraction of soil surface covered by vegetation (as observed from overhead) [-]
fc eff	effective fraction of soil surface covered by vegetation [-]
1 - fc	exposed soil fraction [-]
fw	fraction of soil surface wetted by rain or irrigation [-]
few	fraction of soil that is both exposed and wetted (from which most evaporation occurs) [-]
G	soil heat flux [MJ m-2 day-1]
Gday	soil heat flux for day and ten-day periods [MJ m-2 day-1]
Ghr	soil heat flux for hourly or shorter periods [MJ m-2 hour-1]
Gmonth	soil heat flux for monthly periods [MJ m-2 day-1]
Gsc	solar constant [0.0820 MJ m-2 min-1]
H	sensible heat [MJ m-2 day-1]
HWR	height to width ratio
h	crop height [m]
I	irrigation depth [mm]
IW	irrigation depth for that part of the surface wetted [mm]
J	number of day in the year [-]
Kc	crop coefficient [-]
Kc ini	crop coefficient during the initial growth stage [-]
Kc mid	crop coefficient during the mid-season growth stage [-]
Kc end	crop coefficient at end of the late season growth stage [-]
Kc max	maximum value of crop coefficient (following rain or irrigation) [-]
Kc min	minimum value of crop coefficient (dry soil with no ground cover) [-]
Kcb	basal crop coefficient [-]
Kcb full	basal crop coefficient during mid-season (at peak plant size or height) for vegetation with full ground cover of LAI > 3 [-]
Kcb ini	basal crop coefficient during the initial growth stage [-]
Kcb mid	basal crop coefficient during the mid-season growth stage [-]
Kcb end	basal crop coefficient at end of the late season growth stage [-]
Ke	soil evaporation coefficient [-]
Kp	pan coefficient [-]
Kr	soil evaporation reduction coefficient [-]
Ks	water stress coefficient [-]
Ky	yield response factor [-]
k	von Karman's constant [0.41] [-]
kRs	adjustment coefficient for the Hargreaves' radiation formula [°C-0.5]
Lini	length of initial growth stage [day]
Ldev	length of crop development growth stage [day]
Lmid	length of mid-season growth stage [day]
Llate	length of late season growth stage [day]
LZ	longitude of centre of local time zone [degrees west of Greenwich]
Lm	longitude [degrees west of Greenwich]
LAI	leaf area index [m2 (leaf area) m-2 (soil surface)]
LAIactive	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-1 K-1]
Ra	extraterrestrial radiation [MJ m-2 day-1]
Rl	longwave radiation [MJ m-2 day-1]
Rn	net radiation [MJ m-2 day-1]
Rnl	net longwave radiation [MJ m-2 day-1]
Rns	net solar or shortwave radiation [MJ m-2 day-1]
Rs	solar or shortwave radiation [MJ m-2 day-1]
Rso	clear-sky solar or clear-sky shortwave radiation [MJ m-2 day-1]
ra	aerodynamic resistance [s m-1]
rl	bulk stomatal resistance of well-illuminated leaf [s m-1]
rs	(bulk) surface or canopy resistance [s m-1]
Rs/Rso	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 [%]
RHhr	average hourly relative humidity
RHmax	daily maximum relative humidity [%]
RHmean	daily mean relative humidity [%]
RHmin	daily minimum relative humidity [%]
RO	surface runoff [mm]
Sc	seasonal correction factor for solar time [hour]
SF	subsurface flow [mm]
T	air temperature [°C]
TK	air temperature [K]
TKv	virtual air temperature [K]
Tdew	dewpoint temperature [°C]
Tdry	temperature of dry bulb [°C]
Tmax	daily maximum air temperature [°C]
Tmax, K	daily maximum air temperature [K]
Tmean	daily mean air temperature [°C]
Tmin	daily minimum air temperature [°C]
Tmin, K	daily minimum air temperature [K]
Twet	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]
u2	wind speed at 2 m above ground surface [m s-1]
uz	wind speed at z m above ground surface [m s-1]
W	soil water content [mm]
Ya	actual yield of the crop [kg ha-1]
Ym	maximum (expected) yield of the crop in absence of environment or water stresses [kg ha-1]
Ze	depth of surface soil layer subjected to drying by evaporation [m]
Zr	rooting depth [m]
z	elevation, height above sea level [m]
zh	height of humidity measurements [m]
zm	height of wind measurements [m]
zom	roughness length governing momentum transfer [m]
zoh	roughness length governing heat and vapour transfer [m]
a	albedo [-]
g	psychrometric constant [kPa °C-1]
g psy	psychrometric constant of instrument [kPa °C-1]
D	slope of saturation vapour pressure curve [kPa °C-1]
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 [m3 (water) m-3 (soil)]
q FC	soil water content at field capacity [m3 (water) m-3 (soil)]
q t	threshold soil water content below which transpiration is reduced due to water stress [m3 (water) m-3 (soil)]
q WP	soil water content at wilting point [m3 (water) m-3 (soil)]
l	latent heat of vaporization [MJ kg-1]
l ET	latent heat flux [MJ m-2 day-1]
r a	mean air density [kg m-3]
r w	density of water [kg m-3]
s	Stefan-Boltzmann constant [4.903 10-9 MJ K-4 m-2 day-1]
j	latitude [rad]
w	solar time angle at midpoint of hourly or shorter period [rad]
w 1	solar time angle at beginning of hourly or shorter period [rad]
w 2	solar time angle at end of hourly or shorter period [rad]
w S	sunset hour angle [rad]