Kipp & Zonen LAS, Instruction Manual

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7
1. GENERAL INFORMATION
1.1 INTRODUCTION TO LAS-BET AND LAS-RET SYSTEMS
The
LAS-BET and LAS-RET (ET stands for evapotranspiration) systems are scintillometer based
surface flux monitoring systems, specially intended for earth energy balance and water management
studies. Evapo-transpiration is an important term of the surface energy budget
s v
G E L H Q
+ + = *
[W m
-2
], (1)
where
Q
*
(or R
n
) is the available energy known as the net radiation, H the sensible heat flux, L
v
E the
latent heat flux and G
s
the soil heat flux. In some cases extra storage and/or advective terms can be
added to the surface energy balance. The evapo-transpiration ( ET ), which stands for the evaporation
( E ) from bare soil and the transpiration ( T ) by vegetation, is linked to the latent heat flux ( L
v
E ) as
follows
v
v
L
E L
ET
=
, (2)
where
L
v
is the latent heat of vaporisation (the energy required to evaporate 1 kg of water, L
v
∼
2.45
×
10
6
J kg
-1
). This means that ET can be expressed as an energy flux ( L
v
E in [W m
-2
], mostly used in
used in meteorology) or as a mass flux (ET in [mm day
-1
], mostly used in hydrology). Approximately 1
W m
-2
is equal to 0.0353 mm day
-1
. The latent heat flux or ET is an important term of the soil water
balance
output input
− = ∆
S
, (3)
where
∆
S is the change of storage of water in the soil, which is the result of the amount of input into
the soil (e.g. due to rainfall or irrigation) minus the amount of loss of water (e.g. due to
evapotranspiration, surface-runoff or drainage).
The
LAS-BET and LAS-RET have been developed to provide area-representative surface fluxes of
sensible heat ( H ) and latent heat ( L
v
E ). Most unique feature of the both systems is the line of sight
path-integrating capabilities of the (eXtra) Large Aperture Scintillometer ( LAS / XLAS ). This allows the
user to measure area-averaged surface fluxes of sensible heat at scales of 0.2 to 8 km, over both
homogenous and moderate heterogeneous areas. So far most traditional measurement techniques
(e.g. Eddy-Covariance method, Flux-Profile method, Bowen-ratio method and Lysimeters) are actually
point (
in-situ ) measurements and therefore less representative for large (natural and therefore
heterogeneous) areas.
Basically, the
LAS-BET and LAS-RET system consist of two parts, namely a scintillometer ( LAS or a
XLAS ) and a weather station (developed by Theodor Friedrichs & Co). The structure parameter ( C
n
2
)
measured by the LAS / XLAS and additional meteorological data (air temperature at 2 levels, wind
speed and air pressure) collected by the weather station are used to determine the surface flux of
sensible heat
H . By adding a net radiometer and soil heat flux sensors, the latent heat flux can be
derived, via
s v
G H Q E L
− − =
*
, (4)