Kipp & Zonen LAS Instruction Manual

Contents

Kipp & Zonen LAS Instruction Manual Manual pdf 8 page image

71. GENERAL INFORMATION1.1 INTRODUCTION TO LAS-BET AND LAS-RET SYSTEMSThe LAS-BET and LAS-RET (ET stands for evapotranspiration) systems are scintillometer basedsurface flux monitoring systems, specially intended for earth energy balance and water managementstudies. Evapo-transpiration is an important term of the surface energy budgetsv GELHQ ++=* [W m -2], (1)where Q * (or R n) is the available energy known as the net radiation, H the sensible heat flux, L vE thelatent heat flux and G s the soil heat flux. In some cases extra storage and/or advective terms can beadded 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 vE) asfollowsvvLELET = , (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 vE in [W m-2], mostly used inused in meteorology) or as a mass flux (ET in [mm day -1], mostly used in hydrology). Approximately 1W m-2 is equal to 0.0353 mm day-1. The latent heat flux or ET is an important term of the soil waterbalanceoutputinput −=∆S , (3)where ∆S is the change of storage of water in the soil, which is the result of the amount of input intothe soil (e.g. due to rainfall or irrigation) minus the amount of loss of water (e.g. due toevapotranspiration, surface-runoff or drainage).The LAS-BET and LAS-RET have been developed to provide area-representative surface fluxes ofsensible heat (H) and latent heat (L vE). Most unique feature of the both systems is the line of sightpath-integrating capabilities of the (eXtra) Large Aperture Scintillometer (LAS / XLAS). This allows theuser to measure area-averaged surface fluxes of sensible heat at scales of 0.2 to 8 km, over bothhomogenous and moderate heterogeneous areas. So far most traditional measurement techniques(e.g. Eddy-Covariance method, Flux-Profile method, Bowen-ratio method and Lysimeters) are actuallypoint (in-situ) measurements and therefore less representative for large (natural and thereforeheterogeneous) areas.Basically, the LAS-BET and LAS-RET system consist of two parts, namely a scintillometer (LAS or aXLAS) and a weather station (developed by Theodor Friedrichs & Co). The structure parameter (C n2)measured by the LAS / XLAS and additional meteorological data (air temperature at 2 levels, windspeed and air pressure) collected by the weather station are used to determine the surface flux ofsensible heat H. By adding a net radiometer and soil heat flux sensors, the latent heat flux can bederived, viasv GHQEL −−= * , (4)