4b

PRELIMINARY REVIEWOFREVISEDFAORADIATIONANDTEMPERATURE METHODS

A.Verhoef enR.A. Feddes

RAPPORT 16

Mei1991

Vakgroep Hydrologie, Bodemnatuurkundeen Hydraulica Nieuwe Kanaal 11, 6709PAWageningen ISSN0926-230X

£4b

10-30days Average temperature [°C] Windspeed measured at2m [m/s] Vapour pressure deficit [kPa] Slope ofvapour pressure curve [kPa/°C] Latentheat ofvaporization [MJ/kg] Adiabatic psychrometric constant [kPa/°C] Modifiedpsychrometric constant* [kPa/°C]

This formulawas taken from one of the preliminary reports underlying this investigation. In the final formula the values of 936 and 276 willbe replaced by 925 and 273, respectively. The modifications •rehowever ofminor influence on the evaporation results.

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Net radiation is composed of two parts:net shortwave and net longwave radiation:R„-R^-R^. Netshortwaveradiationcanbedescribedby R„s= (l-o)*Rs

(2)

with Rus a Rs

:Netshortwaveradiation [MJ/m2.d] :Canopyreflectioncoefficient =0.23 overallaverageforgrass :Incomingsolarradiation [MJ/m2.d]

Theincomingsolarradiationisnotmeasuredbutgivenby n Rs= (asn+b sn )*Ra N Rs asn b sn n/N n N Ra

(3)

:Incomingsolarradiation [MJ/m2.d] :Fractionofradiationonovercastdays =0.25 foraverageclimate :Fractionofadditionalradiationoncleardays =0.50 foraverageclimate. :Relativesunshinefraction :Brightsunshinehoursperday [hr] :totaldaylength [hr] :Extraterrestialradiation [MJ/m2.d]

WhereasnetlongwaveradiationispresentedbyEq.(4).

R ^ - (0.9 R„i ed Tmax Tmin a

n h0.1)*(0.34-0.13Ved)*a*0.5*(Tm«*+Tmin*) N

(4)

:Netlongwaveradiation [MJ/m2.d] :Dewpointvapourpressure [kPa] :Maximumtemperature [K] :Minimumtemperature [K] :StefanBoltzmanconstant [MJ/m2.KA.d] =4.903*10-9

Ifatemperaturedependentemissivitycorrelationshouldbeapplied,theterm (0.34-0.139\fed)hastobereplacedbytheso-calledIdsoJacksonEquation: e=-0.02+0.261 exp(-7.77*10-4*Tavg2) Tavg

(5)

:Averagedailytemperature [°C]

The extraterrestial radiation, Ra, can be calculated from the following formulasasgiveninSmith,1991. Ra=37.586*Dr*(n*SIN($)*SIN(5)+COS($)*COS(5)*SIN(fl))

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(6)

Where; $ J Dr

=(