UNIT OF RA(extraterrestrial radiation)

manasmanas Member
edited August 2015 in Freshwater Models
In the table 2.6 of FAO  the unit of RA is given as (MJ/m^2/day).so the value I will be getting  of ETo whether it will be mm/day or not?

Post edited by manas on


  • I want add one more thing. If I use the value of RA in unit of MJ/m2/day and if I get value of ETo in unit of same as RA.. then can I divide the result by a factor Lambda = 2.45.to convert the value of ETo in unit of mm/day?

  • PerrinePerrine Moderator, NatCap Staff
    Hi Manas,

    In the FAO 56 doc, ETo is already expressed in mm/day (Eq. 6).
    The values of RA and Rn that you'd use to compute ETo are in MJ. m^-2.day^-1 (see variables description in Eq. 6)
    Does that help?

  • Yes..it helped.thanks.if  mean monthly temperature is below zero then should I consider Reference evapotranspiration as zero for monthly calculation as in InVEST User Guide, Release 3.1.0, page-125? please tell me whether I have understood it wrongly or not.
  • PerrinePerrine Moderator, NatCap Staff
    That's correct, monthly ETo can be set to 0 when the mean monthly temperature is below zero.
  • ok..but I have observed that if I apply modified Hargreaves for a month in which mean monthly temperature is below zero,then
    also I am getting some positive value.For example; Tmax(monthly)=-.72855 , Tmin= -15.5305, P=60.374, Tavg=-8.12955,TD=14.802,s=40, I am getting ETo=1.40302 and monthly=(31*1.40302)=43.49363 mm. This case happens for some other months also where I have got some positive values though the mean monthly temperature is less than zero.In some other cases where mean monthly temperature is less than zero  I am getting -ve values which is not possible ,so in that case I have considered ETo=0. so please clarify whether I have done any mistake or I have got it wrongly.
  • PerrinePerrine Moderator, NatCap Staff
    Hi Manas, 

    I've checked the development of the modified Hargreaves equation (in Droogers and Allen, 2002): the empirical equation yields negative values (cf. Figure 5) but they do not comment on that in the paper.

    Negative values of ETo should definitely be replaced by 0s, but for months where T<0 and ETo>0, it seems fine to keep the value given by the Hargreaves equation. These values are very small anyway compared to the other months.

    Note that Thornthwaite's method (another commonly-used ETo equation) does yield ETo=0 if T<0, so it's not unreasonable to assume do that in your analysis.



    Droogers, P. & Allen, R.G. 2002. “Estimating reference evapotranspiration under inaccurate data conditions.” Irrigation and Drainage Systems, vol. 16, Issue 1, February 2002, pp. 33–45
  • Thanks @Perrine for your help.I will definitely keep it in mind.
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