[GRASS-user] Clarification on units used in r.sun

Fri Dec 1 16:59:49 EST 2006

Hi again Dylan,

you are using the same sensors than me!!. Just let me a little tricky
suggestion: when you say that the LICOR is returning hourly integrated
data that's not true at all because the sensor gives you the mean flux
over the whole hour, i.e. it counts all the energy incoming on it during
the hour. Then, it returns the equivalent average flux [W/m2]. AFTER THE
MEASURE, you integrate in time the energy flux to obtain the hourly energy
per squared meter. Remember that integration is a sum and, in this case,
like multiply by the measurement period (1 hour).

I really hope not to mislead you

Cheers

Jose

> Dylan,
>
> And you might add for completeness:
>
> daily summed *energy* [Wh/m2] = *energy* [J/m2] (the total amount of
> incoming radiation/energy for the 24-hour period).
>
> Tom
>
>
> Dylan Beaudette wrote:
>> Hi Jose,
>>
>> This makes sense. Thanks again for the sage advice!
>>
>> >From what I have read about our sensor (it is a LiCor LI-200SZ
>> pyranometer) It
>> is returning hourly integrated data. Thus:
>>
>> irradiance [Wm^2] * hour_1 + irradiance [Wm^2] * hour_2 + ... irradiance
>> [Wm^2] * hour_24 = daily summed *energy* [Wh/m^2] .
>>
>> This should now be comparible to the output of r.sun (mode 2) i think.
>>
>> Cheers,
>>
>> Dylan
>>
>> On Friday 01 December 2006 00:14, José Antonio Ruiz Arias wrote:
>>
>>> Hi Dylan,
>>>
>>>
>>>
>>> Probably you have cleared your doubt but I'll try to expound how I see
>>> the
>>> question. The most important thing is you firstly have to think if you
>>> are
>>> dealing with flux or energy.
>>>
>>>
>>>
>>> The flux is something (let say, particles or light photons) coming
>>> through
>>> a normal surface to the beam (for example, persons through a door).
>>> Therefore the unit of flux has to be something like particles per
>>> squared
>>> meter and per second (how many particles have reached the normal
>>> surface in
>>> a second). In case of light (photons) we talk in terms of energy to
>>> count
>>> the particles, so we have J/m2*s = W/m2 (because W = J/s). Then, when
>>> you
>>> measure the solar radiation with a sensor, the measurement is usually
>>> in
>>> units of flux [W/m2]. What does it mean? Let suppose in a given moment
>>> the
>>> sensor reads 500 W/m2. That equals to 500 J/s/m2 = 500 J/m2/s, i.e. in
>>> a
>>> second you have received 500 Joules per squared meter so, in 2 seconds,
>>> you
>>> will have 1000 joules in a squared meter. In 3 seconds you will have
>>> 1500
>>> joules in a squared meter and so on. Now we have sum (or integrated)
>>> the
>>> flux throw the time and the magnitude can be seen as energy per unit of
>>> surface in a certain period of time, let say hour, day, month,

>>>
>>>
>>>
>>> Watts per hour is a measurement of energy used in engineering not in
>>> science. The only reason I think is because the magnitude of its value
>>> is
>>> more appropriate than J or MJ. The equivalence is 1 Wh = (1 J/s)*3600 s
>>> =
>>> 3600 J = 0.0036 MJ
>>>
>>>
>>>
>>> I work with radiometric sensors and sometimes I have used r.sun. The
>>> approach I follow is to express both in J/m2/dia. With r.sun the output
>>> is
>>> in Wh/m2/dia so directly multiply by 0.0036 and you will have MJ/m2/dia
>>> and
>>> will know how much energy you have in a squared meter when the day
>>> finish.
>>> Now let suppose you are getting a measurement every 10 minutes, i.e.
>>> every
>>> 600 seconds. In that lapse of time you will have S[W/m2]*600seconds,
>>> where
>>> S is the measurement. In a day you will have Sday[J/m2/dia] = S1*600s +
>>> S2*600s + S3*600s + 
 and so on for all measurements in the day.
>>>
>>>
>>>
>>> ________________________________
>>>
>>>
>>>
>>> José A. Ruiz Arias
>>>
>>> Departamento de Física
>>>
>>> Escuela Politécnica Superior
>>>
>>> Edificio A-3, Campus Lagunillas
>>>
>>> Universidad de Jaén
>>>
>>> 23071 Jaén Spain
>>>
>>> Tlf. +34 953212474
>>>
>>> Email:  jararias at ujaen.es
>>>
>>> _____________________________________
>>>
>>>
>>>> -----Mensaje original-----
>>>>
>>>> De: grassuser-bounces at grass.itc.it
>>>> [mailto:grassuser-bounces at grass.itc.it]
>>>>
>>>> En nombre de Dylan Beaudette
>>>>
>>>> Enviado el: jueves, 30 de noviembre de 2006 22:55
>>>>
>>>> Para: Glynn Clements
>>>>
>>>> CC: grassuser at grass.itc.it; GRASS devel
>>>>
>>>> Asunto: Re: [GRASS-user] Clarification on units used in r.sun
>>>>
>>>> On Wednesday 29 November 2006 12:31, Glynn Clements wrote:
>>>>
>>>>> Dylan Beaudette wrote:
>>>>>
>>>>>> Quick question on the units used for the ouput of r.sun in mode 2
>>>>>>
>>>> (daily
>>>>
>>>>
>>>>>> sums):
>>>>>>
>>>>>>
>>>>>>
>>>>>> In the manual pages for r.sun, the following 'unit' is included in
>>>>>> the
>>>>>>
>>>>>> description:
>>>>>>
>>>>>> ----------------------------
>>>>>>
>>>>>> The solar radiation maps for given day are computed integrating the
>>>>>>
>>>>>> relevant irradiance between sunrise and sunset times for given day.
>>>>>>
>>>> The
>>>>
>>>>
>>>>>> user can set finer or coarser time step step used for all-day
>>>>>>
>>>> radiation
>>>>
>>>>
>>>>>> calculations. A default value of step is 0.5 hour. Larger steps
>>>>>> (e.g.
>>>>>>
>>>>>> 1.0-2.0) can speed-up calculations but produce less reliable
>>>>>> results.
>>>>>>
>>>> The
>>>>
>>>>
>>>>>> output units are in Wh per squared meter per given day
>>>>>> [Wh/(m*m)/day].
>>>>>>
>>>>>> -------------------------
>>>>>>
>>>>>>
>>>>>>
>>>>>> Is one to interpret this as "watt-hour per square meter per day" ?
>>>>>>
>>>>>> This would seem a little odd, as the the unit 'watt-hour' is not an
>>>>>> SI
>>>>>>
>>>>>> unit.
>>>>>>
>>>>> More odd (to me) is having (different) units of time in both the
>>>>>
>>>>> numerator and denominator. I would have thought it more logical to
>>>>>
>>>>> divide the result by 24 to give Watts per square metre [W/(m^2)].
>>>>>
>>>> I need to check with some local experts, but judging from some recent
>>>>
>>>> tests -
>>>>
>>>> the output from r.sun is comparible to that from a weather station,
>>>> with
>>>>
>>>> *hourly* averaged data in W/(m^2) which i think is analogous to the
>>>>
>>>> Wh/(m^2)
>>>>
>>>> units that r.sun uses.
>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Dylan Beaudette
>>>>
>>>> Soils and Biogeochemistry Graduate Group
>>>>
>>>> University of California at Davis
>>>>
>>>> 530.754.7341
>>>>
>>>>
>>>>
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>>>>
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>>>>
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>>>>
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>>>>
>>>>
>>>>
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>>
>
>
> --
> Thomas E Adams
> National Weather Service
> Ohio River Forecast Center
> 1901 South State Route 134
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