Hello Reinder,
   The 0.5 factor is in the CERES-Maize (1986) book, so it looks like Jones
and Kiniry are the ones who came up with it.
   Whether 0.5 is a fudge factor, I cannot say. It would depend on how they
derived the value.
   I think one of the primary roles of models is to identify knowledge
gaps. Once a gap is identified, a modeler may put in a value to make a
model functional instead of waiting for the knowledge to be developed. The
modeler leaves it up to the next person to do the research and improve the
code.
   I think the questions you raise are worthy of more research.
Sincerely,
Richard

At 10:49 AM 05/21/2003 -0400, you wrote:
>Hello Richard,
>
>Thanks for your reply. Your explanation makes sense to me.
>However it raises another question: why was a factor of 0.5 used , and not
for example 0.4, 0.6 or 0.7? Who calibrated the model and came up with the
factor 0.5? Some people would call this a fudge factor. Any comments?
>
>Sincerely,
>Reinder
>
>>>> [log in to unmask] 05/21/03 12:19am >>>
>Hello Reinder,
>   This is how I interpret the source code you mention below with help from
>the books "Understanding options for agricultural production" and
>"CERES-Maize: a simulation model of maize growth and development" . If
>anyone sees anything incorrect, please let me know.
>   The code you sent describes the upward flow of nitrate from soil layer K
>to layer K-1.
>1. SKGN is the amount of nitrate in a soil layer (kg N/ha).
>2. XMIN is a small amount of nitrate that is never in soil solution, so
>cannot denitrify (kg N/ha that is equivalent to 0.25 ppm nitrate).
>3. FLOW(K-1) is the amount of water leaving layer K to the layer above (K-1).
>4. SW(K) is the volumetric water content in soil layer K.
>5. DLAYR(K) is the depth of soil layer K.
>6. FLOW(K) is the amount of water coming into layer K from the soil layer
>below.
>So,
>(1) SKGN(K)-XMIN is the amount of nitrate in solution.
>(2) FLOW(K-1)/(SW(K)*DLAYR(K)+FLOW(K)) is the proportion of water that
>leaves soil layer K to the layer above.
>(3) multiplying (1) by (2) gives the amount of nitrate that could leave
>layer K to the layer above.
>But, because soil water moves upward by vapor transfer and suction, the
>nitrate does not move as readily with water as when it leaches. So, to
>account for this reduced movement of nitrate, (3) is multiplied by 0.5.
>Sincerely,
>Richard
>
>At 08:17 AM 05/20/2003 -0700, Reinder De Jong wrote:
>> Dear colleagues,
>>
>>In the NFLUX subroutine, which determines nitrogen drainage and leaching,
>I encounter the following line:
>>
>>               OUTN    =MAX((SKGN(K)-XMIN),0.0)*(FLOW(K-1)/
>>     +                  (SW(K)*DLAYR(K)+FLOW(K)))*0.5
>>
>>My questions is, why is the latter part of this equation multplied by 0.5?
>Could somebody explain this to me?
>>
>>Reinder De Jong
>>Agriculture and Agri-Food Canada
>>Ottawa, ON
>>Canada
>>
>