[R] Computing Water Balance using a loop.

[Duncan Murdoch]

On 23/10/2014, 8:33 AM, Frederic Ntirenganya wrote:
> Dear All,
> 
> I want to calculate water balance using the following formula:
> Water balance today = Water balance yesterday + Rainfall − Evaporation
> 
> This is a sample of data I am using:
> 
> head(Wb30)
>   May Rainfall Evaporation Water_Balance
> 1   7        0           5             0
> 2   8       10           5            NA
> 3   9        0           5            NA
> 4  10        0           5            NA
> 5  11        2           5            NA
> 6  12       23           5            NA
> 
> The following is the loop am trying to use but it is not working well.
> 
> #Water balance today = Water balance yesterday + Rainfall − Evaporation
> #If Water balance today < 0 then Water balance today = 0
> #If Water balance today > 100 then Water balance today = 100
> wb=c()
> for (w in 1:length(Wb30$Water_Balance)){
>   if(w<0 & w>100){w<-0 & w<-100}

The line above doesn't make sense.  It is impossible for w to be both
less than 0 and greater than 100, so the condition will never be true.
And if it is true, "w<-0 & w<-100" is not a sensible thing to do.

>   #print (w);
>  for (i in 1:length(Wb30$Rainfall)){
>     for (j in 1:length(Wb30$Evaporation)){
>     }
>   }

Those loops do nothing.

>  wb<-Wb30$Water_Balance[w] + Wb30$Rainfall[i+1]-Wb30$Evaporation[j+1]

Those should be using w-1 in place of w, and w in place of i+1 and j+1.

Duncan Murdoch

> }
> wb
> 
> Any suggest of what I am missing for it to work correctly is welcome.
> 
> 
> Regards,
> 
> Frederic Ntirenganya
> Maseno University,
> African Maths Initiative,
> Kenya.
> Mobile:(+254)718492836
> Email: fredo at aims.ac.za
> https://sites.google.com/a/aims.ac.za/fredo/
> 
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> 
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