Create Function to a RasterStack

Question

This is the function that I want to create:

Erosivity_Factor = 1.74 * log[sum(montly²/annual)]+1.29

I have solved it using a loop:

# Reproducible RasterStack with 12 layers
Monlty<- stack(lapply(1:12, function(i) raster(ncol=20, nrow=20, vals=rnorm(400,30)))) 

# Reproducible Raster
Annual <- raster(ncol=20, nrow=20, vals=rnorm(400,230))

R_month<- list()

for (i in 1:12) {
  R_month[[i]]<- (Monlty[[i]]*Monlty[[i]]/Annual)
          }

R_month_raster <- do.call(stack,R_month)
R_erosivity_factor<- (1.74*log10(calc(R_month_raster,sum))+1.29)

Is there a way to make it with a function?

Answer 1

First, a reproducible example:

library(raster)

r <- raster()

l <- list()

for(i in 1:12){
  l[[i]] <- setValues(r,rep(i,ncell(r)))
}

s <- stack(l)

You need to create your function and then apply it with calc function. Is annual the sum of all month values? or is annual an average? Well, I considered the sum of all monthly values, you can change it later:

Erosivity_Factor <- function(x){1.74*log10((x^2)/sum(x))+1.29}

R_erosivity_factor <- calc(s,Erosivity_Factor)

Since the value of the raster increases every month, results have sense:ç

plot(R_erosivity_factor,zlim=c(-2.1,1.8))

---

For only two raster objects the approach is direct:

R_erosivity_factor <- 1.74*log10((Monlty^2)/Annual)+1.29