aggregate daily rainfall time series to 16 day temporal resolution

fun_aggre16d.R

## x: daily rainfall time series
## lindex: date stamps of the MOD13 VI product time series
## rindex = lindex + 15
## TODO: increase the efficiency by replace the loop use sapply

fun.aggre16d <- function(x, lindex, rindex) {
  v.16d.rain <- vector('numeric', length(lindex))
  for (i in 1:length(lindex)) {
    v.16d.rain[i] <- sum(x[lindex[i]:rindex[i]], na.rm = T)
  }
  return(v.16d.rain)
}