find dry spells

### simulate some dates
dates <- seq.Date(as.Date("2014-01-01"), as.Date("2014-12-31"), by = "1 day")
### simulate some rainfall
rain <- sample(c(0,0,0,2,3), length(dates), replace=T)

data <- data.frame(date = dates, 
                   daily_precip = rain)

# Use a rolling apply function to check the rainfall of the last 3 days including the day in question.
library(zoo)
?rollapply()
# The align is needed to say how the moving window works
precip_prev_3_days <- rollapply(data$daily_precip, width=3, FUN=sum, 
                                align="right", na.rm = TRUE, fill = NA)
# remove the first two days from the data frame - cant have 3 previous days
data <- data[-(1:2),]

index <- precip_prev_3_days==0

dates_to_get <- data[index,"date"]

### Simulate some fdom data
fdom_dates <- seq.POSIXt(as.POSIXct("2014-01-01"), as.POSIXct("2014-12-31"), by = "15 mins")
fdom <- data.frame(date = fdom_dates,
                   fdom = rnorm(length(fdom_dates)))
                   
library(lubridate)

fdom_data_to_look_at <-  as.Date(floor_date(fdom$date, "day")) %in% dates_to_get

plot(fdom[fdom_data_to_look_at,])