Generate series of potential evapotranspiration
evap_est(ts, eMin = 0, eMax = 0)Time series (xts) of potential evapotranspiration totals for the time steps given in same units as eMin and eMax
Dynamic TOPMODEL requires a time series of potential evapotranspiration in order to calculate and remove actual evapotranspiration from the root zone during a run. Many sophisticated physical models have been developed for estimating potential and actual evapotranspiration, including the Priestly-Taylor (Priestley and Taylor, 1972) and Penman-Monteith (Montieth, 1965) methods. These, however, require detailed meteorological data such as radiation input and relative humidities that are, in general, difficult to obtain. Calder (1983) demonstrated that a simple approximation using a sinusoidal variation in potential evapotranspiration to be a good approximation to more complex schemes.
If the insolation is also taken to vary sinusoidally through the daylight hours then, ignoring diurnal meteorological variations, the potential evapotranspiration during daylight hours for each year day number can be calculated (for the catchment's latitude). Integration over the daylight hours allows the daily maximum to be calculated and thus a sub-daily series generated.
Beven, K. J. (2012). Rainfall-runoff modelling : the primer. Chichester, UK, Wiley-Blackwell.
Calder, I. R. (1986). A stochastic model of rainfall interception. Journal of Hydrology, 89(1), 65-71.
## Generating daily PET data for 1970
## the values of eMin and eMax may not by not be realistic
st <- as.POSIXct("1970-01-02 00:00:00",tz='GMT')
fn <- as.POSIXct("1971-01-01 00:00:00",tz='GMT')
daily_ts <- seq(st,fn,by=24*60*60)
dpet <- evap_est(daily_ts,0,1)
## create hourly data for the same period
st <- as.POSIXct("1970-01-01 01:00:00",tz='GMT')
fn <- as.POSIXct("1971-01-01 00:00:00",tz='GMT')
hour_ts <- seq(st,fn,by=1*60*60)
hpet <- evap_est(hour_ts,0,1)
## the totals should eb the same...
stopifnot(all.equal(sum(hpet), sum(dpet)))