The input data has to be a zoo time series constructed with a chron date time object of month/day/year hr:min:sec (i.e., 08/18/70 23:15:00)
sr and ss should be after and before the start and end of the time series, respectively.
ER is calculated as sum Et (i.e, mean nighttime NEP corrected for the difference in daytime temp and average nighttime temp)
GPP is calculated by summing NEP-ERt from sunrise to sunset
NEP=ER+GPP
Tested Against Rivermet spreadsheet (Izagirre 2007). The data from station 1 (7/10 - 7/15/2003) were used with K=0.07 from "Introduced K". ER, NEP, and GPP are in mg/L*d. The results were not identical. When Estimation from rivermet was regressed on estimation from this software, GPP, ER, and NEP intercepts did not differ significantly from 0 and slopes were nearly 1: 0.94, 0.91, and 0.95, respectively. Further testing is greatly appreciated.