predConc only accepts by="unit" or by="day"

[appling]

by='month', by='water year', etc. would all be nice.

[appling]

Also, the error message if you pass in anything else is entirely unhelpful.

# setup
date_limits <- strptime(c("2004/09/01", "2006/09/01"), format="%Y/%m/%d")
lamprey_calib <- UNH_PackersFalls_Field[which(!is.na(UNH_PackersFalls_Field$NO3) & is_between(UNH_PackersFalls_Field$DATE, date_limits)), ]
lamprey_calib$NO3[lamprey_calib$NO3 <= 0] <- NA
lamprey_calib <- lamprey_calib[complete.cases(lamprey_calib[c("DISCHARGE","NO3")]),]
lamprey_est <- USGS_PackersFalls_Interp[which(is_between(USGS_PackersFalls_Interp$DATE, date_limits)), ]
names(lamprey_est)[which(names(lamprey_est)=="DISCHARGE_APPROX")] <- "DISCHARGE"
lamprey_lr <- loadReg(NO3 ~ model(1), data=lamprey_calib, flow="DISCHARGE", dates="DATE", conc.units = "mg/L", time.step="instantaneous") 

# the problematic call
rloadest_conc_months <- predConc(lamprey_lr, transform(lamprey_est, MONTH=month(DATE)), by="MONTH", allow.incomplete=TRUE)
## Error in rownames(retval) <- NULL : object 'retval' not found