R/transmissivity.R

In EcoHydRology: A Community Modeling Foundation for Eco-Hydrology

transmissivity<-function(Tx,Tn, A=0.75, C=2.4, opt="1day", JD=NULL){ 	 
# fraction of direct solar radiation passing through 
# the atmosphere based on the Bristow-Campbell eqn
	#Tx: maximum daily temperature [C]
	#Tn: minimum daily temperature [C]
	if (any(Tx<Tn)) {print("Warning, Tn larger than Tx and generated NAs in the following rows:") 
		print(which(Tx<Tn))
		}
	len<-length(Tx)
	dT <- (Tx-Tn)  # diurnal temperature range just difference between max and min daily temperature
	avDeltaT<-vector(length=len)
	
	if (opt == "2day"){  ## This is the way Bristow-Campbell originally envisioned it 
		for (i in 1:(len-1)){
			dT[i] <- Tx[i] - (Tn[i]+Tn[i+1])/2
		}  
	}
	
	if (opt == "missingdays" & !is.null(JD)){
		for (i in 1:len){
			JDX <- JD[i] + 365
			JDN <- JD[i] - 365
			index1 <- which((JD < (JD[i] + 15) & JD > (JD[i] - 15)) | (JD > (JDX - 15)) |	(JD < (JDN + 15)))
			index2 <- index1[which(index1 > i-14 & index1 < i+15)]
			avDeltaT[i]<-mean(dT[index2])
		}	
	} else if(len<30){ 
		avDeltaT<-mean(dT)
	} else {
		avDeltaT[1:14]<-mean(dT[1:30])
		avDeltaT[(len-14):len]<-mean(dT[(len-30):len])
		for (i in 15:(len-15)){
			avDeltaT[i]<-mean(dT[(i-14):(i+15)])
		}
	}
	B<-0.036*exp(-0.154*avDeltaT)
	return(A*(1-exp(-B*dT^C)))  
}