Calculating the Phosphorus inputs from septic systems is a bit more complicated. First, the number of capita-years must be calculated. This number represents the total amount of time spent by citizens in the watershed. We calculated it by multiplying the average household size by the amount of time spent in the watershed by the total number of homes with septic systems. Because properly functioning holding tanks  do not contribute phosphorus, they were not included in the number of houses. We the number of capita-years was calculated for both seasonal and year round residents.
	The number of capita-years is then multiplied by two rates. The septic loading rate is the amount of phosphorus that one person using a septic system for a year will contribute to the soil. However, because some soils hold on to nutrients more than others, we multiplied by 1 minus the soil retention  rate to give us the total Phosphorus added to the lake. As in the calculations for land use, we used high, low and best estimates for the septic loading and soil retention rates.