Explanation of Limit Plots
In the simulation of the river reach, HSPF computes the fraction of the maximum growth rate of algae allowed by each potentially limiting factor. For example, there may be enough light for algae to grow at half the maximum rate and enough dissolved inorganic nitrogen to grow at 75% of the maximum rate, etc. After computing the limitation fraction for each factor, HSPF compares the fractions and selects the minimum on an hourly basis.
CBPO modified the HSPF code to retrieve the internal variable that specifies the limiting factor. Hourly values were summed by basin and average month to arrive at the data displayed in the charts. For example, the chart at right shows that in the Patuxent River, in April, over the 12 year period of simulation, nitrogen was limiting 8% of the time, phosphorus 13% of the time, and light 79% of the time. Light is generally going to be the limiting factor at least 50% of the time due to night.
The explanation of the legend is as follows:
NONE - no factor was more than 5% limiting (all allowed >95% of the maximum)
DIP - dissolved inorganic phosphorus (PO4)
DIN - dissolved inorganic nitrogen (NH4+NO3)
<.001 - HSPF stipulation that .001 mg/l of DIN and DIP must exist at the end algal growth for that time step. Could be either N or P
Temp - Temperature of the water does not allow algal growth
Low Water - Not enough water for Algal Growth
