SI and the areal depletion curve

The parameter SI and the areal depletion curve are related, therefore initial estimates of both can be made at the same time.
If a lot of data on both the mean water-equivalent of the snow cover and its areal extent were available, SI and the areal depletion curve could be determined by plotting mean areal water-equivalent versus the areal extent of the snow cover for a number of years.
Such an analysis would require a sufficient number of snow courses to represent snow cover conditions over the entire watershed.

Without the detailesysinformation needed to compute SI and the areal depletion curve, the initial estimate of these parameters must be based on the user's knowledge of the typical variability in both snow cover and melt rates over the area.
If the snow cover is very uniform and melts at a uniform rate, the area will remain at 100 percent cover until just before the snow disappears.
In this case, SI would be zero or nearly zero and the shape of the areal depletion curve would be unimportant.