Ex 6.3 - Snowpack-surface water model

1. Why do we model snowmelt as a constant rate rather than a proportion of the available snow? Make this modification to the model and analyze the difference in model performance. Which model seems to be more realistic? 

I don't know why snowmelt is modeled as a constant; perhaps because it's best to start with the simplest model possible and only add complexity when necessary. That said, if we want to make the snowmelt model more realistic, I would think that, in addition to total snowpack, temperature and incident radiation should be incorporated. Nevertheless, making snowmelt a function of the amount of snowpack, such that two percent of the snowpack melts on any day with temp > 0C (melt = 0.02 * snowice/dt) produces more realistic behavior than the constant snowpack model:

Melt as function of snowpack; note decay curve of "SnowIce."

2. Temperature in this model is presented as a random fluctuation over the sin function of time. What alternative methods could be used to generate temperature for this model? 

Weather station data could be used instead of a mathematical model. Incorporating not just average daily temperature, but also minimums and maximums, would likely improve the performance of the model.