1) What are the potential effects of watershed deforestation on water and nutrient flows within the watershed?
Removing trees from the watershed would lessen the bank stability and increase erosion, and without the trees to take up any excess nutrients, it will all run into the stream. Also, it would deprive the river's inhabitants of some allocthonous food sources.
2) In what ways is human water usage affected by agricultural practices? How would water flow within the system potentially be different if an agricultural area was forested instead?
When area is tilled to plant crops, the sediment is looser and more likely to be washed into a stream and be deposited in the streambed. Excess nutrients in the fertilizer used on the farm would also leach into a nearby stream. There is also the matter of drawing water from the stream to irrigate the crops-it could potentially decrease the stream flow.
3) Explain human water usage within the context of the hydrologic cycle as a whole. What factors affect our capacity for water usage?
Some water uses don't return to the landscape from which they were drawn, but are rather evaporated or transpired back into the atmosphere (i.e. agriculture). Some other uses return the water to the stream, albeit contaminated (i.e. most domestic uses).
A major factor is the actual availability of the water for use; if the water just runs off back into the ecosystem, or is frozen as ice, or is not clean enough to use, then it won't be available for human use. Some human populations don't have enough access to all the water they need, and that creates issues.