As most Hydrologists will ...

As most Hydrologists will tell you that runoff is a function  of the Intensity, Duration and Frequency (IDF) characteristics of rainfall, but stream flow is also affected by things such as Base Flow, evaporation and seepage rates.  Seepage rates are determined by substrate material and evaporation by tree cover and cloud cover etc.  However, at the moment I'd like to concentrate on Base Flow.  Again, base flow is influenced by the geological material and seepage rates, but it is also a function of the water content in these materials too.  Increasing tree cover will actually decrease contributions to Base Flow  and runoff enormously as trees can be looked at as large water pumps delivering transpiration to the skies while actively growing. This effect tends to flatten off as the forested areas mature.  Without sufficient recharge to the aquifers on the mountains (and particularly if some of this is used by well use etc.) base flow will decline in times of drought and may not recover for many years.  In dry times the only thing that keeps streams flowing is Base Flow (ground water originating from aqufiers) and if you look at Base Flow as a financial ledger with both credits and debits to its balance sheet, declining credits and and increases debits in the form of increasing dry periods without recharge is a receipy for declining stream flow...perhaps for years into the future.  I am Lippy.  Hear me roar.