the scale at which we consider water blanance

the entire area that drains to a point along a river

any precipitation that falls in the watershed potentially can flow out in a river

any precipitation that falls outside the watershed does not contribute to streamflow within the watershed

there are an infinate number of watersheds that can be identified along any river

any point along the river can be chosen as the outlet to the watershed, and the watershed above that point identified

inputs = outputs + change in storage

P = Q+ET+ΔS

or precipitation = runoff + evapotranspiration + change in storage

the change in storage

any additions or removals of water from soil water or groundwater

used to get an estimate of the possible ET

the amount of water that could be evaporated or transpired from a surface if the water were available

calculated from the air temperature, humidity, and wind speed - all of which are commonly measured at meteorological stations

monthly totals of precipitation (P), potential evapotranspiration (PET), and soil water storage (S), are plotted along with AET and the runnoff (Q).

the AET and S are computed from the measured P and Q and the PET calculated from weather data

a deficiency of precipitation over an extended period of time

ex: a season or more

we lable a period of lower precipitation drought when it affects some activity, group, or ecosystem

how long the precipitation deficiency must go on or how severe the deficiency must be to be labled a drought depends on the region and the system effected

no single standard quantitative measure that defines a drought

a commonly used index to asess drought

computed with a formula from temperature and rainfall info for a region

normal conditions have a PDSI of 0 while whet conditions score positive values (up to 6) and dry conditions score negative values (down to -6)

the more negative the PDSI, the more severe the drought

the strength of the PDSI is that it is standardized to local climate