Reservoirs: Oceans (97.2%), Ice (2.14%), Groundwater (0.61%), Surface Water (0.009%), Soil Moisture (0.005%), Atmosphere (0.001%)

Zone of Aeration or Vadose Zone - pores contain both water and air.

Water Table - top of Zone of Saturation (pores filled with water)

Baseflow - ground water contribution of streams

Inflow equals Outflow plus changes in storage. Law of mass conservation.

Example: Mono Lake suffered significant decreases in water level following diversion of mountain streams (inflow) to supply water to Southern California.

Groundwater discharge into nearby streams (baseflow) will also increase if infiltration of rainwater causes the water table to rise.

Hydrograph - shows the discharge of a river at a single location as a function of time.

Baseflow recession hydrograph - during a period of no excess precipitation, the hydrograph of a stream will decay following an exponential curve. Discharge is entirely from ground water contributions.

Q = Q_oexp(-at)

Where 	Q = flow at some time

Q_o = flow at start of recession

a = recession constant

t = time since recession began

Recession constant depends on topography, drainage pattern and soil/sediment type. Recession constant can be computed if the flow at the start of the recession and at some other point in time are known.

A plot of stream hydrograph with time on a arithmetic scale and discharge on a logarithmic scale yields a straight line.

The total potential ground water discharge is the volume of water discharge during a complete ground water recession

V_tp = Q_ot₁/2.3

Where t₁ is the time at which Q = 0.1Q_o

Amount of potential baseflow after some elapsed time, t, is

V_t = V_tp/10^(t/t1)

Recharge is the difference between the total potential ground water discharge and the potential baseflow remaining at the end of the last recession.