A time lag is a fairly long interval of time between one event and another related event that happens after it.

What is the time lag between the?

the period of time between two closely related events, phenomena, etc., as between stimulus and response or between cause and effect. a time-lag between the declaration of war and full war production.

What is a time lag in geography?

Lag time - the time taken between peak rainfall and peak discharge. Rising limb - shows the increase in discharge on a hydrograph. Falling limb - shows the return of discharge to normal/base flow on a hydrograph. Base flow - the normal discharge of the river.