Based on Handbook of Hydrology, 1993, McGraw-Hill, David R. Maidment, editor
Preface to Homework Number 4
Rainfall Excess and Surface Runoff
When rainfall or snow melt begins, surface runoff does not begin immediately. Some water
may be intercepted by vegetation, some may be stored by the many small depressions in the land
and form puddles, some may be transpired by plants (little during or soon after rainfall, probably,
because it is humid when raining), and the most important delay of surface runoff arises because
of water infiltration into the surface layer of the soil.
Rainfall excess, or direct surface runoff, is water in excess of these deductions from a
rainfall “event”. (So much less depressing than the term “Ithacation”! By the time you graduate
you will know what “Ithacation” really means.) Surface runoff can be very fast, such as that
which forms small rivulets and drains quickly to streams, or somewhat slower, such as that
which flows over the surface of the land but is retarded by vegetation and land surface
Surface runoff is not the total story, of course. Some rain soaks into the ground and then
flows more or less laterally and reappears in streams, perhaps by forming small springs nearer the
streams, or simply by seeping out of the land and into the stream bed. Even though this is not
surface runoff, it contributes to stream flow and potential flooding, particularly after extended
precipitation. Other rain percolates into the deep ground and reappears much later in the streams.
This underground flow, in particular, is why we see many streams continue to flow all summer,
even during quite dry spells (although, admittedly, at a much reduced rate). The water is
rainwater from storms many weeks or months or years previous.