Geoscience Reference
In-Depth Information
Fig. 2.
Hydrological pathways in a coastal watershed.
A watershed is a topographically defined as an area that water enters
through precipitation and leaves as evapotranspiration, surface runoff, and
subsurface water discharge. In the case of a coastal watershed, runoff and
subsurface water discharge enter the sea (Figs. 1 and 2). Rainfall and poten-
tial rate of evapotranspiration are determined by climate. Actual evapotran-
spiration is limited by the climatically controlled potential rate, vegetation
and the wetness of the soil. Soil properties, topography, and the history
of rainfall and evapotranspiration determine runoff and subsurface water
discharge. For the purpose of this discussion, surface runoff includes direct
runoff which occurs as stream flow immediately following a rainstorm, and
drainage of subsurface water into creeks which accounts for stream flow
between streams. The amount of direct runoff generated by a storm depends
on the amount of rainfall and on the moisture condition of the soil. In gen-
eral, more runoff occurs when the soil is initially wet. The subsurface water
discharge can be calculated if rainfall, evapotranspiration, runoff, and the
change in the amount of water stored on the watershed are known.
A link between rising sea level and changes in the water balance is
suggested by the general description of the hydraulics of subsurface water
discharge at the coast. Fresh subsurface water rides up over denser saltwater
in the subsurface system on its way to the sea (Fig. 2), and subsurface
water discharge is focused into a narrow zone that overlaps with the inter-
tidal zone.
The width of the zone of subsurface water discharge measured perpen-
dicular to the coast is indirectly proportional to the discharge rate. The
shape of the water table and the depth to the fresh/saline interface are con-
trolled by the difference in density between freshwater and saltwater, the
rate of freshwater discharge and the hydraulic properties of the subsurface
