Potential Change in Groundwater Discharge as Response to Varying Climatic Conditions – An Experimental Model Study at Catchment Scale - The Baltic Sea Basin - page 394

and

Q = Q S + Q GW (2)

with P precipitation, ET evapotranspiration, Q discharge, Q S surface runoff and
Q GW groundwater discharge. For coastal areas we can assume that the discharge Q
drains directly into the sea. Assuming that (i) the surface and subsurface catchment
areas are equal, (ii) overland flow can be neglected and rivers are fed by groundwa-
ter only and (iii) no other sources or sinks exist, we can reformulate that the total
discharge equals the amount of groundwater recharge R GW (3):

P − ET = Q = R GW (3)

Then the submarine groundwater discharge Q SGD is the difference between
recharge and surface runoff Q S (4):

Q = Q S + Q SGD (4)

Compared to diffuse submarine groundwater discharge, the runoff in streams can
be measured with relative ease and accuracy.

19.2.2 Groundwater Recharge Assessment

Groundwater recharge is a key parameter in groundwater budgets but is difficult
to assess. Scanlon et al. ( 2002 ) offer an excellent review of approaches to quan-
tify groundwater recharge. Methods of groundwater recharge assessment based on
budget considerations provide integral results and thus lack desirable accuracy with
respect to spatial and temporal resolution. Direct measurements, e.g. by lysimeters,
are point supported and require appropriate regionalization if larger catchments are
considered.
The northern part of Germany is characterized by a generally flat topography,
unconsolidated glacial sediments, predominant agricultural land use and humid cli-
mate conditions. Groundwater recharge for these conditions was found to be well
estimated by areal differentiation methods which are based on empirical regression
equations and which can easily be implemented in GIS systems. The method of
Renger and Wessolek ( 1990 ) has proven to provide reliable results of groundwater
recharge as difference between annual precipitation and real evapotranspiration.
Renger and Wessolek ( 1990 ) estimate the annual real evapotranspiration as
follows:

ET r = a · P S + b · P W + c · log W Pl + d · ET p + e (5)

ET r - real evapotranspiration
ET p - potential evapotranspiration
P S - precipitation summer
P W - precipitation winter
W Pl - amount of water useable for plants stored in soil.