Time series of piezometric head implicitly contain information about
groundwater system. This information is reflected in the impulse
response function of precipitation. In order to use these functions as a
signature of the system, they must be determined with appropriate
reliability. In order to investigate the effect of other influences on
the determination of the precipitation response, piezometric data have
been studied from a transect running from dunes along the Dutch North
Sea coast with natural vegetation into a polder area with agricultural
and urban land use. In the dune area, there are several drinking water
abstractions with known abstraction rates or levels. Each time series of
piezometric head has been modeled with precipitation, evaporation and up
to 4 extractions as explanatory variables. The precipitation impulse
response functions from these models have been evaluated per piezometer
and compared to neighboring piezometers. Over-parametrization gives more
problems for the determination of the precipitation impulse response
than the presence of other influences as long as these are not too
correlated with the precipitation. The reliability of the precipitation
response can be assessed using spatial correlation with neighboring
piezometers and the local hydrogeology. Reliable impulse response
functions can be used for interpolation and prediction of heads and for
characterisaton of the hydrogeological settings.