  As part of global climate change an accelerated hydrologic cycle (including
an increase in heavy precipitation) is anticipated. So, it is of great
importance to be able to quantify high-impact hydrologic relationships, for
example, the impact that an extreme precipitation (or temperature) in a
location has on a surrounding region. Building on the Multivariate Extreme
Value Theory we propose a contagion index and a stability index. The contagion
index makes it possible to quantify the effect that an exceedance above a high
threshold can have on a region. The stability index reflects the expected
number of crossings of a high threshold in a region associated to a specific
location i, given the occurrence of at least one crossing at that location. We
will find some relations with well-known extremal dependence measures found in
the literature, which will provide immediate estimators. For these estimators
an application to the annual maxima precipitation in Portuguese regions is
presented.
