What makes water different when it
comes to GIS?
GIS is applied to many fields of endeavor, indeed to
any field in which data can be depicted geospatially.
So, what makes water different? Why do we need
special geographic data models for water? First of
all, water is a subtle substance. It flows from one
place to another on the land surface and through
the subsurface; water evaporates and travels great
distances rapidly in the atmosphere and then
returns to the earth again as precipitation. It flows
in streams and rivers and accumulates in lakes,
bays, and estuaries to form the blue features on
topographic maps. Water movement through the
hydrologic cycle (figure 1.1) is extremely complex
and is still not completely understood.
Using GIS to describe natural water systems
requires a means to describe the connectivity of
water flow through the landscape. It is not enough
to know that there are geographic data layers of
water features like streams, aquifers, and wells. It
also matters which streams contribute water to
a particular aquifer and which wells are drilled
into that aquifer to supply water for domestic
consumption or irrigation. Only in this way can
we understand the inflow and outflow of water,
in particular groundwater systems, and thus
manage these systems wisely. This book, Arc Hydro
Groundwater: GIS for Hydrogeology, describes a geographic
data model for hydrogeology, using the
Edwards Aquifer in Texas as a vehicle for developing
and explaining concepts. The surface water
components are described in the book Arc Hydro:
GIS for Water Resources (Maidment 2002), which
is planned to be updated to reflect the new Arc