on. When two locations are flow-unconnected the
downstream hydrologic distance will be greater than zero in both
directions. A site's downstream hydrologic distance to itself is
equal to zero. The format of the downstream hydrologic distance
matrix is efficient because distance information needed to fit
both the tail-up and tail-down models is only stored once. As an
example, a matrix containing the total hydrologic distance between
sites is easily calculated by adding the downstream distance
matrix to its 