Two sets of hydrologic indices were developed to characterize the wate
r-budget components of nontidal wetlands. The first set consisted of s
ix water-budget indices for input and output variables, and the second
set consisted of two hydrologic interaction indices derived from the
water-budget indices. The indices then were applied to 19 wetlands wit
h previously published water-budget data. Two trilinear diagrams for e
ach wetland were constructed, one for the three input indices and anot
her for the three output indices. These two trilinear diagrams then we
re combined with a central quadrangle to form a Piper-type diagram, wi
th data points from the trilinear diagrams projected onto the quadrang
le. The quadrangle then was divided into nine fields that summarized t
he water-budget information. Two quantitative ''interaction indices''
were calculated from two of the six water-budget indices (precipitatio
n and evapotranspiration). They also were obtained graphically from th
e water-budget indices, which were first projected to the central quad
rangle of a Piper-type diagram from the flanking trilinear plots. The
first interaction index (I) defines the strength of interaction betwee
n a wetland and the surrounding ground- and surface-water system. The
second interaction index (S) defines the nature of the interaction bet
ween the wetland and the surrounding ground- and surface-water system
(source versus sink). Evaluation of these indices using published wetl
and water-budget data illustrates the usefulness of the technique.