Gradual hydrological change, including groundwater decline, altered flow regimes, wetland loss, snow and glacier retreat and salinization, can be normalized within a generation, so that communities, institutions and scientific assessments judge present conditions against progressively altered reference states. This shifting baseline problem has two components that are rarely examined together: the physical reference condition, reconstructed from instrumental, documentary, paleohydrological and Earth observation records, and the perceived reference condition held by water users and decision makers. Human observations can match instrumental records on the direction of change, yet the generational divergence between perceived and reconstructed hydrological baselines is seldom quantified, and the choice of reference period is often treated as a technical default even where multidecadal variability makes any single window unrepresentative.
This session invites contributions that link evidence of long-term hydrological change to the reference conditions perceived or used by people and institutions, including:
(1) reconstruction of hydrological reference conditions from long records, and the sensitivity of trends, impact estimates and restoration targets to the choice of reference period under low-frequency hydroclimatic variability;
(2) quantitative comparison of perceived or reported reference conditions with reconstructed ones, using surveys, interviews, local and Indigenous knowledge, archives or media records, and of how time since the last major event or past water and land-use decisions bias perception;
(3) representation of changing reference conditions, expectations and learning in socio-hydrological and agent-based models, including memory decay rates calibrated with survey, panel or archival data.
(4) evaluation of whether monitoring, visualization, climate services or participatory approaches make gradual change perceptible, slow the decay of awareness and alter decisions, including panel, longitudinal or experimental studies reporting null or adverse effects.
(5) consequences of baseline choice for water allocation, adaptation and restoration, including whose reference conditions are recognized.
Contributions should connect at least one physical record of change with how reference conditions are perceived, modeled or used in decisions. Studies from data-scarce regions are particularly encouraged.