| It is widely accepted that wetlands have a significant influence on the
hydrological cycle. Wetlands have therefore become important elements in water management
policy at national, regional and international level. There are many examples where wetlands
reduce floods, recharge groundwater or augment low flows. Less recognised are the many
examples where wetlands increase floods, act as a barrier to recharge, or reduce low flows.
This paper presents a database of 439 published statements on the water quantity functions
of wetlands from 169 studies worldwide. This establishes a benchmark of the aggregated
knowledge of wetland influences upon downstream river flows and groundwater aquifers.
Emphasis is placed on hydrological functions relating to gross water balance, groundwater
recharge, base flow and low flows, flood response and river flow variability. The functional
statements are structured according to wetland hydrological type and the manner in which
functional conclusions have been drawn. A synthesis of functional statements establishes
the balance of scientific evidence for particular hydrological measures. The evidence
reveals strong concurrence for some hydrological measures for certain wetland types. For
other hydrological measures, there is diversity of functions for apparently similar
wetlands. The balance of scientific evidence that emerges gives only limited support to
the generalised model of flood control, recharge promotion and flow maintenance by
wetlands portrayed throughout the 1990s as one component of the basis for wetland
policy formulation. That support is confined largely to floodplain wetlands, while many
other wetland types perform alternate functions – partly or fully. This paper provides
the first step towards a more scientifically defensible functional assessment system.
Keywords: wetlands, hydrological functions, flood reduction, groundwater recharge, low flows, evaporation |