Better understanding the hydrology of peri-urban ephemeral pools serving as habitats for the Western Chorus Frog

Ephemeral (vernal) pools are small hydrologically isolated wetlands found in a large variety of environments across the world. They provide breeding habitats for amphibians during their spring and early summer period of hydrological activity. Because of their small size and intermittent nature, many questions remain unanswered about their hydrology and concerning best strategies to improve their resilience to human activities and climate change. In the peri-urban area around Montreal City (Monteregie region, Quebec, Canada), ephemeral pools are habitats to the endangered Western Chorus Frog. In an attempt to protect the remaining population and to improve the reproductive success of these frogs, a study was funded by the Quebec Ministry of forest, fauna and parks to understand the processes driving pool hydrology and especially pool hydroperiod (length of hydrologically active period after snowmelt). Fourty-eight pools located in forested, agricultural and low-density housing developments were monitored for water levels since 2015. Seven of these pools were fully characterized for bathymetry and geology (field measurements), groundwater levels (hourly monitoring) and drainage area (LiDAR data). The pools are located on relatively flat land, in shallow irregular basins within generally low permeability Quaternary sediments. The pools hold relatively small volumes of water, with maximum annual water depths between 0.2 and 1.1 m (observed in April or May). Their areas vary between 100 and 5000 m². The hydrologically active periods after snowmelt (starting after the last frost when mean temperature > 5°C over five days and ending when the pool is dry) vary between 15 and 150 days. At some sites, the hydroperiods were shorter than the required length for reproductive success of the Western Chorus Frog, which could explain the population decline. These short hydroperiods appear to be linked to the presence of human impacts in the pool vicinity. Water level reactions after precipitation are indications that pool hydrology is influenced by a relatively small contributing area rather than by their apparent drainage area (ratio Δh/rain < 4). Head gradients between water levels in the nearby sediments and pool water levels underline the role of the shallow groundwater in maintaining humid conditions in the pools and in reactivating the pools during the summer.  Simple water budget models  confirm this groundwater input and show that surface and hypodermic runoff is the main water source to the pools. The models show that, pool hydroperiods will start earlier under a changing climate, because of warmer winters, but will not necessarily be longer due to higher temperatures and increased evapotranspiration. The models will be used to estimate the effects of human interventions (e.g. drainage, residential) on pool hydroperiods, thus contributing to plan pool protection strategies that could help protect the Western Chorus Frog.