Assessing climate change induced declines in ponds in British Columbia's semi-arid grasslands

In British Columbia (BC), the ranching community has expressed a concern with observed declines in the number and surface area of grassland ponds used as drinking water sources for cattle during grazing. This study evaluates the extent of the observed declines in BC’s southern interior grasslands and determines the differences in groundwater – surface water (GW-SW) interactions between perennially and seasonally inundated ponds in the Lac du Bois grasslands. Using remote sensing techniques to compare historic and modern imagery, ponds from eight 100 km2 sites were evaluated. From 1992 – 2012, the total number and surface water area of ponds decreased by 63% and 54%, respectively within the eight sites. Due to the effects of climate change on wetlands worldwide it is expected that changes in climate are responsible for these declines. A climate data analysis of ClimateWNA modelled data for the eight sites showed a significant increase in air temperature and potential evapotranspiration (PET) from 1900 – 2012 implying an increased evaporative demand on surface water. Precipitation also increased significantly over this time period. However, low snowfall is reported for the 1992 – 2012 time period showing that there have been decreased snowmelt inputs available for GW - SW recharge. The field study in Lac du Bois highlighted the importance of GW - SW interactions in maintaining pond surface water. Wells and piezometers were installed in and around two perennially inundated ponds and two seasonally inundated ponds. It was determined that the perennially inundated ponds receive a persistent input from the local groundwater system allowing them to sustain surface water despite high evapotranspiration rates in the summer months. Conversely, groundwater inputs to seasonally inundated ponds are either temporary or non-existent and therefore they are highly dependent on the amount of input from the spring melt and are more vulnerable to summertime evapotranspiration. Our results are consistent with other studies that show that climate change has contributed to significant losses in wetlands worldwide.