Climate change impact on groundwater levels: ensemble modelling of extreme values

This paper presents a first attempt to estimate fu- ture groundwater levels by applying extreme value statis- tics on predictions from a hydrological model. Climate sce- narios for the future period, 2081-2100, are represented by projections from nine combinations of three global climate models and six regional climate models, and downscaled (including bias correction) with two different methods. An integrated surface water/groundwater model is forced with precipitation, temperature, and potential evapotranspiration from the 18 models and downscaling combinations. Extreme value analyses are performed on the hydraulic head changes from a control period (1991-2010) to the future period for the 18 combinations. Hydraulic heads for return periods of 21, 50 and 100 yr (T21 100) are estimated. Three uncertainty sources are evaluated: climate models, downscaling and ex- treme value statistics. Of these sources, extreme value statis- tics dominates for return periods higher than 50 yr, whereas uncertainty from climate models and extreme value statistics are similar for lower return periods. Uncertainty from down- scaling only contributes to around 10 % of the uncertainty from the three sources.