COUPLED GROUNDWATER-SURFACE WATER LEVEL MODELLING ON A CONTINENTAL TO GLOBAL SCALE

Lakes and groundwater are both important for water storage, and can store large volumes of fresh water. Groundwater accounts for almost a third of the world’s fresh water, while lakes and wetlands account for an additional 0.25%. However, the water storage volumes in these reservoirs can change drastically over time and impact global hydrology. For example, around 9500 years ago glacial Lake Agassiz held a large enough volume of water to account for 10 cm of sea level change. Such historic fluctuations in water storage are recorded in strandlines that indicate past lake levels. These strandlines are also indicative of palaeo-groundwater storage, since lake levels also record changes in regional groundwater tables.

We investigate this changing terrestrial water storage through time using a coupled groundwater–surface-water model that can rapidly compute water-table fluctuations on a continental to global scale over time scales from years to thousands of years. Our model outputs the elevation of the water table, either as a depth below the Earth’s surface or, in the case of lakes, as a height above it. By computing water-table elevations at multiple times, we can calculate changes in total terrestrial water storage.

The groundwater model component follows Reinfelder et al. (2013), using Darcy’s Law with a finite-difference approach in a single layer of vertically-integrated hydraulic conductivity to compute groundwater-table evolution. In the surface-water component, surface water either infiltrates to become groundwater, reaches the ocean, or is retained in a local depression to form a lake. Surface water is redistributed following a "fill-spill-merge" methodology: runoff flows into a local depression until it overfills, spilling over and merging with a neighbour.

We will present continental-scale results from this coupled model, showing groundwater and lake evolution over a long time period (from the Last Glacial Maximum, 21,000 years ago, to the present day) and discuss changes in water storage volume over this time period.