Groundwater convergence as a possible mechanism for multi-year persistence in rainfall

[1] Yearly rainfall totals in wet-to-dry climate transition zones show marked multiyear periods of dry and wet anomalies. These are said to originate from large-scale external forcings (e.g. SST anomalies) and enhanced by land-atmosphere coupling through soil moisture, vegetation and surface albedo. We explore the hypothesis that the degree of persistence in rainfall may be partly caused by groundwater convergence, i.e. the confluence of groundwater to discharge zones that remain wet throughout the year to sustain evaporation for longer periods of time. A simple stochastic model of soil moisture-precipitation feedback augmented with a groundwater component shows that groundwater convergence introduces long-term persistence in rainfall. The degree of persistence (as measured by the Hurst exponent of simulated yearly rainfall totals) increases with groundwater residence time. This notion has important ramifications for multiyear drought forecasting, whereby monitoring or concurrent modeling of groundwater stores may play a role in improving predictability.