Prof. Richard Taylor
Department of Geography, University College London
United Kingdom
As the planet warms, substantial uncertainty remains in where, when and how much rain and snow will fall. A consistent, observed impact of global warming is the intensification of precipitation, particularly acute in the tropics, which results in fewer light rain events and more frequent, heavy rainfalls. The consequences of this changing distribution in precipitation globally include reduced soil moisture, more frequent and intense floods as well as longer and more frequent droughts. Here, I review evidence of a possible ‘silver lining’ to these changes in the form of a distinct bias in groundwater replenishment to heavy rainfall, often in association with large-scale controls on climate variability such as El Niño Southern Oscillation and Pacific Decadal Oscillation. This evidence, biased to the tropics and sub-tropics, comes from a range of sources including piezometric records, stable-isotope ratios of O and H, and GRACE satellite data that span a wide range of spatio-temporal scales. This observed relationship between extreme, heavy rainfall and groundwater storage not only informs potential adaptive strategies to improve the resilience of freshwater supplies to climate change but also challenges conventional understanding of terrestrial hydrology represented in large-scale models. Heavy rainfall has been observed to infiltrate through the vadose zone and generate diffuse recharge more rapidly than can be defined by the Richards equation, possibly reflecting the presence and influence of soil macropores. Focused recharge, often derived from runoff generated by heavy rainfall, is not represented in most large-scale models but is observed to be widespread, especially in tropical drylands. The empirical relationship between heavy rainfall and groundwater recharge highlights the need for improved representations of groundwater in large-scale models to better understand terrestrial hydrological responses to climate change informing adaptation, and the heightened vulnerability of groundwater resources to contamination from diffuse and focused recharge. The latter is of particular concern in low-income cities where dependence upon untreated groundwater for domestic water supplies remains high.