The influence of human activities on streamflow reductions during the megadrought in central Chile

Abstract

Abstract. Since 2010, central Chile has experienced a protracted megadrought with annual precipitation deficits ranging from 25 % to 70 %. An intensification of drought propagation has been attributed to the effect of cumulative precipitation deficits linked to catchment memory. Yet, the influence of water extractions on drought intensification is still unclear. Our study assesses climate and water use effects on streamflow reductions during a high-human-influence period (1988–2020) in four major agricultural basins. We performed this attribution by contrasting observed streamflow (driven by climate and water use) with near-natural streamflow simulations (driven mainly by climate) representing what would have occurred without water extractions. Near-natural streamflow estimations were obtained from rainfall–runoff models trained over a reference period with low human intervention (1960–1988). Annual and seasonal streamflow reductions were examined before and after the megadrought onset, and hydrological drought events were characterized for the complete evaluation period in terms of their frequency, duration, and intensity. Our results show that before the megadrought onset (1988–2009) the mean annual deficits in observed streamflow ranged between 2 % and 20 % across the study basins and that 81 % to 100 % of those deficits were explained by water extractions. During the megadrought (2010–2020), the mean annual deficits in observed streamflow were 47 % to 76 % among the basins. During this time, the relative contribution of precipitation deficits on streamflow reduction increased while the contribution of water extractions decreased, accounting for 27 % to 51 % of the streamflow reduction. Regarding drought events during the complete evaluation period, we show that human activities have amplified drought propagation, with almost double the intensity of hydrological droughts in some basins compared to those expected by precipitation deficits only. We conclude that while the primary cause of streamflow reductions during the megadrought has been the lack of precipitation, water uses have not diminished during this time, causing an exacerbation of the hydrological drought conditions and aggravating their impacts on water accessibility in rural communities and natural ecosystems.