Changes in extreme climate event frequencies have markedly different effects on watershed nitrogen exports than do changes in mean temperature and precipitation

Abstract Aquatic reactive nitrogen threatens human health and riverine and coastal ecosystems, yet its behavior under climate extremes remains poorly understood. Here we use a frequently cited SPAtially Referenced Regression On Watershed attributes (SPARROW) model to systematically assess the influence of climate extreme frequencies, defined by type intensity, duration, seasonality and co-occurrence of precipitation and temperature events, on the delivery of total nitrogen to river channels in the contiguous United States. Through multiple recalibrations of the model, we test the explanatory power of a wide range individual and combined mean and extreme climate predictors of total nitrogen flux at 425 US Geological Survey monitoring sites over a two-decade period. We use the Akaike information criterion for final optimized model selection. We find that hot/dry autumns, wet summers, and cool years enhance delivery, while dry summers and hot/wet autumns reduce delivery of total nitrogen. Using the final recalibrated model with temperature and precipitation inputs from three global circulation models, INM-CM4-8, GFDL-CM4, and CanESM5, under Shared Socioeconomic Pathways 245 and 585, we compare the simulated impacts of changes in mean climate alone with changes in a combination of mean and extreme climate occurrence frequencies on total nitrogen flux from current levels to flux levels averaged over 2045–2054. We find that simulations accounting for the effects of extreme climate conditions differ significantly from those which consider only mean temperature (MT) and precipitation changes. Several regions, including some with significant agricultural activity, show a reversal in direction of simulated future total nitrogen flux, from reductions in flux due to a dominant effect of increased MT to an increase in flux due to an overriding effect of more frequent extreme events. These findings suggest that changing extreme climate regimes can substantially alter watershed nitrogen processing and underscore the need to account for them in the future.