Retention portfolios buffer climate impacts on floods, droughts, and water quality

Climate change is intensifying floods, droughts, and nutrient export from agricultural landscapes, threatening water security under increasingly non-stationary hydroclimatic conditions. Although landscape-scale water-retention measures are widely promoted as nature-based solutions, their combined effectiveness under future climates remains poorly constrained, particularly across contrasting hydroclimatic settings. Here, we evaluate the capacity of distributed retention portfolios to buffer climate-driven hydrologic and water-quality change using a harmonized, field-explicit Soil and Water Assessment Tool Plus (SWAT+) framework applied to five European agricultural catchments spanning a pronounced hydroclimatic gradient. Portfolios, co-designed with local stakeholders, were simulated under historical and late-century climates drawn from bias-corrected Coordinated Regional Climate Downscaling Experiment over Europe (EURO-CORDEX) simulations. Impacts were assessed for flood peaks, low flows, soil-moisture drought duration, and nutrient or sediment export. Climate change consistently amplifies hydrologic extremes and pollutant loads, particularly under wetter future conditions. Retention portfolios reduce flood peaks and pollutant export across all catchments but provide only limited improvements in low flows and drought duration. However, portfolio effects rarely offset climate-driven changes, indicating that retention primarily functions as a risk-reduction strategy rather than a means of restoring baseline conditions. Portfolio effectiveness showed weak associations with the catchment dryness index and varied among catchment-specific portfolios. Because hydroclimate, catchment properties, measure composition, and spatial placement covary in this comparative design, their independent effects cannot be separated. The results nevertheless identify measure-pathway alignment and hydrologic connectivity as plausible design considerations. Responses were predominantly co-beneficial across indicators, indicating that portfolios can deliver multiple co-benefits with limited trade-offs. These findings position distributed retention portfolios as a useful but inherently limited component of climate adaptation, underscoring the need to combine retention with complementary strategies to manage residual risks.