On the need to manage long-term diffuse memory controls of hydrological mass loads and water quality

Water management requires understanding and handling of water quality changes and their cause-effect relations, including the source inputs in and hydrological mass transport through catchments that load tracers, nutrients, pollutants and other anthropogenic and geogenic constituents to downstream waters and ecosystems. Costly abatement is often required to protect water resources and ecosystems from excessive nutrient and pollutant loads, and to maintain and restore good ecological status and vital ecosystem services of water systems. But what controls then the magnitudes and dynamics of the hydrological mass loads to the downstream waters and ecosystems? Erroneous understanding of these controls may undermine and mislead resource demanding water management efforts. To support and improve this understanding we have analyzed data from 15-23 year time series of chloride, commonly used as an effective chemical tracer of water movement, in daily rainfall and runoff of two comparative Swedish catchments. We show that long-term catchment memory in form of diffuse internal subsurface sources that have developed from earlier mass inputs controls current load dynamics. In the chloride tracer example the internal memory sources contribute 75-90% of the total stream load, while contemporary source inputs at the surface contribute only 10-25%, with these ranges being consistently determined from scenario analysis of chloride travel time distributions in both catchment cases. While the loading from contemporary surface inputs is hydrologically controlled and dependent on the variability of transport pathways and travel times through a catchment, the average net mass release rate from internal memory sources depends primarily on mean travel time. For the chloride tracer example the release rate is in the range of 1.3*10E-4 - 4.5*10E-3 g/m2/day in both catchment cases. The present quantification approach provides a relatively simple, testable and general management tool for determining how source inputs of various water constituents across multiple spatiotemporal scales combine with the statistics (and, e.g., the climate-driven changes) of precipitation, evapotranspiration, stream discharge, and subsurface solute pathways and travel times to control different hydrochemical management metrics (mass flux, mean and flux-average concentration, cumulative mass load). The quantified existence of load controlling, long-term catchment memory has important implications and must be accounted for in management and abatement strategies, and choices of measures against water resource pollution and ecosystem degradation.