Hydrologic and biogeochemical functioning of intensively managed catchments: A synthesis of top-down analyses

This paper synthesizes a 3-year collaborative effort to characterize the biogeochemical and hydrological features of intensively managed agricultural catchments by combining data analysis, modeling, and preliminary hypothesis testing. The specific focus was on the Midwestern region of the United States. The results suggest that : (1) water management, specifically the homogenization of evapotranspiration losses driven by mono-cultural vegetation cover, and the homogenization of runoff generation driven by artificial drainage, has created engineered, predictable hydrologic systems; (2) nutrient and pesticide management, specifically their regular applications have created two kinds of biogeochemical export regimes: chemostatic (low variability in concentration as exhibited by nitrate) and episodic (high variability in concentration as exhibited by pesticides); (3) coupled mass-balance models for water and solutes reproduce these two regimes as a function of chemical rate constants. Phosphorus transport regimes were found to be episodic at smaller spatial scales, but chemostatic at larger scales. Chemostatic response dominates in transport-limited catchments that have internal sources of the solute to buffer the periodicity in episodic inputs, while episodic response dominates in source-limited catchments. The shift from episodic nitrate export in pristine catchments to chemostatic regimes in managed watersheds was attributed to legacy stores of nitrogen (built from continued fertilizer applications) that buffer interannual variations in biogeochemical processing. Fast degradation kinetics of pesticides prevents the build-up of legacy sources, and leads to episodic export. Analytical expressions were derived for the probability density functions of solute delivery ratio as a function of the stochastics of rainfall-runoff events and biogeochemical controls.