Catchment legacies and time lags: a parsimonious watershed model to predict the effects of legacy storage on nitrogen export

• Published in: PloS one Vol. 10; no. 5; p. e0125971
• Main Authors: Van Meter, Kimberly J, Basu, Nandita B
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 18.05.2015; Public Library of Science (PLoS)
• Subjects: Agricultural Irrigation; Agrochemicals; Anthropogenic factors; Biogeochemistry; Catchment areas; Catchment scale; Catchments; Conservation; Conversion; Denitrification; Environmental aspects; Environmental science; Fertilizer application; Fertilizers; Groundwater; Groundwater - chemistry; Hydrologic models; Hydrology; Land use; Land use management; Landscape; Management; Mathematical models; Models, Theoretical; Nitrogen; Nitrogen Cycle; Nonpoint source pollution; Nutrient concentrations; Nutrients; Nutrients in soil; Parcels; Reduction; Soil chemistry; Soil nutrients; Spatial distribution; System theory; Travel; Travel time; Traveltime; Water conservation; Water Cycle; Water quality; Watershed management; Watersheds
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0125971

Nutrient legacies in anthropogenic landscapes, accumulated over decades of fertilizer application, lead to time lags between implementation of conservation measures and improvements in water quality. Quantification of such time lags has remained difficult, however, due to an incomplete understanding of controls on nutrient depletion trajectories after changes in land-use or management practices. In this study, we have developed a parsimonious watershed model for quantifying catchment-scale time lags based on both soil nutrient accumulations (biogeochemical legacy) and groundwater travel time distributions (hydrologic legacy). The model accurately predicted the time lags observed in an Iowa watershed that had undergone a 41% conversion of area from row crop to native prairie. We explored the time scales of change for stream nutrient concentrations as a function of both natural and anthropogenic controls, from topography to spatial patterns of land-use change. Our results demonstrate that the existence of biogeochemical nutrient legacies increases time lags beyond those due to hydrologic legacy alone. In addition, we show that the maximum concentration reduction benefits vary according to the spatial pattern of intervention, with preferential conversion of land parcels having the shortest catchment-scale travel times providing proportionally greater concentration reductions as well as faster response times. In contrast, a random pattern of conversion results in a 1:1 relationship between percent land conversion and percent concentration reduction, irrespective of denitrification rates within the landscape. Our modeling framework allows for the quantification of tradeoffs between costs associated with implementation of conservation measures and the time needed to see the desired concentration reductions, making it of great value to decision makers regarding optimal implementation of watershed conservation measures.