Abstract
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.
Highlights
High levels of nonpoint source pollution associated with current agricultural practices have contributed to water quality impairment and destruction of aquatic ecosystem habitats at both local and global scales [1,2]
Most existing models such as SWAT and AGNPS [64,65], which are commonly utilized for agricultural landscapes, do not have an explicit mechanism to either account for such legacies or to predict time lags [66]
We have developed a framework that allows for the parsimonious modeling of concentration-reduction benefits over time as a function of spatial patterns of land-use conversion or implementation of conservation measures across the landscape, and the existence of hydrologic and biogeochemical nutrient legacies
Summary
High levels of nonpoint source pollution associated with current agricultural practices have contributed to water quality impairment and destruction of aquatic ecosystem habitats at both local and global scales [1,2]. Watershed management practices to target these non-point source pollutants have in many cases resulted in little or no improvement in water quality, even after extensive implementation of conservation measures [8,9,10]. The lag time between implementation of conservation measures and resultant water quality benefits has recently been recognized as an important factor in their “apparent” failure [8,11]. Conservation measures are often implemented, without explicit consideration of such lag times, and with the expectation that they will lead to immediate benefits. Failure to meet such expectations discourages vital restoration efforts [8].
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