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Variography of Stream Total Phosphorus in an Agricultural Watershed
Citation:
McManus, M., S. Sobat, T. Linscome-Hatfield, AND J. Carson Jr. Variography of Stream Total Phosphorus in an Agricultural Watershed. 2022 Joint Aquatic Sciences Meeting, Grand Rapids, MI, May 14 - 20, 2022.
Impact/Purpose:
We propose to help the states and tribes gain more information from their monitoring data by combining data from multiple water quality monitoring programs so predictions of nutrient concentrations can be made at sites that are not sampled. We combined several different water quality monitoring datasets routinely collected by a state environmental agency for biannual Integrated Reports under the Clean Water Act. As part of our initial efforts with the Indiana Department of Environmental Management, we examined the spatial variation in total phosphorus concentration in the Patoka River watershed. We related that variation in total phosphorus to straight line distances among monitoring sites in the watershed as well as to stream network distances. Based on straight line distances, total phosphorus concentrations were similar among sites up to 10 kilometers apart. However, total phosphorus concentrations among pairs of sites based on upstream-to-downstream distances, indicated that the range of spatially similarity was up to 100 stream kilometers. For flow-unconnected sites, which are on different branches of the stream network, their similarity in total phosphorus concentrations often exceeded what was observed over straight line and flow-connected distances. Accounting for this spatial variation is needed to predict TP concentration on the river and stream network, and such predictions can help environmental agencies prioritize restoration and conservation efforts.
Description:
Environmental agencies use a variety of designs for monitoring rivers and streams in a watershed. Such designs include fixed sites, often near the watershed outlet, randomly selected sites from generalized random-tessellation stratified surveys, and targeted sites, often used to monitor specific sources of pollutants. Indiana Department of Environmental Management (IDEM) has been conducting such monitoring since 1996. We pooled the monitoring datasets for the Patoka watershed to predict total phosphorus (TP) concentration throughout the watershed. We examined the spatial variation, or variography, in TP over Euclidean and stream network distances. A semivariogram cloud identified several sites as spatial outliers having much greater TP concentrations than neighboring sites. A log transformation plus a robust variogram estimator produced a better distribution of semivariances across all pairs of sites in the cloud plot. Randomizations of semivariograms over Euclidean distance indicated spatial autocorrelation in TP concentration at distances less than 10 km apart. However, semivariances over stream network distances for flow-connected sites, which have an upstream-to-downstream relationship, indicated that the range of spatially autocorrelation was up to 100 stream kilometers. For flow-unconnected sites, which are on different branches of the stream network, their semivariances often exceeded what was seen over Euclidean and flow-connected distances. Accounting for this spatial variation is needed to predict TP concentration on the river and stream network. Such predictions can help prioritize restoration and conservation efforts.
