Data Release for Metamodeling and Mapping of Nitrate Flux in the Unsaturated Zone and Groundwater, Wisconsin, USA

Dates

Publication Date: 2018-03-06

Citation

Nolan, B.T., Green, C.T., Juckem, P.F., Liao, Lixia, and Reddy, J.E., 2018, Data Release for Metamodeling and Mapping of Nitrate Flux in the Unsaturated Zone and Groundwater, Wisconsin, USA: U.S. Geological Survey data release, https://doi.org/10.5066/F7ZC81VR.

Summary

Widespread nitrate contamination of groundwater in agricultural areas poses a major challenge to sustainable water resources. Efficient analysis of nitrate fluxes across large regions also remains difficult. This study introduces a method of characterizing nitrate transport processes continuously across regional unsaturated zones and groundwater based on surrogate, machine-learning metamodels of an N flux process-based model. The metamodels used boosted regression trees (BRTs) to relate mappable variables to parameters and outputs of a “vertical flux method” (VFM) applied in the Fox-Wolf-Peshtigo (FWP) area in Wisconsin. In this context, the metamodels are upscaling the VFM results throughout the region, and the VFM parameters and [...]

Summary

Widespread nitrate contamination of groundwater in agricultural areas poses a major challenge to sustainable water resources. Efficient analysis of nitrate fluxes across large regions also remains difficult. This study introduces a method of characterizing nitrate transport processes continuously across regional unsaturated zones and groundwater based on surrogate, machine-learning metamodels of an N flux process-based model. The metamodels used boosted regression trees (BRTs) to relate mappable variables to parameters and outputs of a “vertical flux method” (VFM) applied in the Fox-Wolf-Peshtigo (FWP) area in Wisconsin. In this context, the metamodels are upscaling the VFM results throughout the region, and the VFM parameters and outputs (collectively referred to as “nitrate flux”) are the BRT metamodel response variables: VFM_fcN, nitrate (NO₃⁻) source concentration factor (which determines the local NO₃⁻ input concentrations); VFM_travel_time_yrs, unsaturated zone travel time; NO3_WT_mgL_1980−2020, NO₃⁻ concentration at the water table in 1980, 2000, and 2020 (three separate metamodels); and Zss_N_ext_depth, NO₃⁻ “extinction depth”, the eventual steady state depth of the nitrate front. The metamodels were trained using 129 wells within an active MODFLOW model area of the FWP and 58 mappable predictor variables from a geographic information system, resulting in training and cross-validation testing R² values of 0.52 – 0.86 and 0.22 – 0.38, respectively. The provided metadata file describes all 58 predictor variables considered in metamodel development, whereas the ascii predictor variable grids comprise those in the final metamodels. Metamodel outputs (ascii prediction grids) were compiled as maps of the above metamodel response variables. Relationships between predictor variables and outputs were generally consistent with expectations, e.g. with greater source concentrations and NO₃⁻at the groundwater table in areas of intensive crop use and well drained soils. Shorter unsaturated zone travel time in poorly drained areas indicated possible preferential flow through clay soils and a tendency for fine grained deposits to collocate with areas of shallower water table. Numerical estimates of groundwater recharge may have been a proxy for N input and redox conditions in the northern FWP, which had shallow predicted NO₃⁻ extinction depth. The metamodel results provide proof-of-concept for regional estimation of NO₃⁻ transport processes in a statistical metamodel framework based on mappable GIS input variables.

Contacts

Point of Contact :: Bernard T Nolan
Originator :: Bernard T Nolan, Christopher T Green, Paul F Juckem, Lixia Liao, James E Reddy
Metadata Contact :: Jennifer B Sharpe
Publisher :: U.S. Geological Survey
Distributor :: U.S. Geological Survey - ScienceBase
USGS Mission Area :: Water Resources
SDC Data Owner :: Office of Planning and Programming

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Metamodeling_and Mapping_Nitrate_Flux_Wisconsin_USA_Metadata.xml “Metadata” Original FGDC Metadata	View	69.52 KB	application/fgdc+xml
Metamodeling_Mapping_Nitrate_Flux.zip “Prediction Grids, Predictor Variable Grids, and Metadata”		8.03 MB	application/zip

Related External Resources

Type: Related Primary Publication

Metamodeling and mapping of nitrate flux in the unsaturated zone and groundwater, Wisconsin, USA	https://doi.org/10.1016/j.jhydrol.2018.02.029

Purpose

The prediction grids are the basis of aquifer vulnerability maps, which stand alone but can also provide potentially useful predictor variables (e.g., nitrate concentration at water table) for future regional water quality models.

Data Release for Metamodeling and Mapping of Nitrate Flux in the Unsaturated Zone and Groundwater, Wisconsin, USA

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