Gregory E Schwarz
Science and Products
Low threshold for nitrogen concentration saturation in headwaters increases regional and coastal delivery
River corridors store, convey, and process nutrients from terrestrial and upstream sources, regulating delivery from headwaters to estuaries. A consequence of chronic excess nitrogen loading, as supported by theory and field studies in specific areas, is saturation of the biogeochemically-mediated nitrogen removal processes that weakens the...
Schmadel, Noah; Harvey, Judson; Alexander, Richard; Boyer, Elizabeth W.; Schwarz, Gregory E.; Gomez-Velez, Jesus D.; Scott, Durelle; Konrad, Christopher
An enhanced hydrologic stream network based on the NHDPlus medium resolution dataset
The National Hydrography Dataset Plus, Version 2.1 (NHDPlusV2.1) is an attribute-rich digital stream network for the conterminous United States, serving as a foundational infrastructure for reporting hydrologic information at both regional and national scales. SPAtially Referenced Regressions On Watershed attributes (SPARROW) is a process-based...
Brakebill, John W.; Schwarz, Gregory E.; Wieczorek, Michael E. Phosphorus and nitrogen transport in the binational Great Lakes Basin estimated using SPARROW watershed models
Eutrophication problems in the Great Lakes are caused by excessive nutrient inputs (primarily phosphorus, P, and nitrogen, N) from various sources throughout its basin. In developing protection and restoration plans, it is important to know where and from what sources the nutrients originate. As part of a binational effort, Midcontinent SPARROW (...
Robertson, Dale M.; Saad, David A.; Benoy, Glenn A.; Vouk, Ivana; Schwarz, Gregory E.; Laitta, Michael T Small ponds in headwater catchments are a dominant influence on regional nutrient and sediment budgets
Small ponds—farm ponds, detention ponds, or impoundments below 0.01 km2—serve important human needs throughout most large river basins. Yet the role of small ponds in regional nutrient and sediment budgets is essentially unknown, currently making it impossible to evaluate their management potential to achieve water quality objectives. Here we used...
Schmadel, Noah; Harvey, Judson; Schwarz, Gregory E.; Alexander, Richard; Gomez-Velez, Jesus D.; Scott, Durelle; Ator, Scott W.
Estimates of long-term mean daily streamflow and annual nutrient and suspended-sediment loads considered for use in regional SPARROW models of the Conterminous United States, 2012 base year
Streamflow, nutrient, and sediment concentration data needed to estimate long-term mean daily streamflow and annual constituent loads were compiled from Federal, State, Tribal, and regional agencies, universities, and nongovernmental organizations. The streamflow and loads are used to develop Spatially Referenced Regressions on Watershed...
Saad, David A.; Schwarz, Gregory E.; Argue, Denise M.; Anning, David W. ; Ator, Scott A. ; Hoos, Anne B.; Preston, Stephen D.; Robertson, Dale M.; Wise, Daniel Toward explaining nitrogen and phosphorus trends in Chesapeake Bay tributaries, 1992-2012
Understanding trends in stream chemistry is critical to watershed management, and often complicated by multiple contaminant sources and landscape conditions changing over varying time scales. We adapted spatially-referenced regression (SPARROW) to infer causes of recent nutrient trends in Chesapeake Bay tributaries by relating observed fluxes...
Ator, Scott; Garcia, Ana M.; Schwarz, Gregory E.; Blomquist, Joel; Sekellick, Andrew Adapting a regional water-quality model for local application: A case study for Tennessee, USA
We evaluated whether SPAtially Referenced Regression On Watershed attributes (SPARROW) models calibrated for two adjacent USA regions could be applied at the local scale to support management decisions for streams in Tennessee. Nutrient-source apportionment of load is important for this local-scale application and demands careful...
Hoos, Anne B.; Wang, Sherry H.; Schwarz, Gregory E.
Catchment-level estimates of nitrogen and phosphorus agricultural use from commercial fertilizer sales for the conterminous United States, 2012
Nutrient inputs from commercial agricultural fertilizer, particularly nitrogen and phosphorus, are important factors contributing to the degradation of surface-water quality and the alteration of aquatic ecosystems. Despite this importance, information about the application of fertilizer to agricultural land is not available in a consistent manner...
Stewart, Jana S.; Schwarz, Gregory E.; Brakebill, John W.; Preston, Stephen D. How hydrologic connectivity regulates water quality in river corridors
Downstream flow in rivers is repeatedly delayed by hydrologic exchange with off‐channel storage zones where biogeochemical processing occurs. We present a dimensionless metric that quantifies river connectivity as the balance between downstream flow and the exchange of water with the bed, banks, and floodplains. The degree of connectivity directly...
Harvey, Judson; Gomez-Velez, Jesus; Schmadel, Noah; Scott, Durelle; Boyer, Elizabeth W.; Alexander, Richard; Eng, Ken; Golden, Heather E.; Kettner, Albert; Konrad, Christopher; Moore, Richard; Pizzuto, Jim; Schwarz, Gregory E.; Soulsby, Chris; Choi, Jay The role of baseflow in dissolved solids delivery to streams in the Upper Colorado River Basin
Salinity has a major effect on water users in the Colorado River Basin, estimated to cause almost $300 million per year in economic damages. The Colorado River Basin Salinity Control Program implements and manages projects to reduce salinity loads, investing millions of dollars per year in irrigation upgrades, canal projects, and other...
Schwarz, Gregory; Hirsch, Robert M.; Susong, David; Rumsey, Christine; Miller, Matthew P.; Schwarz, Gregory E.; Hirsch, Robert M.; Susong, David D. Thresholds of lake and reservoir connectivity in river networks control nitrogen removal
Lakes, reservoirs, and other ponded waters are ubiquitous features of the aquatic landscape, yet their cumulative role in nitrogen removal in large river basins is often unclear. Here we use predictive modeling, together with comprehensive river water quality, land use, and hydrography datasets, to examine and explain the influences of more than...
Schmadel, Noah M.; Harvey, Judson; Alexander, Richard; Schwarz, Gregory E.; Moore, Richard; Eng, Ken; Gomez-Velez, Jesus D.; Boyer, Elizabeth W.; Scott, Durelle A hybrid machine learning model to predict and visualize nitrate concentration throughout the Central Valley aquifer, California, USA
Intense demand for water in the Central Valley of California and related increases in groundwater nitrate concentration threaten the sustainability of the groundwater resource. To assess contamination risk in the region, we developed a hybrid, non-linear, machine learning model within a statistical learning framework to predict nitrate...
Ransom, Katherine M.; Nolan, Bernard T.; Traum, Jonathan A.; Faunt, Claudia C.; Bell, Andrew M.; Gronberg, Jo Ann M.; Wheeler, David C.; Zamora, Celia; Jurgens, Bryant C.; Schwarz, Gregory E.; Belitz, Kenneth; Eberts, Sandra M.; Kourakos, George; Harter, Thomas