Disentangling the potential effects of land‐use and climate change on stream conditions Archives

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Disentangling the potential effects of land‐use and climate change on stream conditions

February 20, 2020

Land‐use and climate change are significantly affecting stream ecosystems, yet understanding of their long‐term impacts is hindered by the few studies that have simultaneously investigated their interaction and high variability among future projections. We modeled possible effects of a suite of 2030, 2060, and 2090 land‐use and climate scenarios on the condition of 70,772 small streams in the Chesapeake Bay watershed, United States. The Chesapeake Basin‐wide Index of Biotic Integrity, a benthic macroinvertebrate multimetric index, was used to represent stream condition. Land‐use scenarios included four Special Report on Emissions Scenarios (A1B, A2, B1, and B2) representing a range of potential landscape futures. Future climate scenarios included quartiles of future climate changes from downscaled Coupled Model Intercomparison Project ‐ Phase 5 (CMIP5) and a watershed‐wide uniform scenario (Lynch2016). We employed random forests analysis to model individual and combined effects of land‐use and climate change on stream conditions. Individual scenarios suggest that by 2090, watershed‐wide conditions may exhibit anywhere from large degradations (e.g., scenarios A1B, A2, and the CMIP5 25th percentile) to small degradations (e.g., scenarios B1, B2, and Lynch2016). Combined land‐use and climate change scenarios highlighted their interaction and predicted, by 2090, watershed‐wide degradation in 16.2% (A2 CMIP5 25th percentile) to 1.0% (B2 Lynch2016) of stream kilometers. A goal for the Chesapeake Bay watershed is to restore 10% of stream kilometers over a 2008 baseline; our results suggest meeting and sustaining this goal until 2090 may require improvement in 11.0%–26.2% of stream kilometers, dependent on land‐use and climate scenario. These results highlight inherent variability among scenarios and the resultant uncertainty of predicted conditions, which reinforces the need to incorporate multiple scenarios of both land‐use (e.g., development, agriculture, etc.) and climate change in future studies to encapsulate the range of potential future conditions. DOI link: https://doi.org/10.1111/gcb.14961

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Streamflow Alteration from Impervious Cover: Are All Watersheds Created Equal?

August 6, 2019

Abstract

Very small amounts of impervious cover can impact streamflows. In the Potomac basin, for example, significant streamflow alteration associated with watershed impervious cover >0.5%–2.0% have been found. Since these impacts were found with considerably lower amounts of impervious cover than previously documented in the literature, typically 10%–20%, this study evaluated whether certain watershed characteristics (e.g., watershed area, karst geology, precipitation, soil characteristics, physiographic province, and slope) make a stream reach more susceptible to the impacts of impervious cover than others. The results of this study indicate there are differences in streamflow sensitivity to impervious cover given certain landscape characteristics. The relationships of watershed characteristics with streamflow alteration in flashiness, high flow duration, and low pulse duration were evaluated. Flashiness alteration was positively correlated with impervious cover and influenced by watershed slope and area. High flow duration alteration was negatively correlated with impervious cover and influenced by mean annual precipitation and slope. Low pulse duration was poorly correlated with the watershed characteristics under consideration. These differences may assist in land management efforts and heighten awareness of the environmental impacts of impervious cover.

The full article was published in the Journal of the American Water Resources Association, 54(6), 1169-1374, and is available at https://doi.org/10.1111/1752-1688.12681.

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Creating a stream health baseline for the Chesapeake basin from monitoring and model data

January 11, 2019

This report describes how monitoring and model data were analyzed and combined to generate a preliminary estimate of acceptable stream health in the Chesapeake Bay basin for the 2006 – 2011 baseline period. Streams in about 73% of the basin’s 64,020 sq. miles of drainage area were evaluated with monitoring results, and output from a predictive model was used to estimate stream health in the remaining 27%. Stream health was measured with the bioregion, family-level version of the “Chessie BIBI,” a multi-metric index for stream macroinvertebrate communities. Index scores are normally expressed as one of five index ratings: Excellent or Good (well-functioning), Fair (considered satisfactory), and Poor or Very Poor (stressed or poorly-functioning). Four versions of the predictive model were developed and tested, and the selected version outputs results as three-ratings: Excellent/Good, Fair, and Poor/Very Poor. The five ratings in the monitoring data were re-grouped to match the three ratings of the selected predictive model. The monitoring- and model-based ratings were then area-weighted to reduce bias caused by uneven sample densities and aggregated to the Chesapeake basin scale, with monitoring results given preference. The combined results suggest approximately 60% of the basin’s area had acceptable stream ratings (Excellent, Good, or Fair) during 2006 – 2011. This estimate is a preliminary baseline for the Chesapeake Bay Program’s stream health goal. A final baseline estimate will be produced after a higher resolution stream layer becomes available and acceptable stream health can be estimated as a percent of the basin’s stream miles.

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Biological Surveys of Three Potomac River Mainstem Reaches (2012-2014) with Considerations for Large River Sampling

December 5, 2018

The Interstate Commission on the Potomac River Basin (ICPRB) conducted a study to describe the biological composition of three under-represented reaches in the mainstem Potomac River Basin and determine the effort required to accurately assess large river sites for freshwater mussel and benthic macroinvertebrate populations. Located at Knoxville (MD), Carderock (MD), and Little Falls (MD), these reaches were selected because they are difficult to sample and represent gaps in spatial coverage of the mainstem in the otherwise comprehensive Maryland Department of Natural Resources (MD-DNR) Core Trend Monitoring Program. Data from the Knoxville reach will improve our understanding of the mixing zones below the confluence of the Shenandoah and Potomac rivers and the relative importance of each river at the Potomac water supply intakes downstream. The Carderock and Little Falls reaches are important in identifying stresses on the river’s biological communities that could relate to upstream consumptive losses and water supply withdrawals during severe droughts. The Little Falls reach is in the only stretch of the Potomac River with a minimum flow-by requirement.

Surveys of freshwater mussel and benthic macroinvertebrate populations were conducted during late-summer low-flow periods of 2012, 2013, and 2014. The three years of the study had moderate flows overall and did not experience extreme drought or floods, so managers and researchers should view the results as a characterization of biological communities unaffected by flow extremes. In addition to recording mainstem Potomac species distributions, biological collections underwent post-collection analyses that provided an informed baseline for the collection effort required to achieve sufficiently accurate data in the future.

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