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"HARRISON, JOHN A."
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Greenhouse Gas Emissions from Reservoir Water Surfaces
Collectively, reservoirs created by dams are thought to be an important source of greenhouse gases (GHGs) to the atmosphere. So far, efforts to quantify, model, and manage these emissions have been limited by data availability and inconsistencies in methodological approach. Here, we synthesize reservoir CH4, CO2, and N2O emission data with three main objectives: (1) to generate a global estimate of GHG emissions from reservoirs, (2) to identify the best predictors of these emissions, and (3) to consider the effect of methodology on emission estimates. We estimate that GHG emissions from reservoir water surfaces account for 0.8 (0.5–1.2) Pg CO2 equivalents per year, with the majority of this forcing due to CH4. We then discuss the potential for several alternative pathways such as dam degassing and downstream emissions to contribute significantly to overall emissions. Although prior studies have linked reservoir GHG emissions to reservoir age and latitude, we find that factors related to reservoir productivity are better predictors of emission.
Journal Article
Reactive nitrogen inputs to US lands and waterways: how certain are we about sources and fluxes?
An overabundance of reactive nitrogen (N) as a result of anthropogenic activities has led to multiple human health and environmental concerns. Efforts to address these concerns require an accurate accounting of N inputs. Here, we present a novel synthesis of data describing N inputs to the US, including the range of estimates, spatial patterns, and uncertainties. This analysis shows that human-mediated N inputs are ubiquitous across the country but are spatially heterogeneous, ranging from < 0.1 to 34.6 times the background N input for individual water-resource units (8-digit Hydrologic Unit Codes). The Midwest, Mid-Atlantic, central California, and portions of the Columbia River valley currently receive the highest N loads. Major opportunities to advance our understanding of N sources can be achieved by: (1) enhancing the spatial and temporal resolution of agricultural N input data, (2) improving livestock and human waste monitoring, and (3) better quantifying biological N fixation in non-cultivated ecosystems.
Journal Article
The U.S. consumer phosphorus footprint: where do nitrogen and phosphorus diverge?
Phosphorus (P) and nitrogen (N) are essential nutrients for food production but their excess use in agriculture can have major social costs, particularly related to water quality degradation. Nutrient footprint approaches estimate N and P release to the environment through food production and waste management and enable linking these emissions to particular consumption patterns. Following an established method for quantifying a consumer-oriented N footprint for the United States (U.S.), we calculate an analogous P footprint and assess the N:P ratio across different stages of food production and consumption. Circa 2012, the average consumer's P footprint was 4.4 kg P capita−1 yr−1 compared to 22.4 kg N capita−1 yr−1 for the food portion of the N footprint. Animal products have the largest contribution to both footprints, comprising >70% of the average per capita N and P footprints. The N:P ratio of environmental release based on virtual nutrient factors (kilograms N or P per kilogram of food consumed) varies considerably across food groups and stages. The overall N:P ratio of the footprints was lower (5.2 by mass) than for that of U.S. food consumption (8.6), reinforcing our finding that P is managed less efficiently than N in food production systems but more efficiently removed from wastewater. While strategies like reducing meat consumption will effectively reduce both N and P footprints by decreasing overall synthetic fertilizer nutrient demands, consideration of how food production and waste treatment differentially affect N and P releases to the environment can also inform eutrophication management.
Journal Article
Ultraviolet-C Photoresponsivity Using Fabricated TiO2 Thin Films and Transimpedance-Amplifier-Based Test Setup
We report on fabricated titanium dioxide (TiO2) thin films along with a transimpedance amplifier (TIA) test setup as a photoconductivity detector (sensor) in the ultraviolet-C (UV-C) wavelength region, particularly at 260 nm. TiO2 thin films deposited on high-resistivity undoped silicon-substrate at thicknesses of 100, 500, and 1000 nm exhibited photoresponsivities of 81.6, 55.6, and 19.6 mA/W, respectively, at 30 V bias voltage. Despite improvements in the crystallinity of the thicker films, the decrease in photocurrent, photoconductivity, photoconductance, and photoresponsivity in thicker films is attributed to an increased number of defects. Varying the thickness of the film can, however, be leveraged to control the wavelength response of the detector. Future development of a chip-based portable UV-C detector using TiO2 thin films will open new opportunities for a wide range of applications.
Journal Article
Methanogenesis exceeds CH4 consumption in eutrophic lake sediments
Lakes and reservoirs collectively contribute significant amounts of methane (CH4) to the global atmosphere. If CH4 production were not at least partially balanced by consumption (oxidation) in most of these systems, they could potentially emit an order of magnitude or more CH4. The impacts of environmental drivers such as trophic status, temperature, and latitude on CH4 production, CH4 oxidation, and the balance of the two processes influence current and future patterns of freshwater CH4 emissions. Using CH4 production and oxidation rates measured with a common methodology (incubations) from over 60 different lakes and reservoirs, we provide novel evidence for lower sediment CH4 oxidation efficiency at high sediment CH4 production rates. We also show a strong positive correlation between sediment CH4 production and lake trophic status. Our results suggest that less efficient CH4 consumption at high CH4 production rates could help explain greater surface emissions often observed in eutrophic lakes globally.
Journal Article
The regional and global significance of nitrogen removal in lakes and reservoirs
Human activities have greatly increased the transport of biologically available nitrogen (N) through watersheds to potentially sensitive coastal ecosystems. Lentie water bodies (lakes and reservoirs) have the potential to act as important sinks for this reactive N as it is transported across the landscape because they offer ideal conditions for N burial in sediments or permanent loss via denitrification. However, the patterns and controls on lentie N removal have not been explored in great detail at large regional to global scales. In this paper we describe, evaluate, and apply a new, spatially explicit, annual-scale, global model of lentie N removal called NiRReLa (Nitrogen Retention in Reservoirs and Lakes). The NiRReLa model incorporates small lakes and reservoirs than have been included in previous global analyses, and also allows for separate treatment and analysis of reservoirs and natural lakes. Model runs for the mid-1990s indicate that lentie systems are indeed important sinks for N and are conservatively estimated to remove 19.7 Tg N year⁻¹ from watersheds globally. Small lakes (<50 km²) were critical in the analysis, retaining almost half (9.3 Tg N year⁻¹) of the global total. In model runs, capacity of lakes and reservoirs to remove watershed N varied substantially at the half-degree scale (0-100%) both as a function of climate and the density of lentie systems. Although reservoirs occupy just 6% of the global lentie surface area, we estimate they retain ~33% of the total N removed by lentie systems, due to a combination of higher drainage ratios (catchment surface areailake or reservoir surface area), higher apparent settling velocities for N, and greater average N loading rates in reservoirs than in lakes. Finally, a sensitivity analysis of NiRReLa suggests that, on-average, N removal within lentie systems will respond more strongly to changes in land use and N loading than to changes in climate at the global scale.
Journal Article
Effects of floating vegetation on denitrification, nitrogen retention, and greenhouse gas production in wetland microcosms
Wetlands are biogeochemical hotspots that have been identified as important sites for both nitrogen (N) removal from surface waters and greenhouse gas (GHG) production. Floating vegetation (FV) commonly occurs in natural and constructed wetlands, but the effects of such vegetation on denitrification, N retention, and GHG production are unknown. To address this knowledge gap, we used microcosm experiments to examine how FV affects N and GHG dynamics. Denitrification and N retention rates were significantly higher in microcosms with FV (302 μmol N m⁻² h⁻¹ and 203 μmol N m⁻² h⁻¹, respectively) than in those without (63 μmol N m⁻² h⁻¹ and 170 μmol N m⁻² h⁻¹, respectively). GHG production rates were not significantly different between the two treatments. Denitrification rates were likely elevated due to decreased dissolved oxygen (DO) in microcosms with FV. The balance of photosynthesis and respiration was more important in affecting DO concentrations than decreased surface gas exchange. The denitrification fraction (N₂-N production: N retention) was higher in microcosms with FV (100 %) than those without (33 %) under increased (tripled) N loading. A 5 °C temperature increase resulted in significantly lower denitrification rates in the absence of FV and significantly lowered N₂O production with FV, but did not significantly change CH₄ production or N retention in either treatment. These results suggest that intentional introduction of FV in constructed wetlands could enhance N removal while leaving GHG production unchanged, an insight that should be further tested via in situ experiments.
Journal Article
Reservoir CO2 and CH4 emissions and their climate impact over the period 1900–2060
Reservoirs are essential for human populations, but their global carbon footprint is substantial (0.73–2.41 PgCO
2
-equivalent yr
−1
). Yet the temporal evolution of reservoir carbon emissions and their contribution to anthropogenic radiative forcing remains unresolved. Here we quantify the long-term historical and future evolution (1900–2060) of cumulative global reservoir area, carbon dioxide and methane emissions and the resulting radiative forcing. We show that global reservoir carbon emissions peaked in 1987 (4.4 TmolC yr
−1
) and have been declining since, due largely to decreasing carbon dioxide emissions as reservoirs age. However, reservoir-induced radiative forcing continues to rise due to ongoing increases in reservoir methane emissions, which accounted for 5.2% of global anthropogenic methane emissions in 2020. We estimate that, in the future, methane ebullition and degassing flux will make up >75% of the reservoir-induced radiative forcing, making these flux pathways key targets for improved understanding and mitigation.
Reservoir-induced radiative forcing is increasing globally due to rising methane emissions outweighing declining carbon dioxide emissions, according to modelling based on reservoir surface area observations.
Journal Article