nasa-gesdisc/es-publications-researchareas huggingface dataset:
Global bounds on nitrogen gas emissions from humid tropical forests (2017)

Denitrification and hydrologic leaching are the two major pathways by which nitrogen (N) is lost from the terrestrial biosphere. Humid tropical forests are thought to dominate denitrification losses from unmanaged lands globally, but there is large uncertainty about the range and key drivers of total N gas emissions across the biome. We combined pantropical measures of small watershed stream chemistry with ecosystem modeling to determine total N gas losses and associated uncertainty across humid tropical forests. Our calculations reveal that denitrification in soils and along hydrologic flow paths contributes on average &gt;45% of total watershed N losses. However, when denitrification occurs exclusively in shallow soils, simulations indicate that gas emissions would exceed N inputs and render plants severely N limited, which contradicts observations of widespread N sufficiency in tropical forests. Our analyses suggest an upper bound on soil denitrification of 80% of total external N losses beyond which tropical plant growth would be compromised.

Research areas: Ecosystems
Recent trends and variability in river discharge across northern Canada (2016)

Abstract. This study presents an analysis of the observed inter-annual variability and inter-decadal trends in river discharge across northern Canada for 19642013. The 42 rivers chosen for this study span a combined gauged area of 5.26  106 km2 and are selected based on data availability and quality, gauged area and record length. Inter-annual variability in river discharge is greatest for the eastern Arctic Ocean (coefficient of variation, CV = 16 %) due to the Caniapiscau River diversion into the La Grande Riviere system for enhanced hydropower production. Variability is lowest for the study area as a whole (CV = 7 %). Based on the MannKendall test (MKT), no significant (p > 0.05) trend in annual discharge from 1964 to 2013 is observed in the Bering Sea, western Arctic Ocean, western Hudson and James Bay, and Labrador Sea; for northern Canada as a whole, however, a statistically significant (p < 0.05) decline of 102.8 km3 25 yr1 in discharge occurs over the first half of the study period followed by a statistically significant (p < 0.05) increase of 208.8 km3 25 yr1 in the latter half. Increasing (decreasing) trends in river discharge to the eastern Hudson and James Bay (eastern Arctic Ocean) are largely explained by the Caniapiscau diversion to the La Grande Riviere system. Strong regional variations in seasonal trends of river discharge are observed, with overall winter (summer) flows increasing (decreasing, with the exception of the most recent decade) partly due to flow regulation and storage for enhanced hydropower production along the Hudson and James Bay, the eastern Arctic Ocean and Labrador Sea. Flow regulation also suppresses the natural variability of river discharge, particularly during cold seasons.

Research areas: Land Surface/Agriculture Indicators
Identifying the Influence of Land Cover and Human Population on Chlorophyll a Concentrations Using a Pseudo-Watershed Analytical Framework (2020)

Increasing agricultural development and urbanization exacerbates the degradation of water quality in vulnerable freshwater systems around the world. Advances in remote sensing and greater availability of open-access data provides a valuable resource for monitoring water quality but harmonizing between databases remains a challenge. Here, we: (i) developed a pseudo-watershed analytical framework to associate freshwater lakes with adjacent land cover and human population data and (ii) applied the framework to quantify the relative influence of land cover and human population on primary production for 9313 lakes from 72 countries. We found that land cover and human population explained 30.2% of the variation in chlorophyll a concentrations worldwide. Chlorophyll a concentrations were highest in regions with higher agricultural activities and human populations. While anthropogenic land cover categories equated to only 4 of the 18 categories, they accounted for 41.5% of the relative explained variation. Applying our pseudo-watershed analytical framework allowed us to quantify the importance of land cover and human population on chlorophyll concentration for over 9000 lakes. However, this framework has broader applicability for any study or monitoring program that requires quantification of lake watersheds.

Research areas: Water Quality
Avulsion of the Brahmaputra in Bangladesh during the 18th–19th century: A review based on cartographic and literary evidence (2021)

Avulsions involve switching of fluvial discharge through a new channel from an existing course and occur over varied timeframes ranging from hours to decades. With a peak discharge above 100,000 m3/s, the Brahmaputra (2900 km) is one of the largest rivers in the world. Its lower course through the GangaBrahmaputraMeghna (GBM) Delta of Bangladesh was first mapped in 176473 as a broad eastward-swinging curvature skirting the subsiding Sylhet Wetlands and falling into the Meghna River. Beginning from the late eighteenth century, the river started to avulse into a linear, more direct, southward passage into the Ganga called the Jamuna and decreased its length by 46 km (14.2%) from its former course to the Bay of Bengal. Examination of the established timeline and procedure of the avulsion event using maps and images from 176473, 183034, 185460, 190724, and 2014, in addition to evidence from contemporary literature and tree-ring reconstructed long-term discharge data, connoted certain discrepancies. Our re-evaluation indicated that the Brahmaputra shifted westward and was captured by a parallel distributary of the neighbouring Tista system initiating the switch. This was probably aided by exceptionally high monsoonal discharge between 1786 and 1790 as well as the 1787 break-in of the Tista into the Brahmaputra. Because of its high width:depth ratio and braided planform that prevented fast escape of the discharge, abandonment of the eastern channelthe Old Brahmaputraprogressed gradually; well into the next century with the new channel taking over almost completely by 185560. Comparable switchings of the Brahmaputra occurred six times in the last 11 kyr and significantly influenced development of the GBM Delta. The century-scale timeline of the last avulsion event described here refines its current understanding by proposing river capture as the mechanism of avulsion, possibly triggered by clustering of high-flood years.

Research areas: Land Surface/Agriculture Indicators
River linking in India: Downstream impacts on water discharge and suspended sediment transport to deltas (2018)

To expand agricultural production and address water scarcity, India is moving forward with the National River Linking Project (NRLP), which will connect 44 rivers via 9,600 km of canals. Here, we compile the first complete database of proposed NRLP dams, reservoirs and canals, including operating schedules for Himalayan infrastructure. We evaluate potential NRLP-derived changes to mean annual water discharge for 29 rivers and mean monthly water and sediment discharge for six rivers flowing to five major deltas. Sediment rating curves are used to quantify the impacts of changing water discharge within the rivers, and basin-wide trapping efficiency is established for new reservoirs. Given full implementation of the NRLP, we forecast reductions in annual suspended sediment transport to deltas of 4085% (Mahanadi), 7199% (Godavari) and 6097% (Krishna) due to profound reservoir trapping and peak streamflow reductions. The Ganga before its confluence with the Brahmaputra is projected to experience a 3975% reduction in annual suspended load. The Brahmaputra before its confluence with the Ganga is projected to experience a 925% reduction in suspended load, despite losing only 6% of its annual water flow. We calculate a projected corresponding aggradation decrease for the Ganga-Brahmaputra delta from 3.6 to 2.5 mm y1, which is a large enough change to drive relative sea-level rise at the delta front. At the remaining four deltas, the NRLP will exacerbate current sediment starvation. We reconstruct the annual water transfer volume proposed for the NRLP to be 245 km3 y1, higher than previous estimates due to the inclusion of along-canal usage. If completed, the NRLP will transform watershed boundaries, with more than half of the land in India contributing a portion of its runoff to a new mouth. These impacts may have profound environmental and public health implications, particularly in the context of future climate change.

Research areas: Land Surface/Agriculture Indicators
Long‐term hydrological, biogeochemical, and climatological data from Walker Branch Watershed, East Tennessee, USA (2021)

In 1967, the original Walker Branch Watershed (WBW) project was established to study elemental cycling and mass balances in a relatively unimpacted watershed. Over the next 50+ years, findings from additional experimental studies and long-term observations on WBW advanced understanding of catchment hydrology, biogeochemistry, and ecology and established WBW as a seminal site for catchment science. The 97.5-ha WBW is located in East Tennessee, USA, on the U.S. Department of Energy's Oak Ridge Reservation. Vegetation on the watershed is characteristic of an eastern deciduous, second-growth forest. The watershed is divided into two subcatchments: the West Fork (38.4 ha) and the East Fork (59.1 ha). Headwater streams draining these subcatchments are fed by multiple springs, and thus flow is perennial. Stream water is high in base cations due to weathering of dolomite bedrock and nutrient concentrations are low. Long-term observations of climate, hydrology, and biogeochemistry include daily (1969-2014) and 15-min (1994-2014) stream discharge and annual runoff (1969-2014); hourly, daily, and annual rainfall (1969-2012); daily climate and soil temperature (1993-2010); and weekly stream water chemistry (1989-2013). These long-term datasets are publicly available on the WBW website (https://walkerbranch.ornl.gov/long-term-data/). While collection of these data has ceased, related long-term measurements continue through the National Ecological Observatory Network (NEON), where WBW is the core terrestrial and aquatic site in the Appalachian and Cumberland Plateau region (NEON's Domain 7) of the United States. These long-term datasets have been and will continue to be important in evaluating the influence of climatic and environmental drivers on catchment processes.

Research areas: Ecosystems
Post-fire peatland vegetation recovery: a case study in open rich fens of the Canadian boreal forest (2021)

 Fire plays a major role in the structuring and functioning of boreal ecosystems. As peatlands are important components of boreal forests, the impact of fire upon these wetter ecosystems is increasingly studied, but with the focus on treed peatlands and Sphagnum-dominated bogs so far. Important fires occurring more frequently in the past decade in southern Northwest Territories (Canada) provide the opportunity to assess early post-fire vegetation regeneration in open rich fens (one, two, and five years post-fire) and to better understand early recovery succession. We aimed to (i) evaluate whether and how open rich fens are affected by fire, and (ii) describe short-term vegetation regeneration for both bryophytes and vascular species. A shift was observed between pioneer bryophytes and brown mosses between the second and fifth year post-fire. Vascular plants, especially slow-growing species and the ones reproducing mainly by seeds, recovered partially. The first bryophyte species recovering were pioneer species adapted to colonize burned environments such as Marchantia polymorpha L. or Ceratodon purpureus (Hedw.) Brid. For vascular plant species, the ones previously present and able to regrow rapidly from unburned plant structures (base of tussocks, rhizomes, roots) were represented by species like Betula glandulosa Michx. or Carex aquatilis Wahlenb. The wetter conditions and lower fuel availability of fen depressional biotopes were important factors controlling the resistance and regeneration of species associated with them. 

Research areas: Wildfires
Water environmental pressure assessment in agricultural systems in Central Asia based on an Integrated Excess Nitrogen Load Model (2022)

Agricultural runoff is the main source of water pollution in Central Asia. Excessive nitrogen (N) inputs from overuse of chemical fertilizers are threatening regional water resources. However, the scarcity of quantitative data and simplified empirical models limit the reliability of grey water footprint (GWF), particularly in undeveloped regions. In this study, we developed an Integrated Excess Nitrogen Load Model (IENLM) to calculate excess N load and evaluate its potential water environmental pressure in Central Asia. The model optimized the biological N fixation and atmospheric N deposition modules by involving more environmental variables and human activities. Results showed that N fertilizer application contributed over 60% to total N input and was mainly responsible for 42.9% increase of total GWF from 101.5 to 145.0 billion m3 during 1992 - 2018. Water pollution level (WPL) increased from 0.55 in 1992 to 2.41 in 2018 and the pollution assimilation capacity of water systems has been fully consumed just by N load from agriculture since 2005. GWF intensity and grey water pollution  efficiency types in all Central Asian countries have improved in recent years except for Turkmenistan. N fertilizer application and agricultural economy development were the main driving factors induced N pollution. Results were validated by riverine nitrate concentrations and the estimates from prior studies. In future, combining the N fertilizer reduction with other farm management practices were projected to effectively improve the WPL. The modeling framework is favorable for N pollution research in data-scarce regions and provides a scientific basis for decision-making for agriculture and water resource managements.

Research areas: Water Quality
Quantifying the effects of nutrient enrichment and freshwater mixing on coastal ocean acidification (2019)

The U.S. Northeast is vulnerable to ocean and coastal acidification because of low alkalinity freshwater discharge that naturally acidifies the region, and high anthropogenic nutrient loads that lead to eutrophication in many estuaries. This study describes a combined nutrient and carbonate chemistry monitoring program in five embayments of Buzzards Bay, Massachusetts to quantify the effects of nutrient loading and freshwater discharge on aragonite saturation state (Ω). Monitoring occurred monthly from June 2015 to September 2017 with higher frequency at two embayments (Quissett and West Falmouth Harbors) and across nitrogen loading and freshwater discharge gradients. The more eutrophic stations experienced seasonal aragonite undersaturation, and at one site, nearly every measurement collected was undersaturated. We present an analytical framework to decompose variability in aragonite Ω into components driven by temperature, salinity, freshwater endmember mixing, and biogeochemical processes. We observed strong correlations between apparent oxygen utilization and the portion of aragonite Ω variation that we attribute to biogeochemistry. The regression slopes were consistent with Redfield ratios of dissolved inorganic carbon and total alkalinity to dissolved oxygen. Total nitrogen and the contribution of biogeochemical processes to aragonite Ω were highly correlated, and this relationship was used to estimate the likely effects of nitrogen loading improvements on aragonite Ω. Under nitrogen loading reduction scenarios, aragonite Ω in the most eutrophic estuaries could be raised by nearly 0.6 units, potentially increasing several stations above the critical threshold of 1. This analysis provides a quantitative framework for incorporating ocean and coastal acidification impacts into regulatory and management discussions.

Research areas: Water Quality
Quantifying the social benefits and costs of reducing phosphorus (2021)

Excess phosphorus loading to waterbodies has led to increasing frequency and severity of harmful algal blooms, negatively impacting economic activity and human health. While interventions to improve water quality can create large societal benefits, these investments are costly and the value of benefits is often unknown. Understanding the social and economic impacts of reduced phosphorus loading is critical for developing effective land use policies and for generating public and political support for these initiatives. Here, we quantify the social benefits and costs of improving water quality in Lake Champlain under a range of phosphorus reduction and climate change scenarios between 2016 and 2050. We use statistical models to link water quality outputs from an established integrated assessment model with three categories of benefits: tourism expenditures, property sales, and avoided human health impacts. We estimate the costs of reducing phosphorus loading using data reported by the State of Vermont. We find that under the most aggressive phosphorus reduction scenario, the total benefits of improved water quality are $55 to $60 million between 2016 and 2050. Over this 35 year time horizon, the combined benefits do not outweigh the costs under any scenario. If the time horizon is extended to 2100 or beyond, however, the benefits may exceed the costs if the applied discount rate is less than 3%. Importantly, we almost certainly underestimate the value of clean water, due to the omission of other types of benefits. Despite this uncertainty, our study provides a tractable framework for disentangling the complex relationships between water quality and human well-being, and illuminates the value of reductions in phosphorus loading to society.

Research areas: Water Quality
The interaction of physical structure and nutrient loading drives ecosystem change in a large tropical lake over 40 years (2022)

Many lakes across the world are entering novel states and experiencing altered biogeochemical cycling due to local anthropogenic stressors. In the tropics, understanding the drivers of these changes can be difficult due to a lack of documented historic conditions or an absence of continuous monitoring that can distinguish between intra- and inter-annual variation. Over the last forty years (19802020), Lake Yojoa (Honduras) has experienced increased watershed development as well as the introduction of a large net-pen Tilapia farm, resulting in a dramatic reduction in seasonal water clarity, increased trophic state and altered nutrient dynamics, shifting Lake Yojoa from an oligotrophic (low productivity) to mesotrophic (moderate productivity) ecosystem. To assess the changes that have occurred in Lake Yojoa as well as putative drivers for those changes, we compared Secchi depth (water clarity), dissolved inorganic nitrogen (DIN), and total phosphorus (TP) concentrations at continuous semi-monthly intervals for the three years between 1979 and 1983 and again at continuous 16-day intervals for 20182020. Between those two periods we observed the loss of a clear water phase that previously occurred in the months when the water column was fully mixed. Seasonal peaks in DIN coincident with mixing suggest that an enhanced accumulation of ammonium in the hypolimnion (the bottom layer of a stratified lake) during stratification, and release to the epilimnion (the top layer of a stratified lake) with mixing maintains high algal abundance and subsequently low Secchi depth during what was previously the clear water phase. This interaction of nutrient loading and Lake Yojoa's monomictic stratification regime illustrates a key phenomenon in how physical water column structure and nutrients interact in tropical monomictic lakes. This work highlights the need to consider nutrient dynamics of warm anoxic hypolimnions, not just surface water nutrient concentrations, to understand environmental change in these societally important but understudied ecosystems.

Research areas: Water Quality
Multiple Data Products Reveal Long-Term Variation Characteristics of Terrestrial Water Storage and Its Dominant Factors in Data-Scarce Alpine Regions (2021)

As the Water Tower of Asia and The Third Pole of the world, the QinghaiTibet Plateau (QTP) shows great sensitivity to global climate change, and the change in its terrestrial water storage has become a focus of attention globally. Differences in multi-source data and different calculation methods have caused great uncertainty in the accurate estimation of terrestrial water storage. In this study, the Yarlung Zangbo River Basin (YZRB), located in the southeast of the QTP, was selected as the study area, with the aim of investigating the spatio-temporal variation characteristics of terrestrial water storage change (TWSC). Gravity Recovery and Climate Experiment (GRACE) data from 2003 to 2017, combined with the fifth-generation reanalysis product of the European Centre for Medium-Range Weather Forecasts (ERA5) data and Global Land Data Assimilation System (GLDAS) data, were adopted for the performance evaluation of TWSC estimation. Based on ERA5 and GLDAS, the terrestrial water balance method (PER) and the summation method (SS) were used to estimate terrestrial water storage, obtaining four sets of TWSC, which were compared with TWSC derived from GRACE. The results show that the TWSC estimated by the SS method based on GLDAS is most consistent with the results of GRACE. The time-lag effect was identified in the TWSC estimated by the PER method based on ERA5 and GLDAS, respectively, with 2-month and 3-month lags. Therefore, based on the GLDAS, the SS method was used to further explore the long-term temporal and spatial evolution of TWSC in the YZRB. During the period of 19482017, TWSC showed a significantly increasing trend; however, an abrupt change in TWSC was detected around 2002. That is, TWSC showed a significantly increasing trend before 2002 (slope = 0.0236 mm/month, p < 0.01) but a significantly decreasing trend (slope = 0.397 mm/month, p < 0.01) after 2002. Additional attribution analysis on the abrupt change in TWSC before and after 2002 was conducted, indicating that, compared with the snow water equivalent, the soil moisture dominated the long-term variation of TWSC. In terms of spatial distribution, TWSC showed a large spatial heterogeneity, mainly in the middle reaches with a high intensity of human activities and the Parlung Zangbo River Basin, distributed with great glaciers. The results obtained in this study can provide reliable data support and technical means for exploring the spatio-temporal evolution mechanism of terrestrial water storage in data-scarce alpine regions.

Research areas: Land Surface/Agriculture Indicators
Global Characterization of Inland Water Reservoirs Using ICESat-2 (2020)

Accurate, transparent knowledge of global reservoir levels is a prerequisite for effective management of water resources. However, no complete database exists because gauge data are not globally available and the current generation of satellite radar altimeters resolves only the world's largest reservoirs. Here, we investigate water level changes in global reservoirs using ICESat-2, National Aeronautics and Space Administration (NASA)'s new satellite laser altimetry mission. In just the first 12 months of the mission, we find that ICESat-2 accurately (±14.1 cm) retrieved water level changes for 3,712 global reservoirs having surface areas ranging from &lt;1 to &gt;10,000 km<SUP>2</SUP>. From this new global data set, we identify distinct regional patterns in reservoir level change that can be attributed to both water availability and management strategy. Our findings demonstrate that ICESat-2 will form a crucial component of any global reservoir level inventory and enable new insight into how reservoir management responds to climatic variability and increasing human demand.

Research areas: Land Surface/Agriculture Indicators