Temperature controls production but hydrology regulates export of dissolved organic carbon at the catchment scale

Lateral carbon flux through river networks is an important and poorly understood component of the global carbon budget. This work investigates how temperature and hydrology control the production and export of dissolved organic carbon (DOC) in the Susquehanna Shale Hills Critical Zone Observatory in Pennsylvania, USA. Using field measurements of daily stream discharge, evapotranspiration, and stream DOC concentration, we calibrated the catchment-scale biogeochemical reactive transport model BioRT-Flux-PIHM (Biogeochemical Reactive Transport-Flux-Penn State Integrated Hydrologic Model, BFP), which met the satisfactory standard of a Nash-Sutcliffe efficiency (NSE) value greater than 0.5. We used the calibrated model to estimate and compare the daily DOC production rates (Rp; the sum of the local DOC production rates in individual grid cells) and export rate (Re; the product of the concentration and discharge at the stream outlet, or load). Results showed that daily Rp varied by less than an order of magnitude, primarily depending on seasonal temperature. In contrast, daily Re varied by more than 3 orders of magnitude and was strongly associated with variation in discharge and hydrological connectivity. In summer, high temperature and evapotranspiration dried and disconnected hillslopes from the stream, driving Rp to its maximum but Re to its minimum. During this period, the stream only exported DOC from the organic-poor groundwater and from organic-rich soil water in the swales bordering the stream. The DOC produced accumulated in hillslopes and was later flushed out during the wet and cold period (winter and spring) when Re peaked as the stream reconnected with uphill and Rp reached its minimum. The model reproduced the observed concentration-discharge (C-Q) relationship characterized by an unusual flushing-dilution pattern with maximum concentrations at intermediate discharge, indicating three end-members of source waters. A sensitivity analysis indicated that this nonlinearity was caused by shifts in the relative contribution of different source waters to the stream under different flow conditions. At low discharge, stream water reflected the chemistry of organic-poor groundwater; at intermediate discharge, stream water was dominated by the organic-rich soil water from swales; at high discharge, the stream reflected uphill soil water with an intermediate DOC concentration. This pattern persisted regardless of the DOC production rate as long as the contribution of deeper groundwater flow remained low (\u3c18 % of the streamflow). When groundwater flow increased above 18 %, comparable amounts of groundwater and swale soil water mixed in the stream and masked the high DOC concentration from swales. In that case, the C-Q patterns switched to a flushing-only pattern with increasing DOC concentration at high discharge. These results depict a conceptual model that the catchment serves as a producer and storage reservoir for DOC under hot and dry conditions and transitions into a DOC exporter under wet and cold conditions. This study also illustrates how different controls on DOC production and export - temperature and hydrological flow paths, respectively - can create temporal asynchrony at the catchment scale. Future warming and increasing hydrological extremes could accentuate this asynchrony, with DOC production occurring primarily during dry periods and lateral export of DOC dominating in major storm event

Geochemical evolution of the Critical Zone across variable time scales informs concentration-discharge relationships: Jemez River Basin Critical Zone Observatory

This study investigates the influence of water, carbon, and energy fluxes on solute production and transport through the Jemez Critical Zone (CZ) and impacts on C-Q relationships over variable spatial and temporal scales. Chemical depletion-enrichment profiles of soils, combined with regolith thickness and groundwater data indicate the importance to stream hydrochemistry of incongruent dissolution of silicate minerals during deep bedrock weathering, which is primarily limited by water fluxes, in this highly fractured, young volcanic terrain. Under high flow conditions (e.g., spring snowmelt), wetting of soil and regolith surfaces and presence of organic acids promote mineral dissolution and provide a constant supply of base cations, Si, and DIC to soil water and groundwater. Mixing of waters from different hydrochemical reservoirs in the near stream environment during “wet” periods leads to the chemostatic behavior of DIC, base cations, and Si in stream flow. Metals transported by organic matter complexation (i.e., Ge, Al) and/or colloids (i.e., Al) during periods of soil saturation and lateral connectivity to the stream display a positive relationship with Q. Variable Si-Q relationships, under all but the highest flow conditions, can be explained by nonconservative transport and precipitation of clay minerals, which influences long versus short-term Si weathering fluxes. By combining measurements of the CZ obtained across different spatial and temporal scales, we were able to constrain weathering processes in different hydrological reservoirs that may be flushed to the stream during hydrologic events, thereby informing C-Q relationships

Twenty-three unsolved problems in hydrology (UPH) – a community perspective

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through on-line media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focussed on process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come

Hydrogeomorphological Controls On Stream Chemistry And Aquatic Biota In The Catskill Mountains, New York State

Traditional Darcy-based models have not been providing satisfactory answers for watershed scientists working in complex landscapes and new methods are thus being developed. Ideally these new methods would characterize discharge patterns, estimate stream chemistry, and can be transferrable between complex and heterogeneous watersheds. In this dissertation, we develop two such methods by relating hydrologic and geomorphologic (or "hydrogeomorphologic") properties to stream chemistry, biota and (preferential) flow patterns. The research is carried out in two different, well-studied watersheds in the Catskill Mountains, New York State: Neversink River and Town Brook. The 176 km2 Neversink River watershed has a detailed discharge, chemistry, and biotic data for nested sub-watersheds (0.2 to 160 km2) that are affected by acid rain. The results from the Neversink River watershed showed that baseflow stream acid buffering chemistry (ANC values and Ca2+ concentrations) was reduced in subwatersheds that were steeper or had more stream channels. Although speculative, we believe that long-term flushing of base cations from the shallow soils during storm runoff events reduces the acid buffering chemistry during baseflow. A simple geomorphologic relationship, based on mean slope and stream channels per area, was strongly correlated to populations of aquatic biota (macroinvertebrate, periphytic diatom, and fish) in "ungaged" sub-watersheds where discharge was not measured. Town Brook watershed (2.5 km2) was investigated to determine the sources and flowpaths of water during nine rainfall events from April 2007 to October 2007. A combination of hydrometric, chemical, and isotopic data sets was measured and surface saturation maps were developed. The results suggested that during precipitation events greater than 1 cm, hill side saturation areas caused by groundwater springs and soil pipes were a significant runoff source. The properties commonly used to infer surface saturation areas in Town Brook (i.e. slope, upslope area, and/or soil transmissivity) predicted general spatial patterns, but were insufficient to estimate surface saturation at the smallest scales measured ( less than 100 m2). The success of hydrogeomorphologic properties in estimating stream acid buffering chemistry and watershed saturation patterns in the two Catskill watersheds suggest that simple alternatives to traditional Darcy-based predictions may be applicable under certain conditions

Potential for Changing Extreme Snowmelt and Rainfall Events in the Mountains of the Western United States

The intensity of mountain precipitation is often modified by snow accumulation and melt, yet rainfall-based observations are widely used in planning and design. Comparisons of extreme rainfall versus snowmelt intensities are needed because they have different predictability and hazard implications. Regional warming is expected to intensify not only rainfall and snowfall but also slow snowmelt, which could further challenge intensity duration frequency (IDF) techniques. We use observations from 379 mountain sites across the western U.S. to estimate the 10 and 100year intensity at 1, 2, and 30day durations for historical snowmelt (SM), precipitation occurring during snow cover (SCP), and precipitation during the snow-free period (SFP). At 1day durations, 100year SCP was greater than SM and SFP at 40% of sites, while SM was larger than SCP and SFP at 39% of sites. At 30day durations, SM was greater than SCP and SFP at 95% of sites. The continental sites are generally insensitive to increased water input intensity from SCP occurring as rainfall. In contrast, the maritime mountains are relatively insensitive to changes in SM but have the potential for increased water input intensity from greater SCP occurring as rainfall. Standard precipitation intensity data sets accurately estimated the 100year, 1day SCP and SM but underestimated SM at 78 continental sites where SM was greater than SCP and SFP. These results confirm that snow processes modify IDF estimates and highlight regional sensitivity to increased winter rainfall and slower snowmelt that may necessitate local adaptation strategies. Plain Language Summary Extreme precipitation can cause floods, landslides, and other natural hazards in the mountains of the western United States. Predicting extreme precipitation intensity is therefore a critical tool for protecting life and property. Most of our standard prediction tools do not differentiate snow and rain or track the effects of snowmelt. Deficiencies in standard estimates could be amplified by increased winter rainfall and slowing snowmelt rates that are expected from regional warming. We investigated 379 mountain sites over 30+years to estimate the 100year intensity (i.e., statistically a 1/100 likelihood of occurring in any given year) at 1, 2, and 30day durations. We found that snowmelt and precipitation during the snow cover season were the main drivers of extreme water input intensity. Changes to slower snowmelt rates were more likely to affect extreme water input intensity at continental sites like the southern Rocky Mountains. Conversely, changes to rainfall during the snow cover season were more likely to affect water input intensity at maritime sites like the Cascades. These regional differences give a framework to understand vulnerability to changing extreme water input intensity that local resource managers and planners could use to adapt standard estimates to their areas

A Long-Term Micrometeorological and Hydrological Dataset Across an Elevation Gradient in Sagehen Creek, Sierra Nevada, California

To create a continuous dataset subject to QA/QC and gap fill methodology for Sagehen Creek watershed, which allows for this dataset to be usable for long term hydrological and micrometeorological studies in this watershed.We compile and release ~55 years of daily and ~20 years of hourly Micrometeorological and hydrological data from Sagehen Creek a 28 km〖^2〗 watershed with observation sites spanning 1771 to 2670 m. A USGS gauging station measures streamflow at the catchment outlet. There are three Snow Telemetry (SNOTEL) stations: Independence Camp(2128 m), Independence Creek (1962 m) and Independence Lake (2541 m) that measure hourly precipitation, temperature, soil moisture (at 5, 20, and 50 cm), as well as daily snow water equivalent (SWE) and snow depth. A new method was used to estimate hourly precipitation data using quality controlled daily totals. A NOAA cooperative observer (COOP) station measures daily precipitation, temperature, SWE, and snow depth from 1953-1997 and then measures hourly precipitation, temperature, SWE, and snow depth, relative humidity, and solar radiation from 2001 through 2017 2001-present. There are an additional three towers with data beginning in 2009 measuring snow depth, SWE, solar radiation, barometric pressure, precipitation, relative humidity, and temperature: Tower 1 (1934m), Tower 3 (2114 m), and Tower 4 (2350 m). Wind speed, temperature, and relative humidity measured at 7.6 and 30.5 m at each site. Data from all stations were checked for poor QA/QC and substantial and sophisticated gap-filling techniques were deployed. This dataset holds potential for improving understanding of orographic processes and their implications for streamflow generation in a groundwater-dominated watershed

Sensitivity of soil water availability to changing snowmelt timing in the western US

The ecohydrological effects of changing snowmelt are strongly mediated by soil moisture. We utilize 259 Snow Telemetry stations across the western U.S. to address two questions: (1) how do relationships between peak soil moisture (PSM) timing and the day of snow disappearance (DSD) vary across ecoregions and (2) what is the regional sensitivity of PSM timing to earlier DSD associated with warming and drying scenarios? All western U.S. ecoregions showed significant relationships between the timing of PSM and DSD. Changes in the timing of PSM based on warming predicted for the middle and end of the 21st century ranged from 1 to 9days and from 6 to 17days among ecoregions, respectively. The maritime ecoregions PSM timing were 2-3 times more sensitive to warming and drying versus the interior mountain ecoregions. This work suggests that soil hydrology modifies the effects of earlier snowmelt on regional streamflow response and vegetation water stress

Now You See It Now You Don’t: A Case Study of Ephemeral Snowpacks in the Great Basin U.S.A.

Raw data used for research related to the paper: Now You See It Now You Don’t: A Case Study of Ephemeral Snowpacks in the Great Basin U.S.A.Ephemeral snowpacks, or those that routinely experience accumulation and ablation at the same time and persist for <60 days, are challenging to observe and model. Using 328 site years from the Great Basin, we show that ephemeral snowmelt delivers water earlier than seasonal snowmelt. For example, we found that day of peak soil moisture preceded day of last snowmelt in the Great Basin by 79 days for shallow soil moisture in ephemeral snowmelt compared to 5 days for seasonal snowmelt. To understand Great Basin snow distribution, we used moderate resolution imaging spectroradiometer (MODIS) and Snow Data Assimilation System (SNODAS) data from water years 2005-2014 to map snow extent. During this time period snowpack was highly variable. The maximum seasonal snow cover was 64 % in 2010 and the minimum was 24 % in 2014. We found that elevation had a strong control on snow ephemerality, and nearly all snowpacks over 2500 m were seasonal. Snowpacks were more likely to be ephemeral on south facing slopes than north facing slopes at elevations above 2500 m. Additionally, we used SNODAS-derived estimates of solid and liquid precipitation, melt, sublimation, and blowing snow sublimation to define snow ephemerality mechanisms. In warm years, the Great Basin shifts to ephemerally dominant as the rain-snow transition increases in elevation. Given that snow ephemerality is expected to increase as a consequence of climate change, we put forward several challenges and recommendations to bolster physics based modeling of ephemeral snow such as better metrics for snow ephemerality and more ground-based observations.NASA Space Grant Consortium, USDA NIFA NEV052

Humidity determines snowpack ablation under a warming climate

Climate change is altering historical patterns of snow accumulation and melt, threatening societal frameworks for water supply. However, decreases in spring snow water equivalent (SWE) and changes in snowmelt are not ubiquitous despite widespread warming in the western United States, highlighting the importance of latent and radiant energy fluxes in snow ablation. Here we demonstrate how atmospheric humidity and solar radiation interact with warming temperature to control snowpack ablation at 462 sites spanning a gradient in mean winter temperature from -8.9 to +2.9 degrees C. The most widespread response to warming was an increase in episodic, midwinter ablation events. Under humid conditions these ablation events were dominated by melt, averaging 21% (202 mm/year) of SWE. Winter ablation under dry atmospheric conditions at similar temperatures was smaller, averaging 12% (58 mm/year) of SWE and likely dominated by sublimation fluxes. These contrasting patterns result from the critical role that atmospheric humidity plays in local energy balance, with latent and long-wave radiant fluxes cooling the snowpack under dry conditions and warming it under humid conditions. Similarly, spring melt rates were faster under humid conditions, yet the second most common trend was a reduction in spring melt rates associated with earlier initiation when solar radiation inputs are smaller. Our analyses demonstrate that regional differences in atmospheric humidity are a major cause of the spatial variability in snowpack response to warming. Better constraints on humidity will be critical to predicting both the amount and timing of surface water supplies under climate change

Now you see it, now you don't: a case study of ephemeral snowpacks and soil moisture response in the Great Basin, USA

Ephemeral snowpacks, or those that persist for 25 days in the lowest ( 2500 m) elevations. During this time period snowpack was highly variable. The maximum seasonal snow cover during water years 2005-2014 was 64 % in 2010 and at a minimum of 24 % in 2014. We found that elevation had a strong control on snow ephemerality, and nearly all snow-packs over 2500 m were seasonal except those on south-facing slopes. Additionally, we used SNODAS-derived estimates of solid and liquid precipitation, melt, sublimation, and blowing snow sublimation to define snow ephemerality mechanisms. In warm years, the Great Basin shifts to ephemerally dominant as the rain-snow transition increases in elevation. Given that snow ephemerality is expected to increase as a consequence of climate change, physics-based modeling is needed that can account for the complex energetics of shallow snow-packs in complex terrain. These modeling efforts will need to be supported by field observations of mass and energy and linked to finer remote sensing snow products in order to track ephemeral snow dynamics