Browsing by Author "McGlynn, Brian L"

Watersheds are complex, three dimensional structures that partition water between the components of the water balance and multiple storage pools within the watershed. This central function, however, remains poorly understood in a broadly transferable way despite decades of research. Perhaps one reason for this is the disciplinary bias towards studying pristine, mountainous watersheds with steep terrain and shallow soil. Although the relative simplicity of such systems has made them ideal hydrologic laboratories, understanding how watersheds function globally will require the incorporation of new types of landscapes into the studies of hillslope and watershed hydrology.

The Southern Piedmont region of the United States is situated between the Appalachian mountains and Atlantic coastal plains and stretches from Alabama to Maryland. It’s generally rolling terrain if underlain by deeply weathered and highly stratified soil characterized by relatively shallow argillic Bt horizons while weathered saprolite can extend tens of meters deep. Although it is a low-relief landscape, headwaters are often highly dissected with steep narrow valleys containing temporary streams surrounded by diverse topography. The region represents an ideal opportunity to incorporate more diverse landscapes into our studies of watershed hydrology.

As part of the NSF funded Calhoun Critical Zone Observatory, we intensively instrumented a 6.9 ha headwater (watershed 4, WS4), along with other targeted sensor locations including discharge in the 322 ha watershed that contains it (Holcombe’s Branch, HLCM), a nearby meteorological station, a deep groundwater well on a relatively flat interfluve, and a small network of wells in the buried floodplain. Sensors were continuously monitored for over 3 years while logging at 5 minute intervals. This sensor network allowed us to quantify the timing and magnitude of runoff, precipitation, deep and shallow groundwater levels distributed across a watershed, and soil moisture at multiple depths and hillslope positions. By doing so we were able to 1) describe the interactions between water balance components in WS4, 2) compare these watershed-scale measurements to internal hydrologic dynamics to determine what parts of the watershed are responsible for distinct watershed functions, and 3) explore how headwaters connect to higher order streams.

Using the monthly water balance in WS4, we calculated changes in integrated watershed storage and then derived a cumulative monthly storage time series from its running integral. We found that storage changes within the year by hundreds of millimeters (~25% of annual precipitation) in conjunction with seasonal peaks in evapotranspiration. Additionally, of all the potential variables that correlated to runoff magnitudes at the watershed scale, we found storage to be the best, particularly above a threshold value which remained remarkably consistent across all three years even with substantial differences in precipitation.

However, despite the storage threshold dependence of runoff, when we calculated daily storage we found that while runoff increased primarily in response to major precipitation events and then decreased again shortly thereafter, storage primarily wet up once from its low point at the end of the growing season and then drained starting at the growing season and continuing through the summer. Similarly, individual measurements of internal watershed hydrology like soil moisture or water table level displayed either seasonal or event-scale changes. We determined that measurements taken at watershed positions with more convergent hillslopes, or farther from the watershed divide, or installed deeper in the soil are more likely to display seasonal changes, and vice versa for event-scale changes. These three gradients are essentially proxies for vertical, lateral, and longitudinal distances, and so it appeared that the underlying gradient being measured was actually contributing volume. We determined the functions of different landscape components based on this analysis, and came to understand that storage-linked sites wet up first and then stay consistently so, making conditions for runoff. Subsequently, when runoff-linked sites wet-up, they mobilize significant runoff fluxes either by hydraulic displacement, or interflow, or a transmissivity feedback, or likely some combination of them all. During these times a substantial portion of the watershed is connected before drying down again with the exception of more storage-linked locations.

The result of this threshold setting followed by large runoff events is extremely flashy outputs from WS4. In contrast, we found HLCM to be far less flashy and relatively less sensitive to year to year fluctuations in precipitation. Further, we observed that except in the most extreme storms, surface flow from WS4 across the former floodplain in between it and HLCM always fully infiltrates into the sandy, legacy sediments deposited along the entire former floodplain. These sediments are the legacy of centuries of intensive and poorly managed agriculture across the Southern Piedmont. Wells in these sediments revealed a highly dynamic water table that was very responsive to outflow from WS4. A simple geometric simplification of the shape of these sediments and an estimate of their porosity revealed that these sediments had ~900 m3 of available storage space, space that was constantly filling and draining. Interestingly, that available storage volume level was sufficient to absorb discharge from WS4 on 97% of the days we measured. Through most of WS4 flow states, this storage served to buffer HLCM from flashier runoff coming from WS4, and then subsequently releasing it much more slowly and drawn out as shallow subsurfaceflow. However, when it reached volumes within 15% of maximum, usually in conjunction with large fluxes coming from WS4, runoff in HLCM reacted closely with WS4. So the storage volume in legacy sediments serves as an effective buffer from flashy upstream hydrology, but when the reach or approach saturation they become effective at transmitting surface flow, likely via saturation excess. Although we observed this phenomenon in only one alluvial fan, we have reason to think that such features are quite common locally and regionally, and represent a heretofore underappreciated legacy of historic agriculture.

Taken together, these findings describe a hydrologic system that is much more dynamic than its abundant rainfall and surface water resources would suggest. Further, they indicate that even a century or more after agricultural land abandonment and forest regrowth, legacies of the 18th and 19th century remain in the landforms and soils of the region. We feel that these findings are strong support for continued and expanded hydrologic study at the CCZO and in the Southern Piedmont in general.

Headwater streams serve multiple important biogeochemical, hydrologic, and ecological functions, including: transporting solutes from the terrestrial landscape to downstream fluvial ecosystems; providing a surface for gas evasion to the atmosphere; integrating terrestrial, riparian and aquatic ecosystems, amalgamating surface and groundwater; accumulating and storing sediment; and transforming and retaining solutes. The numerous mechanisms mediating these physical and biological processes remain poorly understood despite their prominent influence on catchment outlet biogeochemical dynamics.

In light of this research need, this study sought to determine the influence of hydrologic, ecological, and biogeochemical processes on solute (specifically carbon and nitrogen) concentrations and fluxes in a paired set of headwater stream networks.

This research was conducted at the Tenderfoot Creek Experimental Forest in Montana. An empirical, field-based approach that combined observational monitoring using a network of high temporal resolution sensors and experimental solute additions was used to quantify carbon and nitrogen uptake, metabolism, and export across the snowmelt and baseflow recession periods.

Based on analysis of this data set, we determined that headwater streams show strong demand for carbon and nitrogen across a range of concentrations from ambient to saturating concentrations; that the variation in uptake kinetics seasonally and between sites is driven by substrate availability; that this retention capacity is linked to the magnitude of metabolic demand; and that through the metabolism of the biological community carbon and nitrogen cycles are coupled. We then demonstrate that these biological processes can have variable roles in mediating carbon and nitrogen export at the catchment scale, but during some periods of the year they can be as influential as physically driven fluxes in mediating watershed export.

This study integrates disparate ecological and hydrologic perspectives to address how energy and macronutrients move through headwater stream networks. We believe the findings presented here begin to reconcile the seemingly incompatible paradigms of streams as highly retentive biogeochemical reactors and streams as “passive pipes” that reflect and integrate the terrestrial landscape.

Ephemeral and intermittent streams comprise the majority of stream channel length worldwide. These non-perennial streams are important landscape features in that they transport materials and solutes from the terrestrial landscape to downstream aquatic systems, provide unique ecological habitats, and transmit, transform, and retain chemical species. Although these streams serve a myriad of hydrological and biogeochemical functions, the mechanisms that drive runoff generation as well as the hydrological and biogeochemical contributions of these non-perennial streams to downstream perennial stream systems are poorly understood. To address this knowledge gap, this dissertation focused on investigating dominant runoff sources, flowpaths, and stream-groundwater interactions across an ephemeral-to-perennial drainage network located in the humid Piedmont landscape of the Duke Forest, North Carolina, USA. Not only does this research work to fill an important knowledge gap, but it was conducted in a low relief landscape, which is a highly understudied landscape type.

A 48.4 hectare research watershed was designed, fully instrumented, and continuously managed to address the objectives of this dissertation. Through this, the timing and magnitude of precipitation, runoff, and the dominant runoff generating flowpaths were monitored across watershed scales at 5 minute intervals for two years. In the first chapter of this dissertation, observations of hydraulic gradients between groundwater and the stream were used to show event- and seasonal-driven bidirectional flow between these two systems. The results of this work indicated that non-perennial headwater streams can both lose to and gain water from the deep groundwater system. Calculations from an annual water balance in the headwater catchment confirmed this temporally shifting flow system that resulted in significant annual stream water loss to the groundwater system. In the second chapter, a data-driven visualization of the bidirectional stream-groundwater interactions across a characteristic hillslope that built on Chapter 1 results was presented. This animation and supporting text provided visualization of a shallow, perched, transient water table that drove runoff generation during periods of losing stream gradients. Chapter 3 built on these results through introduction of a conceptual model of the dominant runoff generating processes in this headwater catchment during ephemeral and intermittent runoff regimes. This chapter focused on two intensively sampled precipitation events to demonstrate that distinct runoff generating mechanisms dominate runoff regimes across different catchment storage states. Finally, in the fourth chapter, observations of dissolved organic carbon fluxes, runoff contributions, active surface drainage network length, and groundwater across different landscape elements (headwaters and lowlands), were used to investigate the balance of longitudinal, lateral, and vertical runoff processes on runoff and carbon export dynamics across watershed scales. Overall, these findings shed light on dominant runoff generation mechanisms in low relief, highly weathered landscapes, which are a globally understudied landscape type. These findings also fill a knowledge gap about the mechanisms that drive runoff generation and stream-groundwater interactions surrounding non-perennial streams and the resulting influences on carbon and water fluxes in downstream, perennial waterways.

Global climate change is largely due to human induced increases in the emission of greenhouse gases to the atmosphere. Although this fact does not directly motivate this research, it does set the backdrop for the impressive increase in research that topic has garnered across disciplines over the past 30 plus years. The goals of the research presented herein were to investigate the spatial and temporal dynamics of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) flux dynamics in a snowmelt dominated, semi-arid watershed in central Montana and to assess if and how these fluxes were related to patterns imposed by the topographic structure of the watershed. In the process, it has become apparent that a conceptual model that incorporates all three of these important GHGs, and their relationships with environmental variables does not exist. This is certainty at least in part due to the high variability of these fluxes within and between ecosystems. However, a concise conceptual model is necessary to compare empirical evidence and test alternative scaling methods across systems. Explicitly incorporating hydrologic processes into a conceptual framework will not only be important, but imperative, to predicting and assessing responses of these biogeochemical fluxes to a changing climate. This will be particularly relevant in locations that are likely to experience a change in the timing of precipitation such as snow versus rain dominated in sub-alpine zones or change in the timing and frequency of rain events.

In this study, we assessed the spatial and temporal dynamics of three major GHGs using a spatially distributed sampling campaign over two growing seasons. Real time sensors (5 locations) and local spatial variability plots (700 m2, n = 7, with 30 samples in each) were nested within a landscape scale sampling design (n=52). The sites that were distributed across the landscape (n = 52) were organized by transects that either exemplified specific landscape elements (e.g. uplands vs riparian area) or crossed significant environmental gradients (e.g. riparian – uplands or clearcut – forest). Total annual precipitation was similar between the two focal years (2012 = 764 mm 2013 = 749 mm). However, the contribution from rain versus snow shifted from 76% snow and 24% rain in 2012 to 56% snow and 44% rain in 2013. The influential rain events in 2013 began on 17 July and were observed through 14 August.

In this study, we observed that the strength of the relationship between soil water content and topographic metrics of water redistribution increased as the average wetness of the watershed declined. Soil water content and CO2 flux (fCO2) exhibited distinct spatial and temporal variability at the plot and landscape scales in 2013. The legacy effects of clearcutting remained prevalent with regards to fCO2 (which was significantly higher in the forest than in the clearcut regrowth), while differences in the spatial and temporal variability of \theta were not evident between the two landcover types.

Relationships between fluxes of CO2, CH4, and N2O and \theta were variable. The relationship between each gas and soil water content was not consistent between riparian and upland landscape elements. Although the transition zone between riparian and upland locations has been a focal point in watershed biogeochemistry, it appears that focusing on the shifting hydrodynamics, or the dominant hydrologic processes themselves, might be more important than focusing on specific, pre-defined locations in the landscape.

We capitalized on the significant relationships between terrain mediated \theta in the uplands and cumulative seasonal flux of CH4 to empirically scale our weekly measurements of CH4 flux to the watershed scale. We determined that incorporating multiple terrain metrics in the model produced the strongest fit between modeled and observed CH4 flux. This scaling exercise showed that the best fit model predicted over twice as much CH4 consumption in the uplands than predicted with an individual topographic wetness index or by extrapolating the mean/median CH4 flux to the watershed. Additionally, we determined that even if we used the maximum value of seasonal CH4 efflux in the riparian area to estimate riparian contributions, the riparian CH4 efflux only constituted 1– 4% of the net watershed CH4 flux (depending on which value of net influx is used).

While searching for the mechanisms that create biogeochemical optima can interesting and valuable, moving forward, it seems equally important to investigate the spatiotemporal dynamics of fluxes (or times/ places) that we expect to exhibit more landscape scale characteristic levels of a given flux/pool/process. It is also critical that we do not treat the hydrologic dynamics that can influencing those pools/fluxes as a “black box”. Field studies that measure these hydrologic dynamics can provide rich data sets to test accepted and proposed conceptual models and provide useful calibration data for process-based models. The combination of these techniques will most certainly advance our understanding of the spatial and temporal dynamics of greenhouse gas fluxes across given systems. However, evolved conceptual models will be key to assessing how each unique field site or modeling exercise contributes to greater process understanding and predictive capacity. Here we contribute to an updated conceptual model of the relationship between the processes that influence these GHG fluxes and soil water content. We hope that these conceptual contributions will spur new research questions that span systems and scales, while the empirical contributions highlight a few ways that this can be done in practice.