Christine E. Hatch

Quantifying surface water–groundwater interactions using time series analysis of streambed thermal records: Method development

[1] We present a method for determining streambed seepage rates using time series thermal data. The new method is based on quantifying changes in phase and amplitude of temperature variations between pairs of subsurface sensors. For a reasonable range of streambed thermal properties and sensor spacings the time series method should allow reliable estimation of seepage rates for a range of at least ±10 m d � 1 (±1.2 � 10 � 2 ms � 1 ), with amplitude variations being most sensitive at low flow rates and phase variations retaining sensitivity out to much higher rates. Compared to forward modeling, the new method requires less observational data and less setup and data handling and is faster, particularly when interpreting many long data sets. The time series method is insensitive to streambed scour and sedimentation, which allows for application under a wide range of flow conditions and allows time series estimation of variable streambed hydraulic conductivity. This new approach should facilitate wider use of thermal methods and improve understanding of the complex spatial and temporal dynamics of surface water–groundwater interactions.

Nitrate dynamics within the Pajaro River, a nutrient-rich, losing stream

Abstract The major ion chemistry of water from an 11.42-km reach of the Pajaro River, a losing stream in central coastal California, shows a consistent pattern of higher concentrations during the 2nd (dry) half of the water year. Most solutes are conserved during flow along the reach, but [NO3−] decreases by ~30% and is accompanied by net loss of channel discharge and extensive surface–subsurface exchange. The corresponding net NO3− uptake length is 37 ± 13 km (42 ± 12 km when normalized to the conservative solute Cl−), and the areal NO3− uptake rate is 0.5 μmol m−2 s−1. The observed reduction in [NO3−] along the reach results from one or more internal sinks, not dilution by ground water, hill-slope water, or other water inputs. Observed reductions in [NO3−] and channel discharge along the experimental reach result in a net loss of 200–400 kg/d of NO3−-N, ~50% of the input load. High-resolution (temporal and spatial) sampling indicates that most of the NO3− loss occurs along the lower part of the reach, where there is the greatest seepage loss and surface–subsurface exchange of water. Stable isotopes of NO3−, total dissolved P concentrations, and streambed chemical profiles suggest that denitrification is the most significant NO3− sink along the reach. Denitrification efficiency, as expressed through downstream enrichment in 15N-NO3−, varies considerably during the water year. When discharge is greater (typically earlier in the water year), denitrification is least efficient and downstream enrichment in 15N-NO3− is greatest. When discharge is lower, denitrification in the streambed appears to occur with greater efficiency, resulting in lower downstream enrichment in 15N-NO3−.

Mapping high-resolution soil moisture and properties using distributed temperature sensing data and an adaptive particle batch smoother

This study demonstrated a new method for mapping high resolution (spatial: 1 m, and temporal: 1 hour) soil moisture by assimilating distributed temperature sensing (DTS) observed soil temperatures at intermediate scales. In order to provide robust soil moisture and property estimates, we first proposed an adaptive particle batch smoother algorithm (APBS). In the APBS, a tuning factor, which can avoid severe particle weight degeneration, is automatically determined by maximizing the reliability of the soil temperature estimates of each batch window. A multiple truth synthetic test was used to demonstrate the APBS can robustly estimate soil moisture and properties using observed soil temperatures at two shallow depths. The APBS algorithm was then applied to DTS data along a 71 m transect, yielding an hourly soil moisture map with meter resolution. Results show the APBS can draw the prior guessed soil hydraulic and thermal properties significantly closer to the field measured reference values. The improved soil properties in turn remove the soil moisture biases between the prior guessed and reference soil moisture, which was particularly noticeable at depth above 20 cm. This high resolution soil moisture map demonstrates the potential of characterizing soil moisture temporal and spatial variability and reflects patterns consistent with previous studies conducted using intensive point scale soil moisture samples. The intermediate scale high spatial resolution soil moisture information derived from the DTS may facilitate remote sensing soil moisture product calibration and validation. In addition, the APBS algorithm proposed in this study would also be applicable to general hydrological data assimilation problems for robust model state and parameter estimation. This article is protected by copyright. All rights reserved.

Impacts of three‐dimensional nonuniform flow on quantification of groundwater‐surface water interactions using heat as a tracer

Use of heat-as-a-tracer is a common method to quantify surface water-groundwater interactions (SW-GW). However, the method relies on assumptions likely violated in natural systems. Numerical studies have explored violation of fundamental assumptions such as heterogeneous streambed properties, two-dimensional groundwater flow fields and uncertainty in thermal parameters for the 1D heat-as-a-tracer method. Few studies to date have modeled complex, fully three-dimensional groundwater flows to address the impacts of non-uniform, 3D flow vectors on use of heat-as-a-tracer to quantify SW-GW interactions. COMSOL Multiphysics was used to model scenarios in a fully three-dimensional flow field in homogeneous, isotropic sand with a sinusoidal temperature upper boundary where vertical flows are deliberately disrupted by large and varied horizontal flows from two directions. Resulting temperature time series from multiple depths were used to estimate vertical Darcy flux and compared with modeled fluxes to assess the performance of the 1D thermal methods to quantify multi-dimensional groundwater flows. In addition, apparent effective thermal diffusivity was calculated from synthetic temperature time series, and compared to model input diffusivity. Both increasingly non-uniform and non-vertical groundwater flow fields resulted in increasing errors for both the temperature-derived flux and temperature-derived effective thermal diffusivity. For losing (downward) flow geometries, errors in temperature-derived effective thermal diffusivity were highly correlated with errors in temperature-derived flux and were used to identify how and when underlying assumptions necessary for heat-as-a-tracer for quantifying groundwater flows were violated. Specifically, non-uniform flow fields (with flow lines that converge or diverge) produced the largest errors in simulated fluxes. This article is protected by copyright. All rights reserved.

Hydrogeological controls on spatial patterns of groundwater discharge in peatlands

Peatland environments provide important ecosystem services including water and carbon storage, nutrient processing and retention, and wildlife habitat. However, these systems and the services they provide have been degraded through historical anthropogenic agricultural conversion and dewatering practices. Effective wetland restoration requires incorporating site hydrology and understanding groundwater discharge spatial patterns. Groundwater discharge maintains wetland ecosystems by providing relatively stable hydrologic conditions, nutrient inputs, and thermal buffering important for ecological structure and function; however, a comprehensive site-specific evaluation is rarely feasible for such resource-constrained projects. An improved process-based understanding of groundwater discharge in peatlands may help guide ecological restoration design without the need for invasive methodologies and detailed sitespecific investigation. Here we examine a kettle-hole peatland in southeast Massachusetts historically modified for commercial cranberry farming. During the time of our investigation, a large process-based ecological restoration project was in the assessment and design phases. To gain insight into the drivers of site hydrology, we evaluated the spatial patterning of groundwater discharge and the subsurface structure of the peatland complex using heat-tracing methods and groundpenetrating radar. Our results illustrate that two groundwater discharge processes contribute to the peatland hydrologic system: diffuse lower-flux marginal matrix seepage and discrete higher-flux preferential-flow-path seepage. Both types of groundwater discharge develop through interactions with subsurface peatland basin structure, often where the basin slope is at a high angle to the regional groundwater gradient. These field observations indicate strong correlation between subsurface structures and surficial groundwater discharge. Understanding these general patterns may allow resource managers to more efficiently predict and locate groundwater seepage, confirm these using remote sensing technologies, and incorporate this information into restoration design for these critical ecosystems.

Use of Distributed Temperature Sensing Technology to Characterize Fire Behavior

We evaluated the potential of a fiber optic cable connected to distributed temperature sensing (DTS) technology to withstand wildland fire conditions and quantify fire behavior parameters. We used a custom-made ‘fire cable’ consisting of three optical fibers coated with three different materials—acrylate, copper and polyimide. The 150-m cable was deployed in grasslands and burned in three prescribed fires. The DTS system recorded fire cable output every three seconds and integrated temperatures every 50.6 cm. Results indicated the fire cable was physically capable of withstanding repeated rugged use. Fiber coating materials withstood temperatures up to 422 °C. Changes in fiber attenuation following fire were near zero (−0.81 to 0.12 dB/km) indicating essentially no change in light gain or loss as a function of distance or fire intensity over the length of the fire cable. Results indicated fire cable and DTS technology have potential to quantify fire environment parameters such as heat duration and rate of spread but additional experimentation and analysis are required to determine efficacy and response times. This study adds understanding of DTS and fire cable technology as a potential new method for characterizing fire behavior parameters at greater temporal and spatial scales.

Proliferating a New Generation of Critical Physical Geographers: Graduate Education in UMass’s RiverSmart Communities Project

To build our collective capacities to engage in CPG research and practice, we need to proliferate a new generation of critical physical geographers. To do this, we must train students who can think, conduct research, and share findings in ways that are critical, open-ended, and transdisciplinary. But what does this look like in practice? This chapter outlines five central themes which supported an education rich in the principles of Critical Physical Geography for two graduate students: an inclusive pedagogy and collaboration, an interdisciplinary setting, an applied project, an acceptance of a dynamic ontology, and an open-ended epistemology. The conclusion section highlights which of these practices were the most critical, as well as the possible limitations to similar research. In the end, the graduate students left this research project with a broader understanding of their own research and armed with a practical and diverse set of skills for a changing job market.

Fluvial geomorphic assessment and river corridor mapping as flood risk management tools in Massachusetts, USA

The role of geomorphic processes in flood risk is understudied in the management context. In the United States, only nine states have explored this role and only two – Vermont and Washington State – have developed and implemented legally binding geomorphic-based flood risk management; both rely on the concept of geomorphic assessment, through which fluvial geomorphic processes are documented and river corridors are mapped. Massachusetts, having incurred substantial damages from landslides, bank failures, bed incision, and sedimentation in recent years, has initiated a programme to examine the inclusion of geomorphic processes into flood risk management. At its core, the programme relies on participation of flood risk management stakeholders representing government (local, state, and federal), non-governmental organisations, consulting agencies, academia, and industry. A series of workshops with these stakeholders over 4 years has culminated in a needs assessment that articulates what must be included in the development of a Massachusetts fluvial geomorphic assessment programme. In this report, we share the results of this needs assessment. We do this in the hope that other jurisdictions incurring flood damages from geomorphic processes may find it to be a useful model as they work to mitigate these damages.