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Salt marshes are globally important ecosystems and thus their resilience to climate change holds societal importance. To date, studies addressing salt marsh responses to climate change have focused on sea‐level rise and biogeochemical feedbacks with increasing inundation. Less is known about how salt marsh sediment temperatures, which impact physical, biological, and chemical ecosystem processes, will respond to climate change. In this study, we present multi‐depth sediment temperature and porewater level data from low‐, mid‐, and high‐marsh sites at a New England salt marsh for a 1‐year period and investigate how salt marsh sediment temperatures respond to atmospheric and oceanic forcing. We use spectral analyses to identify the frequencies at which sediment temperatures vary and link the temperature variations to specific forcing mechanisms. We find that all sites across the marsh responded to air temperature with roughly equal amplitude whereas the responses to radiative heating and ocean tides varied spatially. The high‐marsh site is more sensitive to radiative heating than the mid‐ and low‐marsh sites. The low‐marsh is affected by tidal processes and inundation whereas the high‐ and mid‐marsh sites are not. In addition, we find that the bulk thermal diffusivity of the saturated sediments decreases with distance from the tidal channel. These factors contribute to considerable temporal and spatial variability in sediment temperatures with elevation, distance from the tidal channel, and time of year (season) being most important. [ABSTRACT FROM AUTHOR]

Upward discharge to surface water bodies can be quantified using analytical models based on temperature–depth (T‐z) profiles. The use of sediment T‐z profiles is attractive as discharge estimates can be obtained using point‐in‐time data that are collected inexpensively and rapidly. Previous studies have identified that T‐z methods can only be applied at times of the year when there is significant difference between the streambed–water interface and deeper sediment temperatures (e.g., winter and summer). However, surface water temperatures also vary diurnally, and the influence of these variations on discharge estimates from T‐z methods is poorly understood. For this study, synthetic T‐z profiles were generated numerically using measured streambed interface temperature data to assess the influence of diurnal temperature variations on discharge estimation and provide insight into the suitable application of T‐z methods. Results show that the time of day of data collection can have a substantial influence on vertical flux estimates using T‐z methods. For low groundwater discharge fluxes (e.g., 0.1 m d−1), daily transience in streambed temperatures led to relatively large errors in estimated flow magnitude and direction. For higher discharge fluxes (1.5 m d−1), the influence of transient streambed temperatures on discharge estimates was strongly reduced. Discharge estimates from point‐in‐time T‐z profiles were most accurate when the uppermost point in the T‐z profile was near the bed interface daily mean (two time periods daily). Where temperature time series data are available, daily averaged T‐z profiles can produce accurate discharge estimates across a wide range of discharge rates. Seasonality in shallow groundwater temperature generally had a negligible influence on vertical flow estimates. These findings can be used to plan field campaigns and provide guidance on the optimal application of T‐z methods to quantify vertical groundwater discharge to surface water bodies. [ABSTRACT FROM AUTHOR]

Stream thermal regimes are controlled by the interactions of external and internal energy fluxes with the water in the channel. Solar radiation is typically the dominant driver of stream water temperature, but streambed heat fluxes can be important in forested headwater streams. Past studies have presented seemingly disparate formulae for quantifying streambed heat advection from upwelling groundwater. This note details the sources of the differences in these alternative formulations. The equations illustrate the difficulties of attempting to isolate the thermal influence of groundwater-surface water interactions and highlight future research opportunities. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Groundwater temperature is an important water quality parameter that affects species distributions in subsurface and surface environments. To investigate the response of subsurface temperature to atmospheric climate change, an analytical solution is derived for a one-dimensional, transient conduction-advection equation and verified with numerical methods using the finite element code SUTRA. The solution can be directly applied to forward model the impact of future climate change on subsurface temperature profiles or inversely applied to produce a surface temperature history from measured borehole profiles. The initial conditions are represented using superimposed linear and exponential functions, and the boundary condition is expressed as an exponential function. This solution expands on a classic solution in which the initial and boundary conditions were restricted to linear functions. The exponential functions allow more flexibility in matching climate model projections (boundary conditions) and measured temperature-depth profiles (initial conditions). For example, measured borehole temperature data from the Sendai Plain and Tokyo, Japan, were used to demonstrate the improved accuracy of the exponential function for replicating temperature-depth profiles. Also, the improved accuracy of the exponential boundary condition was demonstrated using air temperature anomaly data from the Intergovernmental Panel on Climate Change. These air temperature anomalies were then used to forward model the effect of surficial thermal perturbations in subsurface environments with significant groundwater flow. The simulation results indicate that recharge can accelerate shallow subsurface warming, whereas upward groundwater discharge can enhance deeper subsurface warming. Additionally, the simulation results demonstrate that future groundwater temperatures obtained from the proposed analytical solution can deviate significantly from those produced with the classic solution. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]