J. A. Leach

Twelve-year interannual and seasonal variability of stream carbon export from a boreal peatland catchment

Understanding stream carbon export dynamics is needed to accurately predict how the carbon balance of peatland catchments will respond to climatic and environmental change. We used a 12year record ...

Observations and modeling of hillslope throughflow temperatures in a coastal forested catchment

A growing body of research on stream thermal regimes has highlighted the importance of heat advection associated with surface water and groundwater interactions, such as hyporheic exchange, groundwater discharge, and hillslope throughflow inputs. Existing catchment models that predict stream temperature use a variety of approaches to estimate throughflow temperatures, but none has been evaluated against field measurements of throughflow temperature. In this study, throughflow temperatures were monitored over two winters at 50 locations adjacent to a headwater stream (11 ha catchment area) located in the rain-on-snow zone of the Pacific Northwest. Existing approaches to estimate throughflow temperature under or overpredicted throughflow temperatures by up to 5°C, or were unable to represent the influence of transient snow cover. Therefore, a conceptual-parametric model that is computationally efficient was developed that simulates hillslope hydrology and throughflow temperatures. The model structure includes an upslope reservoir that drains into a downslope reservoir that, in turn, drains into the stream. Vertical and lateral energy and water fluxes are simulated using simplified process representations. The model successfully predicts throughflow temperatures and highlights the dominant role of throughflow advection and the influence of snow cover on stream thermal regimes during high flow periods and rain-on-snow events.

Evaluating topography‐based predictions of shallow lateral groundwater discharge zones for a boreal lake‐stream system

Groundwater discharge along streams exerts an important influence on biogeochemistry and thermal regimes of aquatic ecosystems. A common approach for predicting locations of shallow lateral groundwater discharge is to use digital elevation models (DEMs) combined with upslope contributing area algorithms. We evaluated a topography-based prediction of subsurface discharge zones along a 1500 m headwater stream reach using temperature and water isotope tracers. We deployed fiber-optic distributed temperature sensing instrumentation to monitor stream temperature at 0.25 m intervals along the reach. We also collected samples of stream water for the analysis of its water isotope composition at 50 m intervals on five occasions representing distinct streamflow conditions before, during, and after a major rain event. The combined tracer evaluation showed that topography-predicted locations of groundwater discharge were generally accurate; however, predicted magnitude of groundwater inflows estimated from upslope contributing area did not always agree with tracer estimates. At the catchment scale, lateral inflows were an important source of streamflow at base flow and peak flow during a major rain event; however, water from a headwater lake was the dominant water source during the event hydrograph recession. Overall, this study highlights potential utility and limitations of predicting locations and contributions of lateral groundwater discharge zones using topography-based approaches in humid boreal regions.

Insights on stream temperature processes through development of a coupled hydrologic and stream temperature model for forested coastal headwater catchments

Stream temperature controls a number of biological, chemical and physical processes occurring in aquatic environments. Transient snow cover and advection associated with lateral throughflow inputs can have a dominant influence on stream thermal regimes for headwater catchments in the rain-on-snow zone. Most existing stream temperature models lack the ability to properly simulate these processes. We developed and evaluated a conceptual-parametric catchment-scale stream temperature model that includes the role of transient snow cover and lateral advection associated with throughflow. The model consists of routines for simulating canopy interception, snow accumulation and melt, hillslope throughflow runoff and temperature, and stream channel energy exchange processes. The model was used to predict discharge and stream temperature for a small forested headwater catchment near Vancouver, Canada, using long-term (1963-2013) weather data to compute model forcing variables. The model was evaluated against four years of observed stream temperature. The model generally predicted daily mean stream temperature accurately (annual RMSE between 0.57 and 1.24 ∘C) although it overpredicted daily summer stream temperatures by up to 3 ∘C during extended low streamflow conditions. Model development and testing provided insights on the roles of advection associated with lateral throughflow, channel interception of snow and surface–subsurface water interactions on stream thermal regimes. This study shows that a relatively simple but process-based model can provide reasonable stream temperature predictions for forested headwater catchments located in the rain-on-snow zone.

What are the contemporary sources of sediment in the Mississippi River

Within the last two centuries, the Mississippi River basin has been transformed by changes in land use practices, dam construction and training of the rivers for navigation. Here we analyze the contemporary patterns of fluvial sediment yield in the Mississippi River basin using all available data in order to assess the influence of regional land condition on the variation of sediment yield within the basin. We develop regional scale relations between specific sediment yield (yield per unit area) and drainage area to reveal contemporary regional sediment yield patterns and source areas of riverine sediments. Extensive upland erosion before the development of soil conservation practices exported large amounts of sediment to the valleys and floodplains [Trimble, 1981; Belmont et al., 2011]. We show that sediment today is sourced primarily along the river valleys from arable land, and from stream bank and channel erosion, with sediment yields from areas dominated by arable land two orders of magnitude greater than that of grassland dominated areas. Comparison with the “T factor”, a commonly quoted measure of agricultural soil resilience suggests that the latter may not reflect contemporary soil loss from the landscape.

Winter stream temperature in the rain-on-snow zone of the Pacific Northwest

Stream temperature dynamics during winter are less well studied than summer thermal regimes, but the winter season thermal regime can be critical for fish growth and development. The winter thermal regimes of Pacific Northwest headwater streams, which provide vital winter habitat for salmonids and their food sources, may be particularly sensitive to changes in climate because they can remain ice-free throughout the year and are often located in rain-on-snow zones. This study examined winter stream temperature patterns and controls in small headwater catchments within the rain-on-snow zone at the Malcolm Knapp Research Forest, near Vancouver, British Columbia, Canada. A diagnostic energy budget analysis highlighted that advective fluxes associated with hillslope throughflow inputs were a dominant control on the winter stream thermal regime. In addition, stream temperatures during rain-on-snow events were generally lower than during rain-on-ground events after controlling for air temperature. Methods for estimating throughflow temperatures embedded in stream temperature models were evaluated against field observations, and were found either not to account for the role of snow or to underor over-predict throughflow temperatures by up to 5 ◦C. Therefore, a conceptual-parametric hillslope throughflow temperature model that coupled hydrologic and thermal processes, and accounted for the role of snow was developed and evaluated against field observations of throughflow temperatures. The hillslope throughflow temperature model was linked to stream energy exchange processes in order to predict stream temperature. The stream temperature model accurately predicted