Jason A. Leach

Bio: Jason A. Leach is an academic researcher from Natural Resources Canada. The author has contributed to research in topics: Environmental science & Riparian zone. The author has an hindex of 13, co-authored 30 publications receiving 588 citations. Previous affiliations of Jason A. Leach include Trent University & University of British Columbia.

Above-stream microclimate and stream surface energy exchanges in a wildfire-disturbed riparian zone

Abstract: Stream temperature and riparian microclimate were characterized for a I-5 km wildfire-disturbed reach of Fishtrap Creek, located north of Kamloops, British Columbia. A deterministic net radiation model was developed using hemispherical canopy images coupled with on-site microclimate measurements. Modelled net radiation agreed reasonably with measured net radiation. Air temperature and humidity measured at two locations above the stream, separated by 900 m, were generally similar, whereas wind speed was poorly correlated between the two sites. Modelled net radiation varied considerably along the reach, and measurements at a single location did not provide a reliable estimate of the modelled reach average. During summer, net radiation dominated the surface heat exchanges, particularly because the sensible and latent heat fluxes were normally of opposite sign and thus tended to cancel each other. All surface heat fluxes shifted to negative values in autumn and were of similar magnitude through winter. In March, net radiation became positive, but heat gains were cancelled by sensible and latent heat fluxes, which remained negative. A modelling exercise using three canopy cover scenarios (current, simulated pre- wildfire and simulated complete vegetation removal) showed that net radiation under the standing dead trees was double that modelled for the pre-fire canopy cover. However, post-disturbance standing dead trees reduce daytime net radiation reaching the stream surface by one-third compared with complete vegetation removal. The results of this study have highlighted the need to account for reach-scale spatial variability of energy exchange processes, especially net radiation, when modelling stream energy budgets. Copyright © 2010 John Wiley & Sons, Ltd.

Stream temperature dynamics in two hydrogeomorphically distinct reaches

Abstract: The objective of this study was to analyse stream temperature variability during summer in relation to both surface heat exchanges and reach-scale hydrology for two hydrogeomorphically distinct reaches. The study focused on a 1·5-km wildfire-disturbed reach of Fishtrap Creek located north of Kamloops, British Columbia. Streamflow measurements and longitudinal surveys of electrical conductivity and water chemistry indicated that the upper 750 m of the study reach was dominated by flow losses. A spring discharged into the stream at 750 m below the upper reach boundary. Below the spring, the stream was neutral to losing on three measurement days, but gained flow on a fourth day that followed a rain event. Continuous stream temperature measurements typically revealed a downstream warming along the upper 750 m of the study reach on summer days, followed by a pronounced cooling associated with the spring, with little downstream change below the spring. Modelled surface energy exchanges were similar over the upper and lower sub-reaches, and thus cannot explain the differences in longitudinal temperature patterns. Application of a Lagrangian stream temperature model provided reasonably accurate predictions for the upper sub-reach. For the lower sub-reach, accurate prediction required specification of concurrent flow losses and gains as a hydrological boundary condition. These findings are consistent with differences in the hydrogeomorphology of the upper and lower sub-reaches. The modelling exercise indicated that substantial errors in predicted stream temperature can occur by representing stream-surface exchange as a reach-averaged one-directional flux computed from differences in streamflow between the upper and lower reach boundaries. Further research should focus on reliable methods for quantifying spatial variations in reach-scale hydrology. Copyright © 2010 John Wiley & Sons, Ltd.

Atmospheric and soil moisture controls on evapotranspiration from above and within a Western Boreal Plain aspen forest

Abstract: The Western Boreal Plain of North Central Alberta comprises a mosaic of wetlands and aspen (Populus tremuloides) dominated uplands where precipitation (P) is normally exceeded by evapotranspiration (ET). As such these systems are highly susceptible to the climatic variability that may upset the balance between P and ET. Above canopy evapotranspiration (ETC) and understory evapotranspiration (ETB) were examined using the eddy covariance technique situated at 25.5 m (7.5 m above tree crown) and 4.0 m above the ground surface, respectively. During the peak period of the growing seasons (green periods), ETC averaged 3.08 mm d−1 and 3.45 mm d−1 in 2005 and 2006, respectively, while ETB averaged 1.56 mm d−1 and 1.95 mm d−1. Early in the growing season, ETB was equal to or greater than ETC once understory development had occurred. However, upon tree crown growth, ETB was lessened due to a reduction in available energy. ETB ranged from 42 to 56% of ETC over the remainder of the snow-free seasons. Vapour pressure deficit (VPD) and soil moisture (θ) displayed strong controls on both ETC and ETB. ETC responded to precipitation events as the developed tree crown intercepted and held available water which contributed to peak ETC following precipitation events >10 mm. While both ETC and ETB were shown to respond to VPD, soil moisture in the rooting zone is shown to be the strongest control regardless of atmospheric demand. Further, soil moisture and tension data suggest that rooting zone soil moisture is controlled by the redistribution of soil water by the aspen root system. Copyright © 2013 John Wiley & Sons, Ltd.

Groundwater inflows control patterns and sources of greenhouse gas emissions from streams

Abstract: Headwater streams can be important sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. However, the influence of groundwater-stream connectivity on the patterns and sources of carb ...

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

Abstract: 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 ...

Browning of freshwaters: Consequences to ecosystem services, underlying drivers, and potential mitigation measures

Abstract: Browning of surface waters, as a result of increasing dissolved organic carbon and iron concentrations, is a widespread phenomenon with implications to the structure and function of aquatic ecosystems. In this article, we provide an overview of the consequences of browning in relation to ecosystem services, outline what the underlying drivers and mechanisms of browning are, and specifically focus on exploring potential mitigation measures to locally counteract browning. These topical concepts are discussed with a focus on Scandinavia, but are of relevance also to other regions. Browning is of environmental concern as it leads to, e.g., increasing costs and risks for drinking water production, and reduced fish production in lakes by limiting light penetration. While climate change, recovery from acidification, and land-use change are all likely factors contributing to the observed browning, managing the land use in the hydrologically connected parts of the landscape may be the most feasible way to counteract browning of natural waters.

River water temperature in the United Kingdom Changes over the 20th century and possible changes over the 21st century

Abstract: Change in river water temperature has important consequences for the environment and people. This review provides a new perspective on the topic by evaluating changes in river water temperature for the UK over the 20th century and possible changes over the 21st century. There is limited knowledge of space-time variability in, and controls on, river temperature at the region scale and beyond over the 20th century. There is historical evidence that UK river temperature has increased in the latter part of the 20th century, but low agreement on the attribution of changes to climatic warming because river temperature is a complex, dynamic response to climate and hydrological patterns moderated by basin properties and anthropogenic impacts. Literature is scarce to evaluate changes to UK river temperature in the 21st century, but it appears as likely as not that UK river temperature will increase in the future. However, there are a number of interlinked sources of uncertainty (related to observations, scenarios,...

Accounting for groundwater in stream fish thermal habitat responses to climate change.

Abstract: Forecasting climate change effects on aquatic fauna and their habitat requires an understanding of how water temperature responds to changing air temperature (i.e., thermal sensitivity). Previous efforts to forecast climate effects on brook trout (Salvelinus fontinalis) habitat have generally assumed uniform air-water temperature relationships over large areas that cannot account for groundwater inputs and other processes that operate at finer spatial scales. We developed regression models that accounted for groundwater influences on thermal sensitivity from measured air-water temperature relationships within forested watersheds in eastern North America (Shenandoah National Park, Virginia, USA, 78 sites in nine watersheds). We used these reach-scale models to forecast climate change effects on stream temperature and brook trout thermal habitat, and compared our results to previous forecasts based upon large-scale models. Observed stream temperatures were generally less sensitive to air temperature than previously assumed, and we attribute this to the moderating effect of shallow groundwater inputs. Predicted groundwater temperatures from air-water regression models corresponded well to observed groundwater temperatures elsewhere in the study area. Predictions of brook trout future habitat loss derived from our fine-grained models. were far less pessimistic than those from prior models developed at coarser spatial resolutions. However, our models also revealed spatial variation in thermal sensitivity within and among catchments resulting in a patchy distribution of thermally suitable habitat. Habitat fragmentation due to thermal barriers therefore may have an increasingly important role for trout population viability in headwater streams. Our results demonstrate that simple adjustments to air-water temperature regression models can provide a powerful and cost-effective approach for predicting future stream temperatures while accounting for effects of groundwater.

Sensitivity of summer stream temperatures to climate variability in the Pacific Northwest

Abstract: Estimating the thermal response of streams to a warming climate is important for prioritizing native fish conservation efforts. While there are plentiful estimates of air temperature responses to climate change, the sensitivity of streams, particularly small headwater streams, to warming temperatures is less well understood. A substantial body of literature correlates subannual scale temperature variations in air and stream temperatures driven by annual cycles in solar angle; however, these may be a low-precision proxy for climate change driven changes in the stream energy balance. We analyzed summer stream temperature records from forested streams in the Pacific Northwest for interannual correlations to air temperature and standardized annual streamflow departures. A significant pattern emerged where cold streams always had lower sensitivities to air temperature variation, while warm streams could be insensitive or sensitive depending on geological or vegetation context. A pattern where cold streams are less sensitive to direct temperature increases is important for conservation planning, although substantial questions may yet remain for secondary effects related to flow or vegetation changes induced by climate change.