Terrestrial Ecology

Mica Creek Experimental Watershed

Terrestrial Ecology
Project Titles and Investigators:

Title:	Cold Air Drainage and Modeled Nocturnal Leaf Water Potential
	Primary Contacts: Jason Hubbart (web), Katy Kavanagh (web), Tim Link (web)

Title:	Persistent Summer Air Temperature Inversions, Soil Temperature, and Seedling Establishment in a Contemporary Working Watershed in the Northern Panhandle of Idaho
	Primary Contacts: Jason Hubbart (web), Tim Link (web), Katy Kavanagh (web)

Project Descriptions:

Cold Air Drainage and Modeled Nocturnal Leaf Water Potential
Summary
Spatial variation in microclimate owing to air temperature inversions plays an important role in determining the timing and rate of many physical and biophysical processes. Recent work has demonstrated that stomata do not close completely at night resulting in an increased opportunity for nocturnal transpiration. The following work was undertaken to develop a better understanding of nocturnal cold air drainage and subsequent impact on spatial variation in the accuracy of predawn leaf water potential (Y_pd) used as a surrogate for soil water potential (Y_s). Results indicate strong nocturnal temperature inversions occurring from the low to upper mid slope of a forested hillslope typically spanning the lower 88 vertical meters. Based on average mean temperatures for both months, inversions resulted in lapse rates of 29.0, 27.0, and 25.0 ºC/km at 00:00, 04:00, and 20:00hrs respectively. At this scale (i.e. <1km), the observed lapse rates resulted in highly variable nighttime vapor pressure deficits (D) over the length of the slope, and subsequently variable impacts on modeled disequilibrium between soil and leaf water potential. In response to cold air drainage, modeled Ψ_pd grew consistently more negative at higher elevations up to -0.3 MPa during nocturnal hours based on mean temperatures. Persistent nocturnal inversions on the lower 88 m of the slope resulted in leaf water potentials that were more negative by at least 30% and 50% from the bottom of the slope to the top of the inversion (88 m) based on mean and maximum temperatures respectively. However, on a cloudy night, with low D, the maximum decrease in Ψ_pd was -0.04 MPa. The results of this work indicate that given persistent cold air drainage, and potentially nocturnally active stomata, serious errors could be made in estimating Ψ_s if utilizing classic methodologies and assumptions. These results hold important implications for development of distributed models that calculate sap flow, and estimate water budgets assuming a global environmental lapse rate, and zero nocturnal water loss by plants.

Primary Contacts: Jason Hubbart (web), Katy Kavanagh (web), Tim Link (web)

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Figure. Environmental lapse rates (ELR) based on mean and maximum temperatures, and corresponding D for July and August 2004 on a northern facing fully forested slope within the confines of the Mica Creek Experimental Watershed, Northern Idaho. Temperature and D represent mean instantaneous temperature at each time interval during the months of July and August 2004.


Project

Persistent Summer Air Temperature Inversions, Soil Temperature, and Seedling Establishment in a Contemporary Working Watershed in the Northern Panhandle of Idaho
Objectives
Recent work has demonstrated that The Mica Creek Experimental Watershed is strongly influenced by persistent summer dry season temperature inversions (cold air drainage) holding important implications for the development of distributed models that estimate, water budgets, sap flow, and plant nocturnal water loss (Hubbart et al. in revision). The following work will identify; a) the persistence of temperature inversions in the MCEW during the summer dry season over multiple years, b) the extent to which persistent temperature inversions affect soil temperature by elevation, c) the potential impacts and implications that these findings may have on post-harvest seedling establishment, and will result in d) a simple model describing air and subsequent soil temperatures with elevation in this region. It is hypothesized that a) both clearcut and forested slopes will continue to exhibit persistent air temperature inversions, thus building on previous findings, b) persistent air temperature inversions will be subsequently manifest in soil temperature (i.e. soil temperature increases with elevation in concert with observed air temperature), and c) findings may prove to be detrimental to seedling establishment for some species of conifer (providing an opening for future work).
Methods
This study will be carried out at the Mica Creek Experimental Watershed (MCEW), over the summer dry season months of June – August 2006 (site description). Ambient air temperature (Ta) will be monitored at three meters, and soil temperature will be monitored at two depths (5cm and 15cm). Temperatures will be sensed at 20m vertical distances over two slopes in the MCEW (figures 1 & 2), including a northerly facing clear-cut slope, and a fully forested control slope (<click> for map of experimental watershed). Thermochron iButtons have been shown to be appropriate for this type of hydrological research (Hubbart et al. 2005), and will thus be utilized for this work.
Conclusions
This work will lead to improved understanding of temperature inversion processes in the atmosphere and lithosphere in this complex mountainous region. It will also result in a better understanding of apparent seedling mortality in post-harvest environment of this region. Finally, this work will result in a simple empirical model describing temperature lapse rates, and soil temperatures dynamics in this region.

Primary Contacts: Jason Hubbart (web), Tim Link (web), Katy Kavanagh (web)

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