Thermal and Hydrologic Attributes of Rock
Glaciers and Periglacial Talus Landforms;
Sierra Nevada, California, USA
Constance Millar, Robert Westfall, and Diane Delany
Fig 1. Rock Glaciers: i‐Button locations LL = Left Lateral; RL = Right Lateral, SN = Snout; ST = Stream
. USDA Forest Service, Pacific Southwest Research Station, Albany, California, 94710 USA
Contact: cmillar@fs.fed.us
Fig 2. Taluses: i‐Button locations L = Low; M = Mid; H = High; T = Tree; F = Forefield
INTRODUCTION
GOALS
Rock glaciers and talus landforms are increasingly a focus of mountain hydrology and periglacial process study. Ice contained in rock glaciers is often long‐persistent and insulated by rock debris, while taluses have high water‐holding capacity, but likely lack persistent ice . While poorly understood, thermal regimes and heat‐transfer processes for these features appear to be at least partially decoupled from external free‐air regimes, with the effect of decreasing the permafrost elevation by as much as 1000 m (rock glaciers) and supporting wetland oases in their forefields (taluses). These features and their thermal processes have been little‐studied in semi‐arid mountains and temperate zones such as the Sierra Nevada, where diminishing annual snowpacks with regional air temperature rises present challenges for mountain flora and fauna as well as for downslope water delivery.
Improve understanding of thermal regimes and heat‐transfer processes in rock glaciers and taluses of temperate, semi‐arid mountains by investigating daily, seasonal, annual, and interannual patterns of temperature at landform surfaces and internal matrices, and by analyzing these relative to adjacent ground‐ and free‐air temperatures.
Fig 3. Map of Study Sites: 8 Rock Glaciers and 8 Talus Fields. Talus fields were divided as low and high elevations; N‐ and S‐facing aspects; granitic and metamorphic substrates.
METHODS
General: We deployed Maxim iButton thermochrons, programmed to record at 4‐hr intervals, on landform surfaces, rock matrices (1 m deep), ground‐level (forefields) and free‐air (3 m height).
Rock Glaciers (Fig 1): For each rock glacier, pairs of iButtons, located on the surface and in the matrix, were placed at Left‐Lateral, Right‐Lateral, and Snout positions. Pairs of iButtons were also positioned in the outlet Stream and adjacent Air at 1 m height. Talus Fields (Fig 2) For each talus, pairs of iButtons, located on the surface and in the matrix, were placed along 2‐4 transects up‐talus (at Low, Mid, High positions), at ground‐level (Forefield position) below each transect, and in 1‐2 free‐air positions (Tree position).
RESULTS
Rock Glaciers
Outlet Streams vs. Air (Fig 4): Outlet stream water temperatures were consistently near 0°C year‐
round (0‐3°C in late spring‐fall, 0°C in winter) whereas air tempera‐
tures had high diurnal fluctuations in late spring‐fall, and stayed near 0°C in winter. This suggests outlet streams did not dessicate, but flowed year‐round, as water in the warm season and frozen in winter when snowpack covered stream and air positions.
Surface vs. Matrix (Fig 5): Annual and warm‐season mean matrix temperatures were signifi‐
cantly colder than surface positions. Warm‐season surface air temper‐
atures had high variance, fluctuating from 6°C‐18°C; matrix variances were significantly less. Transitions from warm‐ to cold‐seasons were abrupt and characterized by rapid drop of daily temperatures below 0°C. The relative order switched in winter, with matrix average temperatures warmer than surface. At all sites and positions a “zero‐curtain” (0°C) occurred in spring and persisted until snowmelt.
Thermal Relations of Surface to Matrix (Fig 6): Resistances (lag) of the matrices to changing surface temperatures during the warm season were described by 3rd
order sigmoid relationships (Fig 6a); the heat‐transfer process was better rendered by an autoregressive model that removed auto‐correlations (Fig 6b, numbers indicate progress of the season). During the snow‐free season, matrix temperature lagged air temperature changes by 2‐3 days both for warming and cooling, in approximate weekly cycles. By late fall, even when daily surface temperatures rose, matrix temperatures remained stably cold. Winter showed no pattern of resistance between surface and matrix positions.
REFERENCES (talus results in prep)
Millar, CI, RD Westfall, DL Delany. 2012. Thermal and hydrologic attributes of rock glaciers and periglacial talus landforms; Sierra Nevada, California, USA. Quaternary International. Preprint published online.
Millar, CI, RD Westfall. 2010. Distribution and climatic relationships of the American pika (Ochotona princeps) in the Sierra Nevada and western Great Basin, USA; Periglacial landforms as refugia in warming climates. Arctic, Antarctic, and Alpine Researh, 42: 76‐88
Talus Fields
Talus Surface vs. Matrix (Fig 7): Patterns were highly consistent for all 8 taluses and similar to trends in rocks glaciers. In the warm season, matrix temperatures were significantly cooler than surfaces with lower daily variances. Forefield mean temperatures were similar to talus surfaces but had greater extremes. Differences (means, variances) between surface and matrix positions were greater for low‐ vs. high‐elevation taluses, for metamorphic vs. granitic substrates, and for south‐ vs. north‐facing sites; low elevation‐ and metamorphic‐ sites had respectively warmer means. Low positions in talus were colder than mid or high positions. Resistance to temperature change and lags between matrix and surface were similar to rock glaciers.
In winter, relative order of temperatures changed among positions, with matrices warmer than surface temperatures, although fluctuating less; forefields were warmest, and stayed near 0°C at sites and years where there was consistent snowpack. Differences among years were apparent: the very light snowpack of winter 2011‐2012 showed effects of exposure not experienced in prior wetter years. The “zero curtains” observed in rock glaciers were not as apparent in taluses (Figs 7, 8).
Talus Surface vs. Free Air (Trees; Fig 8): Annual mean temperatures differed significantly between tree and talus surface
positions. During the warm season, talus surfaces had higher extremes than trees, a trend that grew more pronounced as the season progressed. Winters showed the effect of snowpack, with talus surfaces near 0°C during years and times when snow‐covered. Low‐elevation sites, especially in winter 2011‐
2012, documented the lack of snow and short winter season compared to the long winter duration the prior year.
IMPLICATIONS
Thermal regimes are complex but similar in pattern for rock glaciers and taluses, with seasonal, elevation, aspect, position in talus, and substrate effects. Convection and conduction appear to operate; a zero‐curtain condition develops in spring. Thermal regimes appear partially decoupled from external air, especially in summer. Cool, buffered conditions of internal matrices during the warm season likely contribute to ground‐water storage, persistent seepage, and wetland forefields and lakes. As such these landforms provide important hydrologic reserves and habitat for cool‐dependent flora and fauna as climates warm.