Soil Moisture in the Alpine and Sub-Alpine on Niwot Ridge, Colorado

Transcription

1 Soil Moisture in the Alpine and Sub-Alpine on Niwot Ridge, Colorado Mark Losleben (1), Kurt Chowanski (1), Timothy Bardsley(2) (1 ) Mountain Research Station, University of Colorado, (2)National Resources Conservation Service Snow Survey, Salt Lake City, Utah Introduction Soil moisture conditions are key to a wide range of conditions critical to ecosystems and human populations everywhere and certainly in the Western United States. These conditions include reservoir storage, streamflow, fire frequency and intensity, and controls on vegetation type and density to name a few. This poster presents daily field soil moisture data for two and one half years, and coincident soil temperature and precipitation events, at three locations on Niwot Ridge, Front Range, Colorado; two in the alpine tundra, and one in the sub-alpine forest below. We also present laboratory test results, and explore standardization as a technique to compensate for the instrumental artifact shift between frozen and unfrozen soil states. Scope Of Study This poster presents volumetric soil moisture data from three field sites on Niwot Ridge, Colorado, a laboratory test, and standardization schemes. The volumetric soil moisture values are daily means based on hourly measurements, and the period of study is from July 2002 to January The three sites, C1, Saddle, and D1 are shown in figure 1 below. Laboratory tests demonstrate the shift in measurement output between frozen and thawed soils. Over the course of several days in the lab, hourly soil moisture measurements were taken on soil, with a known soil moisture, as the soil temperature ranged from -15 C to 16 C. Methods

2 Volumetric soil moisture measurements for the field sites are made using Campbell Scientific CS-615 Water Content Reflectometers, and recorded on Campbell Scientific data loggers. These probes derive volumetric soil moisture by measuring the relative permittivity, which is a frequency dependent response of homogeneous matter to excitation by a time harmonic electric field. The laboratory tests, performed by the National Resources Conservation Service (NRCS), measured soil moisture with the Stevens vitel probe. Soil temperature measurements are made using Campbell Scientific CS-107 Soil Temperature Probe. Precipitation measurements are made using a Belfort Weighing Bucket.

3 Figure 2. Volumetric Soil Moisture shown with Precipitation Events greater than 1 mm, and Soil Temperature, at the three field sites, C1, Saddle, and D1. As the soil freezes, measured soil moisture decreases dramatically, an effect of the changing permittivity factor between water (80) and ice (4.5). Figure 3. Laboratory test by the NRCS. Volumetric Soil Moisture plotted against Temperature. As the soil is frozen, the measured soil moisture drops; and as the soil is thawed, the measured soil moisture returns to its actual level, reflecting the effect of state on the instrument output.

4 Figure 4. Volumetric Soil Moisture shown at the three field sites, C1, Saddle, and D1 using two standardizations of the data. Standardizing the data may help to directly compare frozen and unfrozen soil moisture conditions. Standardized 0 uses 0 degrees C as the demarcation between frozen and unfrozen soils; Standardized 2 uses +-2 degrees C. The standardizations show fewer large variations in the spring summer and fall, and more smaller variations in the winter. Figure 5. Volumetric Soil Moisture plotted against two standardizations (the same as in Figure 4) in several days of testing in the lab. Both standardizations have asymptotic tendencies as the

5 soil freezes and thaws, however both standardizations make the frozen and unfrozen soils comparable. Results & Discussion Figure 2 shows average volumetric soil moisture, precipitation events greater than 1 mm, and average soil temperature for C1, Saddle, and D1. At all three sites soil moisture rises during snowmelt, and tapers off throughout the summer. The greatest changes in soil moisture occur in the spring, summer, and fall, with small changes in the winter. Soil moisture is highest in the spring as snow melts and percolates into the soil, and tapers in the summer and fall. Several peaks in soil moisture are evident in the summer and fall resulting from seasonal precipitation events. A large drop in measured soil moisture occurs as the soil freezes reflecting the change in the relative permittivity from 80 in water to 4.5 in ice. This shift is illustrated by the laboratory tests (figure 3) where the reported soil moisture drops as the soil freezes, and rises as the soil thaws, even though the soil moisture remains unchanged. The winter soil moisture shows some smaller variations, which standardization seems to magnify at the C1 and SDL sites, and reduce at the D1 site (figure 4). In the field measurements and the lab test (figure 5), the standardization using +-2 degrees C created larger variations and produced a more distinct asymptote at the freezing point than the standardization using 0 degrees C. Concern over frozen soil moisture conditions is valid because it has been found that moisture is quite mobile even in frozen soils (Robock, Alan, C., Williams & Smith). Frozen soils with temperatures well below 0 deg C have small amounts of mobile liquid water in capillaries and films on soil particles. Temperature gradients cause water to flow towards lower temperatures, and to form ice lenses there. In unsaturated soils, like the three field sites presented here, vapor diffusion also transports moisture (Williams & Smith, 1989). Amount of snow cover, solar radiation, and air temperature effect the near surface soil temperature. This can drive soil moisture up during cold spells, and down as the soils warm a little. The soil moisture of late winter may factor into the rate of water uptake of the soil compared to the amount of runoff. Future Work The present study area can be expanded to include additional field sites of the NRCS. Further calibration work, including a greater range of soil moistures, needs to be done at the three field sites in this study. Installation of new probes that provide a soil moisture profile from the surface to two meters may provide greater insight into moisture transport in frozen and unfrozen soils. More controlled lab tests using identical equipment need to be conducted to further compare the soil moisture of frozen and unfrozen soils, as well as the partially frozen soils. References Robock, Alan, C. Adam Schlosser, Konstatin Ya. Vinnikov, Suxia liu, and Nina A. Speranskaya. 1995: Validation of humidity, moisture fluxes, and soil moisture in GCMS: Report of AMIP Diagnostic Subproject 11, Part 1 - Soil moisture. Proceedings of the First International AMIP Scientific Conference. Williams, Peter. Smith, Michael. The Frozen Earth: Fundamentals of Geocryology. Chapter 8. Cambridge University Press