Publication Date

5-2010

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Hydrologic Sciences

Department

Geosciences

Major Advisor

Shawn G. Benner, Ph.D.

Abstract

This study investigated controls on soil water storage and its effect on vegetation cover in a semi-arid, mountainous environment characterized by warm-dry summers and wet-cold winters. Soil moisture and soil temperature were monitored over 286 days at eight sites spanning four elevations (approx. 1100, 1300, 1500, and 1800 m asl), and paired north and south exposures. These sites span an ecological gradient from grass and shrub land to conifer forest. Measurements of soil texture, soil depth, vegetation cover (normalized difference vegetation index, NDVI), and soil carbon content were made at the same sites. Variables that strongly influence the soil water distribution are topographically-driven and include: mean annual precipitation, which increases by a factor of 1.8, and mean annual temperature, which increases by a factor of 1.5, over the 700 m elevation increase; potential insolation, which is 1.5 to 1.9 times higher from north to south aspect, and by 1.1 to 1.4 times over the elevation gradient on north and south aspects, respectively; and soil depth, which is 1.1 to 2.3 times greater on a north aspect than south aspect at a given elevation, and is 1.4 to 2.3 times greater at higher relative to lower elevations on north and south aspects, respectively. North aspects store from 1.1 to 3.7 times as much water as south aspects at a given elevation, and higher elevations store up to 3 times more water than the lowest elevations at a given aspect; these trends are dictated by both higher average water content and deeper soils on higher elevations and on north facing slopes. Overall, soils are shallow, ranging from 34 to 92 cm deep and underlain by granodioritic bedrock. Due to the shallow profile and coarse texture of study area soils, 6 to 16 cm of water infiltrated into dry soil can exceed the storage capacity and may be lost to vertical or lateral redistribution in one to four weeks. Filling of the soil water reservoir, as indicated by whole-profile hydraulic connectivity and attainment of field capacity at the soil-bedrock interface, was observed at all sites in response to both winter and spring precipitation, with north aspects and higher elevations experiencing longer periods of deep wetting.

Vegetation cover is typically greater on north relative to south aspects, and generally increases with increasing elevation. Maximum (peak) seasonal NDVI values are reached as much as seven weeks earlier on south aspects at a given elevation, and as much as 12 weeks earlier at lower elevations compared to higher elevations. North-facing soils hold 3.5 to 4.2 times as much organic carbon as south-facing soils at all but the highest elevation forested sites, where the south and north aspect soil carbon contents were similar. Both vegetation cover and soil carbon content are largest at sites that retain moisture for a longer portion of the summer period, consistent with a water limited ecosystem. The duration of wet soil conditions during the summer, when vegetation production peaks, is strongly influenced by the magnitude and duration of spring and summer precipitation. These observations suggest that vegetation productivity and soil carbon storage in this environment will be particularly sensitive to climatic changes that alter spring and summer precipitation, which is delivered near, or during, the growing season. In contrast, the ecosystem may be less sensitive to changes in the magnitude of winter precipitation, which recharges the relatively small soil reservoir in the winter season and is generally lost from the soil early into the spring when productivity is depressed by lower temperatures and low insolation.

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