Using Opposing Slope Aspects to Understand Water and Energy Flow Controls on Critical Zone Architecture

Abstract:

A long-standing problem is to understand how climate controls the structure of the critical zone, including the depth of weathering, thickness and character of soils, and morphology of hillslopes. We exploit microclimates on opposing aspects in a watershed in the Boulder Creek CZO to investigate the role of water and energy fluxes on development of critical zone architectures.

The 2.6 km² Gordon Gulch, located at ~2500 m a.s.l. at 40°N latitude, is elongated east-west, and consequently is predominantly composed of north and south-facing soil-mantled slopes, dotted with tors, developed on Precambrian gneiss. The depth to fresh rock ranges from about 8 to 12 m, and is up to 2 m deeper on north-facing slopes. In addition to greater thickness, weathered rock is measurably lower in tensile strength on north-facing slopes. While characteristics of weathered rock vary with aspect, the overlying mobile regolith is relatively uniform in thickness at ~0.5 m across the catchment, and its mineralogy shows only minor chemical alteration from parent rock.

These features of the critical zone architecture arise in the face of systematic differences in energy and water delivery by aspect. About 40-50% of the ~500 mm annual precipitation is delivered as snow. During spring, the south-facing slopes receive up to 50% greater direct solar radiation than the north-facing slopes. Consequently, snow cover is ephemeral in the open Ponderosa forests on south-facing slopes, and soil wetting and drying events are frequent. Frost penetration is shallow, and short lived. On north-facing slopes, less direct radiation and a dense Lodgepole pine forest cover leads to snowpack retention. Soils are colder and soil moisture stays elevated for long periods in spring on these slopes. We postulate that deeper and more sustained frost penetration on north-facing slopes enhances the damage rate by frost cracking. Deeper water delivery further aids this process, and supports chemical alteration processes. The uniformity of mobile regolith depths suggests equal mobility on these slopes despite differing conditions.