Microclimatic Influences on Hillslope Geomorphology, Northeastern Arizona

Climate is commonly assumed to be an important factor influencing weathering, erosion and landscape evolution, but few studies have quantitatively assessed differences in geomorphology associated with minor but persistently different moisture and temperature conditions. In northeastern Arizona, we have studied several small ( ∼0.5 km²), semi-arid eastward-draining basins where topography has induced different microclimates on north- and south-facing canyon slopes. These 60-100 m deep canyons expose lenticular silty sandstones and mudstones of the Jurassic Morrison formation; sandstones, shales, and coals of the Cretaceous Dakota formation; and shales of the Mancos formation. Over one annual cycle (beginning 17 August 2002), we monitored air, surface, and subsurface temperatures as well as soil moisture at four locations at 0.5 h intervals. The annual average air temperature (at ∼10 cm above the surface) was 1.3° C warmer on a south-facing colluvial slope and 2.1° C warmer on a south-facing exposure of weathered sandstone than on north-facing slopes of the same type. Annual average surface temperature was 3.0-3.8° C warmer on south-facing slopes and the temperature at ∼10 cm depth in the sandstone was 5.7° C warmer on the south-facing slope. Soil moisture tension averaged at least 78-100 kPa lower on north-facing slopes at 10 cm depth but only 36 kPa lower at 30 cm depth. Morrison formation sandstones weather primarily by granular disintegration and form a disaggregated mantle on some slopes where weathering exceeds erosion, but also form little-weathered vertical cliffs (dominantly on southerly aspects). Rock strength was estimated at over 400 sites using a Schmidt hammer. Slope angle and aspect, as well as vegetation type and % cover were also recorded at each location. Vegetation covers about 4 times as much area on northeast-facing slopes as on southwest facing slopes. Young's modulus values derived from Schmidt hammer rebound for southwest-facing points averaged 3.1 GPa, whereas northeast-facing slopes averaged 1.3 GPa indicating significantly greater weathering on north-facing slopes. Detailed mapping of weathered, unweathered, and debris-covered slopes shows that weathered exposures cover about half of the north-facing slopes and a third of south-facing slopes. Unweathered slopes comprise only 9-12% of north-facing slopes but 25-30% of south-facing slopes. Slope angles on southeast-facing slopes average 47° whereas northeast-facing slopes average 36° . The lower-angle slopes (<20° ) are dominated by weathered bedrock exposure and minimal erosion whereas on steeper slopes (>40° ) the weathered mantle has been stripped off leaving unweathered bedrock. Debris covers are most common on slopes between 20° and 40° and represent areas that are dominated by transport processes. These trends in slope cover, rock weathering, and slope angle suggest that over longer timescales, subtle temperature and moisture differences are sufficient to produce significant aspect-related changes in hillslope geomorphology.