TOPOCLIMATIC CONTROLS ON HILLSLOPE ENVIRONMENTS: IMPLICATIONS FOR BEDROCK WEATHERINGAND LANDSCAPE EVOLUTION AT WALLULA GAP, WA

This study focuses on characterizing hillslope weathering environments to better understand what processes are important controls on hillslope morphology. Spring Gulch is an east-west trending ephemeral stream located in the Horse Heaven Hills of southeastern Washington state. The nearest weather recording station, 14 km to the west and 463 m ASL (WSU AgWeatherNet station, Eby, WA) receives 245 mm of mean annual precipitation and the mean annual temperature of 10.9°C. There is a pronounced dry season from July to September. We collected internal and external rock temperature and humidity measurements, thin sections, rock strength measurements, and thermal imagery (collected by UAV). We installed iButton sensors to record hourly temperature and humidity starting in September 2020. We selected two outcrops of basalt (from the same flow), one on each aspect, and installed 7 sensors. We installed sensors at the surface, 5 cm, and 15cm within the rock. At all depths on both north and south aspects there are diurnal cycles of temperature and relative humidity, however, the diurnal range decreases with depth. Over the past two years, there is greater moisture content on north-facing slopes and moisture content increases with depth. Additionally, northern aspects remained cooler relative to southern aspects. The rocks on both aspects, at all depths, are fully saturated in the late fall through spring and dry out to differing amounts during the summer. Despite the greater humidity in the winter, the total vapor pressure (an important control on rates of subcritical cracking) is higher during the summer months due to the combined effect of high temperature and humidity. The greater humidity on the north aspect leads to higher vapor pressures relative to southern aspects. Rock thin sections and rock strength tests indicate a greater amount of weathering that has resulted in weaker rock on the north aspects.