COS 284-5 - Characterizing ecologically relevant topoclimates across a small prairie watershed

Abstract: Increasing temperatures and aridity driven by climate change are shifting functional patterns of vegetation and intensifying drought stress. While broader climate patterns can represent regional scale dynamics, finer scale measurements near the plant-atmosphere interface and resulting from topographic variability are needed to characterize vegetation responses to key meteorological forcings such as air temperature, relative humidity, and vapor pressure deficit. Topoclimate gradients in complex terrain have been relatively well-studied, but prairie and cropland areas with lower topographic complexity have received less attention. In this study, we deployed a spatially extensive network of relative humidity and air temperature (RHT) sensors across a 0.56 km² watershed with 29 m of topographic relief in northern Colorado. Continuous data were collected for three years to characterize spatiotemporal variation in topoclimates and assess co-occurring meteorological factors that drive variability across the field and along a 10 m vertical gradient. We observed substantial variability in RHT across the field, with maximum differences between warmest and coldest sensor exceeding 10º C, and relative humidity differences of > 40%, with the more humid and cooler measurements commonly being recorded at the lowest point in the watershed and the driest and warmest being recorded at the highest. The cold air drainage events and inversions that led to this stratification of air masses were most prevalent immediately after sundown and primarily in Sep-Nov, whereas neutral and unstable conditions were more commonly observed during the daytime and throughout the year. When summed across the growing season these temperature differences resulted in a 4% reduction in growing degree days in the lower elevation areas compared to higher areas. A principal components analysis, stratified by atmospheric stability (i.e. inversion, neutral, unstable) revealed important roles of solar radiation, mean wind speed, and aerodynamic profile in explaining within-field variance in RHT. These results have broad relevance to ecological, hydrological, and meteorology fields by improving representations of surface fluxes, leading to better estimations of vegetation responses to climate change, and spatial variability in crop production within fields.