Characterizing ecologically relevant topoclimates across a small prairie watershed

Authors: Barnard, David; MACDONALD, JACOB - Colorado State University; Erskine, Robert - Rob; Green, Timothy; Mahood, Adam

Submitted to: Ecological Society of America (ESA)
Publication Type: Abstract Only
Publication Acceptance Date: 4/21/2023
Publication Date: N/A
Citation: N/A

Interpretive Summary: As climate change increases temperatures and aridity, it is increasingly important to characterize variability in temperature and humidity at smaller scales. While much work has focused on the impact of topography in mountainous terrain, little attention has been paid to less topographically complex areas such as prairies and croplands. In this study, we deployed a network of temperature (T) and relative humidity (RH) sensors across a small prairie site (0.56 km2, < 30 m of topographic relief) to record variability over time and across the field. These measurements were coupled with a vertical gradient of sensors to understand relationships between vertical and horizontal variation in RH and T and atmospheric stability as defined by temperature gradients with height. We found substantial differences in both RH and T across the field, with maximum differences exceeding 10 degrees C and 40% RH. Most of these events were the result of cold air draining from high to low elevations that occurred most commonly after sunset and in the months Sep-Nov. These cold air drainages resulted in lower elevation areas accumulating 5% fewer growing degree days on average than higher areas. Data were further analyzed with principal components analysis to assess environmental factors driving atmospheric stability and the formation of inversion, neutral, or unstable conditions. The analyses revealed a strong impact on wind speed, and solar radiation in driving variability in RH and T over time and across the field. These results have wide relevance to ecological, hydrological, and meteorologic 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.

Technical 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 < 30 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 5% 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 wide relevance to ecological, hydrological, and meteorologic 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.