Climate change is intensifying the hydrologic cycle globally, resulting in more extreme weather events, including an increase in large precipitation events, or deluges. Across semi-arid regions of the Great Plains, deluges are often patchy, with some locations receiving substantially more rainfall than others. In the shortgrass steppe of eastern Colorado, controlled field experiments have determined that the dynamics of canopy phenology (green-up magnitude and duration) are positively and linearly correlated with the deluge event size. But whether these relationships extend to naturally occurring deluges is unknown. We used remotely sensed estimates of canopy greenness and a network of rain gauges to determine if similar patterns between deluge magnitude and canopy phenology exists at the landscape scale across soil and topographic gradients.
We identified a number of large isolated summer rainstorms (deluges) between 1985-2012 from a network of 30 rain gauges located across the Central Plains Experimental Range (CPER), a 62.7 km2 shortgrass steppe ecosystem. We then extracted Landsat 5 imagery from dates immediately before and after these deluge events and assessed the relationship between rainfall amount and responses in the normalized difference vegetation index (NDVI) as a measure of canopy greenness.
Results/Conclusions
Immediately following most deluges, there was substantial green-up in the shortgrass steppe, with maximum greenness typically occurring 2-3 weeks after the event. Moreover, green-up magnitude (peak NDVI) was positively and linearly correlated with the amount of rainfall received at a specific location. In fact, the spatial distribution of rainfall amount accounted for up to 85% of the variability in canopy greenness across the CPER following a deluge event. Seasonally, middle and late summer deluges were more strongly correlated with canopy phenological responses than early summer deluges. This is likely due to high soil moisture and the dominance of cool-season grasses early in the growing season. Some mid-growing season deluges resulted in stronger phenological responses than others, possibly due to differing antecedent conditions. We conclude that a single deluge event can have a substantial impact on NDVI, and therefore on localized plant production in the shortgrass steppe. Thus, results from small-scale, controlled experiments focused on deluge magnitude can be scaled up across topo-edaphic gradients to provide useful insight into the controls on the spatial heterogeneity of productivity in the shortgrass steppe.