Drought Variability over the Conterminous United States for the past century

Abstract:

We examine drought variability over the Conterminous United States (CONUS) over the period 1915-2018 using the Noah-MP land-surface model. In particular. we examine the effect of different model options on drought reconstructions, with a particular focus on optional representation of groundwater and dynamic vegetation phenology in the model. Over our 104-year reconstruction period, we identify 12 Great Droughts that each covered at least 36% of the area of the CONUS and lasted for at least 5 months. The Great Droughts tend to have smaller areas when groundwater and/or dynamic vegetation are included in the model configuration. We detect a small decreasing trend in dry area coverage over CONUS over the past century in all configurations, consistent with previous studies. Groundwater tends to increase the duration of the great droughts, primarily by leading to earlier drought onset (associated with short-lived recovery from a previous drought) or later demise (groundwater anomalies lag precipitation anomalies). In contrast, dynamic vegetation tends to shorten drought duration, primarily by earlier drought demise (water loss from ET is reduced during droughts due to closed stomata or dead vegetation). We also examine drought characteristics in six subregions. We find that the U.S. Southwest (Southeast) has the longest (shortest) drought durations. Dry area coverage in all subregions except the Southwest showed decreasing trends over our study period. The effects of groundwater and dynamic vegetation vary in subregions due to differences in groundwater depths (hence connectivity with the surface) and vegetation types. We examine the moisture deficits in different soil layers and find that dynamic vegetation has a greater influence on soil moisture in root zones where soil and vegetation most actively interact.