Drought Variability over the Conterminous United States for the Past Century.

We examine the drought variability over the conterminousUnited States (CONUS) for 1915-2018 using theNoah-MP land surfacemodel. We examine different model options on drought reconstruction, including optional representation of groundwater and dynamic vegetation phenology. Over our 104-yr reconstruction period, we identify 12 great droughts that each covered at least36% of 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 in all configurations. We identify 45 major droughts in the baseline (with a dry area coverage greater than 23.6% of CONUS) that are, on average, somewhat less severe than great droughts. We find that representation of groundwater tends to increase drought duration for both great andmajor droughts, primarily by leading to earlier drought onset (some due to short-lived recovery froma previous drought) or later demise (groundwater anomalies lag precipitation anomalies). In contrast, representation of dynamic vegetation tends to shortenmajor droughts duration, primarily due to earlier drought demise (closed stoma or dead vegetation reducesET loss during droughts). On a regional basis, the U.S. Southwest (Southeast) has the longest (shortest)major drought durations. Consistent with earlier work, dry area coverage in all subregions except the Southwest has decreased. The effects of groundwater and dynamic vegetation vary regionally due to differences in groundwater depths (hence connectivity with the surface) and vegetation types. [ABSTRACT FROM AUTHOR]