Methodology to quantify the role of intense precipitation runoff in soil moisture scarcity: a case study in the U.S. South from 1980-2020

The northern U.S. Gulf Coast is among the wettest regions in the contiguous United States, with a transition zone from humid to semi-arid climates occurring between the western Gulf Coast and the 100 th meridian. As anthropogenic warming induces more frequent extreme wetting events of greater magnitude, a larger proportion of rainfall runs off unsaturated soils rather than being absorbed and replenishing vegetative water supply. This study introduced novel methodology reliant on reconstructed hourly precipitation intensity data from locations with comprehensive records from the past four decades, incorporating these records into a recursive algorithm measuring daily soil moisture levels. To account for runoff, curtailment multipliers for three different soil classes at each site were applied to 24-hour precipitation totals. Soil moisture balance was then obtained from daily evapotranspiration and infiltrated precipitation, and trends from the autoregressive time series modeling were compared. When runoff quantified by the methodology was considered, average annual soil moisture scarcity trends accelerated for most sample soils, including 13 of the 15 highly-infiltrative soils showing a change relative to the unrestricted infiltration in the reference case. The findings, however, were generally not statistically significant. These results are suggestive, but not conclusive, of a growing role from intense precipitation in drought development for the selected region. The seasonality of evolving rainfall rates in the case study area may explain the limited impact, as intensity rates are growing most quickly during the wintertime, a period when episodes infrequently exceed maximum soil infiltration capacity. The methods introduced here, achieving superior accuracy at precise locations relative to gridded products, are reproducible for global locations with adequate data coverage.