The utilization of a recursive algorithm to determine trends of soil moisture deficits in the Mid-Atlantic United States

Climate change models project declining soil moisture levels across the continental US, even in regions with expected increases in annual average precipitation. Although warmer air has been shown to result in more frequent and severe precipitation events, higher vapor pressure deficits are anticipated to result in evapotranspiration rates that exceed the amount of water hitting the soil surface. Numerous analyses have shown rising degrees of aridity in many US locations, even without declining rainfall. The Mid-Atlantic region has less annual rainfall variability and seasonality relative to other areas, and an analysis is presented here examining trends from 1980 to 2019 to determine if the region has become more arid as temperatures have warmed. A comparison of evapotranspiration and precipitation trends cannot adequately answer this question, as the timing of rainfall and soil moisture levels determines how much water is absorbed into the ground and utilized by vegetation. A recursive algorithm is developed to calculate water deficits based on the previous day’s conditions and amount of precipitation received during that day, excluding rainwater falling on saturated soils from being eligible to recharge the water supply available to vegetation. Trends from the algorithm show that soil moisture levels have increased over the past four decades. This equates to lower overall water demand per hectare in the municipal and agricultural sectors. While the trends may change in coming decades, unlike other US regions, the Mid-Atlantic has become wetter as defined within an agricultural and meteorological context.