Modeling the Effect of Changing Precipitation Inputs on Deep Soil Water Utilization in a Southeast US Loblolly Pine Plantation.

Forests in the southeastern U.S. are predicted to experience future changes in seasonal patterns of precipitation inputs as well as more variable precipitation events. These climate change induced alterations could increase drought and lower water availability. Drought could also stimulate changes in plants to grow deeper roots to access subsurface water resources. Evapotranspiration and soil water storage are linked with precipitation and discharge. To address plant response to drought in both deep rooting and soil water utilization as well as soil drainage, we utilize a throughfall exclusion experiment in a Pinus taeda L. plantation of the southeast US to calibrate and validate a hydrological model. The model was accurately calibrated using 914 days of data under ambient rainfall (R2=0.84 and RMSE = 0.04) and validated using 30% throughfall reduction data (R2=0.67 and RMSE = 0.04). We then test these scenarios: (1) evenly reduced precipitation; (2) less precipitation in summer, more in winter; (3) same amount of precipitation with less frequency, but heavier storms; and (4) shallower rooting depth under the above three scenarios. When less precipitation was received, bottom flux decreased much faster than evapotranspiration. When precipitation was reduced more than 30%, plants relied on soil water to satisfy evapotranspirational demand. Under the scenarios of seasonal precipitation redistribution and heavy storms, both evapotranspiration and bottom flux decreased, while surface runoff increased. Although root biomass measured both before and 4 years after the treatment had no significant difference between treatments, there is additional gain in evapotranspiration with deeper roots under evenly reduced precipitation and seasonal precipitation redistribution scenarios, but not when precipitation frequency was adjusted. The deep soil provides an important buffer capacity when precipitation alone cannot satisfy the evapotranspirational demand.