PS 26-63 - Towards biophysically-informed indicators of impending drought

Background/Question/Methods

Drought, one of the most expensive and environmentally damaging natural disasters on the planet, is projected to increase in both intensity and duration by 2050 in the United States. While current drought metrics are well-equipped to detect drought once it is under way, they do not detect drought at spatial and temporal scales conducive to early detection and mitigation. Biophysical plant-climate interactions, particularly evapotranspiration (ET) and its drivers, change quickly in response to evolving drought conditions. One promising early-warning signal of drought is changes in ET anomalies that indicate the sudden onset of plant stress.



Here, we explore temporal decoupling between plant processes and hydrometeorological conditions on diurnal scales as an indicator of current or impending physiological plant stress. During water stress, the timing of peak ET, a key ecosystem variable controlled by plant transpiration and the evaporation of water, and peak vapor pressure deficit (VPD), an indicator of moisture saturation of the air, becomes decoupled. Concurrently, peak ET occurs earlier in the day, likely attributed to stomatal responses to reduce transpiration. Therefore, we propose the decoupling of ET and VPD may represent physiological plant stress and has the potential to be utilized as an indicator of early drought conditions.

Results/Conclusions

We utilized eddy covariance sites across the Midwest United States to assess the temporal decoupling between ET and its drivers as an indicator of drought conditions. Using anomalously low soil water content as a proxy for dry conditions, we uncovered dry periods during the growing season in each time series. During many of these dry periods, we find that the timing of peak ET shifts to earlier in the day, temporally decoupling from peak VPD and peak solar radiation. Further, this decoupling coincides with a diurnal shift to earlier peak gross primary productivity (GPP). Therefore, we infer the concurrence of earlier diurnal shifts in peak ET and GPP may reflect stomatal behavior during drought conditions. The temporal decoupling signal between peak ET and its drivers as an indicator of drought conditions was present in both mesic forest sites and agricultural sites. We will also discuss the potential for spaceborne remote sensing to leverage diurnal temporal decoupling as a signal of drought conditions, specifically from the Moderate Resolution Imaging Spectroradiometer (MODIS) morning and afternoon overpasses.