Responses of ecosystem hydrology to gypsy moth defoliation in streams and watersheds

Background/Question/Methods The quality and quantity of water resources in the Northeastern United States are reliant upon forested watersheds. In this area, water resources are sourced from shallow aquifers and groundwater storage, which are both closely tied to surface ecosystems. Rates of evapotranspiration are specifically determined by environmental conditions and plant traits of a particular ecosystem. The interconnected nature of water resources to surfaces processes in Southern New England makes understanding interactions between ecosystem disturbance and hydrology particularly important. This study directly compares seasonal water yield during the 2015-2018 gypsy moth outbreak and defoliation to periods of non-defoliation. We hypothesize that decreased evapotranspiration associated with reduced leaf area from defoliation increases water yield and alters flow intensity. We also investigated whether changes persist beyond initial defoliation periods. To test these hypotheses, this research integrated remotely sensed imagery of gypsy moth defoliation severity with data from USGS stream gages to understand and quantify the impact of the 2015-2018 gypsy moth outbreak on water resources in southern New England. Results/Conclusions We found the intensity of defoliation was highly variable by watershed. Watersheds that were highly defoliated had significantly higher discharge, when normalized by precipitation and quantified by calculating the water balance of the system These alterations are associated with a negative change in evapotranspiration. To quantify alterations in the flow regime of the steam itself we investigated whether flow intensities were different in watersheds that were defoliated. We assessed these changes using a flow-duration curve analysis comparing flow duration curves in defoliated and non-defoliated periods to baseline flow values. We conclude that increases in discharge associated with defoliation are most apparent at normal and low-flow values and significantly alter discharge during a defoliated growing season.