Date

2019

Department or Program

Geosciences

Additional Department or Program (if any)

Biological Sciences

Primary Wellesley Thesis Advisor

Jackie Hatala Matthes

Additional Advisor(s)

Dan Brabander

Additional Advisor

Alden Griffith

Additional Advisor

Thomas Hodge

Abstract

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. The gypsy moth is a forest insect whose larvae consume leaves of broad-leaved trees. Outbreaks of this insect cause regional decrease in leaf area, which is related to rates of evapotranspiration. This study directly compares seasonal stream discharge during the 2015-2017 gypsy moth outbreak and defoliation to periods of non-defoliation. I hypothesized that decreased evapotranspiration associated with reduced leaf area from would increase flow intensity and discharge produced in proportion to defoliation. To test these hypotheses, this research integrated remotely sensed imagery of gypsy moth defoliation severity with data from USGS stream gages and Daymet precipitation data to understand and quantify the impact of the 2015-2018 gypsy moth outbreak on water resources in southern New England.

Results/Conclusions

I found the intensity of defoliation varied greatly annually and over the southern New England hydrologic landscape. Additionally, I found that there was a strong association between increased defoliation and an increased proportion of precipitation exiting a watershed as discharge. By definition of the water balance equation, this increased discharge supports changes in evapotranspiration associated with defoliation are measurable at downstream locations. The magnitude of discharge increase was most apparent at normal and low flow conditions. Discharge increase associated with defoliation is a consistent increase in the amount of water measured at the stream gage. In the context of broader literature, the intense defoliation of 2016-2018 raises important questions for the stability of the forest community and spread of gypsy moth. Increased discharge rates similarly alter the sedimentation and nutrient loading in a watershed. This study recommends future work to focus on how discharge, sedimentation, and nutrient flux are altered by a disturbance, particularly in areas newly affected by gypsy moth outbreaks.

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