Simulation and statistical modelling approaches to investigate hydrologic regime transformations following Eastern hemlock decline

Abstract

Ecohydrological processes occurring at or near the Earth's surface are strongly influenced by Eastern hemlock [EH; Tsuga canadensis (L.) Carrière], a foundation tree species of eastern North American forests. EH populations are currently threatened by the invasive hemlock woolly adelgid (HWA; Adelges tsugae Annand). HWA HWA populations have been expanding rapidly throughout the EH's range. Catchmentscale research examining the hydrological consequences of HWA infestation is lacking, and plot-scale studies remain conflicted in their findings. Given the complex relationships between canopy interception, unsaturated and saturated groundwater storage, and root water uptake, it is not immediately clear how EH loss will affect the hydrologic cycle. We investigated the impact of EH mortality on stream discharge characteristics across a regional sample of catchments utilizing both simulation and statistical modelling approaches. We first examined the relationship between various catchment characteristics, including EH health, and three hydrological variables through regression analysis. We then employed a non-parametric statistical test to evaluate differences in hydrologic regime trends between non-infested and infested catchments. Finally, we calibrated a physically based hydrologic model and considered differences in optimal model parameter values and simulated overland runoff between non-infested and infested catchments. HWA presence modified several ecohydrological characteristics and precipitation partitioning between groundwater flows and surface runoff, potentially driving higher stream flashiness and overland flow, lower baseflow contributions and catchment storage, shorter flow-path lengths, and variable source area dilation at infested sites. Our results suggest that EH decline will augment flooding potential associated with the increasing frequency and intensity of Atlantic Basin tropical cyclone events. Further, our physically based simulation provides more determinate results than regression analysis, indicating that a purely statistical methodology, commonly utilized in studying the relationship between landcover characteristics and hydrologic regime, neglects dynamic physical ecohydrologic relationships.

Understanding Catchment‐Scale Forest Root Water Uptake Strategies Across the Continental United States Through Inverse Ecohydrological Modeling

“…The initial abstraction parameter, Se MIN , suggested greater abstractions in SR than DR catchments (Figure 2c; p value = 0.12) possibly indicating increased canopy interception. Several parameters controlling baseflow recession (A1 and B1) and surface runoff (C1, I a , and PI) suggest MS catchments generated less runoff and faster gravity driven baseflow return than SR (Figure 2), possibly indicating forest cover influences on infiltration and subsurface water transport as described in Filoso et al (2017) and Singh et al (2019).…”

“…We simulated daily discharge, soil moisture, snowpack water equivalent, and groundwater storage with a modified version of JoFlo, the lumped model of Archibald et al (2014). This model has previously been applied to evaluate broad patterns of surface runoff (Knighton, Pleiss, et al, 2019) and forest cover change (Singh et al, 2019) across CONUS, as well as in catchment-scale studies of stable water isotope dynamics (Knighton et al, 2017) and nutrient transport (Georgakakos et al, 2018).…”

Hammond Hill Research Catchment: Supporting hydrologic investigations of rooting zone and vegetation water dynamics under climate change

“…Measurements of isotopes in soils and xylem in the HH (Figure 3) and hydrologic modelling suggest that forest species composition change following a loss of hemlock and succession by hardwood tree species (e.g., beech) could lead to a drastically altered hydrologic state, characterised by reduced summer transpiration, increased groundwater storage, and elevated discharge extremes (Knighton, Conneely, & Walter, 2019). Regional observations of hemlock loss have yielded similar conclusions (Kim et al, 2017; Singh, Knighton, Whitmore, Walter, & Lassoie, 2020) indicating that continued monitoring is necessary to better understand the future hazards.…”

Hammond Hill Research Catchment: Supporting Hydrologic Investigations of Rooting Zone and Vegetation Water Dynamics under Climate Change

“…Regional observations of hemlock loss have yielded similar conclusions (e.g. Kim et al 2017; Singh et al, 2020) indicating that continued monitoring is necessary to better understand the future hazards.…”

“…The catchment has been monitored since 2017 for the purpose of understanding how hydrologic conditions will respond to warming air temperatures, changing precipitation patterns, and shifting forest ecology. Hydrologic observations within HH have helped us understand: how changes in forest tree composition could influence discharge regimes across the Northeast US (Knighton et al, 2019a), how ephemeral periods of thawing soils and snowmelt in spring might accelerate "old" soil water through the HH critical zone (Knighton et al, 2019b), and how we can better represent above-ground tree-water storage in ecohydrologic models (Knighton et al, 2019b;2020).…”