Comparison of temporal dependence between phreatophyte health and hydrometeorological variables for three groundwater dependent ecosystems

Groundwater dependent ecosystems (GDEs) exhibit complex spatiotemporal dependency on multiple variables including precipitation, surface and soil water, and groundwater availability. While spatial relationships have been explored in many previous studies, the temporal lag between GDE vegetation health and hydrology is not well understood across a scale of anthropogenic influence. Data from three groundwater dependent ecosystems (Arkansas River in Colorado, USA; South Platte River in Colorado, USA; and the San Pedro River in Arizona, USA), each differing in magnitude of anthropogenic encroachment, are collected and analyzed to discern which hydrologic variables have the strongest correlation in time with phreatophyte health (e.g., Populus fremontii and Populus deltoides). Phreatophyte health serves as a surrogate for overall GDE health. The response variable used to estimate GDE health is a daily time series (2016-2023) of plant health that is quantified by computing the normalized difference vegetation index (NDVI) using Planet Imagery (3-m spatial resolution). The covariates are groundwater depth (meters below land surface), discharge (cubic meter per second), precipitation (mm) and temperature (deg. C). We examine the temporal dependence between the response variable and each covariate by first pre-whitening each data series using a Bayesian hierarchical autoregressive model and then applying a cross-correlation analysis to the residuals. Initial results indicate the correlation in time between NDVI and groundwater depth are highest at time t and t-1, regardless of the magnitude of anthropogenic influence, on a monthly time scale. We anticipate that at higher temporal frequency (i.e., daily) the correlation between the response and the covariates will show distinct patterns owing to alterations in the natural flow regime from agricultural practices and reservoir management. Our research highlights the complex temporal relationship between phreatophyte health and hydrology in groundwater dependent ecosystems encroached by differing magnitudes of anthropogenic influence. This research can aid conservationists in understanding the lagged impact that environmental flows, drought, land use change, and pumping-induced water table decline can have on phreatophyte health. Additionally, this research informs the choice of temporal scale (i.e., daily or monthly) at which to model groundwater dependent ecosystems in spatially distributed parameter modeling schemes.