Fig. 5: Groundwater-related threshold responses are visible at two nature preserves along the Kern River and Santa Clara River.
From: Establishing ecological thresholds and targets for groundwater management
a–h, Site-specific NDVI and DTG trends at two nature preserves owned by Audubon and The Nature Conservancy, respectively, along the Kern River (Populus fremontii; gdeID: 134058, a–d) and the Santa Clara River (Salix laevigata, gdeID: 111026, e–h). a,e, NDVI time series for the entire 1985–2022 study period (grey line) and for the years where DTG data exist (coloured points corresponding to individual years). Drought years are shaded in gold. b,f, DTG time series at the vegetation polygon (DTG at veg) with the mean maximum rooting depth for the associated genus (solid horizontal black line) and the corresponding ZDTG threshold at the vegetation polygon (dashed horizontal red line in d and h, respectively). c,g, NDVI versus DTG (m) at the well associated with the vegetation polygon, where coloured points correspond to individual years (refer to a and e), and the black solid trend line is based on an ordinary least square linear fit. d,h, ZNDVI versus ZDTG with the ZDTG threshold set where the ordinary least square fit intersects ZNDVI = −1. Note that post-drought (after 2016) points for the Santa Clara River are considerably lower than the fitted lines in g and h, indicating a hysteresis response due to incomplete recovery of NDVI even after groundwater depth recovered to pre-drought levels.
