Challenges in mapping and evaluating groundwater dependent ecosystems in California
Christian Braudrick1 and Bruce Orr1
  1. Stillwater Sciences, 2855 Telegraph Ave. Suite 400, Berkeley CA, 94705
As part of California’ Sustainable Groundwater Management Act (SGMA), groundwater sustainability agencies are required to identify groundwater-dependent ecosystems (GDEs) and consider GDEs when developing criteria for sustainability. We mapped GDEs in eight groundwater basins throughout California following procedures outlined by the Nature Conservancy. Challenges to identifying GDEs ranged from varied quality and age of vegetation and wetland inventory maps and a paucity of shallow groundwater measurements. Old or poor-quality vegetation and wetland maps make it challenging to delineate vegetation and wetland boundaries under current conditions and to assess the likely rooting depth of dominant vegetation (a key factor in assessing likely connection of vegetation to shallow groundwater), thereby requiring additional effort to update and refine the mapping using recent aerial photographs. While poor vegetation and wetland maps were a challenge, by far the bigger challenge to identifying GDEs was uncertainty in shallow groundwater depth. Most groundwater wells in the basins we studied were much deeper than 100 ft, well below  the rooting depth of vegetation. In basins with complex aquifer conditions (e.g., frequent clay layers) the lack of shallow groundwater measurements make assessing the connection to groundwater very difficult. Identifying the degree to which other sources of water are important ic, an also be very challenging, particularly where depth to groundwater is uncertain. Other sources of water potentially sufficient to support phreatophytes include agricultural and urban runoff, losing streams (which may be connected to groundwater upstream), and rainfall. One of the advantages of SGMA is that the GDE maps can be revised as more data become available during subsequent monitoring. In several of our projects additional groundwater monitoring is currently being implemented to better understand where and when groundwater is supporting GDEs and interconnected surface water.
Changes to vegetation greenness, as assessed using the normalized difference vegetation index (NDVI ), can be used to for basin-scale monitoring of GDE health through time. For riparian GDEs, changes to vegetation associated with channel migration and avulsion make tracking individual polygons through time difficult, especially in larger and more dynamic rivers. This is particularly problematic in braided rivers where morphological changes following floods can be profound. NDVI analysis that looks at overall changes coupled with changes to individual vegetation units can help to address this uncertainty.