The California Groundwater Units dataset classifies and delineates the State into one of three groundwater based polygon units: (1) those areas defined as alluvial groundwater basins or sub-basins (DWR, 2002), (2) highland areas that are adjacent to and topographically upgradient of groundwater basins, and (3) highland areas not associated with any groundwater basin. In total, 938 Groundwater Units are represented. The Groundwater Unit dataset relates existing groundwater basins with their newly delineated highland areas which can be used in subsequent hydrologic studies. The method used to delineate highland areas is similar to methods used to delineate a contributing area, the difference being that highland areas are constrained to the immediately surrounding upslope (upsteam) area. Highland areas were delineated based upon the surficial contributing area (watershed) immediately adjacent to each groundwater basin. The method of delineation is comparable to the method used for stream segments. However, highland areas are not intended to represent the entire surficial contributing area for a groundwater basin. Upslope (upstream) groundwater basins, which may be contiguous with a downstream basin, are not included in the delineation for the downstream basin because the upstream basins will have their own highland areas delineated. In addition, many groundwater basins in the Central Valley are divided along a river: one groundwater basin along the right bank, and another along the left bank. Therefore, each groundwater basin will have its own highland area delineated for each side of the river. Groundwater units can be combined if desired. Some groundwater basins do not have an associated highland because the basin was completely bordered by other groundwater basins, or a combination of groundwater basins and the ocean.