This dataset is from five boreholes located on an improved grassland hillslope transect within the Pontbren study site in mid-Wales, UK. Groundwater was measured using pressure transducers installed in each of the boreholes with a pressure transducer used to account for variations in barometric pressure. Groundwater monitoring was carried out between 2005-2009 as part of the Pontbren Catchment Study Land Use and Management Multi-Scale Experimental Programme. Boreholes 1 and 2 are located in the improved grassland field immediately above the Bowl study site, borehole 3 is located within the Bowl study site and boreholes 4 and 5 are located in improved grassland field below the Bowl, known as the hillslope with tree shelterbelt. Data is presented in terms of height of water (cm) relative to the soil surface. Groundwater temperature (deg C) is also given. Groundwater height was initially sampled every 10 minutes until October 2006 when it was changed to sampling every 30 minutes. Groundwater was measured in all 5 boreholes up until March 2008 at which point sampling was reduced to measurements taken from borehole 3 up until the end of 2009. Within the 'Pontbren Groundwater' dataset folder there are five other sub-folders which contain the dataset from the respective boreholes. Data are provided in the form of .txt files and generally split into 6 month blocks. Associated with each data point in the .txt file is a quality assurance code, QA code, in the adjacent column. Details of the dataset, the quality assurance coding system and monitoring locations are provided in the supporting documentation. Also provided are the details of the borehole logs at the time they were drilled. Other measurements taken at the bowl and hillslope study site include monitoring runoff from an improved grassland field in the form of overland and drain flow, soil water tension, soil volumetric moisture content, groundwater height and precipitation. Datasets of these other parameters are also provided by the EIDC.