is a standard abbreviation for Soil Water Potential , which measures the energy state of water in soil.
could refer to a 0.5-meter depth reading taken on the sixth day of a field study or experiment. Seasonal Worker Programme (SWP): SWP-0.5-Day6-pc.zip
Based on the individual components of the filename, here are the most probable interpretations: is a standard abbreviation for Soil Water Potential