May NHP Unit NHNS CGWB DWLRs India August January country quality November boreholes key points Piezometers crucial role availability open dug wells telemetry systems specialized wells groundwater levels monitoring stations Allegiance Enterprise high-frequency monitoring National Hydrology Project Allegiance Water Management Groundwater Level Monitoring Digital Water Level Recorders purpose-built bore/tube wells The Central Ground Water Board National Hydrograph Network Stations National Ground Water Monitoring Network