A concept of hybrid terrestrial gravimetry and cosmic ray neutron sensing for investigating hydrological extreme events

While studies on hydrological extremes, and floods in particular, usually take a retrospective approach, the German Helmholtz initiative MOSES (Modular Observation Solutions for Earth Systems) aims at understanding extreme events by observing the flood generation processes directly where they occur: in-situ and during the event. As part of this framework, we present a new concept of monitoring regional water storage changes by combining event-based ad-hoc field campaigns with continuous monitoring, using terrestrial gravimetry for total water storage variations and cosmic ray neutron sensing for near-surface soil moisture variations. In this concept, a key role is taken by a continuously monitoring gravimeter station: the gPhone solar cube. This station is energy self-sufficient and easily deployable at any remote location, hosting a gPhoneX, a full weather station, a GNSS antenna and receiver and a cosmic ray neutron probe. The purpose of this station is i) to provide data describing the longer term hydrological dynamics of the study area including the pre-event conditions and ii) to serve as the reference station for the gravity field campaigns during the event. These field campaigns, triggered by forecasts of extreme weather events, are carried out at least prior and after the event on a network of points across the study site. The locations are chosen with respect to the size of the area of interest, topography and travel times between the points. Measurements at each point include relative gravity with two CG-6 instruments, absolute gravity with a Muquans atom quantum gravimeter (AQG) and near-surface soil moisture using three cosmic ray neutron probes in a mobile rover setup. The same routine is strictly repeated at each point to assure uttermost comparability of the measurements. The AQG is also used to calibrate the permanently installed gPhoneX and, thus, to use the gravity reference station for correcting the high instrumental drift of the CG6 gravimeters. The monitoring concept is expected to be transferable to all areas where a similar interest in water storage dynamics at event time scales is strived for.