The supply, storage, and subsurface transport of magma are some of the most fundamental, yet least understood volcanic processes (Poland et al., 2014). These processes, along with eruptive dynamics, are modulated by the geometry and nature of the pathways connecting magmatic reservoirs (Keating et al., 2008). The geometry and dimensions of individual pathways can be constrained by inverting surface deformation with continuum mechanics based models (e.g., Owen et al., 2000;Montagna & Gonnermann, 2013). However, with multiple reservoirs and a network of magmatic pathways, estimating the dimensions of each pathway directly from deformation can be challenging. Because magma flux is proportional to the hydraulic conductivity of the pathway, and pressure change in a reservoir depends on magma flux, time dependent deformation associated with each reservoir may reveal the connectivity of a multi-reservoir system (e.g., Bato et al., 2018;Le Mével et al., 2016;Reverso et al., 2014). Here we demonstrate that, physics-based models, coupled with Bayesian inversion, can synthesize multi-reservoir conceptual models with geodetic measurements to quantitatively constrain the hydraulic connectivity of magmatic systems.Despite decades of research, the nature of Kīlauea's summit reservoirs and their connectivity to the East Rift Zone remains enigmatic (we reserve "East Rift Zone" for the geographic location and "ERZ" for the reservoir active in the observation period). Efforts to interpret summit deformation in terms of simple reservoir models yielded diverse reservoir locations and geometries (e.g., Baker & Amelung, 2012;Fiske & Kinoshita, 1969). Although modeled reservoirs cluster into two groups -a shallow Halema'uma'u (HMM) and a deeper South Caldera (SC) reservoir (e.g., Cervelli & Miklius, 2003;Poland et al., 2014), it has been suggested that the summit system represents a single irregularly shaped reservoir (Dieterich & Decker, 1975;Ryan, 1988). This ambiguity arises because deformation signals associated with these reservoirs are almost always of the same sign. The configuration of magmatic pathways connecting Kīlauea's summit reservoirs and ERZ is also elusive. Cervelli and Miklius (2003) argue that an "Γ shaped" pathway connecting the deeper SC reservoir to the shallower HMM reservoir, and then to ERZ, is required to explain the drainage Abstract From August 2018 to May 2019, Kīlauea's summit exhibited unique, simultaneous, inflation and deflation, apparent in both GPS time series and cumulative InSAR displacement maps. This deformation pattern provides clear evidence that the Halema'uma'u (HMM) and South Caldera (SC) reservoirs are distinct. Post-collapse inflation of the East Rift Zone (ERZ), as captured by InSAR, indicates concurrent magma transfer from the summit reservoirs to the ERZ. We present a physics-based model that couples pressure-driven flow between these magma reservoirs to simulate time dependent summit deformation. We take a two-step approach to quantitatively constrain Kīlauea's magmatic plumbing system. ...