Preferential flow in hillslope systems through subsurface networks developed from a range of botanical, faunal and geophysical processes have been observed and inferred for decades, and may provide a large component of the bulk transport of water and solutes. However, our dominant paradigm for understanding and modeling hillslope hydrologic processes is still based on the Darcy-Richards matric flow framework, now with a set of additional methods to attempt to reproduce some of the aggregate function of the two-phase system of network and matrix flow. We call for a community effort to design and implement a set of well planned experiments in different natural and constructed hillslopes, coupled with the development of new theory and methods to explicitly incorporate and couple the co-evolution of subsurface flow networks as intrinsic components of hydrological, ecological and geomorphic systems. This is a major community challenge that can now benefit from new experimental infrastructure, renewal of older infrastructure, and recent advances in sensor systems and computational capacity, but will also require a sustained and organized interdisciplinary approach. This article is protected by copyright. All rights reserved.