Open questions regarding the influence of topography on the terrestrial water cycle

Topography affects the distribution and movement of water on Earth, but exciting puzzles remain and new discoveries regarding topographic controls continue to surprise us. In this contribution, we discuss some open questions regarding the influence of topography on the terrestrial water cycle based on a combination of literature review and data synthesis. How will changes in water and energy supply along elevation gradients translate into changes in actual evaporation, and how will this be modulated by plant physiological responses and topographically driven moisture redistribution? What role does groundwater play in sourcing the world's water towers, and how will this role change with melting of snowpacks and glaciers? What is the relative importance of topography (vs. climate and geology) in driving groundwater flow dynamics across scales, and how does topography influence inter-catchment groundwater flow or mountain block recharge? A key feature emerging from these questions is the presence of numerous interacting gradients and contrasts that explain many of the patterns we observe. Studying these interactions, and thus answering at least some of the questions posed above, has the potential to improve our understanding of hydrological systems and how they may evolve in the wake of global change.