Subsurface flow contribution in the hydrological cycle lessons learned and challenges ahead—a review

Subsurface flow to maintain base flow and its contribution to high flow is of high significance. The high contribution of subsurface flow to stream flow has usually been determined based on the application of tracers. However, there are some studies that challenge tracer test applications. These studies have shown that tracer test applications lead to a high percentage of subsurface flow contribution because advection and dispersion effects are not individually considered in the mass balance equation. On the other hand, there is yet no broad consensus on the responsible mechanisms that justify high contributions of underground water to river flows. In this paper, we focus on the contribution of subsurface flow to high flows, although a brief description of their role in low flows is included. We discuss different suggested mechanisms, considering their applicability, strengths and inadequacies. In addition, the application of tracer experiments is elaborated. Finally, the challenges of modeling surface/subsurface flow interactions are addressed, followed by a short description of our future targets.