Explicit consideration of preferential groundwater discharges as surface water ecosystem control points

Heterogeneities in sediment and rock permeability induce preferentialgroundwater flow from the scale of pore networks to large basins. Inthe unsaturated zone, preferential flow is frequently conceptualizedas an infiltration process dominated by macropores, resulting in stron-ger delivery of surface‐derived solute than would be predicted via dif-fuse percolation alone (Beven & Germann, 2013). In the saturatedzone, preferential flow occurs in bedrock fractures and karst, alonggeologic contacts and fault zones, and through unconsolidated mate-rials of relatively high connectivity (Winter, Harvey, Franke, & Alley,1998). Focused flow paths emanate on the land surface as preferentialgroundwater discharges, observed throughout stream, lake, wetland,and estuary systems. The prevalence, and perhaps dominance, of spa-tially focused discharges to surface water contrasts with the spatiallydiffuse flow often assumed in various conceptual and predictiveprocess‐based models. This simplification is not made out of anunawareness of preferential groundwater discharge; rather, the abilityto reliably measure focused flow across a range of scales is hamperedby a reliance on (relatively) sparse point measurements. Additionally,realistic distributions of <1‐ to 100‐m‐scale preferential groundwaterdischarges are computationally expensive to simulate at scales rele-vant to decision making. If we accept that preferential discharge ofgroundwater to surface water is an ubiquitous process, fundamentalquestions facing contemporary hydrogeology include (a) When doesspatially focused groundwater discharge matter to the process wewould like to predict? Followed by (b) If we determine when preferen-tial discharge “matters” and should not be simplified to diffuse inflows,how do we measure it at the spatial and temporal scales needed toinform process‐based models?