Colloquium: Jesus Gomez-Velez

Unpacking the Matryoshka Doll: The Nested Nature of Hydrologic Systems
Mechanistic understanding of watershed and river corridor processes is critical for modeling and
prediction and to sustainably manage water resources under present and future socio-economic and
climatic conditions. Just like a nest of Russian matryoshka dolls, hydrologic systems are characterized
by structures that are buried within one another. As we unpack smaller dolls, we find commonalities
but also important differences that set aside each scale. For more than half-a- century, starting with
Hubbert’s and Tóth’s seminal works, we have conceptualized hydrologic systems as a collection of
nested flow paths ranging from small scale circulation underneath river bedforms to large regional
groundwater flow. This nesting has important implications for flow, storage, and transformations
within hydrologic systems, and therefore it is fundamental to appropriately transfer information across
scales. In this talk, I focus on two processes constrained by nested flow: (i) recharge and runoff
generation in mountain watersheds and (ii) connectivity along river corridors. First, I explore the role
of nesting in mountain watersheds and how it controls the contribution of groundwater to runoff,
residence times, and geochemical evolution of water. In particular, I highlight the role of river network
structure, topography, geology, and climate as key modulating and driving mechanisms.
Understanding the interplay among these mechanisms is fundamental to upscale and assimilate
watershed processes into weather and climate models. Second, I turn my attention to river corridors
and investigate how nesting of flow paths influences key reactions controlling water quality due to
increasing contact time with geochemically and microbially active sediments in hyporheic zones,
floodplain areas, and ponds and lakes. Here I focus on our ability to use local scale understanding to
propose up-scale models that can be used for predictions along river networks over continental scales.
Parsimonious models that take advantage of this understanding will play a key role in the design,
implementation, and evaluation of sustainable management practices that target both water quantity
and quality at the scale of the nation.