Drainage basins are basic geomorphic units in which climate, water, rock, soil and life interact. I am interested in understanding the external forcings and internal controls that steer the evolution of river networks and drainage basin reorganization. My PhD thesis research is focused on quantifying the effects of climate, fresh mineral supply and dust input on the chemical evolution of soil.
Large discontinuities in channel head 𝜒 values across the drainage divide between the headwater catchment of Hei River and its "aggressive" neighbors.
Kai was collecting sediment from an active channel in the Qilian Shan for Be-10 measurement.
How fast do drainage divides migrate?
Drainage divides, backbones of terrestrial landscape, are dynamic features that migrate across Earth's surface. The resulting changes in river networks hold important implications for sediment and nutrient transport across Earth's surface and the geographic connectivity between biotic species.
Divide migration is ultimately driven by differential erosion across the divide. In the Qilian Shan, I documented a near-exponential relationship between cross-divide differences in erosion rates (ΔE), which I measured with cosmogenic Be-10 in stream sediment, and channel-head values of the topographic metric 𝜒 (Δ𝜒). This provides empirical support for the hypothesis that Δ𝜒 can reflect ΔE, which is useful because Δ𝜒 can be measured straightforwardly with topographic data. The exponential relationship in the Qilian Shan is similar to those found in the Southern Appalachians and Ozark dome by Willett et al. (2014) and Beeson et al. (2017), which suggests a common set of processes driving divide migration across diverse mountain ranges. In a simple numeric model, the near-exponential ΔE-Δ𝜒 relationship naturally arises as asymmetric topographic divides approach equilibrium. To learn more about this research, please read Hu et al. 2021. EPSL.
Beyond my work on the divide migration in the Qilian Shan, I am interested in further testing the global prevalence of the near-exponential ΔE-Δ𝜒 relationship by building a database of differential erosion rates and channel-head 𝜒 values across actively migrating divides. I am also interested in exploring the mechanisms of drainage basin organization in more detail, especially the potential feedbacks between divide migration, hillslope erosion, soil chemical weathering and basin hydrology.
How sensitive is silicate weathering to climate?
The sensitivity of silicate weathering to climate is of wide interest because it affects ecosystem nutrient fluxes, Earth’s topographic evolution, and the stability of Earth’s long-term climate. Despite considerable progress in understanding how climate variables affect mineral dissolution rates in theory and in well-controlled laboratory experiments, the climatic control on chemical weathering in nature remains elusive.
To better constrain the sensitivity of silicate weathering to climate, I am measuring soil chemical depletion and chemical erosion rates along a steep elevation gradient that spans nearly 3 km in altitude and ~18 ℃ in mean annual air temperature on the granitic San Jacinto Peak (SJP) in southern California. Here I've deployed climate monitoring stations (click this link to interactively explore some of the soil climate data I've collected at SJP) over the past 3 years to measure soil temperature, soil moisture, air temperature and precipitation at 15-minute intervals along the elevation transect at SJP. To quantify the influence of dust on the soil chemistry, I teamed up with Dr. Sarah Aarons' group at Scripps Institution of Oceanography, UCSD to measure dust flux and composition at 6 of the instrumented sites. Our work here will provide a unique dataset for exploring climatic controls on soil chemical erosion.
Ken Ferrier (left) and Kai were setting up a passive dust collector at site SJP483. The snow-covered peak in the background is the San Jacinto Peak (3283m).