Mountain topography alters the phase, amount, and spatial distribution of precipitation. Past efforts focused on how orographic precipitation can alter spatial patterns in mean runoff , with less emphasis on how time-varying runoff statistics may also vary with topography. Given the importance of the magnitude and frequency of runoff events to fluvial erosion, we evaluate whether orographic patterns in mean runoff and daily runoff variability can be constrained using the global WaterGAP3 water model data. Model runoff data is validated against observational data in the contiguous United States, showing agreement with mean runoff in all settings and daily runoff variability in settings where rainfall-runoff predominates. In snowmelt-influenced settings, runoff variability is overestimated by the water model data. Cognizant of these limitations, we use the water model data to develop relationships between mean runoff and daily runoff variability and how these are mediated by snowmelt fraction in mountain topography globally. A global analysis of topographic controls on hydro-climatic variables using a Random Forest Model were ambiguous. Instead, relationships between topography and runoff parameters are better assessed at mountain range scale. Rulesets linking topography to mean runoff and snowmelt fraction are developed for three mid-latitude mountain landscapes—British Columbia, European Alps, and Greater Caucasus. Increasing topographic elevation and relief together leads to higher mean runoff and lower runoff variability due to the increasing contribution of snowmelt. The three sets of empirical relationships developed here serve as the basis for a suite of numerical experiments in our companion manuscript (Part 2).
Understanding the extent to which climate and tectonics can be coupled requires knowing both the form of topography and erosion rate relationships, but also the underlying mechanistic controls on those forms. The stream power incision model (SPIM) is commonly used to interpret such topography erosion rate relationships, but is limited in terms of probing mechanisms. A promising modification is a stochastic-threshold incision model (STIM) which incorporates both variability in discharge and a threshold to erosion, and in which the form of the topography erosion rate relationship is largely controlled by the variability of runoff. However, as applied STIM assumes temporally variable, but spatially constant runoff generating events, an assumption that is likely broken in regions with complicated orography. In response, we develop a unique 1D STIM based profile model that allows for stochasticity in both time and space and is driven by empirical relations between topography and runoff statistics. Testing the development of steady-state topography using spatial-STIM over a range of uplift rates highlights that coupling between mean runoff, runoff variability, and topography suggest that the development of highly nonlinear topography erosion rates should be expected. Further, we find that whether the daily statistics of runoff generating events are spatially linked or unlinked is a primary control on landscape evolution and the final resulting topography. As many empirical topography – erosion rate datasets likely sample across ranges of linked vs unlinked behavior, it is questionable whether single SPIM relationships fit to those data, without considerations of the hydroclimatology, are meaningful.
The Greater Caucasus (GC) Mountains within the central Arabia-Eurasia collision zone, are an archetypal example of a young collisional orogen. However, the mechanisms driving rock uplift and forming the topography of the range are controversial, with recent provocative suggestions that uplift of the western GC is strongly influenced by an isostatic response to slab detachment, whereas the eastern half has grown through shortening and crustal thickening. Testing this hypothesis is challenging because records of exhumation rates mostly come from the western GC, where slab detachment may have occurred. To address this data gap, we report 623 new, paired zircon U-Pb and (U-Th)/He ages from 7 different modern river sediments, spanning a ~400 km long gap in bedrock thermochronometer data. We synthesize these with prior bedrock thermochronometer data, recent catchment averaged 10Be cosmogenic exhumation rates, topographic analyses, structural observations, and plate reconstructions to evaluate the mechanisms growing the GC topography. We find no evidence of major differences in rates, timing of onset of cooling, or total amounts of exhumation across the possible slab edge, inconsistent with previous suggestions of heterogeneous drivers for exhumation along-strike. Comparison of exhumation across timescales highlight a potential acceleration, but one that appears to suggest a consistent northward shift of the locus of more rapid exhumation. Integration of these new datasets with simple models of orogenic growth suggest that the gross topography of the GC is explainable with traditional models of accretion, thickening, and uplift and does not require any additional slab-related mechanisms.