03 Aug 2023
 | 03 Aug 2023
Status: this preprint is currently under review for the journal HESS.

Technical Note: Testing the Connection Between Hillslope Scale Runoff Fluctuations and Streamflow Hydrographs at the Outlet of Large River Basins

Ricardo Mantilla, Morgan Fonley, and Nicolas Velasquez

Abstract. A series of numerical experiments were conducted to test the connection between streamflow hydrographs at the outlet of large watersheds and the time-series of hillslope-scale runoff yield. We used a distributed hydrological routing model that discretizes a large watershed (~17,000 km2) into small hillslope units (~0.1 km2) and applied distinct surface runoff time-series to each unit that deliver the same volume of water into the river network. The numerical simulations show that distinct runoff delivery time-series at the hillslope scale result in indistinguishable streamflow hydrographs at large scales. This limitation is imposed by space-time averaging of input flows into the river network that are draining the landscape. The results of the simulations presented in this paper show that under very general conditions of streamflow routing (i.e., nonlinear variable velocities in space and time), the streamflow hydrographs at the outlet of basins with Horton-Strahler (H-S) order five or above (larger than 100 km2 in our set up) contain very little information about the temporal variability of runoff production at the hillslope scale and therefore the processes from which they originate. In addition, our results indicate that the rate of convergence to a common hydrograph shape at larger scales (above H-S order 5) is directly proportional to how different the input signals are to each other at the hillslope scale. We conclude that the ability of a hydrological model to replicate outlet hydrographs does not imply that a correct and meaningful description of small-scale rainfall-runoff processes has been provided. Furthermore, our results provide context for other studies that demonstrate how the physics of runoff generation cannot be inferred from output signals in commonly used hydrological models.

Ricardo Mantilla et al.

Status: open (until 07 Oct 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2023-187', Keith Beven, 08 Aug 2023 reply
  • RC2: 'Comment on hess-2023-187', Warrick Dawes, 08 Sep 2023 reply

Ricardo Mantilla et al.

Ricardo Mantilla et al.


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Short summary
Hydrologists strive to “Be right for the right reasons” when modeling the hydrologic cycle, however, the datasets available to validate hydrological models are sparse, and in many cases, they comprise streamflow observations at the outlets of large catchments. In this work, we show that matching streamflow observations at the outlet of a large basin is not a reliable indicator that a correct description of the small-scale runoff processes.