Time-variability of flow recession dynamics: Application of machine
learning and learning from the machine
Abstract
Flow recession analysis, relating discharge Q and its time rate of
change -dQ/dt, has been widely used to understand catchment scale flow
dynamics. However, data points in the recession plot, the plot of -dQ/dt
versus Q, typically form a wide point cloud due to noise and hysteresis
in the storage-discharge relationship, and it is still unclear what
information we can extract from the plot and how to understand the
information. There seem to be two contrasting approaches to interpret
the plot. One emphasizes the importance of the ensembles of many
recessions (i.e., the lower envelope or a measure of central tendency),
and the other highlights the importance of the event scale analysis and
questions the meaning of the ensemble characteristics. In this study, we
examine if those approaches can be reconciled. We utilize a machine
learning tool to capture the point cloud using the past trajectory of
discharge. Our results show that most of the data points can be captured
using 5 days of past discharge. We show that we can learn the catchment
scale flow recession dynamics from what the machine learned. We analyze
patterns learned by the machine and explain and hypothesize why the
machine learned those characteristics. The hysteresis in the plot mainly
occurs during the early time dynamics, and the flow recession dynamics
eventually converge to an attractor in the plot, which represents the
master recession curve. We also illustrate that a hysteretic
storage-discharge relationship can be estimated based on the attractor.