Implication of River Hydrological Regime and Precipitation on Source to Sink Analyses

Abstract

Global modern river analyses shows that average discharge (Q_ave) values represent discharge events of very different frequency and magnitude in rivers with different hydrological regimes. Thus, applying Q_avg in source to sink analysis (e.g., BQART) may cause an error of multiple magnitudes in drainage basin and sediment yield estimates. Only rivers with extremely persistent hydrology have normally distributed discharge where the Q_ave values approach the 50^th percentile occurrence (Q₅₀) and can be assigned a specific frequency and magnitude. In such rivers low and high magnitude discharge events differ little from Q_ave. Such persistent discharge is largely a function of perennial precipitation style that provides a year-round surface water supply, and lacks extreme wet or dry seasons. In variable discharge rivers Q_ave is a transient flow event that inadequately characterizes either base-flow or flood conditions.

Comparison of river hydrographs shows that Q_ave in a river with persistent hydrology can be 100 times larger as compared to a river with variable hydrology with the same drainage basin size. This implies that using Q_ave we are likely to consistently underestimate the drainage basin size, the efficient discharge, and thus the river’s capacity for sediment yield in rivers with variable hydrology. This efficient discharge is also the channel forming discharge (bankful flow) and thus determines channel dimensions that we utilize in the ancient record as a key parameter in drainage basin or sediment yield estimates. Channel-forming discharges (and channel dimensions) have values close to Q_ave only in rivers with persistent hydrological regime. In rivers with variable hydrology, the efficient channel forming discharge is a considerably higher magnitude flow close to Q₉₉ rather than Q_ave.

River hydrology is largely driven by climatic conditions, and thus a function of precipitation and snowmelt, aided by hydrological connectivity and groundwater flow. Also here, we commonly utilize average values in our interpretations of environmental drivers, and there is a tendency to link lower sediment yields to lower average precipitation (increased aridity). However, rivers with most variable hydrology occur in arid climates and produce efficient discharges that may be multiple orders of magnitude higher than their Q_ave and are thus likely to have high sediment yields, especially when paired with increased hydrological connectivity such as reduced vegetation cover.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018