Examining why trends in very heavy precipitation should not be mistaken for trends in very high river discharge

Abstract

It is firmly established in the hydrologic literature that flooding depends on both antecedent watershed wetness and precipitation. One could phrase this relationship as "heavy precipitation does not necessarily lead to high stream discharge", but rarely do studies directly affirm this statement. We have observed several nonhydrologists mistake trends in heavy precipitation as a proxy for trends in riverine flooding. If the relationship between heavy precipitation and high discharge was more often explicitly presented, heavy precipitation may less often be misinterpreted as a proxy for discharge. In this paper, we undertake such an analysis for 390 watersheds across the contiguous U.S. We found that 99th percentile precipitation only results in 99th percentage discharge 36 % of the time. However, when conditioned on soil moisture from the Variable Infiltration Capacity model, 62 % of 99th percentile precipitation results in 99th percentile discharge during wet periods and only 13 % during dry periods. When relating trends in heavy precipitation to hydrologic response, precipitation data should, therefore, be segregated based on concurrent soil moisture. Taking this approach for climate predictions, we found that CMIP-5 atmosphere-ocean global circulation model (AOGCM) simulations for a RCP 6.0 forcing project increases in concurrence of greater than median soil wetness and extreme precipitation in the northern United States and a decrease in the south, suggesting northern regions could see an increase in very high discharges while southern regions could see decreases despite both regions having an increase in extreme precipitation. While the actual outcome is speculative given the uncertainties of the AOGCM's, such an analysis provides a more sophisticated framework from which to evaluate the output as well as historic climate data.

Does nonstationarity in rainfall require nonstationary intensity–duration–frequency curves?

“…The roles of natural variability and multidecadal modes of sea-surface temperature (SST) in modulating Canadian extreme rainfall intensity have already been shown in the past (Gan et al, 2007;Shabbar et al, 1997). Further, a review of the literature suggests that heavy precipitation does not necessarily lead to high stream discharge (Ivancic and Shaw, 2015;Do et al, 2017;Wasko and Sharma, 2017). The analysis of Do et al (2017) reveals that the trend in streamflow is more consistent across the continental scale, and neither the anthropogenic activities such as the presence of dams nor the vegetation cover have any significant effect on the results of trend estimates.…”

“…The analysis of Do et al (2017) reveals that the trend in streamflow is more consistent across the continental scale, and neither the anthropogenic activities such as the presence of dams nor the vegetation cover have any significant effect on the results of trend estimates. Interestingly, the consensus among all three studies (Ivancic and Shaw, 2015;Do et al, 2017;Wasko and Sharma, 2017) is that the catchment size, which regulates the flow response because of the antecedent moisture content, is the most important contributing factor in modulating the nature of a trend in stream discharge. The smaller (especially urban) catchments may have increased flood peaks; in contrast, larger (agricultural and rural) catchments may experience decreased runoff due to lower soil moisture.…”

Future extreme precipitation intensities based on a historic event

“…Different types of flooding may result from extreme precipitation, while the antecedent soil conditions also play a role on stream discharge levels (Ivancic and Shaw, 2015;Wasko and Sharma, 2017). In urban environments, extreme precipitation may lead to localscale inundations, causing damage to houses and infrastructure within a time frame of several hours.…”

Evaluation of Gridded Precipitation Data Products for Hydrological Applications in Complex Topography

“…While there is a clear link between the amount, intensity, and distribution of precipitation to various processes in the ecosystem [3], this relation is nonlinear, and heavy precipitation does not necessarily result in high river discharge [4,5]. Additionally, the link of heavy precipitation and flood occurrence is challenging to assess and quantify [6].…”