Abstract
Abstract. Accurate determination of past flooding characteristics is necessary to effectively predict the future flood disaster risk and dominant controls. However, understanding the effects of environmental forcing on past flooding frequency and magnitude is difficult owing to the deficiency of observations (data available for less than 10 % of the world's rivers) and extremely short measurement time series (<100 years). In this study, a numerical model, HYDROTREND, which generates synthetic time series of daily water discharge at a river outlet, was applied to the Yalu River to (1) reconstruct annual peak discharges over the past 1000 years and estimate flood annual exceedance probabilities and (2) identify and quantify the impacts of climate change and human activity (runoff yield induced by deforestation and dam retention) on the flooding frequency and magnitude. Climate data obtained from meteorological stations and ECHO-G climate model output, morphological characteristics (hypsometry, drainage area, river length, slope, and lapse rate), and hydrological properties (groundwater properties, canopy interception effects, cascade reservoir retention effect, and saturated hydraulic conductivity) form significant reliable model inputs. Monitored for decades, some proxies on ancient floods allow for accurate calibration and validation of numerical modeling. Simulations match well the present-day monitored data (1958–2012) and the literature records of historical flood events (1000–1958). They indicate that flood frequencies of the Yalu River increased during 1000–1940, followed by a decrease until the present day. Frequency trends were strongly modulated by climate variability, particularly by the intensity and frequency of rainfall events. The magnitudes of larger floods, events with a return period of 50 to 100 years, increased by 19.1 % and 13.9 %, respectively, due to climate variability over the last millennium. Anthropogenic processes were found to either enhance or reduce flooding, depending on the type of human activities. Deforestation increased the magnitude of larger floods (100- and 50-year floods) by 19.2 %–20.3 %, but the construction of cascade reservoirs in 1940 significantly reduced their magnitude by 36.7 % to 41.7 %. We conclude that under intensified climate change and human activities in the future, effective river engineering should be considered, particularly for small- and medium-sized mountainous river systems, which are at a higher risk of flood disasters owing to their relatively poor hydrological regulation capacity.
Highlights
Extreme climate events have increased over the last century, threatening human life and property (Cai et al, 2014; UNISDR, 2015; Winsemius et al, 2015)
To validate the model and calibrate its input parameters, we used the annual maximum peak flows at two gauging stations for 1958–2012 accessed from the Hydrological Statistical Yearbook of the Heilongjiang basin
The simulation results of the area of rainfall (Ai) River are slightly inferior to those of the Yalu River based on the generalized extreme-value (GEV), the difference between the two methods is not significant when investigating the impacts of climate change and human activities on flood frequencies (100, 50, 20-year, etc.) (Fig. A2)
Summary
Extreme climate events have increased over the last century, threatening human life and property (Cai et al, 2014; UNISDR, 2015; Winsemius et al, 2015). Research has been focused on the physical and statistical characteristics of flood events, estimating flood probability and flooding frequency variability in response to urbanization, climate change, and other factors (Sambrook Smith et al, 2010; Munoz et al, 2015, 2018; Kettner et al, 2018; Zhang et al, 2018). Only shortterm (< 100-year) fluvial gauge data exist for most rivers globally, and the existing observational data are largely affected by human activities (Milliman and Farnsworth, 2013) These relatively short records lead to large uncertainties in the predictions of future flood disasters and are problematic in discerning whether changes in flood frequency and magnitude are in response to climate change or human activities (Holmes and Dinicola, 2010; Yang and Yin, 2018). To improve the understanding of the main controlling factors of the flooding frequency and magnitude under the impact of climate change and human activities, the forward hydrological model HYDROTREND is applied here
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