Historical and future runoff changes in the Yangtze River Basin from CMIP6 models constrained by a weighting strategy

Annual-mean globally averaged surface air temperature has increased at an average rate of 0.08 °C per decade since 1880 (0.18 °C per decade since 1981) (NOAA National Centers for Environmental Information 2021), which has enormous impacts on the global water cycle (Sohoulande et al 2016). As one of the key elements of the hydrologic cycle, runoff has changed significantly over the past years (Nam et al 2018). Since runoff impacts people, agriculture, industries and ecosystems, an enhanced understanding of the spatiotemporal variations in runoff at a river basin and their response to climate change is of great significance for effective flood prevention, ecological environment protection and policy formulation (Xiao et al 2018, Xing et al 2018).

The Yangtze River Basin (YRB), the largest river basin in China and the 12th largest river basin in the world, plays a crucial role in regional water cycle, economic and social development (Tao et al 2021). The middle and lower reaches of the YRB (MLYRB) have the most abundant water source in China, with the highest nationwide density and number of lakes (Li et al 2019). There has been considerable research on spatiotemporal variation of runoff in the YRB/MLYRB based on short-term observations and simulations by various land surface models (LSMs) or global hydrological models (GHMs) (Chen et al 2014, Tang et al 2018, Xiao et al 2018, Gao et al 2020). However, previous studies mostly focused on the seasonal, interannual variability or long-term trend of the runoff in the MLYRB, while less attention has been paid to its interdecadal variability. 2020 summer saw the worst flooding disasters in the MLYRB since 1998 (another recorded severe flood occurred in 1983), which severely impacted the lives and activities of the local people (Wang et al 2021). This implies an interdecadal variation in the runoff at the MLYRB. To develop adaption strategies for climate risk reduction, it is necessary to analyze the historical decadal changes of summer runoff in the MLYRB, to reveal its linkage to atmospheric circulation and to project future runoff changes in this region.

Due to the short observed runoff data, model outputs are indispensable for runoff projection. To obtain long-term runoff data, previous studies have commonly adopted the LSMs/GHMs driven by bias-corrected precipitation data, which is estimated by general circulation models (GCMs) (Koirala et al 2014, Schewe et al 2014, Nam et al 2019). However, obtaining simulated runoff data in accordance with the above procedure will face great challenges. For example, due to our limited knowledge of the highly complicated climate system, large uncertainties exist in the precipitation predictions from GCMs (Kim et al 2008, Hawkins and Sutton 2011), which contribute to the major source of uncertainties in runoff simulations (Wilby et al 2008). And the runoff simulated by LSMs/GHMs might be heavily dependent on the model used, the bias correction and the downscaling method applied to the model input data (Hagemann et al 2013, Schewe et al 2014). Therefore, it is meaningful to use as many models as possible to explore the future runoff changes in the study region. In recent years, with a rapid increase in computational power and the great development of model structures and spatial resolutions, GCMs have been extensively applied to climate projection (Kitoh et al 2009, Mizuta et al 2012). Currently, scientists commonly utilize multi-model ensembles to provide a range of plausible outcomes that a region may experience (Merrifield et al 2020). The ensemble projection allows researchers to analyze the consistencies and discrepancies of multiple models in the ensemble to evaluate the overall level or the individual model's ability in projecting long-term climate change.

As the most comprehensive collection of climate simulations produced to date, the Coupled Model Intercomparison Project Phase 6 (CMIP6) archive (Eyring et al 2016) contains the latest outputs of more than 30 GCMs developed at different institutes around the world (Jiang et al 2020). Compared with the CMIP5, the CMIP6 has designed new scenarios, i.e., the shared socioeconomic pathway (SSP) 1–2.6, SSP2–4.5, SSP3–7.0, and SSP5–8.5, whose new framework filled in the critical gaps of intermediate forcing levels in CMIP5 (Eyring et al 2016, O'Neill et al 2016). It is therefore necessary to use CMIP6 GCMs to project runoff changes in the MLYRB under these new scenarios. Given the existing large model systematic errors, biases need to be constrained within the ensemble runoff projection. However, in most of the previous studies, each simulation/projection is taken to be equally important and the equal-weighted ensemble mean becomes a common way to reduce the uncertainties caused by internal variability and models' biases. However, the equal-weighted method has a fundamental drawback because an individual model has distinct performance and skill (Chen et al 2020, Jiang et al 2020, Zhou et al 2020). Some studies specifically selected those models that have better performance to calculate the equal-weighted mean. However, such a method has not considered the model independence (Lee and Wang 2014, Wang et al 2014). Additionally, some studies have suggested that both the model skill and the inter-dependence of multi-model archives are essential to constrain the uncertainties in climate simulations and projections (Knutti et al 2013, Sanderson et al 2015, 2017). Specifically, given a set of models, it is important to evaluate the value of adding another model to this set for projection. Taking an extreme case, for example, adding a duplicated model to the original set would not provide any new information; rather, it would even bias the combination of existing models' results toward the results of the duplicated model (Caldwell et al 2014). In the CMIP6 archive, some models are highly similar to each other because they are from the same institution or they share a large number of codes. In that case, these simulations and projections cannot be seen as independent samples of runoff changes. Therefore, it is essential to consider both the model skill and the model independence to effectively constrain the model biases and provide a more reliable projection of runoff in the MLYRB.

Therefore, in this study, we will first use the observational/reanalysis data to investigate the interdecadal variation of the summer (June, July and August) runoff in the MLYRB and reveal its linkage to large-scale atmospheric circulation. The results of this part are described briefly in section 3.1 and more details can be found in supplementary information (available online at stacks.iop.org/ERL/17/024015/mmedia). Subsequently, the CMIP6 archive will be processed by a model weighting scheme utilized by Sanderson et al (2017) to constrain the uncertainties in the historical simulation and projection of summer runoff in the MLYRB. The model weighting scheme considers both the model skill and the model independence, which can reduce respectively the influence of relatively poor-performing models and the influence of duplicated models as much as possible. It is developed based on the assumption that if a model has skills as good as other models but has higher independence, the model should have greater potential to provide new and more useful information. Finally, based on the multi-model weighted mean projections, we provided a comprehensive picture of future runoff changes under different climate change scenarios. The results of this study could provide insights into the effects of climate change on hydrological processes and provide a valuable reference for future flood prevention in the MLYRB.

2.1. Observations and CMIP6 archive

2.2. Statistical methods

2.3. Model weighting scheme

3.1. Interdecadal changes of runoff in the reanalysis data

Figures 1(a) and (b) show the observed first leading EOF mode and the responding principal component (PC1) for summer runoff over southern China. In this study, we refer the domain of (27°–32°N, 108°–120°E) as the MLYRB region, where summer runoff has the dominant interdecadal variability (figure 1(a)). The PC1 has well captured the abnormal hydrology events over the MLYRB, such as the extreme floods in 1983, 1998 and 2020 (solid red curve in figure 1(b)). Additionally, the nine year low-pass filtered series of the PC1 (red dashed curve in figure 1(b)) in 1981–2020 exhibits an obvious interdecadal oscillation. To further interpret the results of the EOF analysis, we compare the PC1 with the standardized area-mean summertime runoff over the MLYRB region (SRI, blue curves in figure 1(b)). Obviously, the changes of SRI match the PC1 well at both the interannual and decadal time scales, with the correlation coefficient of 0.97 and 0.98, respectively. Given this consistency, we adopt the area-averaged summer runoff over the MLYRB to the following model assessment and projections. The result of the moving t-test technique shown in figure 1(c), which is used for shift year detection, shows 1991 and 2000 are the shift years. Considering both the shift years and the nine year low-pass filtered series of the PC1 in figure 1(b), we identify the years from 1991–1999 as the high runoff period and the years from 2003–2013 as the low runoff period.

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Previous studies have shown that the changes in annual runoff in the MLYRB are affected by many meteorological factors including precipitation and temperature (Xiao et al 2018). In summer, according to the composites during the two periods (supplementary figure 1), precipitation is the dominant factor that directly leads to the interdecadal oscillation of summertime runoff in the MLYRB. Since precipitation in the MLYRB can be significantly influenced by weather patterns (Xiao et al 2015, Su et al 2017, Li et al 2021), we have found a close linkage between runoff changes in this region and the atmospheric circulation (supplementary figure 2). We have confirmed the significant contributions of the anomalous anticyclone in the western North Pacific and the strengthening of zonal westerly in the upper troposphere to the observed runoff changes in the MLYRB through their impacts on the East Asian summer monsoon. It has been discussed in supplementary information in detail. The result agrees with that of Tang et al (2018) and it indicates that climate change plays an important role in the interdecadal variation of summer runoff in the MLYRB. On this basis, we attempt to further investigate the long-term decadal variation characteristics of summer runoff under climate change with 28 global climate models participating in CMIP6.

3.2. Ensemble-mean of historical simulations by CMIP6 models

In this section, utilizing runoff from 28 GCMs in the CMIP6 archive, we aim to objectively present the overall performance of these models in simulating historical summer runoff in the MLYRB during the baseline period. First of all, the multi-model mean (MME) has also shown decadal variation in the runoff of the MLYRB, however, the simulated evolution (orange and purple curves in figure 1(b)) is much different from the observation. Even though the climate models have well represented the human-induced effects on climate and the long-term increase of precipitation extremes in the MLYRB (figures not shown), the MME of the 28 CMIP6 GCMs have not well reproduced the observed climatology and trend of summer runoff in the MLYRB due to large model systematic errors (supplementary figure 3 and supplementary information). Note that the simulated runoff trend is important for future projections in worldwide climate impact studies, and the MME is obviously not appropriate for an impact study of future climate change.

To quantitatively evaluate the models' performance in simulating the temporal variation of summer runoff in the MLYRB, we calculate anomaly correlation coefficient (ACC) and RMSE between the observed area-averaged runoff and simulations in individual GCMs and the MME at both interannual and decadal time scales (figure 2). It is clear that the CMIP6 models have diversities in reproducing temporal evolution of runoff changes. At the interannual time scale (figure 2(a)), (a) the ACCs in individual models range from −0.49 to 0.5 and most are centered around zero, only one model (ACCESS-ESM1-5) has well simulated the observed runoff changes (ACC passes the 95% significance level); and (b) the RMSEs range from 1.5–3.98. At decadal time scale (figure 2(b)), the simulations also have large biases from the observation with RMSEs ranging from 0.86–3.85 and ACCs ranging from −0.8 to 0.8. Because of the large model spread at both scales, the MME, a skill-weighted ensemble to some degree (Sanderson et al 2017), cannot well represent the interannual and decadal changes of summer runoff in the MLYRB (figures 1(b) and 2). The MME's ACC is −0.08 (−0.3) and the MME's RMSE is 1.95 (1.81) at the interannual (decadal) time scale over the MLYRB region. This means that the MME's projection may be potentially biased and cannot be used directly.

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3.3. Constraining multi-model biases through a weighting strategy

In this section, we attempt to implement the weighting strategy (section 2.3) to constrain the model biases. Our goal is to obtain a weighted runoff projection that is more reliable and representative than that from the commonly used MME. The single set of weights provided in this section can be applied to a wide range of runoff projections.

3.3.1. Inter-model distance matrix

As the basis for the model weighting strategy, the multivariate inter-model distance matrix $\delta$ is illustrated in figure 3(a). Since the magnitude of the RMSE distance may be sensitive to the object being evaluated, for each object each model's distances were divided by the mean distance across all models to ensure the mean inter-model distance is equal to one and each object gets about the same weight, like Sanderson et al (2017) or Knutti et al (2017). Compared with the mean pairwise inter-model distance, it can be found that some individual models (ACCESS-CM2, ACCESS-ESM1-5) have larger distances than any other model in the archive. This implies that these models are likely to be unique in future projections. However, in some previous studies, some of these unique models were abandoned only because their performance for the simulation of the present climate is poor. In addition, in several subsets of the archive (Institute of Numerical Mathematics Climate Model (INMCM) variants, Norwegian Earth System Model version 2 (NorESM2) variants), the models in a single subset may have smaller distances from each other, whereas they all have larger distances from other models outside the subset. In this situation, using the traditional method used in the previous studies (e.g. MME) to project runoff can lead to poor accuracy and poor estimation of uncertainty because well-performed models or poor-performed models together with their duplicates all are used, which would bias the ensemble projection toward to the results of the duplicated models. These two kinds of problems mentioned above are closely related to the model independence, which can be solved effectively in our weighting scheme through calculating the RMSE distances across the models in the archive to determine the independence weight of each model. The results of the independence weight scheme will be discussed in the following section.

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3.3.2. Independence weight

The key point for estimating the independence weight is the determination of the appropriate value of the radius of similarity ${D_{\text{u}}}$ (equation (3) in the supplementary information). It is a free parameter that determines the distance scale over which the model's independence weight should be reduced. ${D_{\text{u}}}$ is sampled by considering the distribution of inter-model distances. Here we determine its value based on several typical individual models or subsets in the CMIP6 archive mentioned in section 3.3.1. Figure 3(b) shows the relationship between ${D_{\text{u}}}$ and their independence weights. As mentioned in section 3.3.1, we expect that those models that have no obvious duplicates (i.e. ACCESS-CM2 and ACCESS-ESM1-5), should not be down-weighted by the method. In addition, for some subsets in which there are several closely related variants submitted from the same institution (i.e. INMCM and NorESM2), it is appropriate to select a value of ${D_{\text{u}}}$ that can produce a weight of about $1/n$ (n is the number of variants submitted) for each variant (Sanderson et al 2017). Finally, we set ${D_{\text{u}}}$ to be 0.67 times the distance between the best-performing model and observation, and then, the independence weight of each model in the archive can be estimated (supplementary figure 6).

3.3.3. Skill weight

According to the RMSEs between each model and observations, the model skill weight can be determined. A larger RMSE corresponds to a smaller skill weight, and vice versa. Similar to independence weight, the radius of model quality ${D_{\text{q}}}$ needs to be determined appropriately (equation (6) in the supplementary information). Once ${D_{\text{q}}}$ is determined, the model's skill weight and overall weight would finally be produced. In this paper, we provide ${D_{\text{q}}}$ with a relatively appropriate value considering the errors both in present and future climate changes reproduced by a multi-model weighted average. Keep ${D_{\text{u}}}$ invariant, which has been determined in section 3.3.2, and sample a range of ${D_{\text{q}}}$, figure 3(c) shows the corresponding changes in multivariate errors from a multi-model weighted mean relative to that from the simple ensemble mean for the baseline period. It is obvious that taking ${D_{\text{q}}}$ as approximately 0.55 times the distance between the best-performing model and observations could reduce the most in-sample error by about 25%–30% under comprehensive consideration (the reduction of the RMSE of the precipitation simulation can be up to about 32%). However, in-sample error is not the only factor considered. That is, even if one model in the archive can be seen as a good-performing model for the baseline period simulation, we still cannot guarantee that its projections are reliable for adaptation activities.

Thus, in order to determine whether our weighting scheme can efficiently improve the performance of the multi-model ensemble runoff projection under climate change, we conduct a 'perfect model test' following the method outlined by Sanderson et al (2017). It is introduced in the supplementary information in detail. In this method, a single model is selected from the archive to represent the truth and the remaining models are used to project it. For a specific range of ${D_{\text{q}}}$, the RMSEs of the weighted mean projection of each perfect model's projection of summer runoff changes relative to 1981–2014 are compared with the MME. Results are shown in figure 3(d). Out-of-sample errors in runoff change projections can be reduced most when taking ${D_{\text{q}}}$ as approximately 65% of the distance between the best-performing model and observation. Considering model skills during historical, baseline and future periods, we finally determine ${D_{\text{q}}}$ to be 0.7 times the smallest distance between models and observation, and then, each model's skill weight and overall weight can be estimated (supplementary figure 6).

It is obvious that our weighting scheme can effectively constrain the multi-model biases and really improve the performance of the multi-model ensemble runoff simulation over the MLYRB during the baseline period (figure 2 and supplementary figures 3(b) and (e)). This is mainly reflected in the reduction of the RMSE compared with the MME. Although we take the MLYRB region as a whole and just consider its area-mean runoff variations in the weighting scheme, results show significant improvement in the weighted mean simulation of the spatial distribution of two key runoff features:the climatology and trend over the MLYRB (supplementary figures 3(b) and (e)). The RMSEs of the two features have been reduced by 76.8% and 88.6%, respectively, compared with the MME. Figure 2 shows the improvement in the simulation of both interannual and decadal variations. The weighted mean has an ACC of 0.34 (passes the 95% significance level) (0.88) and an RMSE of 1.44 (1.12) for the interannual (decadal) time scale. It has obviously better skills than almost all individual models and the MME. Considering the improvement and good performance (especially in simulating runoff's decadal variations), the set of overall model weights produced in this section are finally used to investigate the characteristics of interdecadal variation of summer runoff in the MLYRB during the future period (2015–2100) under climate change.

3.4. Future projection with constraints of model biases

In this section, after the multi-model biases during the future period are constrained using the model weighting scheme, we aim to provide a relatively objective and reasonable runoff projection in the MLYRB. In this study, we focus on interdecadal variation characteristics of mean summer runoff in the MLYRB under the SSP1-2.6, SSP2–4.5, SSP3–7.0 and SSP5–8.5 scenarios. Figure 4 shows interannual and decadal series (blue lines) of runoff projected by multi-model weighted mean during 2015–2100. For the convenience of comparison, the simulated series (red lines) during 1981–2014 are also presented. Following Yao et al (2020), all series are standardized by the values of 1981–2010, which represents the World Meteorological Organization climatological standard normal (Kappelle et al 2021). It can be seen that the weighted mean can well simulate the observed interdecadal variation during 1981–2020, which demonstrates that our model weighting scheme is applicable and reliable. It can also be reflected by evaluating the skill of the weighted and unweighted mean simulation of climatology, trend and variation of area-average summertime runoff over the MLYRB (supplementary table 2). After the model biases are constrained with the weighting scheme, great improvement can be found in not only past simulations, but also in the future projection during 2015–2020 under different climate scenarios. This is due to the consideration of the model's future performance by conducting the 'perfect model test' in the scheme. It shows a certain degree of credibility of the runoff projection presented in this paper. Under the climate change scenario, positive standardized values indicate that summer runoff in the MLYRB tends to be larger in the future than the mean state during the present day. It is worth noting that the decadal series of runoff under the four SSPs all indicate that the peak value occurs in 2020, which to some extent can explain the rainstorm and flood disasters that occurred in the MLYRB in that year. According to the changes in runoff and precipitation during 2021–2040 relative to the mean state during 1981–2010 (supplementary figure 7), we projected that the MLYRB and its surrounding regions are going to experience a new pluvial period in the coming 20 years. Under the direct impacts of the precipitation, summertime runoff in the middle and south of the MLYRB tends to increase significantly under all economy pathways. The high consistency of the spatial patterns of runoff and precipitation changes demonstrates that precipitation is still the major climate variable influencing summertime runoff in the south of China under future climate change.

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This study reveals that the summer runoff in the MLYRB experienced an obvious interdecadal oscillation during 1981–2020 with significantly high runoff during 1991–1999 and low runoff during 2003–2013. According to the results of the composite and correlation analysis, we found that the appearance of the western North Pacific anomalous anticyclone and the strengthening of zonal westerly in the upper troposphere are main climate causes through their impacts on precipitation, which is confirmed to be the direct factor leading to decadal changes of summer runoff in the MLYRB. Because of its close linkage with climate change, long-term decadal changes in summer runoff over the MLYRB were then evaluated with 28 GCMs participating in CMIP6.

Due to large multi-model biases and wide model spread, the MME poorly simulates the climatology, trend, interannual and decadal variations of summer runoff in the MLYRB during the baseline period. In this situation, no matter whether an individual model or the MME is selected for runoff projection, the result will have considerable biases and cannot provide a valuable reference. Therefore, in this paper, a model weighting scheme is conducted with the simulations of 28 GCMs from the CMIP6 archive. Following the framework outlined by Sanderson et al (2017), each model is weighted on the basis of its skill in simulating the historical observations as well as its independence from the other models. The computational process determines that this scheme can constrain the multi-model biases during both the historical and future period under different climate change scenarios. It is confirmed that the weighted mean has much better performance in simulating the historical observed climatology, trend, interannual and especially decadal variations of summer runoff in the MLYRB compared with the MME or individual models in the CMIP6 archive. In that case, the multi-model weighted mean runoff simulation and projection were finally used to detect the decadal variations of summer runoff in the MLYRB during a longer term under climate change.

Compared with the mean state during 1981–2010, summer runoff in the MLYRB is projected to be higher in the future. It is worth noting that the peaks of summer runoff in the MLYRB under different scenarios all occur in 2020, which is likely to be followed by increased summer runoff until 2040. Specifically, the weighted projections indicate that in the following 20 years, the MLYRB and its surrounding regions are projected to experience a new pluvial period. And in the future extreme precipitation is still the major climate hazard impacting south of China. The weighting scheme in this paper can be reliable for runoff projections in other regions at multiple time scales.

It should be noted that there are several limitations of the current research. First, only the linkage between the interdecadal variation of summer runoff in the MLYRB and atmospheric circulation anomalies during 1981–2020 is analyzed. It is necessary to use available long-term observations or long-term reconstruction data to obtain more samples and further investigate whether the relationship could last for a longer time during the historical period. Second, the problem of trade-off between model skill and model independence outlined by Sanderson et al (2017) still exists. That is, in the CMIP6 archive, those models with higher model skill weights also tend to have more duplicates and thus have lower model independence weights. Due to the unavoidable trade-off problem of the CMIP6 archive, in this paper the model skill and independence are likely to have compensating effects in the weighting scheme, which leads to there still being much room to further reduce the model biases and uncertainties. Researchers should use more scientific ensemble methods to deal with the trade-off problem in future runoff projections. Moreover, the weighting scheme just considers local runoff and its direct meteorological impact factor (i.e. precipitation). In fact, it has been pointed out by many previous studies that summer runoff in the MLYRB is significantly influenced by individual remote circulation patterns through their impacts on the East Asia summer monsoon (e.g. El Niño/Southern Oscillation, Pacific Decadal Oscillation) (Xiao et al 2015, Su et al 2017). We propose that in future studies, more optimal and physically relevant variables as well as the climate patterns should be selected and their local and nonlocal effects should be considered to acquire better runoff projections. Finally, human activities can have a great impact on river hydrology in many catchments. With the increasing population, dam capacity and gross domestic product, together with the warming trend induced by human activities, runoff may be influenced significantly. However, in the weighting scheme we did not consider any impact of human factors. To some satisfaction, runoff projected by this scheme tends to increase under global warming and to be larger under stronger warming forcing except for SSP1-2.6 (figure not shown). Additionally, Chen et al (2014) indicated that the annual runoff in the YRB has shown little response to human activities since the late 1970s. This implies that the characteristics of runoff variability at annual and seasonal time scale are distinct, which should be paid more attention in future studies.

This research is supported by the National Natural Science Foundation of China (Grant Nos. 42088101, 41875118).

All data that support the findings of this study are included within the article (and any supplementary files).

The authors declare no conflict of interest.

Wang H J and He S P designed the research. He S P and Zhao J Z performed research. Zhao J Z prepared the manuscript with contributions from all co-authors.