Stoch Environ Res Risk Assess (2014) 28:491–504 DOI 10.1007/s00477-013-0767-1 ORIGINAL PAPER The comparison of sensitivity analysis of hydrological uncertainty estimates by GLUE and Bayesian method under the impact of precipitation errors Lu Li • Chong-Yu Xu Published online: 15 August 2013 Springer-Verlag Berlin Heidelberg 2013 Abstract The input uncertainty is as significant as model error, which affects the parameter estimation, yields bias and misleading results. This study performed a comprehensive comparison and evaluation of uncertainty estimates according to the impact of precipitation errors by GLUE and Bayesian methods using the Metropolis Hasting algorithm in a validated conceptual hydrological model (WASMOD). It aims to explain the sensitivity and differences between the GLUE and Bayesian method applied to hydrological model under precipitation errors with constant multiplier parameter and random multiplier parameter. The 95 % confidence interval of monthly discharge in low flow, medium flow and high flow were selected for comparison. Four indices, i.e. the average relative interval length, the percentage of observations bracketed by the confidence interval, the percentage of observations bracketed by the unit confidence interval and the continuous rank probability score (CRPS) were used in this study for sensitivity analysis under model input error via GLUE and Bayesian methods. It was found that (1) the posterior distributions derived by the Bayesian method are narrower and sharper than those obtained by the GLUE under precipitation errors, but the differences are quite small; (2) Bayesian L. Li (&) Uni Climate, Uni Research, Bergen, Norway e-mail: lu.li@uni.no L. Li Bjerknes Centre for Climate Research, Bergen, Norway C.-Y. Xu Department of Geosciences, University of Oslo, Oslo, Norway C.-Y. Xu Department of Earth Sciences, Uppsala University, Uppsala, Sweden method performs more sensitive in uncertainty estimates of discharge than GLUE according to the impact of precipitation errors; (3) GLUE and Bayesian methods are more sensitive in uncertainty estimate of high flow than the other flows by the impact of precipitation errors; and (4) under the impact of precipitation, the results of CRPS for low and medium flows are quite stable from both GLUE and Bayesian method while it is sensitive for high flow by Bayesian method. Keywords GLUE Bayesian Precipitation error Uncertainty Sensitivity Hydrological model 1 Introduction Hydrological models are important tools for improving our understanding of catchment dynamics, supporting water resources management, and predicting hydrologic impacts produced by future environmental changes. However, model calibrations and subsequent predictions will be subject to uncertainty, which arises in that no rainfallrunoff model is a true reflection of the processes involved, and is impossible to specify the initial and boundary conditions required by the model with complete accuracy. Hydrologists have been exploring many methodologies to better treat uncertainty and applied to various models and catchments. According to Montanari (2007), all of these methods can be broadly classified into four types: (1) approximate analytical methods; (2) techniques based on the statistical analysis of model errors; (3) approximate numerical methods/sensitivity analyses; (4) non-probabilistic methods. Numerous approaches for quantifying the validation uncertainty for the model output have been proposed. In which, two kinds of method have been widely 123 492 used, i.e. the Generalized Likelihood Uncertainty Estimation (GLUE) method (Beven and Binley 1992; Beven and Freer 2001) and the Bayesian method (Engeland et al. 2005; Krzysztofowicz 1999; Thiemann et al. 2001; Vrugt et al. 2009; Li et al. 2013a). The generalized likelihood uncertainty estimation (GLUE) method was first proposed by Beven and Binley (1992) in order to quantify the parameters uncertainty. The Bayesian method which needs to understanding the mathematics and statistics is difficult to implement since it is hard to find the proper statistic model which fits the data. However, numerous approaches for quantifying the uncertainty in hydrological modelling based on Bayesian method have been proposed (Blasone et al. 2008a, b; Vogel et al. 2008; Vrugt et al. 2003a, 2009; Yang et al. 2008). Generally speaking, there are four important sources of uncertainties in hydrological modelling, i.e. uncertainties in input data, uncertainties in output data used for calibration, uncertainties in model parameters and uncertainties in model structure (Refsgaard and Storm 1996). However, it will be difficult to study the modeling uncertainty considering all of the four error sources simultaneously, partly because they are not all independent of each other. The problem can be simplified by considering these uncertainties separately based on certain assumptions. Various methods have been developed to analyze uncertainty sources in hydrological modelling (Beven and Binley 1992; Refsgaard et al. 2006; Vrugt et al. 2003b; Alvisi et al. 2012; Shen et al. 2013). Recently, input uncertainty has been considered more and more important in hydrological modelling. Kavetski et al. introduced the Bayesian total error analysis (BATEA) framework to estimate the input uncertainty and parameters (Kavetski et al. 2002, 2006a, b). Bayesian model averaging is put forward to analyse the structural deficiencies of the specific model (Duan et al. 2007; Marshall et al. 2007; Tsai 2010). And it was developed to a new framework called integrated Bayesian uncertainty estimator (IBUNE) which is aimed to distinguish the various sources of uncertainty including parameters, input and model structure uncertainty (Ajami et al. 2007). Renard et al. (2009) used a set of probabilistic calibration methods to analyse input and structure errors quantitatively. Some criteria have been proposed to evaluate the performance of uncertainty analysis methods: (1) indices for compare the efficiency of the posterior distribution based on the Nash–Sutcliffe (NS) coefficient or other objective functions (Jin et al. 2010; Yang et al. 2008); (2) indices used to compare the sharpness of the model uncertainty intervals, which include the width of 95 % confidence interval (95CI), and the average distance between the upper and the lower limits of 95 % confidence intervals (95PPU) (Li et al. 2009; Xiong et al. 2009; Yang et al. 2008); and (3) indices to compare the reliability of the uncertainty 123 Stoch Environ Res Risk Assess (2014) 28:491–504 estimates, i.e. the percentage of observations bracketed by the 95CI (PCI) and the continuous rank probability score (CRPS) (Engeland et al. 2010; Li et al. 2013a, b). However, none of the above applications included the sensitivity analysis of the indices in evaluation of uncertainty assessment method, especially for different level of flows, i.e. low flow, medium flow and high flow. It is common knowledge that the uncertainty of hydrological simulation is dependent on the input accuracy. However, the question remains how the precipitation errors impact on the performance of the uncertainty estimate methods and how indices of uncertainty evaluation responses to different precipitation errors via GLUE and Bayesian methods. The objective of this paper is to attempt to partly fill this gap. This study performed a comprehensive comparison and evaluation of uncertainty estimates according to the impact of precipitation errors by GLUE and Bayesian methods using the Metropolis Hasting (MH) algorithm in a validated conceptual hydrological model (WASMOD). It seeks to explain the sensitivity and differences between the GLUE and Bayesian method applied to hydrological model under precipitation errors with constant multiplier parameter and random multiplier parameter. In water and snow balance modelling system (WASMOD) (Xu 2002; Kizza et al. 2013; Li et al. 2010, 2011, 2013b), the 95CI of monthly discharge in low flow, medium flow and high flow were selected for comparison. Four indices, i.e. the average relative interval length (ARIL), the percentage of observations bracketed by the confidence interval (PCI), the percentage of observations bracketed by the unit confidence interval (PUCI) and the CRPS were used in this study for sensitivity analysis under model input error via GLUE and Bayesian methods. Finally, the paper tries to give some suggestions on the choice of uncertainty estimate method and evaluation indices with input uncertainty in hydrological model applications. 2 Method 2.1 Perturbing error This paper considered the impact of precipitation uncertainty by perturbing method which is assessed by using observed precipitation, which is assumed as ‘‘perfect’’, then perturb that precipitation by multiplying parameter (e.g. Monte Carlo method to create a range of different values of precipitation within a selected interval) and see what impact it has on. These multipliers are considered to be systematic error and stochastic error in precipitation. Rainfall depth multipliers which are considered to be latent variables to the system are introduced by Kavetski et al. Stoch Environ Res Risk Assess (2014) 28:491–504 (2002). They put forward an explicit term to the likelihood function to estimate both variables and model parameters by the probabilistic calibration procedure which is called the BATEA. Besides, Ajami et al. (2007) developed a random multiplier ut which is normally distributed with mean equals to l and variance equals to r2l at time step t which is expressed as follows: r~t ¼ ut rt u N l; r2l ; ð1Þ 493 subjectively determined and this was discussed a lot before (Freer et al. 1996; Li et al. 2010; Smith et al. 2008). In this study, the standardize NS value was chosen as the likelihood function: T P Robs;t Rsim;t Lðhi jY Þ ¼ 1 t¼1 T P Robs;t Robs 2 2 ¼1 r2i r2obs r2i \r2obs t¼1 ð3Þ where r~t represents true precipitation depth at time step t; rt is observed precipitation depth at time step t. Compared with BATEA, it contains less variables, i.e. l and r2l , which are needed to be estimated. However, it still added two more parameters in calibration procedure. In this paper, it assumed that the observed precipitation depth is true and which will be perturbed by adding either a systematic multiplier or a random multiplier by following form: 0 ut rt u N ðl; r2 Þ rt ¼ ð2Þ ut rt u cons tan t where Lðhi jY Þ is the likelihood measure; Robs;t is the observed discharge; Rsim;t is the simulated discharge, which is depending on the model parameter hi; Robs is the average value of Robs;t ; r2i is the variance of errors for the given parameter set hi and the observed discharge data set Y; and r2obs is the variance of the observed data set. 2.3 Bayesian method 2.3.1 Bayesian inference GLUE and Bayesian method are used to estimate the model and parameter uncertainty under the impact of precipitation errors. The Bayesian theorem is expressed as follows: 2.2 GLUE where / ¼ fh; xg, in which h represents the hydrological model parameters and x represents statistical parameters. The posterior density pð/jgÞ can be derived from the prior density f(/) and the likelihood function f ðgj/Þ. Variable g is a transformed variable from the original space. The GLUE methodology was proposed by Beven and Binley (1992), in which hundreds and thousands of model runs are made with randomly chosen parameter values from a priori probability distribution. The acceptability of each run is evaluated against observed values. The run is considered to be ‘‘non-behavioral’’ and to be removed from further analysis if the acceptability is below a certain subjective threshold. A threshold is either defined in terms of a certain allowable deviation of the highest likelihood value in the sample, or sometimes as a fixed percentage of the total number of simulations (Blasone et al. 2008a). The fixed percentage method was used in the study, which is named as the acceptable sample rate (ASR) (Li et al. 2010). According to the research results of Li et al. (2010), there is a good linear relation between threshold values and ASR for hydrological models and ASR has been tested as a main subjective factor for the uncertainty estimation result of GLUE. Li et al. (2010) found that for monthly WASMOD, when ASR is bigger than 1 %, the 95CI of discharge is much less sensitive to the change in ASR. So in this study, the ASR is chosen to be 1 % of the sampling size that is 200,000. In the GLUE method, the likelihood values serve as relative weights of each parameter set or simulated value. It is noted that the likelihood function and the threshold are pð/jgÞ ¼ R f ðgj/Þ f ð/Þ ; f ðgj/Þ f ð/Þd/ ð4Þ 2.3.2 Likelihood function In this study, Box_Cox transformation was used to transform the observed and simulated discharges and the resulting residuals are normally distributed, which has been used a lot in hydrological uncertainty analyses before (Engeland and Gottschalk 2002; Engeland et al. 2005; Jin et al. 2010; Wang et al. 2009; Yang et al. 2007a). It can be expressed as: k y 1 k 6¼ 0 : k gðy; kÞ ¼ ð5Þ lnðyÞ k ¼ 0 In the study of Li et al. (2011), the Lilliefors test (Lilliefors 1967) was used to verify the normality of the distribution of the residuals of the Box–Cox transformed variable for the monthly WASMOD in Chao River basin. They found the Box–Cox transformation yields residuals closest to normal for values of k = 0.2 in monthly WASMOD. So in this study, the parameter k is chosen to be 0.2. And the likelihood function is the key issue in the Bayesian method, which is defined by the distribution of 123 494 Stoch Environ Res Risk Assess (2014) 28:491–504 M residuals. nt represents the residual of gt ðyt Þ and gM as t yt follows: M ð6Þ nt ¼ gt ðyt Þ gM t yt ; where yM t and yt represent the simulated and observed variables in original space, respectively; gM t and gt are transformed variables. It is needed to check whether nt is independent. If not, an AR (1) model is used to make the residuals independent (Eq. 7). nt ¼ ant1 þ b þ et : ð7Þ The Jarque–Bera test (Carlos and Bera 1980) was used to check the independence of residuals, and the results showed that an AR (1) model was needed to make the transformed residuals independent. The resulting likelihood function is: Y T 1 n 1 nt ant1 b f ðnj/Þ ¼ q 0 q : ð8Þ r r r r t¼1 In which / represents all parameters; T represents the time; q represents the normal density operator. Assuming that there is no systematic error in monthly residuals, which results in the following likelihood function: Y T 1 n 1 nt ant1 f ðnj/Þ ¼ q 0 q : ð9Þ r r r r t¼1 All the notations are as defined above. 2.3.3 Posterior density There is no information about the distribution of parameters. Parameter r is an unknown model standard deviation and with Jeffreys’ uninformative prior it is proportional to r-1 (Bernardo and Smith 1994; Yang et al. 2007b). Besides, the prior probability densities of other parameters are generally taken as non-informative multi-uniform distribution in hydrological applications (Engeland et al. 2005; Jin et al. 2010; Liu et al. 2005; Yang et al. 2008). The prior densities and intervals of all parameters in the hydrological models are shown in Table 1. With the priori density f ð/Þ considered to be uniform, the posterior density pð/jnÞ is given as follows: f ðfjuÞ f ðuÞ : f ðfjuÞ f ðuÞdu Substituting Eq. 9 into Eq. 10 results in: Q T n0 nt ant1 1 q q f ð/ Þ r r r t¼1 pð/jnÞ ¼ T R 1 n0 Q nt ant1 q q f ð/Þd/ r r r pðujfÞ ¼ R ð10Þ ð11Þ Table 1 The prior distributions of all parameters in WASMOD Parameter a1 a2 a3 rb Prior distributionsa U[0, 1] U[0, 1] U[0, 1] r-1 a U ½a; b means the prior distribution of the parameter is uniform over the interval½a; b b / r1 means the prior density of the parameter at value r is proportional to r1 2.4 Metropolis hasting algorithm A Markov Chain Monte Carlo (MCMC) methodology, called MH algorithm (Hastings 1970), was used to get the posterior distributions and estimate the parameters (Chib and Greenberg 1995; Engeland and Gottschalk 2002; Kuczera and Parent 1998; Li et al. 2010). Furthermore, the pffiffiffi scale reduction score R (Gelman and Rubin 1992) was used to check the convergence of Monte Carlo chains. 2.5 Ninety-five percentage confidence intervals of discharge Discharge values for each month were obtained by running the hydrological model with all parameter sets from the MH samples. The 95 % confidence intervals for discharge due to parameter uncertainty are estimated by these discharge samples. The 2.5 % percentile and 97.5 % percentile of discharge will be derived by sorting ascending of all discharge samples at each month. The intervals in GLUE method consider weighted values, while is not in the Bayesian method (Engeland et al. 2005). The detail steps of calculation can be seen in the studies of Li et al. (2010, 2011). 2.6 Criteria for comparison In this study, four indices were used to compare the derived 95CI of monthly discharge: the ARIL (Jin et al. 2010; Li et al. 2010) is used for measuring sharpness, PCI (Li et al. 2009) is used for reliability, the percentage of observations bracketed by the PUCI (Li et al. 2011) and the CRPS (Hersbach 2000) are used for measuring efficiency which combine sharpness and reliability. 1 X LimitUpper;t;p LimitLower;t;p ARIL ¼ : ð12Þ T Robs;t LimitUpper;t;p and LimitLower;t;p are the upper and lower boundary values of the p confidence interval, T is the number of time steps, Robs;t is the observed discharge. t¼1 in which r [ 0, a 2 ½0; 1Þ: 123 PCI ¼ NQin;p : T ð13Þ Stoch Environ Res Risk Assess (2014) 28:491–504 495 NQin;p is the number of observations which are contained within the p confidence interval. PCI was plotted as a function of p and should be close to the 45 diagonal line. PUCI ¼ ð1:0 AbsðPCI 0:95ÞÞ=ARIL: ð14Þ Abs means absolute value. The PUCI is only used in 95CI evaluation, ranges from zero to infinity, and the upper boundary is not clear (Li et al. 2011). The crps for one time step for an ensemble with m members is calculated as: m m X m X 1X yi y 1 yi y j crpsðF; yÞ ¼ ð15Þ 2 m i¼1 2m i¼1 j¼1 where y is the observed value and yi is a sample member and the || indicates the absolute distance. The calculations above can be done faster by approximating the last double sum with a single sum: crpsðF; yÞ ¼ m m X 1X 1 yi y yi yiþ1 : m i¼1 2ðm 1Þ i¼1 CRPS ¼ T 1X crpsðFt ; yt Þ: T t¼1 ð17Þ The CRPS is averaged over the whole time series. The minimal value zero of CRPS is only achieved when the empirical distribution is identical to the predicted distribution, that is, in the case of a perfect deterministic forecast (Hersbach 2000; Yang et al. 2008). An ideal uncertainty analysis technique would lead to a 95 % probability band that is as narrow as possible while still being a correct estimate under the statistical assumptions of the technique. The ARIL and CRPS should be as small as possible while the PCI should be as close to 0.95 as possible. The larger the PUCI the lower the uncertainty of 95CI of discharge is. 3 Study area and hydrological models 3.1 Study area ð16Þ To get the CRPS for a time series, take the average of all crps: The GLUE and Bayesian methods have been applied to the Chao River basin upstream of the Miyun reservoir with a drainage area of 5,300 km2 (Fig. 1). Available Fig. 1 Main river and meteorological stations and discharge station of Chao River basin in North China Table 2 The main equations in WASMOD Actual evapotranspiration et ¼ minfðsmt1 þ pt Þð1 expða1 ept Þ; ept Þg 2 0 a1 1 Slow flow st ¼ a2 ðsmt1 Þ o a2 1 Fast flow ft ¼ a3 ðsmt1 Þðpt ept ð1 expðpt =maxðept ; 1ÞÞÞÞ o a3 1 Total flow d t ¼ s t þ ft Water balance smt ¼ smt1 þ pt et dt In which, t is t th month, pt is precipitation, ept is potential evaporation, smt is t th monthly soil moisture, SCt–1 is channel storage at time step t–1 123 496 Table 3 The scenarios of uncertainty estimates by GLUE and Bayesian method for WASMOD with different precipitation errors Stoch Environ Res Risk Assess (2014) 28:491–504 The describe of scenarios Abbreviation of scenarios GLUE under systematic error with ut a equals to 1.1 gsa GLUE under systematic error with ut equals to 1.05 gsb GLUE under systematic error with ut equals to 1.0 gsc GLUE under systematic error with ut equals to 0.95 gsd GLUE under systematic error with ut equals to 0.9 gse b gra GLUE under random error with ut from N (1.05, 0.005) grb GLUE under random error with ut from N (1.0, 0.005) grc GLUE under random error with ut from N (1.1, 0.005) GLUE under random error with ut from N (0.95, 0.005) grd GLUE under random error with ut from N (0.9, 0.005) gre Bayesian under systematic error with ut equals to 1.1 bsa Bayesian under systematic error with ut equals to 1.05 bsb Bayesian under systematic error with ut equals to 1.0 bsc Bayesian under systematic error with ut equals to 0.95 bsd Bayesian under systematic error with ut equals to 0.9 Bayesian under random error with ut from N (1.1, 0.005) bse bra ut is multiplier parameter Bayesian under random error with ut from N (1.05, 0.005) brb N (1.1, 0.005) means the normal distribution with variance equals to 0.005 and mean equals to 1.1 Bayesian under random error with ut from N (1.0, 0.005) brc Bayesian under random error with ut from N (0.95, 0.005) brd Bayesian under random error with ut from N (0.9, 0.005) bre Fig. 2 The box-plots of posterior samples for parameter a1 estimated by GLUE and Bayesian methods under systematic errors and random errors. The subscripts S and R in subtitle represent the systematic error and random error, respectively; the error scenatios of 1, 2, 3, 4 and 5 means that the multiplier parameter is 1.1, 1.05, 1.0, 0.95 and 0.9 for systematic error and is normally distributed with variance value is 0.005 and mean values are 1.1, 1.05, 1.0,0.95 and 0.9 for random error, respectively GLUES GLUER 0.8 0.8 Value b Value a 0.6 0.4 0.4 1 2 3 4 5 1 Error scenarios 2 MHS 4 5 MHR 0.8 Value Value 3 Error scenarios 0.8 0.6 0.4 0.6 0.4 1 2 3 4 Error scenarios monthly hydrological data were between 01/1976 and 12/1983, which have undergone serious and strict quality control measures in previous studies (Wang 2005; Li et al. 2010). The models were calibrated against observed discharges at the watershed outlet of Xiahui station during 123 0.6 5 1 2 3 4 5 Error scenarios the same period. The mean annual precipitation is 494 mm, of which 80 % occurs in the rainy season from June to September. The mean runoff coefficient of the catchment is 0.19. There is rarely little precipitation in the winter. Stoch Environ Res Risk Assess (2014) 28:491–504 GLUER 0.8 0.8 0.6 0.6 Value Value GLUES 0.4 0.2 0 0.2 1 2 3 4 Error scenarios 0 5 1 2 3 4 Error scenarios 0.8 0.6 0.6 Value 0.8 0.4 0.4 0.2 0 0 1 2 3 4 Error scenarios 5 1 2 3 4 Error scenarios GLUE S 0.4 Value Value 5 GLUE R 0.4 0.2 0.2 0 0 1 2 3 4 5 1 Error scenarios 2 3 4 5 Error scenarios MHS MHR 0.4 Value 0.4 Value 5 MHR 0.2 Fig. 4 The box-plots of posterior samples for parameter a3 estimated by GLUE and Bayesian methods under systematic errors and random errors. The subscripts S and R in subtitle represent the systematic error and random error, respectively; the error scenarios of 1, 2, 3, 4 and 5 means that the multiplier parameter is 1.1, 1.05, 1.0, 0.95 and 0.9 for systematic error and is normal distributed with variance value is 0.005 and mean values are 1.1, 1.05, 1.0,0.95 and 0.9 for random error, respectively 0.4 MHS Value Fig. 3 The box-plots of posterior samples for parameter a2 estimated by GLUE and Bayesian methods under systematic errors and random errors. The subscripts S and R in subtitle represent the systematic error and random error, respectively; the error scenarios of 1, 2, 3, 4 and 5 means that the multiplier parameter is 1.1, 1.05, 1.0, 0.95 and 0.9 for systematic error and is normal distributed with variance value is 0.005 and mean values are 1.1, 1.05, 1.0,0.95 and 0.9 for random error, respectively 497 0.2 0.2 0 0 1 2 3 4 Error scenarios 3.2 WASMOD A simple conceptual water balance model, monthly WASMOD was used (Xu 2002; Jin et al. 2010). The input data to 5 1 2 3 4 5 Error scenarios the model include precipitation, potential evapotranspiration, while the output data include fast flow, slow flow, actual evapotranspiration, and soil moisture. The main equations are shown in Table 2 (Xu 2002; Li et al. 2010). 123 498 Stoch Environ Res Risk Assess (2014) 28:491–504 4 Results The error of observing precipitation come from many sources, i.e. wind effect and evaporation (Wolff et al. 2013). According to the technical standard for observations of precipitation the systematic error of observing precipitation is around 4–15 % in general (Ren et al. 2003), while the random error is very hard to quantify. Ajami et al. (2007) have studied the input uncertainty by adding a random multiplier which is under normal distribution with mean equal to [0.9,1.1] and variance equal to [1e–5, 1e–3]. According to pervious knowledge, in this study the Perturbing Error method was used and the multiplier parameters under system input errors are fixed to be 1.0, 0.9, 0.95, 1.05 and 1.1, while the multiplier parameters under random input errors are normal distribution with variance equals to 0.005 and mean equals to 1.0, 0.9, 0.95, 1.05 and 1.1. GLUE and Bayesian method are used to estimate the parameter and model uncertainty under these system errors and random errors. So there are 20 scenarios used in the study for uncertainty estimates, which are shown in Table 3. 4.1 Parameter estimate by GLUE and Bayesian method under precipitation errors The box-plots of posterior distributions of parameters a1, a2 and a3 that estimated by GLUE and Bayesian methods are shown in Figs. 2, 3 and 4, respectively. It can been seen from the figures that (1) the posterior distributions estimated by GLUE under input system errors and random errors are nearly the same, while by Bayesian method, the interval of posterior distributions of parameters under system errors are slightly narrower than those under random error; and (2) the posterior distributions derived by the Table 4 The character of parameters estimated by GLUE and Bayesian method for WASMOD under precipitation errors with constant multiplier parameter Method Parameter Multiplier parameter Min Max Mean Variance Skewness GLUE a1 1.1 0.472 0.857 0.609 6.42E-03 0.512 1.05 0.491 0.853 0.622 6.17E-03 0.551 1.0 0.506 0.855 0.633 5.65E-03 0.530 0.95 0.522 0.853 0.644 4.94E-03 0.466 0.9 0.543 0.854 0.661 4.44E-03 0.451 1.1 0.038 0.737 0.362 2.55E-02 0.108 1.05 0.048 0.754 0.381 2.58E-02 0.112 1.0 0.057 0.766 0.397 2.58E-02 0.070 0.95 0.073 0.786 0.415 2.59E-02 0.061 0.9 1.1 0.099 0.018 0.793 0.262 0.426 0.131 2.56E-02 2.76E-03 0.134 0.200 1.05 0.024 0.295 0.149 3.56E-03 0.175 1.0 0.029 0.337 0.173 4.63E-03 0.149 0.95 0.043 0.381 0.203 5.82E-03 0.145 0.9 0.049 0.434 0.231 6.92E-03 0.122 1.1 0.697 0.807 0.749 3.32E-04 0.258 1.05 0.683 0.805 0.755 3.61E-04 -0.288 1.0 0.704 0.814 0.76 3.33E-04 0.012 0.95 0.719 0.825 0.764 2.47E-04 0.217 0.9 0.72 0.828 0.774 3.07E-04 0.192 1.1 0.083 0.23 0.15 4.87E-04 0.178 1.05 0.093 0.281 0.157 6.47E-04 0.616 1.0 0.099 0.264 0.166 6.42E-04 0.433 0.95 0.104 0.251 0.174 5.35E-04 0.049 0.9 1.1 0.101 0.026 0.242 0.105 0.176 0.061 5.61E-04 1.37E-04 -0.186 0.292 1.05 0.032 0.127 0.071 1.99E-04 0.582 1.0 0.037 0.163 0.084 2.97E-04 0.416 0.95 0.025 0.202 0.101 3.68E-04 0.374 0.9 0.048 0.204 0.118 5.62E-04 0.283 a2 a3 Bayesian a1 a2 a3 123 Stoch Environ Res Risk Assess (2014) 28:491–504 499 Bayesian method are slightly narrower and sharper than those obtained by the GLUE method, which means less uncertainty in parameters. This is confirmed by the variances of parameters samples given in Tables 4 and 5, which show the posterior summary statistics for all parameters estimated by GLUE and Bayesian methods with precipitation errors, including system errors and random errors, respectively. The tables show that (1) the variances obtained by the Bayesian method are the smaller compared with those obtained by the GLUE method; (2) the skewness coefficients of the posterior distribution of parameters are strongly impacted by the precipitation errors in Bayesian method while which is not seen in GLUE; (3) the differences of the variance and skewness of posterior samples between system errors and random errors are quite small in GLUE method; (4) the medium value of posterior distribution for parameters a1, a2 and a3 estimated by GLUE and Bayesian methods are decreasing with the precipitation errors increasing. It might because that the evapotranspiration, fast flow and slow flow will increase with the precipitation increasing, which results in all parameters decrease in order to maintain original water balance; and (5) in Bayesian method, the random error with the N(1.0, 0.005) distributed multiplier parameter will slightly increase the variances of parameters. 4.2 Ninety-five percentage confidence interval of discharge by GLUE and Bayesian methods under precipitation errors Figure 5 shows the 95 % confidence intervals of monthly discharge (1976/1–1983/12) due to parameter and model uncertainty and observed discharge estimated by GLUE and Bayesian method. It is seen that the 95CI estimated Table 5 The character of parameters estimated by GLUE and Bayesian method for WASMOD under precipitation errors with normally distributed multiplier parameter Method Parameter Multiplier parameter Min Max Mean Variance Skewness GLUE a1 N (1.1, 0.005) 0.472 0.836 0.608 6.38E-03 0.512 N (1.05, 0.005) 0.491 0.850 0.622 6.09E-03 0.546 N (1.0, 0.005) 0.508 0.855 0.633 5.59E-03 0.528 N (0.95, 0.005) 0.522 0.842 0.643 4.92E-03 0.471 N (0.9, 0.005) 0.543 0.846 0.660 4.44E-03 0.442 N (1.1, 0.005) 0.038 0.737 0.362 2.53E-02 0.113 N (1.05, 0.005) 0.048 0.754 0.380 2.55E-02 0.110 N (1.0, 0.005) 0.068 0.766 0.396 2.54E-02 0.081 N (0.95, 0.005) 0.073 0.786 0.416 2.61E-02 0.064 N (0.9, 0.005) N (1.1, 0.005) 0.099 0.023 0.793 0.263 0.427 0.133 2.55E-02 2.85E-03 0.122 0.200 N (1.05, 0.005) 0.028 0.295 0.151 3.62E-03 0.169 N (1.0, 0.005) 0.029 0.338 0.175 4.73E-03 0.153 N (0.95, 0.005) 0.043 0.386 0.205 5.91E-03 0.139 N (0.9, 0.005) 0.055 0.440 0.235 7.18E-03 0.143 N (1.1, 0.005) 0.684 0.798 0.747 3.00E-04 N (1.05, 0.005) 0.697 0.805 0.756 3.15E-04 N (1.0, 0.005) 0.710 0.821 0.761 3.59E-04 0.183 N (0.95, 0.005) 0.718 0.824 0.765 2.73E-04 0.203 N (0.9, 0.005) 0.73 0.83 0.775 3.01E-04 0.291 N (1.1, 0.005) 0.094 0.228 0.151 4.73E-04 0.154 N (1.05, 0.005) 0.09 0.248 0.157 5.72E-04 0.484 N (1.0, 0.005) 0.092 0.251 0.165 6.48E-04 0.167 N (0.95, 0.005) 0.103 0.25 0.174 6.32E-04 0.184 N (0.9, 0.005) N (1.1, 0.005) 0.11 0.025 0.243 0.12 0.176 0.062 7.24E-04 1.34E-04 0.01 0.385 N (1.05, 0.005) 0.029 0.136 0.071 1.99E-04 0.522 N (1.0, 0.005) 0.031 0.162 0.084 3.06E-04 0.293 N (0.95, 0.005) 0.038 0.186 0.101 3.87E-04 0.386 N (0.9, 0.005) 0.049 0.203 0.118 5.64E-04 0.247 a2 a3 Bayesian a1 a2 a3 0.069 -0.17 123 500 Stoch Environ Res Risk Assess (2014) 28:491–504 80 (a) 95% confidence interval observed Simulated Q(mm) 60 40 20 0 1976-01 1977-01 1978-01 1979-01 1980-01 1981-01 1982-01 1983-01 Month (Y-M) 80 95% confidence interval observed Simulated (b) Q(mm) 60 40 20 0 1976-01 1977-01 1978-01 1979-01 1980-01 1981-01 1982-01 1983-01 Month (Y-M) Fig. 5 The 95 % confidence intervals of monthly discharge (1976/1–1983/12) due to parameter uncertainty and model uncertainty (grey bands) and observed discharge (red spots). a GLUE (ARIL is 1.445 and P-95CI is 69.8 %); b Bayesian (ARIL is 2.487 and P-95CI is 96.9 %) 1 (b) gsa 0.6 gsd gse 0.4 1 gra gsb gsc 0.8 PCI of all flow (a) PCI of all flow Fig. 6 Proportion of observation inside confidence intervals (PCI) for WASMOD of monthly runoffs in Chao River basin by a GLUE with systematic error, b GLUE with random error, c Bayesian with systematic error and d Bayesian with random error. Legend details can be seen in Table 3 0.2 0.8 grb 0.6 grc grd gre 0.4 0.2 0 0 0 0.2 0.4 0.6 0.8 1 0 Confidence Interval PCI of all flow (d) 1 bsa 0.6 bse 0.2 1 brb brc 0.6 brd bre 0.4 0.2 0 0 0 0.2 0.4 0.6 0.8 Confidence Interval 123 0.8 1 0.8 bsd 0.4 0.6 bra bsb bsc 0.8 0.4 Confidence Interval PCI of all flow (c) 0.2 1 0 0.2 0.4 0.6 0.8 Confidence Interval 1 Stoch Environ Res Risk Assess (2014) 28:491–504 501 from GLUE is slightly narrower than that from Bayesian method, which can be seen more clearly in the comparison of PCI in Figure 6, which shows the PCI for WASMOD of monthly runoffs in Chao river basin by GLUE and Bayesian method with systematic or random error. It can be seen that the uncertainty interval derived from Bayesian method is more reliable than that from GLUE since the points in Fig. 6c and d are more closer to the 45 diagonal lines. Figure 7 shows the ARIL for WASMOD of monthly runoffs in Chao river basin by GLUE and Bayesian methods with precipitation errors. It can be seen the same results as Fig. 5 that the uncertainty interval derived by Bayesian is slightly wider than that from GLUE. Besides, the impact of precipitation errors are more obvious in Bayesian method than GLUE method since the lines of ARIL are more various in Fig. 7c and d than Fig. 7a and b. 4.3 Sensitivity analysis of uncertainty estimates by GLUE and Bayesian methods in response to precipitation errors The sensitivity analysis will be applied by a quantitatively comparison of 95CI of WASMOD monthly runoff for three 3 (b) ARIL of all flow 2.5 gsb 2 gsc 1.5 gsd 1 3 2.5 gsa ARIL of all flow (a) gse 0.5 gra grb grc grd gre 2 1.5 1 0.5 0 0 0 0.2 0.4 0.6 0.8 1 0 Confidence Interval (d) 3 2.5 bsa bsb 2 bsc 1.5 bsd 1 0.2 0.4 0.6 0.8 1 Confidence Interval ARIL of all flow (c) ARIL of all flow Fig. 7 Average relative interval length (ARIL) for WASMOD of monthly runoffs in Chao River basin by a GLUE with systematic error, b GLUE with random error, c Bayesian with systematic error and d Bayesian with random error. Legend details can be seen in Table 3 levels of flows, i.e. low flow, medium flow, high flow, which are accounting for 10, 70 and 20 % of total runoff, respectively and all flows, obtained by GLUE and Bayesian methods under different precipitation errors. There are four indices, i.e. ARIL, PCI, PUCI and CRPS, used for evaluation of 95 % confident intervals of runoffs estimated by GLUE and Bayesian methods. Figure 8 shows the variation of four indices for uncertainty results of four level flows according to 20 scenarios which are a combination of different uncertainty estimate methods with different precipitation errors. It can be seen that (1) for the ARIL, Bayesian method is obviously more sensitive than GLUE in response to different precipitation errors, especially for low flows, which means that there is more risk for Bayesian method in uncertainty estimate of sharpness, especially for the low flows; (2) according to PCI, the 95CI of discharge from GLUE is much more sensitive than those from Bayesian method, especially for low and medium flows, which indicates that there is more risk in reliability for uncertainty estimate by GLUE method than Bayesian method; (3) the PCI is not sensitive in high flows according to the precipitation errors by both GLUE and Bayesian methods, which means that the risk of reliability in bse 0.5 3 2.5 bra 2 brb brc 1.5 brd bre 1 0.5 0 0 0 0.2 0.4 0.6 0.8 Confidence Interval 1 0 0.2 0.4 0.6 0.8 1 Confidence Interval 123 gsa gsb gsc gsd gse gra grb grc grd gre bsa bsb bsc bsd bse bra brb brc brd bre PUCI gsa gsb gsc gsd gse gra grb grc grd gre bsa bsb bsc bsd bse bra brb brc brd bre PCI gsa gsb gsc gsd gse gra grb grc grd gre bsa bsb bsc bsd bse bra brb brc brd bre ARIL (a) (b) (c) (d) 6 5 Low High 3.0 123 Medium All 4 3 2 1 0 Scenarios 1.2 1.0 0.8 0.6 0.4 Low Medium 0.2 High 0.0 All Scenarios 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Low Medium High All Scenarios 6.0 5.0 4.0 Low Medium High 2.0 All 1.0 0.0 Scenarios Fig. 8 The uncertainty evaluation of 95 % confident intervals of monthly discharge in Chao River basin in 20 scenarios by four indices, including a ARIL, b PCI, c PUCI and d CRPS. The definition of scenarios can be seen in Table 3 uncertainty estimates for high flows is not as large as the other flows by both GLUE and Bayesian method under the impact of precipitation errors; (4) the PUCI shows that both GLUE and Bayesian method are more sensitive in uncertainty estimate of high flow than other flows according to precipitation errors; and (5) the CRPS of low, medium flows are quite stable under impact of bsb 0.55 1.44 bsa PUCI CRPS 2.16 0.95 0.46 1.16 ARIL PCI PUCI CRPS 1.17 0.35 0.98 2.80 0.56 0.79 1.19 0.40 0.96 2.49 bsc 1.45 0.56 0.75 1.42 gsc 1.21 0.40 0.96 2.48 bsd 1.47 0.56 0.70 1.35 gsd 1.19 0.38 0.97 2.59 bse 1.49 0.57 0.68 1.28 gse 1.16 0.48 0.95 2.09 bra 1.43 0.55 0.79 1.53 gra 1.15 0.42 0.96 2.38 brb 1.43 0.57 0.79 1.49 grb 1.18 0.39 0.97 2.51 brc 1.44 0.57 0.76 1.41 grc 1.22 0.45 0.94 2.18 brd 1.47 0.55 0.69 1.34 grd 1.22 0.45 0.95 2.24 bre 1.48 0.57 0.68 1.27 gre ARIL the average relative interval length, PCI the percent of observations bracketed by the 95 % confident interval, PUCI the percentage of observations bracketed by the unit confidence interval, CRPS the continuous rank probability score Bayesian 1.44 0.80 PCI 1.50 1.55 ARIL gsb GLUE gsa Scenarios Indices Method Table 6 The evaluation results of 95 % confident interval of discharge for WASMOD in Chao River basin by GLUE and Bayesian method under different precipitation errors (20 scenarios) gsa gsb gsc gsd gse gra grb grc grd gre bsa bsb bsc bsd bse bra brb brc brd bre CRPS 502 Stoch Environ Res Risk Assess (2014) 28:491–504 Stoch Environ Res Risk Assess (2014) 28:491–504 precipitation errors and it only changes in uncertainty estimate of high flow in Bayesian method by the impact of precipitation errors. The quantitative results of ARIL, PCI, PUCI and CPRS estimated by GLUE and Bayesian method due to different model precipitation errors are also summarized in Table 6. 503 Acknowledgments This study was supported by the Research Council of Norway, Research Project-JOINTINDNOR 203867, Department of Science and Technology, Govt. of India, Project 190159/V10 (SoCoCA) and Project NORINDIA 806793. The second author was also supported by Program of Introducing Talents of Discipline to Universities—the 111 Project of Hohai University. References 5 Conclusion In this study, a comprehensive evaluation and quantification of the effect of precipitation errors, i.e. multiplier parameter is constant or multiplier parameter is a random number with normal distribution, in GLUE on the parameter and model uncertainty was performed, and the results were compared with a formal Bayesian method using the MH algorithm for a conceptual hydrological model. The 95 % confidence intervals of monthly discharge for low flow, medium flow and high flow were selected for comparison by four indices, i.e. ARIL, PCI, PUCI and CRPS. The following conclusions are drawn from this study. • • • • • • The posterior distributions of parameters derived by the Bayesian method are slightly narrower and sharper than those obtained by the GLUE method for both with and without precipitation errors, though the differences are quite small. Bayesian method performs more sensitive in uncertainty estimates of discharge than GLUE according to the impact of precipitation errors. In terms of the PCI, the impact of precipitation errors on the 95CI of WASMOD monthly runoffs in Chao river basin obtained by Bayesian is very small, especially for high flows, which indicates that the Bayesian method is a more reliable approach for application especially for regions which might be with large uncertainty, e.g. ungauged basins. In terms of the ARIL, the Bayesian method is more sensitivity than GLUE method according to precipitation errors, especially for low flows. The values of PUCI show that the GLUE and Bayesian methods are more sensitive in uncertainty estimate of high flow than the other flows by the impact of precipitation errors. Under the impact of precipitation errors, the results of CRPS for low and medium flows are quite stable from both GLUE and Bayesian method while it is sensitive for high flow by Bayesian method. It is worth noting that the index of CRPS is very sensitive for high flows, which is strongly recommended to use in uncertainty evaluation for extreme events, i.e. flood. It should be noted that although the study yielded interesting findings, the generality of the results requires testing of the method in other areas using other models in future research. 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