2.1 VIC model
The variable infiltration capacity (VIC) model is adopted to simulate flows in this study. The VIC model is a macro-scale distributed hydrological model based on a soil-vegetation-atmosphere transfer scheme. It considers both energy and water balances and explicitly represents the effects of multiple vegetation covers on water and energy budgets. The VIC model also incorporates the representation of subgrid spatial variability of precipitation with the representation of spatial variability of infiltration. In addition, it includes both the saturation and infiltration excess runoff processes in a model grid cell with a consideration of the subgrid-scale soil heterogeneity and the frozen soil processes for cold climate conditions (Liang et al. 1994; Guo et al. 2009). Three types of evaporation: evaporation from wet canopy, evapotranspiration from dry canopy and evaporation are considered. The one dimensional Richard equation is used to describe the vertical soil moisture movement and the moisture transfer between soil layers obeys the Darcy law. The routing model represented by the unit hydrograph method for overland flow and the linear Saint-Venant method for channel flow, allow runoff to be predicted (Zhou and Guo, 2013). The VIC model has been tested and applied to various basins of different scales with good performance (Su and Xie 2003;Yuan et al. 2004༛Guo et al. 2009).
2.2 Marginal distributions of observed and simulated flows
The probability distributions of daily observed and simulated flows used in this study refer to the flow duration curve, and the daily streamflow is assumed to be a random variable (Castellarin et al. 2004). The Gamma and Log-normal are the two commonly used distributions in hydrology to fit the daily streamflow series in the literatures (Xiong et al. 2015), thus are selected as candidate probability models for observed and simulated flows in this study. The probability density function of the Gamma and Log-normal are as follows.
where α and β represent the parameters of the Gamma distribution.
where \({\mu _Y}\) and \({\sigma _Y}\) represent the parameters of the Log-normal distribution.
L-moment method was used to estimate the distribution parameters for given data series (Hosking 1990). The Kolmogorov- Smirnov statistic D is the measure of the goodness of fit between the hypothesized distribution and the empirical distribution obtained by Weibull plotting position formula (Liu et al. 2016). In this study, the 5% significance level was selected to reject or accept a fitted distribution. The probability distribution which provides the smaller D value is chosen as the better fitting distribution.
2.3 Joint distributions of observed and simulated flows
The dependence between observed and simulated flows is modelled with the copula function. In this study we concentrate on the bivariate case. Using a copula function, the joint distribution of observed flow H and simulated flow S is given by
$${F_{H,S}}(h,s)={C_\theta }(G(h),F(s))={C_\theta }(u,v)$$
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where \(u=G(h)\) and \(v=F(s)\) represent the marginal distribution functions; θ is the dependence parameter of copula.
Three widely used bivariate Archimedean copulas in hydrology, including the Gumbel-Hougaard (G-H), Frank and Clayton copulas described in Table 1, are adopted as candidates (Nelsen 2006). The relationship between their dependence parameters and Kendall correlation coefficient tau (τ) are also presented in Table 1. The p-value associated to the Cramér-von Mises test statistic Sn is computed based on the parametric bootstrap technique to formally assess whether the selected model is suitable (Dung et al. 2015). The selected copula should have the lower value of the statistic Sn with an admissible p-value (i.e. larger than 0.05).
2.4 Copula-based Bayesian processor
Let H be the observed flow whose realization h is being simulated. Let estimator S be the output flow generated by a deterministic rainfall-runoff model whose realization s constitutes a point estimate of H. The posterior density function of conditional on a deterministic simulated flow \(S=s\), is derived via Bayes’ theorem:
$$\phi (h|s)=\frac{{f(s|h) \cdot g(h)}}{{\int_{{ - \infty }}^{{+\infty }} {f(s|h) \cdot g(h)} dh}}$$
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The prior density used in the CBP is directly the probability density function of H which expressed as \(g(h)\). It can be estimated based on historical data of observed flows using the procedure in section 2.2 in the original space. Using the copula function, the likelihood function is defined as (Nelsen 2006)
$$f(s|h)={c_\theta }(u,v) \cdot f(s)$$
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where \({c_\theta }(u,v)={\partial ^2}{C_\theta }(u,v)/\partial u\partial v\) is the density function of \({C_\theta }(u,v)\); \(f(s)\) is the PDF of . Given \(S=s\), the likelihood function of can be calculated by Eq. (5).
The posterior density function \(\phi (h|s)\) can be rewritten as follows
$$\phi (h|s)=\frac{{{c_\theta }(u,v)}}{{\int_{0}^{1} {{c_\theta }(u,v)du} }} \cdot g(h)$$
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The Monte Carlo sampling technique is applied to estimate the numerical solution of the posterior density function \(\phi (h|s)\). The corresponding posterior distribution function \(\Phi (h|s)\) is defined by
$$\Phi (h|s)=\int_{{ - \infty }}^{h} {\phi (h|s)dh}$$
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Once \(\Phi (h|s)\) has been determined, various information and products (e.g., quantiles with specified exceedance probabilities, simulation intervals with specified inclusion probabilities, and probabilities of exceedance for specified thresholds) could be provided for decision makers.
2.5 Performance criteria used
For deterministic simulations, the Nash-Sutcliffe efficiency (NSE) and Relative Error (RE) are used to evaluate the performance. NSE can range from −∞ to 1. Essentially, the closer the model efficiency is to 1, the more accurate the model is (Chen et al. 2015). A value of RE closes to zero indicates a good agreement between observed and simulated runoff volume.
For probabilistic simulations, the simulative quantile-quantile (QQ) plot, α-index (reliability), π-index (resolution) and continuous rank probability score (CRPS) are adopted to evaluate probabilistic simulations (Laio and Tamea 2007; Thyer et al. 2009; Madadgar et al. 2014; Liu et al. 2018). The metric α-index assesses the reliability of simulations and varies between 0 (worst reliability) and 1 (perfect reliability), while the π-index indicates the resolution (sharpness) of the simulative distribution, and greater value of π-index indicates greater resolution (lower uncertainty) of simulations. The smaller the CRPS value is, the better the simulation performance. For a deterministic simulation, the CRPS reduces to the mean absolute error (MAE) (Pappenberger et al. 2015). Its minimal value of zero is only achieved in the case of a perfect deterministic simulation.