the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Design water demand of irrigation for a large region using a highdimensional Gaussian copula
Yiliang Du
Vijay P. Singh
Xiaohong Chen
Kairong Lin
Haiou Wu
Spatial and frequency distributions of precipitation should be considered in determining design water demand of irrigation for a large region. In Guangdong Province, South China, as a study case, an eightdimensional joint distribution of precipitation for agricultural subregions was developed. A design procedure for water demand of irrigation for a given frequency of precipitation of the entire region was proposed. Water demands of irrigation in the entire region and its subregions using three design methods, i.e., equalized frequency (EF), typical year (TY) and mostlikely weight function (MLW), were compared. Results demonstrated that the Gaussian copula efficiently fitted the highdimensional joint distribution of eight subregional precipitation values. The Kendall frequency was better than the conventional joint frequency to analyze the linkage between the frequency of precipitation of the entire region and individual subregions. For given frequencies of precipitation of the entire region, design water demands of irrigation of the entire region among the MLW, EF and TY methods slightly differed, but those of individual subregions of the MLW and TY methods fluctuated around the demand lines of the EF method. The alterations of design water demand in subregions were more complicated than those in the entire region. The design procedure using the MLW method in association with a highdimensional copula, which simulated individual univariate distributions, captured their dependences for multivariables, and built a linkage between regional frequency and subregional frequency of precipitation, is recommended for design water demand of irrigation for a large region.
Water demand of irrigation in a region is associated with exploitation and utilization of land and water resources, such as farm area, cultivation pattern, category of crops, canal technology, etc., and is also impacted by natural factors, for example precipitation volume and soil properties (Tarjuelo et al., 2005; Griffin, 2006; Leenhardt et al., 2004, 2011; Wriedt et al., 2009; Davidson and Hellegers, 2011). Among the many factors, precipitation, which is regarded as stochastic, is an uncertain factor influencing irrigation (Wisser et al., 2008; Thomas, 2008; Cai et al., 2011; Meza et al., 2012). In general, the more the precipitation, the less the water demand of irrigation, and vice versa. In practical regional water resources planning, water demands of irrigation are estimated in association with various frequencies of regional precipitation (Gohari et al., 2013).
For design water demand in a large region, it is important that the heterogeneity of those factors in geographical space is considered (Lankford, 2010; Leenhardt et al., 2011). Hence, water demand should generally be analyzed in separate subregions in which the factors are homogenous, and can eventually be summed up. However, for a given precipitation frequency of the entire region in the design water demand of irrigation, the difficulty is how to obtain a reasonable combination of precipitation frequencies of multiple subregions, even though other factors influencing irrigation have previously been demonstrated in water resources planning.
A method, named typical year (TY), has been used for design water demand of irrigation in China (Cai et al., 2001). A combination of observed subregional precipitation in a certain year in which the precipitation frequency for the entire region weighted by individual subregions was the nearest to the given frequency, was selected and was zoomed in or out. Due to the limited observed samples, the representation of the typical year has been questionable. In actuality, the frequency of regional precipitation in a large region corresponds to various combinations of subregional frequencies.
Moreover, design water demand of irrigation in a large region not only presents the stochastic characteristic of precipitation in the entire region and its subregions, but also addresses the relationship between subregional precipitation and heterogeneity. The multivariable statistical simulation and joint design theory can be considered to improve the selection of an appropriate combination. Due to the multivariate dependence and the flexibility of unbounded marginal distributions, copula functions have been widely applied in the simulation and design of hydrological multivariables, e.g., extreme properties of heavy precipitation (Zhang et al., 2011, 2013; Abdul Rauf and Zeephongsekul, 2014), floods (Zhang and Singh, 2006; Chowdhary et al., 2011; Zhang et al., 2015a) and droughts (Ganguli and Reddy, 2014; Zhang et al., 2015b; Tu et al., 2016). In recent years, they have been used to analyze floods or droughts encountered in multiple hydrological regions in China (Yan et al., 2010a; Xie et al., 2012; Tu et al., 2017b). These studies on multivariate hydrology mostly focused on bivariate and trivariate issues, but less on higher dimensional hydrological statistical analyses (Chen et al., 2015). A highdimensional metaGaussian copula beyond three variables has been applied in other fields, e.g., economic analysis (Aussenegg and Cech, 2012; Creal and Tsay, 2015).
As a multidimensional copula and its marginal distributions are determined according to observed samples, the joint frequency and probability density for any combination of precipitation frequencies of subregions can be calculated. In a practical design combination, a large quantity of combinations can be randomly generated using the Monte Carlo method on the basis of the determined copula. It is clear that a given joint frequency corresponds to quite a number of combinations. In order to select one from various combinations, a simple method, based on the equalized frequency (EF) method, which refers to all marginal frequencies being identical, is used for design flood peak and volume (Liu et al., 2015).
Another improved method is that the combination can be exclusively determined by using the mostlikely weight function (MLW) in association with the products of joint and marginal probability densities (Salvadori et al., 2011). The mostlikely weight design method has been applied for the design combination of hydrological multivariables, such as flood and drought properties (Gräler et al., 2013; Zhang et al., 2015c), flood and tide or heavy precipitation and tide in coastal rivers (Corbella and Stretch, 2012; Lian et al., 2013; Zheng et al., 2013; Tu et al., 2017a), and the precipitation or streamflow of multiple regions (Yan et al., 2010b).
Furthermore, in practical water resources planning, the main concern is that water demand of irrigation of the entire region and that of subregions are designed for a given frequency of precipitation for the entire region, not for a given joint frequency of precipitation of subregions. The relationship between regional frequency and joint frequency of precipitation is also required to be investigated in determining design water demand of irrigation in a large region.
This paper considered water demand of irrigation of paddies in Guangdong Province, South China, as a case study of a large region. Annual precipitation data of eight agricultural subregions and their net quotas of irrigation in association with precipitation frequency were used. A highdimensional metaGaussian copula and several conventional univariate distributions were applied to fit the joint and marginal distributions of subregional precipitation, respectively. Combinations of cumulative frequency for subregional precipitation were generated by using the Monte Carlo simulation method on the basis of the determined copula. The link between the joint frequency of subregional precipitation and the frequency of precipitation of the entire region was established. The methods, i.e., typical year, equalized frequency, and the mostlikely weight function, were used for design combinations of subregional precipitation for given frequencies of precipitation of the entire region. Water demand of irrigation of the entire region and individual subregions among design methods was compared in order to improve design water demand of irrigation in a large region.
2.1 Water demand of irrigation of paddy
In a paddy field, water demand of irrigation per unit area, i.e., the net quota of irrigation, is determined by various factors, such as crop and soil types, cultivation pattern and precipitation process, etc. In regional planning of water resources, as other factors are demonstrated in advance, the net quota of irrigation mainly changes with annual precipitation due to the stochastic property of precipitation. In practice, the net quota of irrigation per unit area q(u) in association with the frequency of precipitation is determined via field experiments. Then, for a given frequency of annual precipitation, u, water demand of irrigation, W(u), which refers to water withdrawn from river or other water sources engineering, can be calculated as
where A and q(u) refer to the paddy area and the net quota of irrigation, respectively, and η is the utilization coefficient of irrigation, which refers to the ratio of net water supply in the field to water withdrawn from river or other water sources engineering.
In a large region, for example Guangdong Province, China, the differences of annual precipitation and the net quota of irrigation among individual agricultural regions are required to be considered. Then, for a given regional frequency of annual precipitation, u_{0}, for a large region with subregions, water demand of irrigation, W(u_{0}), can be deduced as follows:
where A_{i} refers to the irrigation area of the ith subregion and q_{i}(u_{i}) is the net quota of irrigation per unit area at the frequency of precipitation of the ith subregion, u_{i}.
Therefore, the key point to design water demand of irrigation for a given precipitation frequency of the entire region, u_{0}, is how to determine a combination of subregional precipitation frequencies, $\left\{{u}_{\mathrm{1}},\mathrm{\cdots},{u}_{d}\right\}$.
2.2 Annual precipitation distribution
If a large region consists of d subregions, let X_{i} be the series of annual precipitation in which $i=\mathrm{1},\mathrm{\cdots},d$ and $j=\mathrm{1},\mathrm{\cdots},n$ refer to subregion d and n years, respectively. The theoretical cumulative distribution of annual precipitation in a subregion, ${F}_{{X}_{i}}\left(x\right)$, can be fitted by several conventional three parameter univariate distributions, which have been widely used in hydrology, such as generalized extreme value (GEV), generalized logistic (GLO), Pearson III (PIII) and generalized normal (GNO). Their cumulative distribution and probability density functions are presented in Table 1.
The Kolmogorov–Smirnov (K–S) statistic, D, for the goodnessoffit test of annual precipitation distribution of a subregion was computed as (Dobric and Schmid, 2007; Massey, 2012; Tu et al., 2016)
The parameters of all recommended univariate distributions were estimated by the Lmoment method. The critical values at the significance level of 0.05 for the goodnessoffit test of all distributions were obtained by the Monte Carlo method with 5000 or more simulations. If the K–S statistic, D, which was computed from the samples, was less than the corresponding critical value, the tested distribution was accepted. The optimal distribution was selected from the accepted distributions by comparing their rootmeansquare error (RMSE) and Akaike information criterion (AIC) values. In addition, empirical and theoretical distributions were compared to evaluate the goodness of fit of the observed samples of precipitation. In hydrological practice, the empirical distribution functions are defined and transformed by Gringorten (1963).
Subsequently, based on the areal weight method, the annual precipitation in the entire region, X_{0}(j), was calculated as
Where α_{i} refers to the areal ratio of the ith subregion to the entire region. Then, the theoretical distribution of annual precipitation for the entire region, ${F}_{{X}_{\mathrm{0}}}\left(x\right)$, can also be fitted by using the above recommended univariate distributions and the KS test method.
2.3 Conventional design methods
2.3.1 Typical year method (TY)
In practical design, water demand of irrigation for a large region, a combination of observed subregional precipitation in a certain year, in which the precipitation of the entire region weighted by individual subregions is the nearest to that of a given frequency for the entire region, has been the only selection (Cai et al., 2001). The selected year was called the typical year corresponding to the given frequency of precipitation. Let ${\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}$ be a given cumulative distribution frequency (CDF) of precipitation of the entire region. Then, the corresponding precipitation can be calculated by using the inverse function of the frequency distribution as follows:
Further, the relative alteration, $R\left(j\right)j=\mathrm{1},\mathrm{\cdots},n$, of the observed precipitation in each year compared to the precipitation of the given frequency, was defined as
Then,
where J is the selected rank of a certain year, which corresponds to the typical year for the given frequency ${\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}$.
In addition, due to the limited length of annual precipitation observations, the relative alteration R(J) of the typical selected year might be large. Namely, annual precipitation of the entire region weighted by subregional precipitation in a typical selected year, in terms of magnitude, may differ from that of the given frequency. Therefore, a scaling method was applied to zoom in or out for the subregional precipitation in the typical selected year as follows:
where β is a scale coefficient and X_{i}(J) and ${\stackrel{\mathrm{\u0303}}{X}}_{i}\left(J\right)$ refer to the ith subregional precipitation before and after zooming in or out, respectively.
For a given precipitation frequency of the entire region, ${\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}$, individual subregional frequencies, ${\stackrel{\mathrm{\u0303}}{u}}_{i}\left(J\right)i=\mathrm{1},\mathrm{\cdots},d$, can eventually be deduced by their frequency distributions as follows:
2.3.2 Equalized frequency method (EF)
In order to get a combination of subregional frequencies for a given precipitation frequency of the entire region, the equalized frequency method (Liu et al., 2015) is also used for downscaling precipitation for a large region. As the name implies, the EF assumes that the frequencies of subregional precipitation are identical. That is, for a given precipitation frequency ${\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}$, let ${\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{1}}=,\mathrm{\cdots},={\stackrel{\mathrm{\u0303}}{u}}_{d}$, and then ${\stackrel{\mathrm{\u0303}}{u}}_{i}$ can be found as follows:
where ${F}_{{X}_{\mathrm{0}}}^{\mathrm{1}}\left({\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}\right)$ and ${F}_{{X}_{i}}^{\mathrm{1}}\left({\stackrel{\mathrm{\u0303}}{u}}_{i}\right)$ refer to the precipitation in the entire region and subregions calculated by using the inverse function of individual frequency distributions, respectively. In practical design, the subregional frequency ${\stackrel{\mathrm{\u0303}}{u}}_{i}$ is determined by applying the method of successive search approximation within an available range.
2.4 Joint design based on copula function
2.4.1 Multidimensional joint distribution
For a large region, the difference of precipitation between any two subregions is associated with geographical location. Though annual precipitation in any subregion is regarded as stochastic, there exists dependence between two subregions, in particular for adjacent subregions due to similar geographic and climate conditions. Herein, a multidimensional copula function was used for modeling the joint distribution of subregional precipitation. Assume that annual precipitation of each subregion, ${X}_{i},i=\mathrm{1},\mathrm{\cdots},d$, is a continuous random variable with a ddimensional joint distribution $H({X}_{i},\mathrm{\cdots},{X}_{d})$ and individual marginal distribution functions ${F}_{{X}_{i}}\left(x\right),i=\mathrm{1},\mathrm{\cdots},d$. Then, on the basis of the Sklar theorem (Nelsen, 2006), the joint distribution, $H({X}_{\mathrm{1}},\mathrm{\cdots},{X}_{d})$, can be defined as
where $C\left({u}_{\mathrm{1}},\mathrm{\cdots},{u}_{d}\right)$ is the ddimensional copula function which is the joint distribution function of standard uniform random variables, and ${u}_{i}={F}_{{X}_{i}}\left(x\right),i=\mathrm{1},\mathrm{\cdots},d$, refer to individual CDFs of subregional precipitation.
The copula function, which accommodates different marginal distributions of individual variables and captures their dependence, has been widely applied in multivariate hydrology. More details on the theoretical properties of various copula families can be found in Nelsen (2006). Owing to its flexibility, accessibility and simple copula parameters in association with a correlation coefficient matrix, a ddimensional metaGaussian copula was selected for modeling the joint distribution of multiple subregional precipitations. Its theoretical cumulative distribution function, $C\left({u}_{\mathrm{1}},\mathrm{\cdots},{u}_{d}\right)$, and density function, $c\left({u}_{\mathrm{1}},\mathrm{\cdots},{u}_{d}\right)$, were deduced as follows (Genest et al., 2007):
where
where ${b}_{\mathrm{1}}={\mathrm{\Phi}}^{\mathrm{1}}\left({u}_{\mathrm{1}}\right),\mathrm{\cdots},{b}_{d}={\mathrm{\Phi}}^{\mathrm{1}}\left({u}_{d}\right)$, in which ${\mathrm{\Phi}}^{\mathrm{1}}\left(\cdot \right)$ refers to the inverse function of the standard normal distribution. $\mathit{\omega}={\left[{\mathit{\omega}}_{\mathrm{1}},\mathrm{\cdots},{\mathit{\omega}}_{d}\right]}^{T}$ and $\mathit{\zeta}={\left[{b}_{\mathrm{1}},\mathrm{\cdots},{b}_{d}\right]}^{T}$ are the matrices of variables in the integrand. The correlation coefficient matrix Σ was expressed as
where, ${\mathit{\rho}}_{ij}\in \left[\mathrm{1},\mathrm{1}\right]$ refers to the correlation coefficient between any two subregional precipitations.
For the goodnessoffit test of multidimensional metaGaussian copula, the Cramér–von Mises test statistic on the basis of the Rosenblatt transform was used (Rosenblatt, 1952; Genest et al., 2009). For the joint distribution of subregional precipitation with a d dimension, the goodnessoffit test statistics, ${S}_{n}^{B}$, were formulated as
where ${E}_{i},\mathrm{i}=\mathrm{1},\mathrm{\cdots},\mathrm{d}$, refers to the pseudoobservations of individual subregional precipitation. Let E_{i}=u_{1}, and ${E}_{i},i=\mathrm{2},\mathrm{\cdots},d$ be assigned as (Rosenblatt, 1952; Dobric and Schmid, 2007)
A parametric bootstrap procedure for ${S}_{n}^{B}$, deduced from the literature, is addressed in Appendix D (Genest et al., 2009). In multivariate practice, the joint empirical distribution functions are defined (Dobric and Schmid, 2007; Genest et al., 2009; Tu et al., 2016) and transformed by Gringorten (1963).
In addition, the Kendall function, which is a univariate expression of multivariate information (Genest and Rivest, 1993; Barbe et al., 1996; Salvadori et al., 2011, 2013), has been shown to be an appropriate tool for calculating the copulabased joint frequency of multivariate events (Nappo and Spizzichino, 2009; Salvadori et al., 2011) and is widely applied in discussing the joint probability or return period of hydrological multivariables (Salvadori and Michele, 2004; Michele et al., 2013). The Kendall CDF, ${F}_{{K}_{c}}$, which was transformed from the joint CDF of eight subregional precipitation and was used in comparing it with the frequency of entire regional precipitation, was estimated as
where $q\in \left(\mathrm{0},\mathrm{1}\right)$ is the probability level and n refers to the length of observed or simulated samples. The function I(⋅) is an indicator function, which is equal to 1 when the enclosed expression is true, and 0 otherwise.
2.4.2 Mostlikely weight function (MLW)
In multivariate design practice, using sample data of annual precipitation of all subregions, the univariate distribution of the entire region, joint and marginal distributions of subregions, and parameters of all distributions can be determined by the recommended univariate models and Gaussian copula. The Monte Carlo method can be used to simulate new combinations of CDFs of precipitation using the determined distributions and parameters. Then, the CDFs of precipitation of the entire region corresponding to each combination of subregional CDFs can be achieved. However, there are a large number of combinations which lead to the CDFs of the entire region, which can be almost identical within a predefined small difference. That is, a given CDF of precipitation of the entire region can correspond to many combinations of subregional CDFs with enough further simulations. The design realization using the mostlikely weight function was proposed by Salvadori et al. (2011) as follows:
where $\left[{\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{1}},\mathrm{\cdots},{\stackrel{\mathrm{\u0303}}{u}}_{d}\right]$ is eventually selected as the design combination of CDFs of subregional precipitation for a given CDF of the entire region, ${\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}$. $f\left({x}_{\mathrm{1}},\mathrm{\cdots},{x}_{d}\right)$ refers to the product of joint probability densities, $c\left({u}_{\mathrm{1}},\mathrm{\cdots},{u}_{d}\right)$, and their individual marginal probability densities, $f\left({x}_{i}\right)i=\mathrm{1},\mathrm{\cdots},d$. Therefore, the procedure for design combination of subregional precipitation for a given CDF of the entire region was as follows.

The joint and marginal distributions and parameters of subregional precipitation were determined by the goodness of fit of the recommended ddimensional metaGaussian copula and univariate distributions for the observed samples.

Using the Monte Carlo method according to the determined ddimensional joint distribution, large quantities of combinations of CDFs of subregional precipitation, $\left[{u}_{\mathrm{1}}\left(j\right),\mathrm{\cdots},{u}_{d}\left(j\right)\right]j=\mathrm{1},\mathrm{\cdots},m$, with the number of simulations, m, were generated, and the corresponding combinations of subregional precipitation, $\left[{X}_{\mathrm{1}}\left(j\right),\mathrm{\cdots},{X}_{d}\left(j\right)\right]j=\mathrm{1},\mathrm{\cdots},m$, and precipitation and CDFs of the entire region, ${X}_{\mathrm{0}}\left(j\right)j=\mathrm{1},\mathrm{\cdots},m$ and ${u}_{\mathrm{0}}\left(j\right)j=\mathrm{1},\mathrm{\cdots},m$, were calculated.

For a given CDF of precipitation of the entire region, ${\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}$, the allowable relative error, was defined as
$$\begin{array}{}\text{(22)}& \left{\displaystyle \frac{{u}_{\mathrm{0}}\left(j\right){\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}}{{\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}}}\right\le {R}_{\mathrm{e}},\end{array}$$where R_{e} refers to the threshold value of allowable relative error. Then, in the simulated u_{0}(j), those which satisfied Eq. (22) were selected. That is, ${u}_{\mathrm{0}}\left(k\right)k=\mathrm{1},\mathrm{\cdots},l$ with the length of l were found to satisfy ${u}_{\mathrm{0}}\left(k\right)\cong {\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}$ and the selected combinations, $\left[{u}_{\mathrm{1}}\left(k\right),\mathrm{\cdots},{u}_{d}\left(k\right)\right]k=\mathrm{1},\mathrm{\cdots},l$ and $\left[{X}_{\mathrm{1}}\left(k\right),\mathrm{\cdots},{X}_{d}\left(k\right)\right]k=\mathrm{1},\mathrm{\cdots},l$, corresponded to the given ${\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}$.

For all selected combinations, the products of the probability densities, $f\left[{x}_{\mathrm{1}}\left(k\right),\mathrm{\cdots},{x}_{d}\left(k\right)\right]k=\mathrm{1},\mathrm{\cdots},l$, on the basis of the joint probability densities, $c\left[{u}_{\mathrm{1}}\left(k\right),\mathrm{\cdots},{u}_{d}\left(k\right)\right]k=\mathrm{1},\mathrm{\cdots},l$ and marginal $f\left[{x}_{\mathrm{1}}\left(k\right)\right],\mathrm{\cdots}f\left[{x}_{d}\left(k\right)\right]k=\mathrm{1},\mathrm{\cdots},l$, respectively, were calculated, and $\left[{\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{1}},\mathrm{\cdots},{\stackrel{\mathrm{\u0303}}{u}}_{d}\right]$ with the maxima of the product from $\left[{u}_{\mathrm{1}}\left(k\right),\mathrm{\cdots},{u}_{d}\left(k\right)\right]k=\mathrm{1},\mathrm{\cdots},l$ was the design combination for the given CDF of precipitation of the entire region, ${\stackrel{\mathrm{\u0303}}{u}}_{\mathrm{0}}$.
In addition, in order to better present the relationship of the Kendall CDF of subregional precipitation and the CDF of precipitation of the entire region, a confidence interval (CI) was defined by the distance which deviated from the diagonal (Serinaldi, 2013; Volpi and Fiori, 2014) and transformed herein by the normal distribution. The CI with a probability of 1−α was involved.
The Guangdong Province as a study case is located in South China with a land area of 158.57×10^{3} km^{2} (illustrated in Fig. 1). The entire region mostly belongs to the monsoon climate zone, varying from the tropic to southern subtropic. Annual precipitation is abundant, but uneven in terms of spatial and temporal distribution. Since China's reform and opening up in the late 1970s, the water demand of the province has been increasing with the rapid development of the regional socioeconomy. In 2015, the total water consumption of the province accounted for 44.31 billion m^{3}, in which approximately onehalf was for irrigation.
According to the report of the Irrigation Quota of Guangdong Province, the entire region marked by A0, in terms of agriculture on the basis of climate, soil type, cropping system and other management measures, was zoned into eight subregions marked by A1–A8 in Fig. 1. Areal data of subregions and their paddy fields were used (see Table 2). Annual precipitation data of eight subregions for the period of 1953–2013, in terms of multisite average, were transformed from 25 hydrometeorological stations.
The net quotas of irrigation of individual subregions in association with precipitation frequency resulted from the previous field experiments of irrigation in the late twentieth century. According to the distribution and statistical properties on the basis of field experiments in a research report entitled Annual irrigation Quota in Guangdong Province (1999), the net quotas of irrigation per unit area in the paddy fields of individual subregions in association with the precipitation frequency are illustrated in Fig. 2. They ranged from 7221 to 8520 m^{3} hm^{−2} in terms of annual average with coefficients of variation of 0.205–0.293, and precipitation was regarded to follow the Pearson III distribution whose three parameters were transformed using the given mean values and coefficients of variation marked in Fig. 2.
The utilization coefficient of irrigation of Guangdong Province has been at a low level, which approximately accounted for 0.46 in terms of average value in 2000 according to the Water Resources Comprehensive Planning of Guangdong Province. In order to respond to tough national water management measures of China, the utilization coefficient is expected to be no less than 0.51 by 2020 in the Tough Water Management Assessing Performance of Guangdong Province. Therefore, the utilization coefficients of eight agricultural subregions were uniformly predefined as a fixed value of 0.51 in this paper.
4.1 Univariate properties and distribution of precipitation
Annual precipitation in the entire region and individual subregions remarkably changed and was typically random. Precipitation in terms of average varied mostly from 1500 to 2000 mm, except for the A5 subregion with a larger value of 2789 mm (see Table 2). The regional maximum of precipitation accounted for 4071 mm that occurred in 1973 in the A5 subregion, and the regional minimum of precipitation less than 800 mm occurred in 1963 in the A6 subregion. In general, the average precipitation of the entire region accounted for 1835 mm, with a maximum of 2421 mm and a minimum of 1152 mm. The general statistical characteristics of annual precipitation are illustrated in Fig. 3. All values in the range of extended boundaries were generally regarded as normal, and otherwise abnormal. The box plot shows that most samples of precipitation fell within the extended boundaries, except for several samples from the A1 and A6 subregions.
The statistics of the goodnessoffit test of four alternative univariate distributions were smaller than those of the significance level of 0.05 (see Table 3), which implied that the GEV, GLO, PIII and GNO distributions fitted annual precipitation of the subregions and the entire region. The RMSE and AIC values among the distributions slightly differed and those of the GNO distribution were the smallest for most regions. Then, as illustrated in Fig. 4, all lines of the theoretical CDF almost overlapped the points of the empirical CDF. They demonstrated that the GNO satisfactorily fitted the frequency distributions of annual precipitation for the entire region and subregions. As shown in Table 4, the shape parameters of the GNO in the A1 and A2 subregions were clear minus, that in the A5 subregion was clear positive and others were close to zero, which implied significantly leftskewed, rightskewed and normal distributions, respectively.
4.2 Eightdimensional joint distribution of subregional precipitation
A matrix of correlation coefficients between subregional precipitations is illustrated in Fig. 5. Due to the geographical distance and direction, the coefficients varied from different pairs of subregions, but most of them were quite large except between A8 and other subregions, such as both A8–A6 and A8–A4, which were less than 0.1. In actuality, the correlation coefficient implied dependence between subregional precipitation values. The larger the coefficient was, the larger the dependence was.
The QQ plot of empirical and theoretical joint CDFs showed that the sample points fell near the diagonal of 1:1, though more fell in the lower tail (see Fig. 6). For the goodnessoffit test of the eightdimensional metaGaussian copula, the pvalue accounted for 0.262 beyond the significance level of 0.05. This demonstrated that the highdimensional Gaussian copula better fitted the joint distribution of precipitation of eight subregions. However, eightdimensional joint CDFs of subregional precipitation on the basis of observed data were limited and the maximum was less than 0.75.
When the conventional joint CDF of subregional precipitation was transformed into the Kendall CDF, the CDF was indeed enlarged (see Fig. 7a). For example, using observed data, the minimum of 0.00005 and the maximum of 0.71957 for the conventional joint CDF corresponded to 0.00916 and 0.99084 for the Kendall CDF, respectively. Using the Hessian axes in which the scales of dual axes were transferred following the standard normal distribution (see Fig. 7b), the conventional joint CDF and the Kendall CDF showed a linear relationship, which demonstrated that it was appropriate to use the Kendall instead of the conventional joint CDF.
4.3 Relationship between subregional joint and entire region CDFs of precipitation
According to the determined eightdimensional Gaussian copula, a million combinations of CDFs of subregional precipitation were generated by using Monte Carlo simulation. Correspondingly, the conventional joint and Kendall CDFs of subregions and the CDFs of the entire region were achieved (illustrated in Fig. 8). Comparing the conventional joint and the entire region CDFs (see Fig. 8a), the combination points tended to happen in the northwest and had an upconvex lower boundary. When the CDF of the entire region was given a certain value, the corresponding joint CDFs varied within the limited upper bound on which the joint CDFs were less than the given value of the entire region.
Using the Kendall CDF instead of the conventional joint CDF (see Fig. 8b), the combination points scattered near the diagonal of 1:1 with a concave upper lower boundary. It also showed that most observed samples fell within the envelope of CI with a probability of 0.95. In addition, between the Kendall CDF and the CDF of the entire region, there were large correlation coefficients of 0.9221 and 0.9153 for the observed and simulated samples, respectively. These showed that the Kendall CDF instead of the conventional CDF was convenient to analyze the relationship of the CDF between the entire region and the subregional joint CDF.
4.4 Design combinations of the CDF of subregional precipitation
The given CDFs of precipitation of the entire region can be predefined to change in the range from 0.05 to 0.95 with a step of 0.05, which refers to the alteration of regional precipitation from extreme dry to extreme wet. Considering the uncertainty of Monte Carlo simulation, those simulated combinations (see grey points in Fig. 9), in which the allowable relative error of their calculated CDFs of the entire region compared to a given CDF was less than 0.05 %, were selected to be an alternate for further design combination.
Using the EF method (see the blue dashed line in Fig. 9), design points consisted of a better smooth curve which mostly fell within 0.5 of the CI. The Kendall CDF was very close to the CDF of the entire region, but that of the former was larger than that of the latter, with lower CDFs being less than 0.55 in the study case, and vice versa in larger CDFs. Using the TY method (see the red circles in Fig. 9), design points were irregularly scattered on the two sides of the diagonal of 1:1, and even several points were beyond 0.95 of the CI, for example for the given CDFs of 0.8 and 0.9. Using the MLW method (see the blue triangles in Fig. 9), design points fell within the range between 0.5 and 0.95 of the CI, and design Kendall CDFs were larger than the given CDFs.
In general, if the CDF of subregional precipitation was equalized, the differences between the design Kendall CDF and the given CDF of the entire region were almost no more than a CI of 0.5. On the basis of maximum joint probability density, design Kendall CDFs were larger than the given CDFs and preferred to the upper limited bound. By zooming in or out according to the typical year, design points almost fell within 0.95 of the CI, but they were relatively scattered in comparison to other methods.
In addition, between individual CDFs of eight subregional precipitations and the CDF of the entire region, the design points maintained a smooth curve and were undifferentiated for all subregions when using the EF method (see the blue dashed line in Fig. 10). The design individual CDFs of subregions were very close to the CDF of the entire region, but the former were larger than the latter in the lower CDFs and vice versa in the larger CDFs. Using TY and MLW methods, design points were scattered on the two sides of the diagonal of 1:1. Design CDFs differed from subregions, but their differences were undetermined. However, as seen from the envelope of design points of individual subregions, the ranges of the MLW design were comparatively narrow and concentrated around the diagonal of 1:1 except for the A8 subregion, but those of the TY design were much wider, in particular for the A4, A7 and A8 subregions.
4.5 Design water demand of irrigation
For given CDFs of precipitation of the entire region, the water demand of irrigation of the entire region for all selected simulations and design points are illustrated in Fig. 11a. As the given CDF changed from 0.05 to 0.95, the average value of water demand decreased from 22.79 to 12.78 billion m^{3}, correspondingly representing the range from extremely dry to extremely wet (see the black dashed line in Fig. 11a). The difference between the maximum and minimum demands for a given CDF (see the blue and red dashed lines in Fig. 11a) varied in the range of 1.15–1.72 billion m^{3}. Design demands of the MLW and EF methods (see the blue and black solid lines in Fig. 11a) were slightly smaller than the average values, but those of the TY method (see the red line in Fig. 11a) fluctuated around the line of average value. Compared to the average values (see Fig. 11b), the maximum and minimum demands increased and decreased by 3.0 %–7.5 % and 2.5 %–3.8 %, respectively. Design demands of the MLW and EF methods decreased within 1.4 % and 2.1 %, respectively, and those of the TY method increased or decreased within 2.8 % or 2.1 %, respectively. These values demonstrated that the differences of water demand among three design methods for the entire region were quite small.
Design water demands of irrigation in individual subregions are illustrated in Fig. 13. As the given CDF changed from 0.05 to 0.95, subregional demands of the EF method smoothly decreased from 2.16–3.73 to 1.16–2.29 billion m^{3}, correspondingly representing the range from extremely dry to extremely wet (see the black lines in Fig. 12). Then, the demands of the TY and MLW methods fluctuated around the lines of the demand of the EF method, and the fluctuations of the former were remarkably larger than those of the latter (see the red and blue lines in Fig. 12). Compared to the EF method (see Fig. 13), the increase or decrease in the water demand of the MLW design accounted for less than 13 % in most subregions, except for the A8 subregion with a maximum of 26.1 %, but that of the TY method accounted for 15.4 %–24.3 % for the A1, A2, A3 and A5 subregions and 39.8 %–45.7 % for the A4, A6, A7 and A8 subregions. These values demonstrated that the alterations of design water demand in subregions were more complicated in comparison with those in the entire region.
Using Guangdong Province of South China as a case study of a large region, a highdimensional metaGaussian copula was applied for fitting the joint distribution of multiple regional precipitation. A large number of combinations of CDFs of precipitation of eight subregions were generated by using the Monte Carlo method. The relationship among the CDFs of the entire region, the conventional joint CDF, and the Kendall CDF of subregions was determined. Three design methods, including the EF and TY design methods, and a new design procedure of the MLW method in association with the joint probability density, were used for design combinations of subregional CDFs for given CDFs of precipitation of the entire region. Then, design water demands of irrigation of the entire region and individual subregions were compared. The main conclusions of this study are as follows.

The frequency distributions of annual precipitation of the entire region and of subregions were fitted well by the GNO distribution. The shape parameters in the A1 and A2 subregions were a clear minus, those in the A5 subregion were a clear positive, and others were close to zero, which implied significantly leftskewed, rightskewed and normal distributions, respectively. The eightdimensional Gaussian copula satisfactorily fitted the joint distribution of subregional precipitation.

There was a clear linear dependence between the conventional joint and Kendall CDFs of subregional precipitation when both of them were transferred by the standard normal distribution. Comparing the Kendall CDFs of subregions and the CDF of the entire region, most observed samples fell within the envelope of CI of a probability of 0.95 around the diagonal of 1:1, and there were greater dependences between them with correlation coefficients of 0.9221 and 0.9153 for the observed and simulated samples, respectively. The use of the Kendall CDF instead of the conventional joint CDF can better link the joint frequency of subregions and the univariate frequency of the entire region. However, any one given CDF of the entire region corresponded to a large number of joint CDFs varying from very small to limited large CDFs. That is, there was an upper bound in larger values of the joint CDFs of subregions corresponding to given CDFs of the entire region.

For given CDFs of precipitation of the entire region, design Kendall CDFs and individual CDFs of eight subregions of the EF method maintained a smooth curve and were very close to their diagonal of 1:1. The design Kendall CDFs of the MLW method which were larger than the given CDFs of the entire region fell between 0.5 and 0.95 probabilities for the CI far from the diagonal, and those of the TY method were irregularly scattered on the two sides of the diagonal. Then, design CDFs of individual subregions of the MLW and TY methods were also scattered on the two sides of the diagonal, but they differed for individual subregions. The change ranges of the MLW design were comparatively narrow and were concentrated around the diagonal, but those of the TY design were much wider.

For given CDFs varying from 0.05 to 0.95 representing the range from extremely dry to extremely wet, the simulated water demand of irrigation of the entire region in terms of the average value accounted for from 22.79 to 12.78 billion m^{3}. Design demands of the MLW and EF methods were slightly smaller than the average values, and those of the TY method fluctuated around the average values. Compared to the average demand, design demands of the MLW, EF and TY methods decreased or increased, respectively, within 1.4 %, 2.1 %, and 2.8 %, which demonstrated that the differences of design demand of the entire region among the three methods were quite small.

For given CDFs varying from 0.05 to 0.95 representing the range from extremely dry to extremely wet, design water demands of individual subregions of the EF method decreased smoothly from 2.16–3.73 to 1.16–2.29 billion m^{3}, and those of the MLW and TY methods fluctuated around the demand lines of the EF method, but the fluctuations of the TY method were remarkably larger than those of the MLW method. Compared to the EF method, the increase or decrease in the water demand of the MLW design accounted for less than 13 % in most subregions, except for the A8 subregion with a maximum of 26.1 %, but those of the TY method accounted for 39.8 %–45.7 % for the A4, A6, A7 and A8 subregions. These values demonstrated that the alterations of design water demand in subregions were more complicated in comparison with those in the entire region.
All in all, in practical planning of regional water resources, using the EF method can realize water demand of irrigation for the entire region and its subregions for a given frequency of precipitation, but it is arbitrary that the series of subregional precipitation are regarded to be undifferentiated for a large region, for example the case region. The TY method was constrained by the limited observed data of precipitation and it cannot be chosen when several different combinations of subregional precipitation made the frequency of precipitation of the entire region approximately identical. Therefore, a design procedure using the MLW method in association with a highdimensional copula, which simulated individual univariate distributions, captured their dependences for multivariables and built a linkage between regional frequency and subregional frequency of precipitation, is recommended for design water demand of irrigation for a large region.
Precipitation data used in this study were obtained from the Surface Climate Daily Data Set of China and are available upon request. Address inquiries to the corresponding author.
XJT conceived this study and developed the method and wrote the paper. YLD conducted modelling experiments and performed the analysis. VPS revised the manuscript. XHC and KRL contributed comments on the method and writing. HOW conducted data analysis.
The authors declare that they have no conflict of interest.
Support by the National Key R&D Program of China (2017YFC0405900), the
National Natural Science Foundation of China (51479217; 51879288), and the
Outstanding Youth Science Foundation of NSFC (51822908) is gratefully
acknowledged.
Edited by: Luis
Samaniego
Reviewed by: two anonymous referees
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