# Temporal and Spatial Variations of Precipitationδ18O and Controlling Factors on the Pearl River Basin and Adjacent Regions

Authors: Yunfeng Ruan; Zhaofei Liu; Zhijun Yao; Rui Wang
Journal: Advances in Meteorology
 (2018)

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## Abstract

Based on the precipitationδ18O values from the datasets of the Global Network of Isotopes in Precipitation (GNIP), the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data, and previous researches, we explored the temporal and spatial variations of precipitation  δ18O in a typical monsoon climate zone, the Pearl River basin (PRB), and adjacent regions. The results showed that the temporal variations of precipitation  δ18O for stations should be correlated with water vapor sources, the distance of water vapor transport, the changes in location, and intensity of the intertropical convergence zone (ITCZ) rather than “amount effect.” Meanwhile, local meteorological and geographical factors showed close correlations with mean weighted precipitation  δ18O values, suggesting that “altitude effect” and local meteorological conditions were significant for the spatial variations of precipitation  δ18O. Moreover, we established linear regression models for estimating the mean weighted precipitation  δ18O values, which could better estimate variations in precipitation  δ18O than the Bowen and Wilkinson model in the PRB and adjacent regions.


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## Body

## 1. Introduction

Meteoric precipitation is a significant part of the circulation of natural water, and the composition of stable isotopes is closely related to meteorological factors (e.g., temperature, relative humidity, and precipitation amount) and geographical factors (e.g., latitude, altitude, and distance from the moisture transport source), which have shown a sensitive response to environmental change [1–3]. The stable isotopic composition in precipitation can be used as a tracer to reveal the moisture sources [4], retrieve the atmospheric processes [5, 6], and reflect the characteristics of regional climate [7].Based on the Global Network of Isotopes in Precipitation (GNIP) established by the International Atomic Energy Agency (IAEA) and the World Meteorological Organization, a number of scholars have explored the characteristics of precipitationδ18O and factors controlling precipitation  δ18O variations [2, 8–10], revealed the original moisture sources of precipitation [11, 12], calculated the contribution ratio of every moisture source to precipitation [9], found significant impacts of climatic events on precipitation  δ18O [13], and utilized models to predict the temporal and spatial distribution of precipitation  δ18O [3, 14–16]. Previous studies have shown that precipitation  δ18O variation for the mid and high-latitude regions is mainly controlled by “temperature effect” and in the low-latitude regions is mainly controlled by “amount effect” [2, 8]. However, not all studies showed “amount effect” in low-latitude regions, but the significance of variation in moisture sources on the precipitation  δ18O variation [17, 18] and different models have represented different simulation accuracy on precipitation  δ18O [3, 14–16]. In China, many studies have explored the temporal and spatial variations of precipitation  δ18O in different geographical zones [15, 16, 19–21], river basins [22–24], and cities [18, 25, 26]. Different moisture sources, which originate from the Arabian Sea, the Bay of Bengal, the South China Sea and the Western Pacific Ocean, and the westerly winds, have remarkable influence on the stable isotopic variations in China [3, 15, 16]. However, few studies have focused on the Pearl River basin (PRB) and adjacent regions, which have the closer distance to the moisture transport sources for being located in the south of China. This may reveal the more complicated patterns of temporal and spatial distribution of precipitation  δ18O and controlling factors. Therefore, this study utilizes precipitation  δ18O data from the GNIP and previous studies to (1) understand the temporal and spatial variations of precipitation  δ18O in the PRB and adjacent regions; (2) explore the controlling factors on the variation of precipitation  δ18O in the PRB and adjacent regions; and (3) establish appropriate spatial models for predicting precipitation  δ18O in the PRB and adjacent regions.


## 2. Study Area, Data, and Methods

### 2.1. Study Area

The PRB is situated in the south of China, located from 97.65°E to 117.30°E and 3.68°N to 29.25°N. The Pearl River is the second largest Chinese river in terms of streamflow, with a drainage area of 4.42× 105 km2 (Pearl River Water Resources Committee (PRWRC), 1991). The mean annual temperature and precipitation range from 14 to 22°C and from 1200 to 2200 mm. In general, the elevation mainly increases from the southeast (southeast delta area) to the northwest (Yunnan–Guizhou Plateau) (Figure 1).Figure 1
Location of the GNIP and previous studies stations in the PRB and adjacent regions.


### 2.2. Data

The precipitationδ18O data in this study mainly come from the GNIP database (http://isohis.iaea.org) and previous studies by Chinese scholars. The stations in the GNIP utilized in this study are Hong Kong (114.17°E, 22.32°N), Guangzhou (113.32°E, 23.13°N), Guilin (110.08°E, 25.07°N), and Liuzhou (109.40°E, 24.35°N) in the PRB and Haikou (110.35°E, 20.03°N), Guiyang (106.72°E, 26.58°N), Kunming (102.68°E, 25.02°N), and Fuzhou (119.28°E, 26.08°N) in the adjacent regions of the PRB. The data included the monthly temperature, precipitation amount, vapor pressure, and precipitation  δ18O. Data from previous studies came from the stations of Liangfengdong (108.05°E, 25.27°N) [27], Guilin (110.08°E, 25.07°N) [28], and Huanjiang (108.33°E, 24.74°N) [21] which are located in the PRB, and the stations of Ailaoshan (101.03°E, 24.55°N) [21] and Mengzi (103.23°E, 23.23°N) [30] which are located adjacent to the PRB. The spatial distributions of the stations are shown in Figure 1, and the basic information of sampling stations is shown in Table 1.Table 1
Basic information of sampling stations.
Sample site Latitude (°N) Longitude (°E) Altitude (m) Study periods Data sources Hong Kong 22.32 114.17 66 1961–2014 GNIP Haikou 20.03 110.35 15 1988–2000 GNIP Guangzhou 23.13 113.32 7 1986–1989 GNIP Guilin 25.07 110.08 170 1983–1990 GNIP Guilin 25.07 110.08 170 2010 Wu et al. [28] Liuzhou 24.35 109.40 97 1988–1992 GNIP Guiyang 26.58 106.72 1071 1988–1992 GNIP Kunming 25.02 102.68 1892 1986–2003 GNIP Fuzhou 26.08 119.28 16 1985–1992 GNIP Liangfengdong 25.27 108.05 620 2003-4~2003-9, 2004-3~2004-5 Luo et al. [27] Huanjiang 24.74 108.33 400 2005–2010 Liu et al. [21] Ailaoshan 24.55 101.03 2481 2005–2010 Liu et al. [21] Mengzi 23.23 103.23 1302 2009–2011 Li et al. [30]


### 2.3. Methods

#### 2.3.1. Precipitationδ18O

All precipitationδ18O data are on a per mil (‰) basis, and  δ notation is relative to the Vienna Standard Mean Ocean Water (VSMOW) standard. The  δ18O are calculated as follows: (1)δ18O=RSRV-SMOW-1×1000,where RS and RV-SMOW represent the precipitation samples and R (18O/16O) of VSMOW, respectively.In this study, the sampling period is divided into the summer monsoon period (May to September), nonsummer monsoon period (October to April), and the annual time scale (January to December).The mean precipitationδ18O values are the precipitation amount-weighted  δ18O values, which are calculated as follows [29]:(2)δ18O¯=∑δ18Oi·Pi∑Pi,where δ18Oi and Pi represent each monthly precipitation  δ18O value and the corresponding precipitation amount, respectively.


#### 2.3.2. OLR and Vertically Integrated Moisture Transport

The monthly mean variations of the outgoing longwave radiation (OLR) and vertically integrated moisture transport were adopted from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data (https://www.esrl.noaa.gov/psd/) during 1980 and 2011.


#### 2.3.3. Linear Correlation Analysis

Monthly precipitationδ18O values for the summer monsoon, nonsummer monsoon periods, and the annual time scale are used to explore the linear correlations with precipitation (mm) and temperature (°C). The correlation is explored for each station alone.


#### 2.3.4. Simulation of the Spatial Distribution of Precipitationδ18O

(1) Nonlinear Regression Model. The Bowen and Wilkinson (BW) model was used as the nonlinear regression model, which was established by Bowen and Wilkinson [14], and has been successfully applied to estimate the spatial distribution of precipitation  δ18O [16]. The model considers that the isotopic composition of precipitation is affected by the temperature of driving rainout and the regional patterns of the origin and delivery of moisture. Therefore, the latitude and altitude of the precipitation observation stations are substituted for the temperature effect as the geographic parameters to estimate the isotopic composition of precipitation [14]. The modeled precipitation  δ18O has the following form:(3)δ18O=aLAT2+bLAT+cALT+d,where  δ18O is an observation of the mean weighted value of the oxygen isotopic composition of precipitation and a, b, c, and d are regression parameters. LAT and ALT represent the latitude and altitude, respectively.(2) Linear Regression Model. Variations in precipitation  δ18O are cocontrolled by different meteorological and geographical factors [3, 21, 31]. Meteorological factors in this study come from the China meteorological data sharing service system and include AP, S, WS, P, RH, VP, and T, which represent atmospheric pressure (hPa), sunshine duration (h), wind speed (m/s), precipitation (mm), relative humidity (%), vapor pressure (hPa), and air temperature (°C), respectively, and altitude (m) (ALT) is used as the geographical factor. In order to compare the factors controlling precipitation  δ18O values among stations under different time scales in the PRB and adjacent regions, linear correlations between the mean weighted precipitation  δ18O values of all stations and their corresponding mean meteorological values and altitude were calculated on the three time periods, while stations of Liangfengdong, Huanjiang, Ailaoshan, and Mengzi were only calculated for the annual time scale.The modeled precipitationδ18O has the following form:(4)δ18O=aX1+bX2+cX3+⋯+nXn,where  δ18O is an observation of the mean weighted value of the oxygen isotopic composition of precipitation, X1,X2,X3,…,nXn represent different meteorological and geographical factors that are correlated with the mean weighted precipitation  δ18O value at P value < 0.05 or 0.01, and a,b,c,…,n are regression parameters.


## 2.1. Study Area

The PRB is situated in the south of China, located from 97.65°E to 117.30°E and 3.68°N to 29.25°N. The Pearl River is the second largest Chinese river in terms of streamflow, with a drainage area of 4.42× 105 km2 (Pearl River Water Resources Committee (PRWRC), 1991). The mean annual temperature and precipitation range from 14 to 22°C and from 1200 to 2200 mm. In general, the elevation mainly increases from the southeast (southeast delta area) to the northwest (Yunnan–Guizhou Plateau) (Figure 1).Figure 1
Location of the GNIP and previous studies stations in the PRB and adjacent regions.


## 2.2. Data

The precipitationδ18O data in this study mainly come from the GNIP database (http://isohis.iaea.org) and previous studies by Chinese scholars. The stations in the GNIP utilized in this study are Hong Kong (114.17°E, 22.32°N), Guangzhou (113.32°E, 23.13°N), Guilin (110.08°E, 25.07°N), and Liuzhou (109.40°E, 24.35°N) in the PRB and Haikou (110.35°E, 20.03°N), Guiyang (106.72°E, 26.58°N), Kunming (102.68°E, 25.02°N), and Fuzhou (119.28°E, 26.08°N) in the adjacent regions of the PRB. The data included the monthly temperature, precipitation amount, vapor pressure, and precipitation  δ18O. Data from previous studies came from the stations of Liangfengdong (108.05°E, 25.27°N) [27], Guilin (110.08°E, 25.07°N) [28], and Huanjiang (108.33°E, 24.74°N) [21] which are located in the PRB, and the stations of Ailaoshan (101.03°E, 24.55°N) [21] and Mengzi (103.23°E, 23.23°N) [30] which are located adjacent to the PRB. The spatial distributions of the stations are shown in Figure 1, and the basic information of sampling stations is shown in Table 1.Table 1
Basic information of sampling stations.
Sample site Latitude (°N) Longitude (°E) Altitude (m) Study periods Data sources Hong Kong 22.32 114.17 66 1961–2014 GNIP Haikou 20.03 110.35 15 1988–2000 GNIP Guangzhou 23.13 113.32 7 1986–1989 GNIP Guilin 25.07 110.08 170 1983–1990 GNIP Guilin 25.07 110.08 170 2010 Wu et al. [28] Liuzhou 24.35 109.40 97 1988–1992 GNIP Guiyang 26.58 106.72 1071 1988–1992 GNIP Kunming 25.02 102.68 1892 1986–2003 GNIP Fuzhou 26.08 119.28 16 1985–1992 GNIP Liangfengdong 25.27 108.05 620 2003-4~2003-9, 2004-3~2004-5 Luo et al. [27] Huanjiang 24.74 108.33 400 2005–2010 Liu et al. [21] Ailaoshan 24.55 101.03 2481 2005–2010 Liu et al. [21] Mengzi 23.23 103.23 1302 2009–2011 Li et al. [30]


## 2.3. Methods

### 2.3.1. Precipitationδ18O

All precipitationδ18O data are on a per mil (‰) basis, and  δ notation is relative to the Vienna Standard Mean Ocean Water (VSMOW) standard. The  δ18O are calculated as follows: (1)δ18O=RSRV-SMOW-1×1000,where RS and RV-SMOW represent the precipitation samples and R (18O/16O) of VSMOW, respectively.In this study, the sampling period is divided into the summer monsoon period (May to September), nonsummer monsoon period (October to April), and the annual time scale (January to December).The mean precipitationδ18O values are the precipitation amount-weighted  δ18O values, which are calculated as follows [29]:(2)δ18O¯=∑δ18Oi·Pi∑Pi,where δ18Oi and Pi represent each monthly precipitation  δ18O value and the corresponding precipitation amount, respectively.


### 2.3.2. OLR and Vertically Integrated Moisture Transport

The monthly mean variations of the outgoing longwave radiation (OLR) and vertically integrated moisture transport were adopted from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data (https://www.esrl.noaa.gov/psd/) during 1980 and 2011.


### 2.3.3. Linear Correlation Analysis

Monthly precipitationδ18O values for the summer monsoon, nonsummer monsoon periods, and the annual time scale are used to explore the linear correlations with precipitation (mm) and temperature (°C). The correlation is explored for each station alone.


### 2.3.4. Simulation of the Spatial Distribution of Precipitationδ18O

(1) Nonlinear Regression Model. The Bowen and Wilkinson (BW) model was used as the nonlinear regression model, which was established by Bowen and Wilkinson [14], and has been successfully applied to estimate the spatial distribution of precipitation  δ18O [16]. The model considers that the isotopic composition of precipitation is affected by the temperature of driving rainout and the regional patterns of the origin and delivery of moisture. Therefore, the latitude and altitude of the precipitation observation stations are substituted for the temperature effect as the geographic parameters to estimate the isotopic composition of precipitation [14]. The modeled precipitation  δ18O has the following form:(3)δ18O=aLAT2+bLAT+cALT+d,where  δ18O is an observation of the mean weighted value of the oxygen isotopic composition of precipitation and a, b, c, and d are regression parameters. LAT and ALT represent the latitude and altitude, respectively.(2) Linear Regression Model. Variations in precipitation  δ18O are cocontrolled by different meteorological and geographical factors [3, 21, 31]. Meteorological factors in this study come from the China meteorological data sharing service system and include AP, S, WS, P, RH, VP, and T, which represent atmospheric pressure (hPa), sunshine duration (h), wind speed (m/s), precipitation (mm), relative humidity (%), vapor pressure (hPa), and air temperature (°C), respectively, and altitude (m) (ALT) is used as the geographical factor. In order to compare the factors controlling precipitation  δ18O values among stations under different time scales in the PRB and adjacent regions, linear correlations between the mean weighted precipitation  δ18O values of all stations and their corresponding mean meteorological values and altitude were calculated on the three time periods, while stations of Liangfengdong, Huanjiang, Ailaoshan, and Mengzi were only calculated for the annual time scale.The modeled precipitationδ18O has the following form:(4)δ18O=aX1+bX2+cX3+⋯+nXn,where  δ18O is an observation of the mean weighted value of the oxygen isotopic composition of precipitation, X1,X2,X3,…,nXn represent different meteorological and geographical factors that are correlated with the mean weighted precipitation  δ18O value at P value < 0.05 or 0.01, and a,b,c,…,n are regression parameters.


## 2.3.1. Precipitationδ18O

All precipitationδ18O data are on a per mil (‰) basis, and  δ notation is relative to the Vienna Standard Mean Ocean Water (VSMOW) standard. The  δ18O are calculated as follows: (1)δ18O=RSRV-SMOW-1×1000,where RS and RV-SMOW represent the precipitation samples and R (18O/16O) of VSMOW, respectively.In this study, the sampling period is divided into the summer monsoon period (May to September), nonsummer monsoon period (October to April), and the annual time scale (January to December).The mean precipitationδ18O values are the precipitation amount-weighted  δ18O values, which are calculated as follows [29]:(2)δ18O¯=∑δ18Oi·Pi∑Pi,where δ18Oi and Pi represent each monthly precipitation  δ18O value and the corresponding precipitation amount, respectively.


## 2.3.2. OLR and Vertically Integrated Moisture Transport

The monthly mean variations of the outgoing longwave radiation (OLR) and vertically integrated moisture transport were adopted from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data (https://www.esrl.noaa.gov/psd/) during 1980 and 2011.


## 2.3.3. Linear Correlation Analysis

Monthly precipitationδ18O values for the summer monsoon, nonsummer monsoon periods, and the annual time scale are used to explore the linear correlations with precipitation (mm) and temperature (°C). The correlation is explored for each station alone.


## 2.3.4. Simulation of the Spatial Distribution of Precipitationδ18O

(1) Nonlinear Regression Model. The Bowen and Wilkinson (BW) model was used as the nonlinear regression model, which was established by Bowen and Wilkinson [14], and has been successfully applied to estimate the spatial distribution of precipitation  δ18O [16]. The model considers that the isotopic composition of precipitation is affected by the temperature of driving rainout and the regional patterns of the origin and delivery of moisture. Therefore, the latitude and altitude of the precipitation observation stations are substituted for the temperature effect as the geographic parameters to estimate the isotopic composition of precipitation [14]. The modeled precipitation  δ18O has the following form:(3)δ18O=aLAT2+bLAT+cALT+d,where  δ18O is an observation of the mean weighted value of the oxygen isotopic composition of precipitation and a, b, c, and d are regression parameters. LAT and ALT represent the latitude and altitude, respectively.(2) Linear Regression Model. Variations in precipitation  δ18O are cocontrolled by different meteorological and geographical factors [3, 21, 31]. Meteorological factors in this study come from the China meteorological data sharing service system and include AP, S, WS, P, RH, VP, and T, which represent atmospheric pressure (hPa), sunshine duration (h), wind speed (m/s), precipitation (mm), relative humidity (%), vapor pressure (hPa), and air temperature (°C), respectively, and altitude (m) (ALT) is used as the geographical factor. In order to compare the factors controlling precipitation  δ18O values among stations under different time scales in the PRB and adjacent regions, linear correlations between the mean weighted precipitation  δ18O values of all stations and their corresponding mean meteorological values and altitude were calculated on the three time periods, while stations of Liangfengdong, Huanjiang, Ailaoshan, and Mengzi were only calculated for the annual time scale.The modeled precipitationδ18O has the following form:(4)δ18O=aX1+bX2+cX3+⋯+nXn,where  δ18O is an observation of the mean weighted value of the oxygen isotopic composition of precipitation, X1,X2,X3,…,nXn represent different meteorological and geographical factors that are correlated with the mean weighted precipitation  δ18O value at P value < 0.05 or 0.01, and a,b,c,…,n are regression parameters.


## 3. Results and Discussion

### 3.1. Correlations of Precipitationδ18O for Stations with Temperature and Precipitation Amount

The positive correlation between isotopic composition and temperature was called “temperature effect,” while negative correlation between isotopic composition and precipitation amount was called “amount effect” [8, 32]. The correlations between monthly precipitation  δ18O of each station and precipitation amount, temperature, and the slopes dδ18Op/dP and dδ18Op/dT are shown in Table 2. Precipitation  δ18O shows significant negative correlations with temperature in all stations under the annual time scale and in partial stations under the summer monsoon and nonsummer monsoon periods, which shows opposite result to the “temperature effect” of precipitation  δ18O, suggesting the “temperature effect” is rarely found in the summer monsoon areas with low latitudes but mainly occurs in mid-high latitudes, especially near the poles [32], and in nonsummer monsoon areas [11]. Except for Haikou under the annual time scale, precipitation  δ18O has stronger negative correlations with the precipitation amount, especially at the stations of Hong Kong and Kunming, with correlation coefficients of −0.61 and −0.62 and P values < 0.01 and 0.01, respectively, which suggests that precipitation  δ18O is significantly influenced by the “amount effect” in the monsoon regions [3, 8]. However, only three stations have correlations with the precipitation amount under the summer monsoon and nonsummer monsoon periods, suggesting the “amount effect” mainly occurs under the annual time scale. Except for Fuzhou under the summer monsoon period, the slopes of dδ18Op/dP for Guiyang and Kunming are significantly higher than other stations in the PRB and adjacent regions (Table 2), and stations of rδ-P at P value < 0.05 or 0.01 for the annual time scale show significant correlations (P value < 0.01) with altitude (Figure 2). This is mainly because the higher altitude areas experience greater temperature drop for wet air-parcels as the decrease in absolute humidity, resulting in greater slopes of dδ18Op/dP [2].Table 2
Correlations of monthly precipitationδ18O values at different stations with precipitation amount and temperature for three time periods in different stations of the PRB and adjacent regions. Note: ∗ and ∗∗ are represent the correlations at P values < 0.01 and 0.05, respectively. The units of dδ18O/dP and dδ18O/dT are 100 mm and 1°C, respectively.
Station Precipitation amount Temperature r δ - P dδ18O/dP r δ - T dδ18O/dT Summer monsoon period Haikou 0.10 0.17 - 0.47 ∗ ∗ - 1.02 Hong Kong - 0.29 ∗ ∗ - 0.27 - 0.24 ∗ ∗ - 0.38 Guangzhou - 0.28 - 0.41 - 0.13 - 0.26 Guilin 0.26 0.44 - 0.32 ∗ - 0.34 Liuzhou - 0.07 - 0.12 - 0.27 - 0.23 Guiyang - 0.25 - 1.08 - 0.32 - 0.50 Kunming - 0.37 ∗ ∗ - 1.28 0.09 0.03 Fuzhou - 0.59 ∗ ∗ - 1.09 0.19 0.14  Nonsummer monsoon period Haikou - 0.18 - 0.41 - 0.37 - 0.26 Hong Kong - 0.42 ∗ ∗ - 0.21 - 0.34 ∗ ∗ - 0.21 Guangzhou - 0.25 - 0.68 - 0.38 - 0.24 Guilin 0.11 0.32 - 0.34 ∗ ∗ - 0.16 Liuzhou - 0.12 - 0.71 - 0.35 0.00 Guiyang - 0.36 ∗ - 3.26 - 0.31 - 0.20 Kunming - 0.67 ∗ ∗ - 7.15 - 0.12 0.06 Fuzhou 0.02 0.05 - 0.29 - 0.21  The annual time scale Haikou - 0.20 - 0.43 - 0.65 ∗ ∗ - 0.50 Hong Kong - 0.61 ∗ ∗ - 0.92 - 0.67 ∗ ∗ - 0.38 Guangzhou - 0.44 ∗ ∗ - 0.80 - 0.50 ∗ ∗ - 0.84 Guilin - 0.17 ∗ - 0.45 - 0.71 ∗ ∗ - 0.29 Liuzhou - 0.33 ∗ - 1.34 - 0.60 ∗ ∗ - 0.26 Guiyang - 0.48 ∗ ∗ - 2.46 - 0.57 ∗ ∗ - 0.28 Kunming - 0.62 ∗ ∗ - 4.37 - 0.44 ∗ ∗ - 0.53 Fuzhou - 0.35 ∗ ∗ - 0.89 - 0.38 ∗ ∗ - 0.16Figure 2
Correlations ofdδ18Op/dP (rδ-P at a P value < 0.05 or 0.01) with altitude for the annual time scale.


### 3.2. Precipitationδ18O Variations at Stations in the PRB and Adjacent Regions

The mean monthly precipitation amount, air temperature, and mean weighted precipitationδ18O values in the sampling period for each precipitation station of the GNIP are shown in Figure 3. The precipitation  δ18O values present “V-shaped” patterns in all stations, which show a decreasing trend as the temperature and the precipitation amount increase and an increasing trend as the temperature and the precipitation amount decrease (Figure 3). Precipitation  δ18O values at the stations show significant seasonal variation, with more depleted mean precipitation  δ18O values in the summer monsoon period and more enriched  δ18O values in the nonmonsoon period (Figure 3). Our results reveal significantly negative correlation between precipitation  δ18O values for each station and precipitation amount in the annual time scale (Table 2), which shows “amount effect” for each station. However, the most depleted precipitation  δ18O value for each station does not occur in the month with the most precipitation amount. This phenomenon has also been observed in other monsoon regions [18, 21, 33, 34]. Mean weighted precipitation  δ18O values (the annual time scale, the summer monsoon, and the nonsummer monsoon periods) are more enriched at the stations of Guangzhou (−5.83‰, −6.27‰, and −4.09%), Haikou (−6.09‰, −7.80‰, and −4.12‰), Hong Kong (−6.59‰, −7.16‰, and −4.09‰), Liuzhou (−6.37‰, −8.09‰ and, −3.74‰), Guilin (−6.12‰, −7.76‰, and −3.56‰), Fuzhou (−6.6‰, −7.73‰, and −4.44‰), and Huanjiang (−6.16‰) with the lower altitude comparison with the more depleted values at the stations of Guiyang (−8.32‰, −9.42‰, and −5.55‰), Kunming (−10.11‰, −10.67‰, and −8.00‰), Liangfengdong (−7.3‰), Mengzi (−7.26‰), and Ailaoshan (−9.22‰) with higher altitude (Huanjiang, Liangfengdong, Mengzi, and Ailaoshan do not have the mean weighted values for the summer monsoon and nonsummer monsoon periods), suggesting differences in spatial distribution characteristics of precipitation  δ18O. By comparing these values with the mean annual precipitation amount Figure 3, the result shows that the most depleted mean annual precipitation  δ18O values do not occur in the station with the most mean annual precipitation amount. Both of the seasonal and spatial characteristics of variations show that there exist other factors controlling the precipitation  δ18O variations rather than “amount effect.”Figure 3
Variations of mean monthly weighted precipitationδ18O, temperature, and precipitation amount at Haikou (a), Hong Kong (b), Guangzhou (c), Guilin (d), and Liuzhou (e) stations in the PRB and at Guiyang (f), Kunming (g), and Fuzhou (h) adjacent to the PRB. Note: the shadow area represents the monsoon period (May to September).
(a) (b) (c) (d) (e) (f) (g) (h)


### 3.3. Controlling Factors on the Precipitationδ18O Variation

#### 3.3.1. Effects of Changes in Water Vapor Sources and Vapor Transport on Seasonal Precipitationδ18O Variation

Water vapor sources and vapor transport influence significantly the precipitationδ18O variations [3, 15, 16, 18, 31]. The intertropical convergence zone (ITCZ) is a major convergence zone of the troposphere wind field that always closely tracks with monsoon activity and moisture source for precipitation [18]. Outgoing long wave radiation (OLR) is closely related to convection activities, with low OLR values reflecting strong convection activities, conversely, reflecting weak convection activities. The distribution area of low OLR values can accurately reflect the location and intensity changes of the ITCZ to reveal the moisture sources for the summer monsoon precipitation [35]. Therefore, the average OLR values for each month from 1980 to 2011 were calculated (Figure 4). In addition, the average vertically integrated moisture transport for each month from 1980 to 2011 which was calculated by multiplying the zonal and meridional winds by specific humidity from the surface to the 300 hPa level (Figure 5) was used to explore the effects of changes in moisture sources and vapor transport on precipitation  δ18O.Figure 4
Variations of mean OLR for each month from 1980 to 2011 over Pearl River basin (a) and adjacent regions. Note: the unit of the color scale is W/m2.Figure 5
Vertically integrated mean moisture transport (kg m−1 s−1) for each month from 1980 to 2011 over Pearl River basin (a) and adjacent regions. Note: the unit of the color scale is kg m−1 s−1.In spring, the ITCZ is mainly distributed near the equator regions in March and April, with a weak intensity in the ITCZ and a long distance from the water vapor sources to the PRB and adjacent regions (Figure4). The PRB and adjacent regions are dominated by a westerly system, with a small water vapor transport (Figure 5), resulting in enriched precipitation  δ18O (Figure 3) contributed by local evaporation and inland evaporation taken by the westerly system. In May, the ITCZ gradually moves to the Bay of Bengal, with a strengthening in the intensity of the ITCZ that suggests the onset of the Asian summer monsoon (Figure 4). The prevailing wind direction gradually shifts from a westerly system to a south wind (Figure 5). Large amounts of moisture are brought from the Bay of Bengal and strong convection occurs in the water vapor sources. The vapor transportation depletes the  δ18O in water vapor, which causes the shape decrease in the precipitation  δ18O for May in comparison to April (Figure 3).In summer, the convection center of the ITCZ moves further to the north with the rapidly strengthening intensity in the Bay of Bengal, the South China Sea, and the Arabian Sea in June (Figure4), and the prevailing wind direction is dominated by a southwest wind (Figure 5). As large amounts of moisture are brought to the PRB and adjacent regions by the southwest wind, a stronger convection occurs in the water vapor sources and vapor transportation, resulting in a continuous decrease in the precipitation  δ18O in the PRB and adjacent regions. In July, the prevailing wind direction and water vapor transport do not change too much compared with that in June, but the strong intensity convention for the ITCZ is enlarged further in the Bay of Bengal and the Indo-China Peninsula, depleting the  δ18O in the water vapor and continuously decreasing the precipitation  δ18O in the PRB and adjacent regions. In August, by comparison with July, the prevailing wind direction in the PRB and adjacent regions becomes jointly controlled by southwest and southeast winds (Figure 5). The scope of the convection in the ITCZ is narrowed in the Bay of Bengal, but the intensity of the convection increases in the western Pacific Ocean (Figure 4), which increases the distance for water vapor transport to the PRB and adjacent regions. These changes contribute to the depleted precipitation  δ18O values in the PRB and adjacent regions (Figure 3).In autumn, the prevailing wind direction does not change much in September as compared with August (Figure5). However, the convective centers of the ITCZ in the Bay of Bengal and the South China Sea move southward (Figure 4), which increases the distance for water vapor transport to the PRB and adjacent regions. As a result, the stations experience decreasing precipitation  δ18O values, with Guilin and Liuzhou obviously reaching their most depleted precipitation  δ18O values during this month. However, Haikou and Guangzhou do not reach their most depleted precipitation  δ18O values in September but have significantly higher values than the most depleted values obtained in August (Figure 3). As shown in Figure 5, the water vapor sources originate from the Bay of Bengal and South China Sea and also originate from the adjacent seas in September. Moreover, Haikou and Guangzhou are all coastal stations in the south of PRB, which results in higher precipitation  δ18O values than in August due to the relatively shorter distance for water vapor transport. From October to November, the prevailing wind direction changes from southwest and southeast to west, and the ITCZ moves further southward to the vicinity of the equator, suggesting that the summer monsoon has disappeared. The water vapor sources mainly originate from the inland [3], resulting in enriched precipitation  δ18O values in the PRB and adjacent regions (Figure 3).In winter, water vapor mainly originates from local evaporation and inland evaporation due to the westerly system (Figure5), resulting in enriched precipitation  δ18O values in the PRB and adjacent regions (Figure 3).


#### 3.3.2. Local Meteorological and Geographical Factors Controlling Spatial Precipitationδ18O Variation

In order to explore the factors controlling spatial distribution of precipitationδ18O in the PRB and adjacent regions, we compared the correlations between mean weighted precipitation  δ18O values and meteorological and geographical factors among stations, showing that mean values for atmospheric pressure, temperature, water vapor pressure, precipitation amount, and altitude show significant correlations with mean weighted precipitation  δ18O values in the annual time scale of the PRB and adjacent regions, with correlation coefficients of 0.929, 0.787, 0.786, 0.589, and −0.908, respectively, with P values < 0.01 for atmospheric pressure, temperature, water vapor pressure, altitude, and 0.05 for precipitation amount (Figures 6(a), 6(b), 6(c), 6(e), and 6(d)). This suggests that they are the main factors controlling spatial precipitation δ18O variation. The slope of dδ18Op/dT and dδ18Op/dP is 0.34‰/°C (Figure 6(b)) and 1.7‰/100 mm (Figure 6(d)), respectively. The slope of dδ18Op/dT is higher than the northeast regions of China (0.27‰/°C) and slightly lower than the northwest regions of China (0.37‰/°C) [21]. The slope of dδ18Op/dALT is −0.15‰/100 m (Figure 6(e)), which indicates the “altitude effect” on the weighted mean annual precipitation  δ18O values, which is lower than that found for the Tibet Plateau (−0.3‰/100 m) by Liu et al. [21] and for the globe (−0.28‰/100 m) by Poage and Chamberlain [36]. However, this is similar to the whole country scale found for China (−0.15‰/100 m and −0.13‰/100 m) by Liu et al. [16] and Liu et al. [21], respectively.Figure 6
Correlations of mean weighted precipitationδ18O values for stations with meteorological and geographical factors which are at P value < 0.05 or 0.01. Note: (a), (b), (c), (d), and (e) represent the mean annual values; (f), (g), (h), and (i) represent the mean values for the summer monsoon period; (j), (k), and (l) represent the mean values for the nonsummer monsoon period.
(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l)In the summer monsoon period, the controlling factors for spatial precipitationδ18O variation is atmospheric pressure, temperature, vapor pressure, and altitude, exhibiting correlation coefficients of 0.930, 0.889, 0.913, and −0.921, respectively, with all the P values < 0.01 (Figures 6(f), 6(g), 6(h), and 6(i)). The slope of dδ18Op/dT and dδ18Op/dALT is 0.35‰/°C (Figure 6(g)) and −0.17‰/100 m (Figure 6(i)), respectively, which is similar to that for the annual time scale.In the nonsummer monsoon period, the controlling factors for the precipitationδ18O values are sunshine duration, atmospheric pressure, and altitude, with correlation coefficients of −0.776, 0.959, and −0.957, respectively, explaining 60.2%, 91.9%, and 91.6% of the variance, respectively, with P values < 0.05, 0.01, and 0.01 (Figures 6(j), 6(k), and 6(l)). The dδ18Op/dALT is −0.20‰/100 m (Figure 6(l)), suggesting that the “altitude effect” for the nonsummer monsoon period is the most significant among the three time periods.However, mean temperature shows significantly positive correlation with mean weighted precipitationδ18O in the annual time scale and the summer monsoon period (Figures 6(b) and 6(g)). Meanwhile, mean precipitation also shows significantly positive correlation with mean weighted precipitation  δ18O values (Figure 6(d)). Moreover, both temperature and precipitation show significantly negative correlations with altitude (Figure 7). This indicates that the correlations of precipitation and temperature may reflect the result of the “altitude effect,” suggesting that altitude should be the more significant factor for spatial precipitation  δ18O variation in the PRB and adjacent regions.Figure 7
Correlations of mean annual temperature (a), mean summer monsoon temperature (b), and mean annual precipitation (c) for stations with altitude.
(a) (b) (c)


### 3.4. Simulation of Precipitationδ18O in the PRB and Adjacent Regions

#### 3.4.1. Nonlinear Regression Model and Linear Regression Model

The Bowen and Wilkinson model has been applied widely for simulating the precipitationδ18O in monsoon climatic zones [3, 10, 16]. No significant interaction effect of latitude and altitude has been found since the stations are mostly situated below 200 m [14]. The interaction effect of latitude and altitude occurs when stations are both below 200 m and above 200 m [16], and the elevation of our research area ranges from 0 to 2852 m. Therefore, we adopt a nonlinear fit to consider the correlation between latitude (LAT) and altitude (ALT) to establish the mean weighted precipitation  δ18O patterns under the three time scales: the annual time scale, summer monsoon, and nonsummer monsoon periods. The model equations are as follows:(5)δ18O=-0.062LAT2+2.792LAT-0.001ALT-37.251(6)δ18O=-0.072LAT2+3.315LAT-0.002ALT-45.091(7)δ18O=-0.007LAT2+3.383LAT-0.002ALT-9.056,where (5), (6), and (7) represent the mean weighted precipitation  δ18O patterns for the annual time scale, summer monsoon, and nonsummer monsoon periods, respectively.According to the above results (Figure6), we establish linear regression models using those meteorological and geographical variables for modeling in the three time periods. The resulting models are as follows:(8)δ18O=0.015AP+0.271T-0.173VP-0.0006P-0.0004ALT-22.429(9)δ18O=0.034AP-0.646T+0.222VP+0.0004ALT-30.177(10)δ18O=-0.002S+0.098AP+0.009ALT-101.567,where (8), (9), and (10) represent the mean weighted precipitation  δ18O patterns for the annual time scale, summer monsoon, and nonsummer monsoon periods, respectively. The parameters, S, VP, T, AP, P, and ALT represent sunshine duration (h), vapor pressure (hPa), air temperature (°C), atmospheric pressure (hPa), precipitation amount (mm), and altitude (m), respectively.By comparing the results of the Bowen and Wilkinson model with other researches, the regression coefficients of the second-order term (−0.062) (see (5)) are significantly more negative than that modeled on a global scale (−0.0051) [14] or modeled for the United States (−0.0057) [37] and China (−0.0073) [16]. This may be caused by the smallest area of our study. And the PRB and adjacent regions are significantly influenced by both the southeast and southwest monsoons, and precipitation  δ18O depleted as moisture is transferred from the oceans to the PRB and adjacent regions. Moreover, the Bowen and Wilkinson model estimates the best precipitation  δ18O patterns for nonsummer monsoon period (Figure 8). Zhao et al. [3] found that the Bowen and Wilkinson model could estimate precipitation  δ18O well only in winter for China. This suggests that there are differences in modeling the precipitation  δ18O by the Bowen and Wilkinson model for different regional scales and time scales in the monsoon climatic regions.Figure 8
Linear correlations between measured amount-weighted precipitationδ18O and modeled weighted precipitation  δ18O. Note: (a), (b), and (c) represent (5), (6), and (7) by the Bowen and Wilkinson model, respectively; (d), (e), and (f) represent (8), (9), and (10) by the linear regression model, respectively.
(a) (b) (c) (d) (e) (f)By comparing the results of the Bowen and Wilkinson model with the linear regression model, the results show that the linear regression model also estimates the best precipitationδ18O patterns for the nonsummer monsoon period (Figure 8). The amount of variance explained by the linear regression model is higher (93.5%, 92.8%, and 99.0%, resp.) than those (85.8%, 90%, and 92.4%, resp.) (Figure 8) by the Bowen and Wilkinson model for the annual time scale, summer monsoon, and nonsummer monsoon periods, respectively. Moreover, the mean errors explained by the linear regression model are lower (0.27‰ and 0.24‰, resp.) than those (0.48‰ and 0.35‰, resp.) by the Bowen and Wilkinson model for the annual time scale and the summer monsoon period. There is a higher mean error (0.41‰) in the linear regression model than that in the Bowen and Wilkinson model (0.32‰) for the nonsummer monsoon period, but the linear regression model explains more variance with a lower standard deviation of mean error (99.0‰ and 0.19‰, resp.) than does the Bowen and Wilkinson model (92.4‰ and 0.22‰, resp.). The results suggest that precipitation  δ18O can be estimated more accurately by the linear regression model than by the Bowen and Wilkinson model in the PRB and adjacent regions.


### 3.5. Spatial Distribution of Precipitationδ18O in the PRB

Spatial distribution of precipitationδ18O values from 1980 to 2011 for the annual time scale, summer monsoon, and nonsummer monsoon periods of the PRB are presented based on (8), (9), and (10), respectively. The data utilized come from the meteorological data network of China (Figure 9(a)). Precipitation  δ18O values are gradually more depleted (Figure 9) from the south and east to the north and northwest, especially in the annual time scale and the summer monsoon period, which are mainly caused by “altitude effect.” Precipitation  δ18O values are more depleted in the summer monsoon period and more enriched in the nonsummer monsoon period (Figures 9(c) and 9(d)). However, spatial distribution of precipitation  δ18O reflect only the results of the linear regression model based on limited quantities and years of observation stations. As a result, more of the long-time observation stations should be established for more accurate estimation of precipitation  δ18O variations in the monsoon climatic regions.Figure 9
Spatial distribution of mean weighted precipitationδ18O values over Pearl River basin of China based on the Kriging method from 1980 to 2011. Note: (a) represents meteorological stations in the Pearl River basin; (b) represents spatial distribution of the mean weighted annual precipitation  δ18O values by (8); (c) represents spatial distribution of mean weighted precipitation  δ18O values for the summer monsoon period by (9); (d) represents spatial distribution of mean weighted precipitation  δ18O values for the nonsummer monsoon period by (10).
(a) (b) (c) (d)


## 3.1. Correlations of Precipitationδ18O for Stations with Temperature and Precipitation Amount

The positive correlation between isotopic composition and temperature was called “temperature effect,” while negative correlation between isotopic composition and precipitation amount was called “amount effect” [8, 32]. The correlations between monthly precipitation  δ18O of each station and precipitation amount, temperature, and the slopes dδ18Op/dP and dδ18Op/dT are shown in Table 2. Precipitation  δ18O shows significant negative correlations with temperature in all stations under the annual time scale and in partial stations under the summer monsoon and nonsummer monsoon periods, which shows opposite result to the “temperature effect” of precipitation  δ18O, suggesting the “temperature effect” is rarely found in the summer monsoon areas with low latitudes but mainly occurs in mid-high latitudes, especially near the poles [32], and in nonsummer monsoon areas [11]. Except for Haikou under the annual time scale, precipitation  δ18O has stronger negative correlations with the precipitation amount, especially at the stations of Hong Kong and Kunming, with correlation coefficients of −0.61 and −0.62 and P values < 0.01 and 0.01, respectively, which suggests that precipitation  δ18O is significantly influenced by the “amount effect” in the monsoon regions [3, 8]. However, only three stations have correlations with the precipitation amount under the summer monsoon and nonsummer monsoon periods, suggesting the “amount effect” mainly occurs under the annual time scale. Except for Fuzhou under the summer monsoon period, the slopes of dδ18Op/dP for Guiyang and Kunming are significantly higher than other stations in the PRB and adjacent regions (Table 2), and stations of rδ-P at P value < 0.05 or 0.01 for the annual time scale show significant correlations (P value < 0.01) with altitude (Figure 2). This is mainly because the higher altitude areas experience greater temperature drop for wet air-parcels as the decrease in absolute humidity, resulting in greater slopes of dδ18Op/dP [2].Table 2
Correlations of monthly precipitationδ18O values at different stations with precipitation amount and temperature for three time periods in different stations of the PRB and adjacent regions. Note: ∗ and ∗∗ are represent the correlations at P values < 0.01 and 0.05, respectively. The units of dδ18O/dP and dδ18O/dT are 100 mm and 1°C, respectively.
Station Precipitation amount Temperature r δ - P dδ18O/dP r δ - T dδ18O/dT Summer monsoon period Haikou 0.10 0.17 - 0.47 ∗ ∗ - 1.02 Hong Kong - 0.29 ∗ ∗ - 0.27 - 0.24 ∗ ∗ - 0.38 Guangzhou - 0.28 - 0.41 - 0.13 - 0.26 Guilin 0.26 0.44 - 0.32 ∗ - 0.34 Liuzhou - 0.07 - 0.12 - 0.27 - 0.23 Guiyang - 0.25 - 1.08 - 0.32 - 0.50 Kunming - 0.37 ∗ ∗ - 1.28 0.09 0.03 Fuzhou - 0.59 ∗ ∗ - 1.09 0.19 0.14  Nonsummer monsoon period Haikou - 0.18 - 0.41 - 0.37 - 0.26 Hong Kong - 0.42 ∗ ∗ - 0.21 - 0.34 ∗ ∗ - 0.21 Guangzhou - 0.25 - 0.68 - 0.38 - 0.24 Guilin 0.11 0.32 - 0.34 ∗ ∗ - 0.16 Liuzhou - 0.12 - 0.71 - 0.35 0.00 Guiyang - 0.36 ∗ - 3.26 - 0.31 - 0.20 Kunming - 0.67 ∗ ∗ - 7.15 - 0.12 0.06 Fuzhou 0.02 0.05 - 0.29 - 0.21  The annual time scale Haikou - 0.20 - 0.43 - 0.65 ∗ ∗ - 0.50 Hong Kong - 0.61 ∗ ∗ - 0.92 - 0.67 ∗ ∗ - 0.38 Guangzhou - 0.44 ∗ ∗ - 0.80 - 0.50 ∗ ∗ - 0.84 Guilin - 0.17 ∗ - 0.45 - 0.71 ∗ ∗ - 0.29 Liuzhou - 0.33 ∗ - 1.34 - 0.60 ∗ ∗ - 0.26 Guiyang - 0.48 ∗ ∗ - 2.46 - 0.57 ∗ ∗ - 0.28 Kunming - 0.62 ∗ ∗ - 4.37 - 0.44 ∗ ∗ - 0.53 Fuzhou - 0.35 ∗ ∗ - 0.89 - 0.38 ∗ ∗ - 0.16Figure 2
Correlations ofdδ18Op/dP (rδ-P at a P value < 0.05 or 0.01) with altitude for the annual time scale.


## 3.2. Precipitationδ18O Variations at Stations in the PRB and Adjacent Regions

The mean monthly precipitation amount, air temperature, and mean weighted precipitationδ18O values in the sampling period for each precipitation station of the GNIP are shown in Figure 3. The precipitation  δ18O values present “V-shaped” patterns in all stations, which show a decreasing trend as the temperature and the precipitation amount increase and an increasing trend as the temperature and the precipitation amount decrease (Figure 3). Precipitation  δ18O values at the stations show significant seasonal variation, with more depleted mean precipitation  δ18O values in the summer monsoon period and more enriched  δ18O values in the nonmonsoon period (Figure 3). Our results reveal significantly negative correlation between precipitation  δ18O values for each station and precipitation amount in the annual time scale (Table 2), which shows “amount effect” for each station. However, the most depleted precipitation  δ18O value for each station does not occur in the month with the most precipitation amount. This phenomenon has also been observed in other monsoon regions [18, 21, 33, 34]. Mean weighted precipitation  δ18O values (the annual time scale, the summer monsoon, and the nonsummer monsoon periods) are more enriched at the stations of Guangzhou (−5.83‰, −6.27‰, and −4.09%), Haikou (−6.09‰, −7.80‰, and −4.12‰), Hong Kong (−6.59‰, −7.16‰, and −4.09‰), Liuzhou (−6.37‰, −8.09‰ and, −3.74‰), Guilin (−6.12‰, −7.76‰, and −3.56‰), Fuzhou (−6.6‰, −7.73‰, and −4.44‰), and Huanjiang (−6.16‰) with the lower altitude comparison with the more depleted values at the stations of Guiyang (−8.32‰, −9.42‰, and −5.55‰), Kunming (−10.11‰, −10.67‰, and −8.00‰), Liangfengdong (−7.3‰), Mengzi (−7.26‰), and Ailaoshan (−9.22‰) with higher altitude (Huanjiang, Liangfengdong, Mengzi, and Ailaoshan do not have the mean weighted values for the summer monsoon and nonsummer monsoon periods), suggesting differences in spatial distribution characteristics of precipitation  δ18O. By comparing these values with the mean annual precipitation amount Figure 3, the result shows that the most depleted mean annual precipitation  δ18O values do not occur in the station with the most mean annual precipitation amount. Both of the seasonal and spatial characteristics of variations show that there exist other factors controlling the precipitation  δ18O variations rather than “amount effect.”Figure 3
Variations of mean monthly weighted precipitationδ18O, temperature, and precipitation amount at Haikou (a), Hong Kong (b), Guangzhou (c), Guilin (d), and Liuzhou (e) stations in the PRB and at Guiyang (f), Kunming (g), and Fuzhou (h) adjacent to the PRB. Note: the shadow area represents the monsoon period (May to September).
(a) (b) (c) (d) (e) (f) (g) (h)


## 3.3. Controlling Factors on the Precipitationδ18O Variation

### 3.3.1. Effects of Changes in Water Vapor Sources and Vapor Transport on Seasonal Precipitationδ18O Variation

Water vapor sources and vapor transport influence significantly the precipitationδ18O variations [3, 15, 16, 18, 31]. The intertropical convergence zone (ITCZ) is a major convergence zone of the troposphere wind field that always closely tracks with monsoon activity and moisture source for precipitation [18]. Outgoing long wave radiation (OLR) is closely related to convection activities, with low OLR values reflecting strong convection activities, conversely, reflecting weak convection activities. The distribution area of low OLR values can accurately reflect the location and intensity changes of the ITCZ to reveal the moisture sources for the summer monsoon precipitation [35]. Therefore, the average OLR values for each month from 1980 to 2011 were calculated (Figure 4). In addition, the average vertically integrated moisture transport for each month from 1980 to 2011 which was calculated by multiplying the zonal and meridional winds by specific humidity from the surface to the 300 hPa level (Figure 5) was used to explore the effects of changes in moisture sources and vapor transport on precipitation  δ18O.Figure 4
Variations of mean OLR for each month from 1980 to 2011 over Pearl River basin (a) and adjacent regions. Note: the unit of the color scale is W/m2.Figure 5
Vertically integrated mean moisture transport (kg m−1 s−1) for each month from 1980 to 2011 over Pearl River basin (a) and adjacent regions. Note: the unit of the color scale is kg m−1 s−1.In spring, the ITCZ is mainly distributed near the equator regions in March and April, with a weak intensity in the ITCZ and a long distance from the water vapor sources to the PRB and adjacent regions (Figure4). The PRB and adjacent regions are dominated by a westerly system, with a small water vapor transport (Figure 5), resulting in enriched precipitation  δ18O (Figure 3) contributed by local evaporation and inland evaporation taken by the westerly system. In May, the ITCZ gradually moves to the Bay of Bengal, with a strengthening in the intensity of the ITCZ that suggests the onset of the Asian summer monsoon (Figure 4). The prevailing wind direction gradually shifts from a westerly system to a south wind (Figure 5). Large amounts of moisture are brought from the Bay of Bengal and strong convection occurs in the water vapor sources. The vapor transportation depletes the  δ18O in water vapor, which causes the shape decrease in the precipitation  δ18O for May in comparison to April (Figure 3).In summer, the convection center of the ITCZ moves further to the north with the rapidly strengthening intensity in the Bay of Bengal, the South China Sea, and the Arabian Sea in June (Figure4), and the prevailing wind direction is dominated by a southwest wind (Figure 5). As large amounts of moisture are brought to the PRB and adjacent regions by the southwest wind, a stronger convection occurs in the water vapor sources and vapor transportation, resulting in a continuous decrease in the precipitation  δ18O in the PRB and adjacent regions. In July, the prevailing wind direction and water vapor transport do not change too much compared with that in June, but the strong intensity convention for the ITCZ is enlarged further in the Bay of Bengal and the Indo-China Peninsula, depleting the  δ18O in the water vapor and continuously decreasing the precipitation  δ18O in the PRB and adjacent regions. In August, by comparison with July, the prevailing wind direction in the PRB and adjacent regions becomes jointly controlled by southwest and southeast winds (Figure 5). The scope of the convection in the ITCZ is narrowed in the Bay of Bengal, but the intensity of the convection increases in the western Pacific Ocean (Figure 4), which increases the distance for water vapor transport to the PRB and adjacent regions. These changes contribute to the depleted precipitation  δ18O values in the PRB and adjacent regions (Figure 3).In autumn, the prevailing wind direction does not change much in September as compared with August (Figure5). However, the convective centers of the ITCZ in the Bay of Bengal and the South China Sea move southward (Figure 4), which increases the distance for water vapor transport to the PRB and adjacent regions. As a result, the stations experience decreasing precipitation  δ18O values, with Guilin and Liuzhou obviously reaching their most depleted precipitation  δ18O values during this month. However, Haikou and Guangzhou do not reach their most depleted precipitation  δ18O values in September but have significantly higher values than the most depleted values obtained in August (Figure 3). As shown in Figure 5, the water vapor sources originate from the Bay of Bengal and South China Sea and also originate from the adjacent seas in September. Moreover, Haikou and Guangzhou are all coastal stations in the south of PRB, which results in higher precipitation  δ18O values than in August due to the relatively shorter distance for water vapor transport. From October to November, the prevailing wind direction changes from southwest and southeast to west, and the ITCZ moves further southward to the vicinity of the equator, suggesting that the summer monsoon has disappeared. The water vapor sources mainly originate from the inland [3], resulting in enriched precipitation  δ18O values in the PRB and adjacent regions (Figure 3).In winter, water vapor mainly originates from local evaporation and inland evaporation due to the westerly system (Figure5), resulting in enriched precipitation  δ18O values in the PRB and adjacent regions (Figure 3).


### 3.3.2. Local Meteorological and Geographical Factors Controlling Spatial Precipitationδ18O Variation

In order to explore the factors controlling spatial distribution of precipitationδ18O in the PRB and adjacent regions, we compared the correlations between mean weighted precipitation  δ18O values and meteorological and geographical factors among stations, showing that mean values for atmospheric pressure, temperature, water vapor pressure, precipitation amount, and altitude show significant correlations with mean weighted precipitation  δ18O values in the annual time scale of the PRB and adjacent regions, with correlation coefficients of 0.929, 0.787, 0.786, 0.589, and −0.908, respectively, with P values < 0.01 for atmospheric pressure, temperature, water vapor pressure, altitude, and 0.05 for precipitation amount (Figures 6(a), 6(b), 6(c), 6(e), and 6(d)). This suggests that they are the main factors controlling spatial precipitation δ18O variation. The slope of dδ18Op/dT and dδ18Op/dP is 0.34‰/°C (Figure 6(b)) and 1.7‰/100 mm (Figure 6(d)), respectively. The slope of dδ18Op/dT is higher than the northeast regions of China (0.27‰/°C) and slightly lower than the northwest regions of China (0.37‰/°C) [21]. The slope of dδ18Op/dALT is −0.15‰/100 m (Figure 6(e)), which indicates the “altitude effect” on the weighted mean annual precipitation  δ18O values, which is lower than that found for the Tibet Plateau (−0.3‰/100 m) by Liu et al. [21] and for the globe (−0.28‰/100 m) by Poage and Chamberlain [36]. However, this is similar to the whole country scale found for China (−0.15‰/100 m and −0.13‰/100 m) by Liu et al. [16] and Liu et al. [21], respectively.Figure 6
Correlations of mean weighted precipitationδ18O values for stations with meteorological and geographical factors which are at P value < 0.05 or 0.01. Note: (a), (b), (c), (d), and (e) represent the mean annual values; (f), (g), (h), and (i) represent the mean values for the summer monsoon period; (j), (k), and (l) represent the mean values for the nonsummer monsoon period.
(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l)In the summer monsoon period, the controlling factors for spatial precipitationδ18O variation is atmospheric pressure, temperature, vapor pressure, and altitude, exhibiting correlation coefficients of 0.930, 0.889, 0.913, and −0.921, respectively, with all the P values < 0.01 (Figures 6(f), 6(g), 6(h), and 6(i)). The slope of dδ18Op/dT and dδ18Op/dALT is 0.35‰/°C (Figure 6(g)) and −0.17‰/100 m (Figure 6(i)), respectively, which is similar to that for the annual time scale.In the nonsummer monsoon period, the controlling factors for the precipitationδ18O values are sunshine duration, atmospheric pressure, and altitude, with correlation coefficients of −0.776, 0.959, and −0.957, respectively, explaining 60.2%, 91.9%, and 91.6% of the variance, respectively, with P values < 0.05, 0.01, and 0.01 (Figures 6(j), 6(k), and 6(l)). The dδ18Op/dALT is −0.20‰/100 m (Figure 6(l)), suggesting that the “altitude effect” for the nonsummer monsoon period is the most significant among the three time periods.However, mean temperature shows significantly positive correlation with mean weighted precipitationδ18O in the annual time scale and the summer monsoon period (Figures 6(b) and 6(g)). Meanwhile, mean precipitation also shows significantly positive correlation with mean weighted precipitation  δ18O values (Figure 6(d)). Moreover, both temperature and precipitation show significantly negative correlations with altitude (Figure 7). This indicates that the correlations of precipitation and temperature may reflect the result of the “altitude effect,” suggesting that altitude should be the more significant factor for spatial precipitation  δ18O variation in the PRB and adjacent regions.Figure 7
Correlations of mean annual temperature (a), mean summer monsoon temperature (b), and mean annual precipitation (c) for stations with altitude.
(a) (b) (c)


## 3.3.1. Effects of Changes in Water Vapor Sources and Vapor Transport on Seasonal Precipitationδ18O Variation

Water vapor sources and vapor transport influence significantly the precipitationδ18O variations [3, 15, 16, 18, 31]. The intertropical convergence zone (ITCZ) is a major convergence zone of the troposphere wind field that always closely tracks with monsoon activity and moisture source for precipitation [18]. Outgoing long wave radiation (OLR) is closely related to convection activities, with low OLR values reflecting strong convection activities, conversely, reflecting weak convection activities. The distribution area of low OLR values can accurately reflect the location and intensity changes of the ITCZ to reveal the moisture sources for the summer monsoon precipitation [35]. Therefore, the average OLR values for each month from 1980 to 2011 were calculated (Figure 4). In addition, the average vertically integrated moisture transport for each month from 1980 to 2011 which was calculated by multiplying the zonal and meridional winds by specific humidity from the surface to the 300 hPa level (Figure 5) was used to explore the effects of changes in moisture sources and vapor transport on precipitation  δ18O.Figure 4
Variations of mean OLR for each month from 1980 to 2011 over Pearl River basin (a) and adjacent regions. Note: the unit of the color scale is W/m2.Figure 5
Vertically integrated mean moisture transport (kg m−1 s−1) for each month from 1980 to 2011 over Pearl River basin (a) and adjacent regions. Note: the unit of the color scale is kg m−1 s−1.In spring, the ITCZ is mainly distributed near the equator regions in March and April, with a weak intensity in the ITCZ and a long distance from the water vapor sources to the PRB and adjacent regions (Figure4). The PRB and adjacent regions are dominated by a westerly system, with a small water vapor transport (Figure 5), resulting in enriched precipitation  δ18O (Figure 3) contributed by local evaporation and inland evaporation taken by the westerly system. In May, the ITCZ gradually moves to the Bay of Bengal, with a strengthening in the intensity of the ITCZ that suggests the onset of the Asian summer monsoon (Figure 4). The prevailing wind direction gradually shifts from a westerly system to a south wind (Figure 5). Large amounts of moisture are brought from the Bay of Bengal and strong convection occurs in the water vapor sources. The vapor transportation depletes the  δ18O in water vapor, which causes the shape decrease in the precipitation  δ18O for May in comparison to April (Figure 3).In summer, the convection center of the ITCZ moves further to the north with the rapidly strengthening intensity in the Bay of Bengal, the South China Sea, and the Arabian Sea in June (Figure4), and the prevailing wind direction is dominated by a southwest wind (Figure 5). As large amounts of moisture are brought to the PRB and adjacent regions by the southwest wind, a stronger convection occurs in the water vapor sources and vapor transportation, resulting in a continuous decrease in the precipitation  δ18O in the PRB and adjacent regions. In July, the prevailing wind direction and water vapor transport do not change too much compared with that in June, but the strong intensity convention for the ITCZ is enlarged further in the Bay of Bengal and the Indo-China Peninsula, depleting the  δ18O in the water vapor and continuously decreasing the precipitation  δ18O in the PRB and adjacent regions. In August, by comparison with July, the prevailing wind direction in the PRB and adjacent regions becomes jointly controlled by southwest and southeast winds (Figure 5). The scope of the convection in the ITCZ is narrowed in the Bay of Bengal, but the intensity of the convection increases in the western Pacific Ocean (Figure 4), which increases the distance for water vapor transport to the PRB and adjacent regions. These changes contribute to the depleted precipitation  δ18O values in the PRB and adjacent regions (Figure 3).In autumn, the prevailing wind direction does not change much in September as compared with August (Figure5). However, the convective centers of the ITCZ in the Bay of Bengal and the South China Sea move southward (Figure 4), which increases the distance for water vapor transport to the PRB and adjacent regions. As a result, the stations experience decreasing precipitation  δ18O values, with Guilin and Liuzhou obviously reaching their most depleted precipitation  δ18O values during this month. However, Haikou and Guangzhou do not reach their most depleted precipitation  δ18O values in September but have significantly higher values than the most depleted values obtained in August (Figure 3). As shown in Figure 5, the water vapor sources originate from the Bay of Bengal and South China Sea and also originate from the adjacent seas in September. Moreover, Haikou and Guangzhou are all coastal stations in the south of PRB, which results in higher precipitation  δ18O values than in August due to the relatively shorter distance for water vapor transport. From October to November, the prevailing wind direction changes from southwest and southeast to west, and the ITCZ moves further southward to the vicinity of the equator, suggesting that the summer monsoon has disappeared. The water vapor sources mainly originate from the inland [3], resulting in enriched precipitation  δ18O values in the PRB and adjacent regions (Figure 3).In winter, water vapor mainly originates from local evaporation and inland evaporation due to the westerly system (Figure5), resulting in enriched precipitation  δ18O values in the PRB and adjacent regions (Figure 3).


## 3.3.2. Local Meteorological and Geographical Factors Controlling Spatial Precipitationδ18O Variation

In order to explore the factors controlling spatial distribution of precipitationδ18O in the PRB and adjacent regions, we compared the correlations between mean weighted precipitation  δ18O values and meteorological and geographical factors among stations, showing that mean values for atmospheric pressure, temperature, water vapor pressure, precipitation amount, and altitude show significant correlations with mean weighted precipitation  δ18O values in the annual time scale of the PRB and adjacent regions, with correlation coefficients of 0.929, 0.787, 0.786, 0.589, and −0.908, respectively, with P values < 0.01 for atmospheric pressure, temperature, water vapor pressure, altitude, and 0.05 for precipitation amount (Figures 6(a), 6(b), 6(c), 6(e), and 6(d)). This suggests that they are the main factors controlling spatial precipitation δ18O variation. The slope of dδ18Op/dT and dδ18Op/dP is 0.34‰/°C (Figure 6(b)) and 1.7‰/100 mm (Figure 6(d)), respectively. The slope of dδ18Op/dT is higher than the northeast regions of China (0.27‰/°C) and slightly lower than the northwest regions of China (0.37‰/°C) [21]. The slope of dδ18Op/dALT is −0.15‰/100 m (Figure 6(e)), which indicates the “altitude effect” on the weighted mean annual precipitation  δ18O values, which is lower than that found for the Tibet Plateau (−0.3‰/100 m) by Liu et al. [21] and for the globe (−0.28‰/100 m) by Poage and Chamberlain [36]. However, this is similar to the whole country scale found for China (−0.15‰/100 m and −0.13‰/100 m) by Liu et al. [16] and Liu et al. [21], respectively.Figure 6
Correlations of mean weighted precipitationδ18O values for stations with meteorological and geographical factors which are at P value < 0.05 or 0.01. Note: (a), (b), (c), (d), and (e) represent the mean annual values; (f), (g), (h), and (i) represent the mean values for the summer monsoon period; (j), (k), and (l) represent the mean values for the nonsummer monsoon period.
(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l)In the summer monsoon period, the controlling factors for spatial precipitationδ18O variation is atmospheric pressure, temperature, vapor pressure, and altitude, exhibiting correlation coefficients of 0.930, 0.889, 0.913, and −0.921, respectively, with all the P values < 0.01 (Figures 6(f), 6(g), 6(h), and 6(i)). The slope of dδ18Op/dT and dδ18Op/dALT is 0.35‰/°C (Figure 6(g)) and −0.17‰/100 m (Figure 6(i)), respectively, which is similar to that for the annual time scale.In the nonsummer monsoon period, the controlling factors for the precipitationδ18O values are sunshine duration, atmospheric pressure, and altitude, with correlation coefficients of −0.776, 0.959, and −0.957, respectively, explaining 60.2%, 91.9%, and 91.6% of the variance, respectively, with P values < 0.05, 0.01, and 0.01 (Figures 6(j), 6(k), and 6(l)). The dδ18Op/dALT is −0.20‰/100 m (Figure 6(l)), suggesting that the “altitude effect” for the nonsummer monsoon period is the most significant among the three time periods.However, mean temperature shows significantly positive correlation with mean weighted precipitationδ18O in the annual time scale and the summer monsoon period (Figures 6(b) and 6(g)). Meanwhile, mean precipitation also shows significantly positive correlation with mean weighted precipitation  δ18O values (Figure 6(d)). Moreover, both temperature and precipitation show significantly negative correlations with altitude (Figure 7). This indicates that the correlations of precipitation and temperature may reflect the result of the “altitude effect,” suggesting that altitude should be the more significant factor for spatial precipitation  δ18O variation in the PRB and adjacent regions.Figure 7
Correlations of mean annual temperature (a), mean summer monsoon temperature (b), and mean annual precipitation (c) for stations with altitude.
(a) (b) (c)


## 3.4. Simulation of Precipitationδ18O in the PRB and Adjacent Regions

### 3.4.1. Nonlinear Regression Model and Linear Regression Model

The Bowen and Wilkinson model has been applied widely for simulating the precipitationδ18O in monsoon climatic zones [3, 10, 16]. No significant interaction effect of latitude and altitude has been found since the stations are mostly situated below 200 m [14]. The interaction effect of latitude and altitude occurs when stations are both below 200 m and above 200 m [16], and the elevation of our research area ranges from 0 to 2852 m. Therefore, we adopt a nonlinear fit to consider the correlation between latitude (LAT) and altitude (ALT) to establish the mean weighted precipitation  δ18O patterns under the three time scales: the annual time scale, summer monsoon, and nonsummer monsoon periods. The model equations are as follows:(5)δ18O=-0.062LAT2+2.792LAT-0.001ALT-37.251(6)δ18O=-0.072LAT2+3.315LAT-0.002ALT-45.091(7)δ18O=-0.007LAT2+3.383LAT-0.002ALT-9.056,where (5), (6), and (7) represent the mean weighted precipitation  δ18O patterns for the annual time scale, summer monsoon, and nonsummer monsoon periods, respectively.According to the above results (Figure6), we establish linear regression models using those meteorological and geographical variables for modeling in the three time periods. The resulting models are as follows:(8)δ18O=0.015AP+0.271T-0.173VP-0.0006P-0.0004ALT-22.429(9)δ18O=0.034AP-0.646T+0.222VP+0.0004ALT-30.177(10)δ18O=-0.002S+0.098AP+0.009ALT-101.567,where (8), (9), and (10) represent the mean weighted precipitation  δ18O patterns for the annual time scale, summer monsoon, and nonsummer monsoon periods, respectively. The parameters, S, VP, T, AP, P, and ALT represent sunshine duration (h), vapor pressure (hPa), air temperature (°C), atmospheric pressure (hPa), precipitation amount (mm), and altitude (m), respectively.By comparing the results of the Bowen and Wilkinson model with other researches, the regression coefficients of the second-order term (−0.062) (see (5)) are significantly more negative than that modeled on a global scale (−0.0051) [14] or modeled for the United States (−0.0057) [37] and China (−0.0073) [16]. This may be caused by the smallest area of our study. And the PRB and adjacent regions are significantly influenced by both the southeast and southwest monsoons, and precipitation  δ18O depleted as moisture is transferred from the oceans to the PRB and adjacent regions. Moreover, the Bowen and Wilkinson model estimates the best precipitation  δ18O patterns for nonsummer monsoon period (Figure 8). Zhao et al. [3] found that the Bowen and Wilkinson model could estimate precipitation  δ18O well only in winter for China. This suggests that there are differences in modeling the precipitation  δ18O by the Bowen and Wilkinson model for different regional scales and time scales in the monsoon climatic regions.Figure 8
Linear correlations between measured amount-weighted precipitationδ18O and modeled weighted precipitation  δ18O. Note: (a), (b), and (c) represent (5), (6), and (7) by the Bowen and Wilkinson model, respectively; (d), (e), and (f) represent (8), (9), and (10) by the linear regression model, respectively.
(a) (b) (c) (d) (e) (f)By comparing the results of the Bowen and Wilkinson model with the linear regression model, the results show that the linear regression model also estimates the best precipitationδ18O patterns for the nonsummer monsoon period (Figure 8). The amount of variance explained by the linear regression model is higher (93.5%, 92.8%, and 99.0%, resp.) than those (85.8%, 90%, and 92.4%, resp.) (Figure 8) by the Bowen and Wilkinson model for the annual time scale, summer monsoon, and nonsummer monsoon periods, respectively. Moreover, the mean errors explained by the linear regression model are lower (0.27‰ and 0.24‰, resp.) than those (0.48‰ and 0.35‰, resp.) by the Bowen and Wilkinson model for the annual time scale and the summer monsoon period. There is a higher mean error (0.41‰) in the linear regression model than that in the Bowen and Wilkinson model (0.32‰) for the nonsummer monsoon period, but the linear regression model explains more variance with a lower standard deviation of mean error (99.0‰ and 0.19‰, resp.) than does the Bowen and Wilkinson model (92.4‰ and 0.22‰, resp.). The results suggest that precipitation  δ18O can be estimated more accurately by the linear regression model than by the Bowen and Wilkinson model in the PRB and adjacent regions.


## 3.4.1. Nonlinear Regression Model and Linear Regression Model

The Bowen and Wilkinson model has been applied widely for simulating the precipitationδ18O in monsoon climatic zones [3, 10, 16]. No significant interaction effect of latitude and altitude has been found since the stations are mostly situated below 200 m [14]. The interaction effect of latitude and altitude occurs when stations are both below 200 m and above 200 m [16], and the elevation of our research area ranges from 0 to 2852 m. Therefore, we adopt a nonlinear fit to consider the correlation between latitude (LAT) and altitude (ALT) to establish the mean weighted precipitation  δ18O patterns under the three time scales: the annual time scale, summer monsoon, and nonsummer monsoon periods. The model equations are as follows:(5)δ18O=-0.062LAT2+2.792LAT-0.001ALT-37.251(6)δ18O=-0.072LAT2+3.315LAT-0.002ALT-45.091(7)δ18O=-0.007LAT2+3.383LAT-0.002ALT-9.056,where (5), (6), and (7) represent the mean weighted precipitation  δ18O patterns for the annual time scale, summer monsoon, and nonsummer monsoon periods, respectively.According to the above results (Figure6), we establish linear regression models using those meteorological and geographical variables for modeling in the three time periods. The resulting models are as follows:(8)δ18O=0.015AP+0.271T-0.173VP-0.0006P-0.0004ALT-22.429(9)δ18O=0.034AP-0.646T+0.222VP+0.0004ALT-30.177(10)δ18O=-0.002S+0.098AP+0.009ALT-101.567,where (8), (9), and (10) represent the mean weighted precipitation  δ18O patterns for the annual time scale, summer monsoon, and nonsummer monsoon periods, respectively. The parameters, S, VP, T, AP, P, and ALT represent sunshine duration (h), vapor pressure (hPa), air temperature (°C), atmospheric pressure (hPa), precipitation amount (mm), and altitude (m), respectively.By comparing the results of the Bowen and Wilkinson model with other researches, the regression coefficients of the second-order term (−0.062) (see (5)) are significantly more negative than that modeled on a global scale (−0.0051) [14] or modeled for the United States (−0.0057) [37] and China (−0.0073) [16]. This may be caused by the smallest area of our study. And the PRB and adjacent regions are significantly influenced by both the southeast and southwest monsoons, and precipitation  δ18O depleted as moisture is transferred from the oceans to the PRB and adjacent regions. Moreover, the Bowen and Wilkinson model estimates the best precipitation  δ18O patterns for nonsummer monsoon period (Figure 8). Zhao et al. [3] found that the Bowen and Wilkinson model could estimate precipitation  δ18O well only in winter for China. This suggests that there are differences in modeling the precipitation  δ18O by the Bowen and Wilkinson model for different regional scales and time scales in the monsoon climatic regions.Figure 8
Linear correlations between measured amount-weighted precipitationδ18O and modeled weighted precipitation  δ18O. Note: (a), (b), and (c) represent (5), (6), and (7) by the Bowen and Wilkinson model, respectively; (d), (e), and (f) represent (8), (9), and (10) by the linear regression model, respectively.
(a) (b) (c) (d) (e) (f)By comparing the results of the Bowen and Wilkinson model with the linear regression model, the results show that the linear regression model also estimates the best precipitationδ18O patterns for the nonsummer monsoon period (Figure 8). The amount of variance explained by the linear regression model is higher (93.5%, 92.8%, and 99.0%, resp.) than those (85.8%, 90%, and 92.4%, resp.) (Figure 8) by the Bowen and Wilkinson model for the annual time scale, summer monsoon, and nonsummer monsoon periods, respectively. Moreover, the mean errors explained by the linear regression model are lower (0.27‰ and 0.24‰, resp.) than those (0.48‰ and 0.35‰, resp.) by the Bowen and Wilkinson model for the annual time scale and the summer monsoon period. There is a higher mean error (0.41‰) in the linear regression model than that in the Bowen and Wilkinson model (0.32‰) for the nonsummer monsoon period, but the linear regression model explains more variance with a lower standard deviation of mean error (99.0‰ and 0.19‰, resp.) than does the Bowen and Wilkinson model (92.4‰ and 0.22‰, resp.). The results suggest that precipitation  δ18O can be estimated more accurately by the linear regression model than by the Bowen and Wilkinson model in the PRB and adjacent regions.


## 3.5. Spatial Distribution of Precipitationδ18O in the PRB

Spatial distribution of precipitationδ18O values from 1980 to 2011 for the annual time scale, summer monsoon, and nonsummer monsoon periods of the PRB are presented based on (8), (9), and (10), respectively. The data utilized come from the meteorological data network of China (Figure 9(a)). Precipitation  δ18O values are gradually more depleted (Figure 9) from the south and east to the north and northwest, especially in the annual time scale and the summer monsoon period, which are mainly caused by “altitude effect.” Precipitation  δ18O values are more depleted in the summer monsoon period and more enriched in the nonsummer monsoon period (Figures 9(c) and 9(d)). However, spatial distribution of precipitation  δ18O reflect only the results of the linear regression model based on limited quantities and years of observation stations. As a result, more of the long-time observation stations should be established for more accurate estimation of precipitation  δ18O variations in the monsoon climatic regions.Figure 9
Spatial distribution of mean weighted precipitationδ18O values over Pearl River basin of China based on the Kriging method from 1980 to 2011. Note: (a) represents meteorological stations in the Pearl River basin; (b) represents spatial distribution of the mean weighted annual precipitation  δ18O values by (8); (c) represents spatial distribution of mean weighted precipitation  δ18O values for the summer monsoon period by (9); (d) represents spatial distribution of mean weighted precipitation  δ18O values for the nonsummer monsoon period by (10).
(a) (b) (c) (d)


## 4. Conclusions

Based on the precipitationδ18O values from the datasets of the Global Network of Isotopes in Precipitation (GNIP), the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data, and previous researches, we explored the temporal and spatial variations of precipitation  δ18O in the PRB and adjacent regions. The principal conclusions are as follows:(1)
There was no “temperature effect” for the precipitationδ18O for each station in the PRB and adjacent regions. “Amount effect” has been found at all stations (except for Haikou) for the annual time scale, while only three stations show the “amount effect” in the summer monsoon and nonsummer monsoon periods.(2)
Temporal variations show that the most depleted mean monthly precipitationδ18O value does not occur in the month with the most mean monthly precipitation amount, which should be correlated with water vapor sources, the distance of water vapor transport, the changes in location, and intensity of the intertropical convergence zone (ITCZ). Spatial variations show that the most depleted mean precipitation  δ18O values does not occur in the station with the most mean precipitation amount for the annual time scale. Meanwhile, “altitude effect” has been significantly found among stations and precipitation δ18O values among stations show close correlation with local meteorological conditions, which can account for the characteristics in the spatial variations. This indicates that water vapor sources and changes in location and intensity of the ITCZ, “altitude effect” as well as local meteorological conditions should be taken significantly into consideration when interpreting the temporal and spatial variations of precipitation  δ18O rather than “amount effect” in the PRB and adjacent regions.(3)
We established linear regression models combining multiple meteorological variables for estimating the mean weighted precipitationδ18O in the PRB for the annual time scale, summer monsoon, and nonsummer monsoon periods. The amounts of variances explained by the linear regression model are all more than 92%, which are higher than those explained by the Bowen and Wilkinson model, suggesting that the linear regression models can estimate more accurately than the Bowen and Wilkinson model in the PRB and adjacent regions.(4)
This research can provide useful information for rebuilding temporal and spatial precipitationδ18O in the monsoon climatic regions. Meanwhile, more of the long-time observation stations should be established for more accurate estimation of precipitation  δ18O variations in the monsoon climatic regions.



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*Source: 1419326-2018-02-20.xml*