Innovative trend analysis of annual and seasonal precipitation in Ningxia,China

YANG Huiling,XIAO Hui,GUO Chunwi,SUN Yu, n GAO RuinKy Lortory of Clou Pripittion Physis n Svr Storms&Cntr of Disstr Rution,Institut of Atmosphri Physis,Chins Amy of Sins,Bijing,Chin; Institut of Urn Mtorology,Chin Mtorologil Aministrtion,Bijing,Chin; Environmntl Mtorology Forst Cntr of Bijing-Tinjin-Hi,Bijing,Chin; Univrsity of Chins Amy of Sins,Bijing,Chin; Ningxi Climt Cntr,Yinhun,Chin

ABSTRACT Based on daily rainfall data from 26 station records,spatial and temporal variations in annual and seasonal precipitation of different rainfall intensities from 1961 to 2018 in Ningxia, China are investigated using the innovative trend analysis (ITA) method. The results show that annual precipitation increases on the northern plain but decreases in the southern mountainous area.The increase in regional annual precipitation is mainly due to an increase in weak precipitation,while the decrease in regional annual rainfall is a result of a reduction in heavy precipitation.Lowintensity precipitation shows an upward trend, while high-intensity precipitation shows a downward trend.The variation trend of extreme precipitation is more obvious.The contributions of different types of extreme precipitation vary by season.During spring,the increase in regional rainfall is mainly caused by the increase in heavy precipitation, while the decrease in regional precipitation is mainly caused by the decrease in weak precipitation.During summer and autumn,the increase in regional precipitation is caused by the increase in light precipitation, while the reduction in regional rainfall is caused by the decrease in heavy precipitation.This study provides support for water resources planning and addressing droughts and floods.

KEYWORDS ITA;precipitation;trend analysis;Ningxia

1. Introduction

Global warming has a significant impact on hydrological cycles(Chen et al.2015).Extreme weather events such as floods,droughts,and other disasters are associated with climate warming,resulting in increasing social and economic losses (Holmes, Cook, and Yang 2009; Lehmann,Coumou,and Frieler 2015;Swain et al.2018).China has experienced a significant increase in extreme precipitation, particularly in Northwestern and Southeastern China (Xu et al. 2011). Understanding the spatial and temporal variabilities of precipitation is important not only for climate scientists but also for a wide range of relevant decision makers (Brunsell 2010). Precipitation changes caused by climate change may greatly vary from one region to another(Yuan,Hu,and Qing 2019).

Studies have found that there has been an obvious warming trend in Northwestern China since 1951(Wang,Yang, and Zhang 2007). Some studies have suggested that the annual total precipitation has increased in Northwestern China (Sui, Jiang, and Tian 2013), where water resources are scarce,drought is serious,ecologically fragile areas are widely distributed, and the ecological environment is highly dependent on precipitation(Wang et al. 2014). Ningxia is in the eastern part of Northwestern China and is a typical ecotone of agriculture and animal husbandry lands and an ecologically fragile area. A variety of climate types occur in this region, and extreme climate events frequently occur(Li and Li 2001).

Many studies have used different methods to analyze the spatial variability of precipitation,such as the Mann-Kendall (MK) test and wavelet analysis (Hamed 2008).However, these previous methods cannot describe the changes in different rainfall intensities well.The innovative trend analysis (ITA) method recently introduced by Sen (2012, 2014) has been used to detect trends in environmental, hydrological, and meteorological variables (Ay and Kisi 2015; Onyutha 2016). It has been shown that the ITA index agrees well with other statistical methods, proving that it is a feasible and effective method for performing trend analysis (Wu and Qian 2017). Compared to other nonparametric methods, the ITA method has universal applicability,regardless of the distribution hypothesis, series correlation, seasonal period, and time series size. In addition, some of the unidentified trends detected using the MK test can be identified using the ITA method, and it can identify important hidden sub-trends within a graph.

Because of the uneven distribution of meteorological stations in Northwestern China,calculating the regional average precipitation will inevitably lead to a large error(Chen et al. 2012). Many previous studies have focused on analyzing the overall precipitation trend of this region and do not analyze the changes in different precipitation intensities. To better inform decision-making for regional economic development, there is an urgent need for an objective and detailed analysis of the precipitation change in Ningxia.The purpose of this study is to use the ITA method to study the annual and seasonal trends of different precipitation intensities in Ningxia.

Figure 1.(a) Locations of the selected meteorological stations,the type of topography,and the distribution of rivers and lakes.(b)Schematic diagram of the ITA method.

2. Study area and methods

Ningxia can be divided into three regions: the northern plain area, the central arid region, and the southern mountainous area. Daily precipitation data from 26 stations in Ningxia for the 58-year period from 1961-2018 were obtained from the China Meteorological Data Network. The locations of the selected stations are shown in Figure 1(a). Five stations lack too much observational data,so the other 21 stations are analyzed.

The four seasons include spring (March-May), summer(June-August),autumn(September-November),and winter(December-February).The precipitation intensity is separated into five categories according to percentiles: light rain(0-20%), low rain (20%-40%), moderate rain (40%-60%),high rain (60%-80%), and heavy rain (80%-100%). The ITA method(Figure 1(b))used in this study is the same as that employed in previous studies (Sen 2012, 2014; Ay and Kisi 2015;Wu and Qian 2017).The position of significance level 0.1 is represented by a blue-dashed line in the ITA Schematic diagram(Figure 1(b)).

3. Results and discussion

3.1. Spatial-temporal variation in rainfall

Figure 2(a)shows the average monthly rainfall in Ningxia.Precipitation is mainly concentrated during June, July,August, and September, accounting for more than 80%of the annual precipitation. A time series of the annual rainfall changes calculated using the linear filtering method is shown in Figure 2(b).Results indicate that the precipitation in the whole region shows a downward trend before 2010 and a weak upward trend after 2010. Figure 2(c,d)presents the spatial distribution of annual cumulative precipitation in 1961-2018 from the gridded daily dataset with the resolution of 0.5°latitude by 0.5°longitude(CN05)and the station data, respectively. The station data is interpolated by the inverse distance weighted method.Except for a few regions in southern Ningxia, the distribution of annual cumulative precipitation obtained from station data and grid data is very close. The annual cumulative precipitation decreases from south to north.

Figure 2. (a) Averaged monthly rainfall in Ningxia. (b) The time series of annual rainfall trends obtained by using smoothing and filtering methods. Spatial distribution of cumulative precipitation in 1961-2018 (units: mm) from (c) CN05 and (d) station data. (e)Annual variation trend of precipitation in Ningxia obtained by ITA method.Annual variation trend of precipitation in five categories in Ningxia obtained by using ITA method:(f)light rain,(g)low rain,(h)moderate rain,(i)high rain,and(j)heavy rain.

3.2. Annual trends

3.2.1. Overall trends

Figure 2(e)shows the annual variation trend of total precipitation obtained using the ITA method. Figure 2(f-j)shows the annual variation trend of different precipitation intensities. G1, G2, G3, G4, and G5 represent light, low,moderate,high,and heavy precipitation,respectively.The results indicate that there are obvious regional differences in the variation in annual precipitation. The increase in precipitation mainly occurs on the northern plain,where the annual average precipitation is relatively low, while the decrease in precipitation mainly occurs in the southern mountainous area.The eastern part of the central arid area has an increasing trend, while the western part has a decreasing trend.The incidence of drought in the southern mountainous area is higher than that in other areas.The annual variation trend of total precipitation obtained using the ITA method is consistent with that obtained using other methods in previous studies(Wang et al.2014). Considering that the southern mountainous areas are more prone to drought,additional storage capacity is needed to prevent drought.

We can better understand the reasons for the change in total precipitation by analyzing the changes in different precipitation levels.From Figure 2(f-j),it can be seen that the variation trend of moderate precipitation is nearest that of total precipitation. Light and low precipitation show an obvious increasing trend in the northern and central regions and a weak decreasing trend in the southern mountainous areas. High precipitation and heavy precipitation show a clear decreasing trend in the entire region(except for a small number of areas in the eastern part of the central areas), particularly in the southern mountainous areas. The increase in light precipitation is the most obvious,while the decrease in heavy precipitation is the most apparent.Therefore,the decrease in total precipitation is mainly due to the decrease in high and heavy precipitation.The increase in precipitation is mainly because of the increase in light and low precipitation,that is,the increase in weak precipitation.The variation trend of extreme precipitation is clearer.

Figure 3.Annual variation trend of precipitation at nine stations in Ningxia obtained by using the ITA method.

3.2.2. Trends in annual rainfall at various stations

Three stations are selected for each region: stations 3, 5,and 6 represent the northern plain;stations 14,15,and 16 represent the central arid area;and stations 20,22,and 26 represent the southern mountainous area.Figure 3 shows the distribution of annual precipitation for the nine stations using the ITA method.The results show that the precipitation trends in the three regions have both similarities and differences.The similarity is that low-intensity precipitation has an increasing trend, while high-intensity precipitation has a decreasing trend. The characteristics of the three stations in the same region are similar.The northern plain area is termed‘south of the Yangtze River’because of the irrigation of the Yellow River. Light, low, and moderate precipitation in this region have a certain increasing trend,particularly light and low precipitation,which exceed the 0.1 significance level. However, high and heavy precipitations have a decreasing trend, particularly the decrease in heavy precipitation, which has a significance level greater than 0.1.In the central arid area,vegetation is scarce and land cover is mostly sand.There is a very slight increase in light and low precipitation in this area,which do not exceed the 0.1 significance level, and the change in moderate precipitation is very small.High and heavy precipitation has a significant decreasing trend. In particular,the decrease in heavy precipitation is significantly greater than that on the northern plain, far exceeding the 0.1 significance level. There is a significant decrease in heavy precipitation in this region. The southern mountainous area is the famous‘Xihaigu’poor area;Liupan Mountain is in this area,and precipitation has a very important impact on farming.Only a very slight increase in light precipitation has been recorded in this region.Precipitation at all other intensities has a decreasing trend.The decrease in high and heavy precipitation is greater than the 0.1 significance level. The precipitation decrease in this region is more obvious than that in the previous two regions. Therefore,the low precipitation intensities in the three regions have an increasing trend,the high precipitation intensities have a decreasing trend, and the variation trend of extreme precipitation is more obvious. However, the variation trend of precipitation in different regions and at different precipitation intensities is different. The increase in lowlevel precipitation is more apparent on the northern plain,and the decrease in high-level precipitation is more obvious in the central arid area.Except for light precipitation,the other precipitation levels in the southern mountainous area have obviously decreased.

3.3. Seasonal trends

Figure 4(a-d) shows the seasonal variation trend of total precipitation obtained using the ITA method.Figure 4(e-x)shows the seasonal variation trend of different precipitation intensities.The variation trend of total precipitation is as follows: the variation trend of spring precipitation is nearest that of annual precipitation, which increases on the northern plain,decreases in the southern mountainous area,increases in the eastern part of the central arid region,and decreases in the western part of the central arid region.Except for a small number of areas in the central arid region, precipitation during summer and autumn has a decreasing trend; the trend of precipitation during autumn is more obvious than that during summer.Precipitation during winter has a clear increasing trend.

At the same time,the advantage of the ITA method is that the contribution of different precipitation levels to the change in total precipitation can be obtained. The variation trend of different levels of precipitation(Figure 4(e-x))is as follows:The level of G1 precipitation decreases during spring and increases during summer. During autumn,there is an increasing trend for G1 in the entire region except for the southern mountainous area, and during winter, there is an increasing trend in the entire region except for the northeast plain. The level of G2 precipitation has an increasing trend during spring,summer,autumn,and winter on the northern plain and in the central arid region,while there is a slight decreasing trend in the southern mountainous area. During spring, the level of G3 precipitation increases on the northern plain but decreases in the central arid region and the southern mountainous area. During summer, G3 in most of the northern plain and the central arid region has an increasing trend, while the southern mountainous area has a slight decreasing trend.During autumn,a small number of areas on the northern plain and the eastern part of the central arid region have an increasing trend, while the western part of the central arid region and southern mountainous areas have a decreasing trend for G3.During spring, the level of G4 precipitation increases on the northern plain and in the central arid areas, while it decreases in the southern mountainous area.During summer and autumn,except for a small portion of the eastern part of the central arid area, the entire area has a decreasing trend for G4.G3 and G4 in the entire region have an increasing trend during winter. The variation trend of the level of G5 precipitation is similar to that of G4,but the decreasing trend is more pronounced during summer and autumn,particularly autumn.Therefore,the increase of total precipitation in spring on the northern plain and in the eastern part of the central arid region is mainly caused by the contributions of G4 and G5. The decrease in total precipitation in the southern mountainous area during spring is mainly caused by the decreases in G1 and G2.The increase in precipitation in the eastern part of the central arid area during summer is mainly caused by the increases in G1 and G2. The decrease in precipitation in most other areas is mainly caused by the contributions of G4 and G5.The increase in precipitation in the eastern part of the central arid area during autumn is mainly because of the increases in G2 and G3. The decrease in precipitation in the other regions is mainly caused by the decreases in G3,G4,and G5.Moderate and heavy precipitation plays major roles in the reduction in precipitation.Winter precipitation increases in the entire region, and all the precipitation levels increase. Weak precipitation(G1,G2)increases are more apparent;weak precipitation plays a greater role in increasing winter precipitation.During spring,the increase in regional rainfall is mainly caused by the increase in heavy precipitation,while the decrease in regional precipitation is mainly a result of the decrease in weak precipitation. During summer and autumn, the enhanced regional precipitation is dominated by the increase in weak precipitation,while the reduction in regional rainfall is controlled by the decrease in heavy precipitation. During winter, the increase in weak precipitation provides a great contribution to the increase in precipitation.

Figure 4.The seasonal variation trend of total precipitation and the seasonal variation trend of five kinds of precipitation in Ningxia are analyzed by using the ITA method.Columns 1-4 represent spring,summer,autumn,and winter,respectively;Rows 1-6 represent total rainfall,light rain,low rain,moderate rain,high rain,and heavy rain,respectively.

3.4. Comparisons to previous studies

(1) Previous studies paid more attention to the overall variation trend of precipitation; few studies have studied the variation trend of different precipitation intensities. This study found that different types of extreme precipitation play different roles in decreasing or increasing overall precipitation.

(2) Previous studies have focused on a wide range of study areas,such as Northwestern China;few studies have studied the characteristics of local precipitation in Ningxia.

4. Conclusions

The ITA method was used to detect the annual and seasonal variation trends of five different precipitation intensities in Ningxia. The major conclusions are as follows:

(1) Overall trends.Total precipitation decreases from south to north, and rainfall shows a downward trend.

(2) Annual trends. The increase in precipitation mainly occurs on the northern plain, while the decrease in precipitation occurs in the southern mountainous area.The eastern part of the central arid area has an increasing trend, while the western part of the central arid area has a decreasing trend. The increase in regional annual precipitation is mainly due to the increase in weak precipitation, while the decrease in regional annual rainfall is a result of the reduction in heavy precipitation. Low-intensity precipitation has an increasing trend, while high-intensity precipitation has a decreasing trend. The variation trend of extreme precipitation is more obvious.

(3) Seasonal trends. The results show that the contributions of different types of extreme precipitation vary by season. During spring, the variation trend of precipitation is similar to that of the annual trend. The increase in regional rainfall is mainly caused by the increase in heavy precipitation, while the decrease in regional precipitation is mainly caused by the decrease in weak precipitation.During summer and autumn,precipitation has a decreasing trend for the entire region, and the decreasing trend of precipitation during autumn is more obvious than that during summer.The enhanced regional precipitation is dominated by the increase in weak precipitation,while the reduction of regional rainfall is controlled by the decrease in heavy precipitation. During winter, precipitation increases in the entire region.The increase in weak precipitation has a great contribution to the increase in winter precipitation.

(4) The results of this study provide a more comprehensive understanding of precipitation changes in Ningxia, China, which is beneficial for policymakers and managers to manage resources under changing climatic conditions.The precipitation in the southern mountainous area obviously decreases. In particular, the change in extreme precipitation requires special attention from managers.This information will also provide guidance for future research in other parts of China. The spatial variations in precipitation trends imply that water resource managers should implement measures to adapt to different regions within the context of global warming.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This study was supported by the National Key Research and Development Plan of China [Grant No. 2016YFE0201900-02],the National Natural Science Foundation of China [Grant Nos.41575037 and 41205099],the National Basic Research Program of China[973 Program,Grant No.2014CB441403],and Guizhou Province Scientific Research Joint Project [Grant No. G[2013]4001].