Spatiotemporal Variations of Precipitation over Iran Using the High-Resolution and Nearly Four Decades Satellite-Based PERSIANN-CDR Dataset

: Spatiotemporal precipitation trend analysis provides valuable information for water management decision-making. Satellite-based precipitation products with high spatial and temporal resolution and long records, as opposed to temporally and spatially sparse rain gauge networks, are a suitable alternative to analyze precipitation trends over Iran. This study analyzes the trends in annual, seasonal, and monthly precipitation along with the contribution of each season and month in the annual precipitation over Iran for the 1983–2018 period. For the analyses, the Mann–Kendall test is applied to the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Climate Data Record (PERSIANN-CDR) estimates. The results of annual, seasonal, and monthly precipitation trends indicate that the significant decreases in the monthly precipitation trends in February over the western (March over the western and central-eastern) regions of Iran cause significant effects on winter (spring) and total annual precipitation. Moreover, the increases in the amounts of precipitation during November in the south and southeast regions lead to a remarkable increase in the amount of precipitation during the fall season. The analysis of the contribution of each season and month to annual precipitation in wet and dry years shows that dry years have critical impacts on decreasing monthly precipitation over a particular region. For instance, a remarkable decrease in precipitation amounts is detectable during dry years over the eastern, northeastern, and southwestern regions of Iran during March, April, and December, respectively. The results of this study show that PERSIANN-CDR is a valuable source of information in low-density gauge network areas, capturing spatiotemporal variation of precipitation.


Introduction
Precipitation is one of the important climatic variables whose variability has impacts on floods, droughts [1], and water availability [2]. This is especially important for arid and semi-arid regions such as Iran, with limited water availability [3] and many natural disasters including floods and droughts [4]. Therefore, investigating the variations in precipitation is necessary for decision-makers and stakeholders to manage water resources and natural hazards.
Investigating precipitation variability requires reliable, long-term, and high spatiotemporal resolution precipitation information. Several studies have been conducted to investigate global In arid and semi-arid regions, the temporal distribution of annual precipitation is crucial for water resources management and irrigation planning. For this purpose, over Iran, Khalili et al. [38] calculated a precipitation concentration index (PCI), which describes the degree of inter-annual precipitation variability. PCI does not include any information about the contribution of each month and season to annual precipitation separately and no study to date has been carried out to investigate this issue in Iran. So, the overarching goals of this study are as follows: (1) to investigate the trends in annual, seasonal, and monthly precipitation over Iran, and (2) to investigate the trends in the contribution of each season and month in the total annual precipitation for the period of 1983-2018 over Iran. The PERSIANN-CDR dataset with 0.25° spatial resolution is adapted as precipitation estimation and the Mann-Kendall non-parametric test is used for trend investigation.
The remainder of this paper is organized as follows. Sections 2 and 3 provide information on the study area and the methodology, respectively. Sections 4 and 5 present the trend analysis results and discussion, respectively. The main conclusions of this study are summarized in Section 6.

Study Area
Iran is located in southwest Asia (44•05′E-63•19′E; 25•03′N-39•47′N) and covers an area of about 1,648,000 km 2 , of which approximately 88% is arid and semi-arid [4] (Figure 1a). Iran is bordered to the north by the Caspian Sea and to the south by the Persian Gulf and the Gulf of Oman. The climate of Iran is affected by different meteorological systems including Siberian high-pressure, Mediterranean, and low-pressure southern systems [38]. These systems are affected by the high spatial variation of elevation of two mountain ranges-Alborz in the north and Zagros in the west [44]-leading to the temporal and spatial variation of precipitation over the country. On the basis of the PERSIANN-CDR datasets (1983-2018), the mean annual precipitation of Iran is 264.7 mm.

Data
The Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Climate Data Record (PERSIANN-CDR) product was developed by the Center for Hydrometeorology and Remote Sensing (CHRS) at the University of California, Irvine (UCI). The PERSIANN-CDR dataset provides daily near-global precipitation estimates with 0.25° x 0.25° spatial resolution from 1983 to the present. The PERSIANN-CDR algorithm uses an artificial neural network (ANN) model to convert gridded satellite (GridSat-BI) infrared information [48,49] as an input into the rain rates retrieved. The National Centers for Environmental Prediction (NCEP) Stage IV hourly precipitation [50,51] is utilized for creating the non-linear regression parameters of the ANN model. The estimates of the ANN model are bias-adjusted with the Global Precipitation Climatology Project (GPCP) monthly product with 2.5° x 2.5° spatial resolution at a monthly time scale. The PERSIANN-CDR dataset is available via the NOAA National Centers for Environmental Information (NCEI) Program (https://www.ncdc.noaa.gov/cdr) and via the CHRS Data Portal (http://chrsdata.eng.uci.edu/, [52]). A comprehensive description of the PERSIANN-CDR algorithm can be found in Ashouri et al. [14]. In the current study, the PERSIANN-CDR product for the period ranging from 1983 to 2018 was used for the trend analysis.

Data Processing
As shown in Figure 1b, there are 5604 pixels of PERSIANN-CDR within Iran's boundary. The trend analysis is done for each pixel and different temporal scales, including annual, seasonal, and monthly precipitation. For this purpose, the monthly datasets of each pixel are downloaded, then converted to the seasonal and annual data. The seasons are defined as follows: winter (December-February), spring (March-May), summer (June-August), and fall (September-November). To understand the seasonal variations, we investigate the trend in the contribution of each season and month to annual precipitation. These contributions are calculated by dividing the seasonal and monthly precipitation values by the corresponding annual precipitation value, respectively. Moreover, for precise investigation, the average percentage of each seasonal and monthly contribution in annual precipitation is analyzed in wet and dry years separately. Wet and dry years are selected with respect to the years with a 12-month SPI (standardized precipitation index) of >1 and <−1, respectively [53].

Trend Analysis
The nonparametric Mann-Kendall [54,55] test has been widely applied to investigate the trends in precipitation using different datasets [7,38,40,43]. This test is a useful method for this purpose because it allows for missing values and the data are not required to be from a normal distribution. According to this test and under the null hypothesis (H0 ), data are independent and there is no trend. The alternative hypothesis, H1, is that the data have a trend. The Mann-Kendall test starts with calculating the statistic S. In this study, the time series of precipitation in each pixel is used to calculate the statistic S as follows: where n represents the total number of data in time series, and are the j th and k th value of the precipitation time series in chronological order, and the sign function is defined as follows: when n ≥ 8, the distribution of statistic S approaches the normal form with mean (E(S)) and variance (Var(S)) defined as follows: The second part of Equation (4) (∑ ( − 1)(2 + 5) ) is used only when data series contain successive data with same value (tied values). In this equation, m is the number of tie group and is the length of m th group. Finally, the standardized test statistic Z is computed using Equation (1) and Equation (4) as follows: The statistic Z is used as a measure of singnificant of trend by testing the null hypothesis. The null hypothesis H0 is rejected when | | > and, in this hypothesis, the existence of a significant trend is accepted. α denotes the chosen significant level, which is considered as 0.05 in this study. Moreover, the negative and positive values of Z indicate the decreasing and increasing trend, respectively. Figure 2 shows the spatial distribution map of annual precipitation trends using the PERSIANN-CDR estimates for the period from 1983 to 2018. In this figure, the statistically significant (95%) positive and negative trends are in blue and red, respectively, and non-statistically significant trends are shown in white. There are limited numbers of pixels showing statistically significant changes in the annual precipitation over Iran. The annual precipitation decreased in the western (including Khuzestan, Ilam, Lorestan, Kermanshah, Hamadan, and Kurdistan provinces) and eastern (including some regions over Khorasan-jonoubi, Kerman, and Razavi Khorasan provinces) regions. On the other hand, the annual precipitation has increased significantly in small regions of northwestern Iran (including some area around Ardabil) and in a few pixels in the central part of the country.

Seasonal Trend Analysis
Figure 3a displays the spatial distribution of the mean seasonal precipitation over Iran for the period 1983 to 2018 using the PERSIANN-CDR estimates. The results indicate that mountainous areas of Alborz, the coast of the Caspian Sea in Northern Iran, and western mountainous regions of the country (The Zagros Mountains) are the wettest regions. On the other hand, central, eastern, and southern regions of Iran are classified as the driest regions. Across the whole country, the highest amounts of precipitation occur during winter, while relatively identical precipitation patterns with lower rain rates are received during spring. Summer and fall are dry seasons, particularly in the central, eastern, and southern regions. The monsoon precipitation that occurs during summer in the arid region of the south-east is well captured by PERSIANN-CDR estimates. Figure 3b shows the spatial distribution of trends in the seasonal precipitation based on the PERSIANN-CDR estimates for the period from 1983 to 2018. The figure shows that winter precipitation is decreasing in small regions over western Iran, and this trend is statistically significant. Similarly, spring precipitation exhibits a statistically downward trend in small areas over the western and central-eastern regions of the country. The spatial pattern of the Mann-Kendall test for summer displays an upward trend in precipitation for different parts of Iran. In addition, a statistically significant upward trend is detectable over the south-eastern regions during the fall. Positive trend

Negative trend
Positive trend Negative trend Figure 4a indicates the spatial contribution map of each season to the annual precipitation based on the climate data record of PERSIANN-CDR. The map is calculated by averaging the precipitation contribution of each season in annual rainfall for the period from 1983 to 2018. The results display that central, southern, and western Iran receive most of the annual rainfall during winter. On the contrary, northern and northwest Iran receive the same amounts of rainfall during fall, spring, and winter. The spatial pattern of the trend in the contribution of each season to annual precipitation is shown in Figure  4b. The statistically downward trend is only detectable over small regions in central Iran during spring. In contrast, the statistically upward trend can be seen along the Zagros Mountain and northeast Iran during summer and over the south-eastern regions during fall. Moreover, the south-eastern regions of Iran, whose contribution during fall is about 5% to 15% of the total annual precipitation, have a statistically significant upward trend. Moreover, except for a few pixels, no statistically significant trends can be observed in the contribution of the winter season to annual precipitation.   Figure 5 illustrates the spatial contribution map for each season to annual precipitation based on climate data record of PERSIANN-CDR for dry and wet years. The results show that mostly the central, southern, and eastern regions receive more than half of winter rainfall for both dry and wet years. However, the northeast of Iran receives more than half of the annual rainfall during winter in dry years and during spring in the wet years. The contribution of precipitation occurrence of both winter and spring is important for the total rainfall over the northwest of the country.

Monthly Trend Analysis
The mean monthly precipitation in each pixel is shown in Figure 6a. The figure shows that, same as seasonal precipitation, a mountain range in the west and north gets most of the monthly precipitation. The regions with the highest precipitation have stretched over the southwest-northwest from December to May. Moreover, the precipitation occurs over the north and southeast of Iran from July to September. The blue and red pixels in Figure 6b refer to positive and negative statistically significant trends of mean monthly precipitation at the 95% confidence levels, respectively. Statistically negative significant trends occur in four months (December, January, February, and March). The eastern region is characterized by significant negative trends in December and January. Moreover, a downward significant trends is shown in the western region (including Kermanshah, Kordestan and parts of Orumieh provinces) in February. Furthermore, the results of March precipitation trends test show significant negative trends in the western (including Ilam, Lorestan, Hamadan and some regions of Arak provinces) and eastern (including some region of Khorasanjonoubi and Yazd provinces) regions of Iran. In all summer months, the positive trend is detected in different regions like the north and central area in July. The results of the Mann-Kendall test of fall months show that, in most parts of Iran, except the south (in September), central, and southeast region (in October), and south southeast regions of Iran (including Hormozgan, Kerman, Sistan, and Baluchestan, and part of Fars provinces) (in November), have a significant increase. Figure 7a presents the spatial distribution of the average percentage of each month's contribution to annual precipitation. The temporal distribution of precipitation in each pixel is different. For instance, the uniform distribution of precipitation occurs from November to May over the northern regions of Iran, while for other regions, the majority of precipitation falls in one or two months. Figure 8

Discussion
To investigate the trends in annual, seasonal, and monthly precipitation over Iran, PERSIANN-CDR dataset is used as precipitation estimation. The comparison results of the annual trend with seasonal trend indicated that the trends for winter and spring precipitation are similar to the annual trend, where we observe a statistically downward trend in western and central-eastern Iran. Moreover, a statistically significant upward trend over the south-eastern regions during the fall leads to a positive annual trend over a limited amount of pixels. Meanwhile, the positive trend in summer precipitation does not have any effect on annual precipitation. Furthermore, some monthly trends affect the trend in seasonal and annual precipitation. For instance, negative significant trends in the western region (including Kermanshah, Kordestan, and parts of Orumieh provinces) in February cause significant decreases in precipitation during winter, as well as in the annual precipitation in these regions. On the contrary, downward trends in December and January over the eastern region do not affect the trend in precipitation during winter (Figure 3b). In addition, as no significant trends are detected during the spring months except March over these regions, the downward spring significant trend is caused by March decreasing precipitation.
Here, we compare our results with previous studies. The negative trend of annual precipitation in the western part of Iran was also reported by Tabari and Talaee [40]. Unlike the findings of some previous studies [43,46], which showed negative trends in other parts of Iran such as northwestern Iran (including Tabriz, Oroomieh, and Saghez station), no significant trends are detected by our results over these regions. This discrepancy might be owing to the difference between the study periods. Furthermore, when comparing our results to those of previous studies, it must be pointed out that no comparison can be made over the regions that do not have any weather stations (such as trend positive over northwestern), because previous study results are limited to regions with weather stations. Moreover, according to Table 1, there are inconsistency between our results and pervious research at seasonal ( [45]) and monthly ( [39,41,44]) precipitation, which might be because of different study periods as well as different types of precipitation datasets (pixel-based and point gauge) used for trend analysis. Indeed, rain gauge measure precipitation in single point, while satellite product estimates precipitation over an area (pixel area). Investigation of the trends in the contribution of each season and month in the total annual precipitation indicated that the positive trend in the contribution of fall to the annual precipitation over the south-eastern regions of Iran causes significant increasing precipitation in the fall season because both contributions for the fall in annual precipitation and fall seasonal precipitation have identical significant trend patterns. However, trends in the contribution of other season to annual precipitation do not affect the seasonal precipitation. Furthermore, there is no pixel over Iran significantly increasing in the contribution of a particular season to annual precipitation and a simultaneously significant decrease in other seasons, and vice versa. So, the contribution of any season to annual precipitation does not significantly shift to another season.

Conclusions
Given the high spatial and temporal variations of precipitation in semi-arid regions like Iran, it is necessary for water decision-makers to analyze the precipitation patterns in these regions. In this study, the high spatial and temporal resolution PERSIANN-CDR dataset was used to analyze the precipitation at annual, seasonal, and monthly time scales over Iran for the period 1983-2018. Furthermore, trend analysis for the contribution of each season and month to the annual precipitation was conducted. The Mann-Kendall non-parametric test was applied for the trend analysis.
Both statistically upward and downward trends were observed in monthly precipitation. However, most of months do not have a significant effect on the seasonal and annual precipitation. Indeed, just February and March cause significant decreases in precipitation during winter and spring, respectively, and in the annual precipitation. Moreover, the upward fall significant trend is caused by November decreasing precipitation, while it does not have a remarkable effect on the annual precipitation. Seasonal trend analysis showed that the summation of a significant downward trend in winter and spring causes significant decreasing annual precipitation, while summer and fall trends do not display remarkable effects on annual precipitation. Finally, the result of the annual trend, which is affected by the monthly and seasonal trend, indicated that some parts of Iran have statistically significant negative trends, including the western regions (including Khuzestan, Ilam, Lorestan, Kermanshah, Hamadan, and Kurdistan provinces) and eastern regions of Iran (including some regions over South Khorasan, Kerman, and Razavi Khorasan provinces), while the northwest (small area of Ardabil province) and a few pixels in the central part show positive trends.
In general, spatiotemporal investigation of the contribution of each season and month in annual precipitation indicated that small regions in central Iran have a significant upward trend in spring, particularly in March. In addition, there are significant upward trends in summer and fall over a different part of Iran, including Zagros Mountain and northeast of Iran in summer (especially in June and July) and over south-eastern regions during the fall (especially in November). However, trends of contribution of season to annual precipitation, except for fall, do not have a significant effect on the seasonal precipitation. In addition, the contribution of any season to annual precipitation does not significantly shift to another season. Moreover, the results showed that the contribution of each season and month to annual precipitation in some parts of Iran varies between wet and dry years. The results indicated that precipitation that occurs over the eastern, northeast, and southwest regions of Iran during March, April, and December, respectively, could decrease in dry years.
Overall, the PERSIANN-CDR dataset with its relatively high spatial and temporal resolution covering nearly four decades proves to be a useful dataset for spatiotemporal investigation of precipitation, especially over regions like Iran with sparse spatial distribution of rain gauges.