Climate change has a direct impact on water resources and consequently on society and the economy 12. It has a diverse effect on the environment and may impact temperature, rainfall, and ecosystem functions 345. Rainfall is the main climate factor that rules the hydrologic cycles and determining its change is essential in water resources management 267. It becomes more important in arid and semi-arid regions where water scarcity is a common issue and regional agriculture is being affected by long-term precipitation changes 89. Therefore, analyzing rainfall trends become important in such regions to better manage water scarcity or reservoir overflow and their derivative problems 10.

It’s crucial to determine precipitation trend change in each region to develop and improve regional strategies for water resources management 611. Rainfall trend analysis has received much attention and has been investigated by many researchers all over the world in the past century 12. Its trend has been investigated in global 13141516 and regional scales 1718919 by parametric 202122 and non-parametric tests 2324252627. Non-parametric tests have been used more frequently than parametric tests since they are distribution-free 28, therefore in this study the non-parametric Mann-Kendall test, which is the most frequently used non-parametric test in this regard 29, has been used to determine annual rainfall trend in Khorasan Razavi province of Iran.

Several studies have been investigated or mentioned the climate change impacts on Khorasan Razavi province since it is the second populated province in Iran. The climate change effect on crops yield 30, maize production and growth stages 313233, assessment and adaption strategies for wheat and saffron yield 313435, buildings energy demand 36, temperature trend 37 and vegetation cover trend 38 has been investigated. Recently, this province has encountered water shortage problems and most cities have encountered water supply problems that may be due to climate change effect and miss management. So, climate change effect on the province precipitation can be assumed as a research gap that will be investigated and analyzed in this study. The present study aims to (i) Analyze the long-term rainfall data; (ii) Determine the rainfall trend; and, (iii) Determine the median rate of change in rainfall trend in Khorasan Razavi province.

The homogeneity tests revealed that four stations were considered non-homogeneous by only one test and three stations were considered non-homogeneous by two tests. Therefore, all stations were considered homogeneous by at least two tests and so determined homogeneous; Thus, all 20 stations are involved in the trend study to detect the rainfall trend of each station. The total stations that are considered homogeneous by each homogeneity test are reported in Table 1.

The long-term average rainfall of Khorasan Razavi province for the thirty-year period is 242 mm and each year rainfall to the long-term average depicted in Figure 3. It indicates the annual precipitation for 16 years was more than the long-term average which shows that by true management, the excess water in rainy years can be saved to use in drought times. Moreover, it shows that the water shortage problems are due to mismanagement since the long-term average doesn’t show a significant trend. It should be noted that the problem of mismanagement is not limited to Khorasan Razavi province and in most parts of the country, we see this problem in the field of water. Another reason for the water shortage problem that has led to the water crisis in the study region is the excessive and uncontrolled withdrawal of groundwater resources. Rainfall penetrates the aquifers of the study area and recharges the groundwater resources, but over-exploitation from this source, usually with unauthorized/unlicensed wells, causes a decrease in the aquifer levels. Rapid population growth in the study area and migration from small towns to Khorasan Razavi province, which is the second-most populous province in Iran, are other causes of water shortage in the study region. Therefore, it is observed that the survey of rainfall trends can clarify many probable issues/problems in the field of water resources management.

The maximum and minimum rainfall in the stations are reported in Table 2 within the mean and standard deviations. The analysis revealed that the maximum and minimum annual rainfall belongs to Moghan station in 1992 by 646 mm and Azadvar station in 1990 by 28 mm. The rainiest station is Moghan in the center of the province by the annual long-term average of 398 mm, and the Mahane station has the least long-term annual average precipitation of 158 mm in the south of the study province.

The station's data time series trend has been found out by using the nonparametric Mann-Kendall test. Only two stations had a significant trend at 95% confidence which is shown in Figure 4. The Sen’s slope estimator was used to determine the median change of trend at these stations and was -2.86 for Mahane and -3.18 for Torbat-e heydariyeh station

Rainfall is a key component in the hydrological process, especially in arid/semi-arid regions such as Iran. Given that the study of rainfall is of particular importance in the management of water resources, such a study seemed necessary in Khorasan Razavi province, the second-most populous region in Iran located in the northeast of the country. Investigating the trend of average rainfall over a period of thirty years (1989-2019), it was observed that the trend of average rainfall has not changed much. However, in this region, we are witnessing many problems, including water shortages, one of the main reasons for which is the lack of proper management (i.e., mismanagement) of water resources. Also, a more detailed study of the synoptic and rain gauge stations in the study area from the perspective of rainfall trend showed that except for two stations that had a decreasing rainfall trend, no special trend was observed in other stations. As a result, the general framework used in the present study, including evaluating the homogeneity of time series data and then using the Mann-Kendall and Sen's slope estimator methods together, is a good option for the purposes of evaluating rainfall trends.

No potential conflict of interest was reported by the authors.