Rainfall trend analysis using Mann-Kendall and Sen’s slope test method in Nanded district of Maharashtra state

Introduction

The primary physical environment component that supports economic development in each place is its climate, which also influences agriculture, industry, and all other economic development variables. It establishes how farmers' cropping patterns are distributed and performed. The hydrological cycle and the regional agricultural calendar may be significantly impacted by changes in rainfall patterns brought on by climate change (Mirza and Hussain 2003; Abrol et al., 2004; David Chikodzi and Linda Yeukai Mapfaka, 2018) [8,1,2] . Rainfall is one of the decisive weather parameters for the detection of climate change (Sahu and Khare, 2015) [11] . Numerous scholars have examined the seasonal and annual patterns of rainfall in India on a regional level (Naidu et al., 1999 [9,5,4,6,10] . The majority of crops grown in the Marathwada region are rainfed, and crop productivity is contingent upon the amount and distribution of rainfall that occurs each month. Because of the sustainability of rainfed agriculture and the financial consequences of rainfall-dominated operations, it is crucial to have acute knowledge of both yearly and monthly rainfall and its pattern (Galkate et al., 1999) [3] . From the perspective of water management, recent trends in rainfall are more trustworthy since longer data sets also lessen the trend's relevance. In the twenty-first century, managing the region's crop water supplies effectively requires a spatial and temporal understanding of the rainfall that occurs there. In order to establish a connection between rainfall and other hydrological components, it is also necessary to detect patterns of rainy days in the area. This information can be useful in making decisions regarding the management of water resources as well as offering insights into potential changes in the hydrologic cycle of the area. The problem has gotten worse in recent years due to climate change, and it has been discovered that Maharashtra state, and the Marathwada region in particular, have recorded the greatest number of farmer suicides in 2014. By keeping this in view, present study was undertaken for trend analysis of the annual and monthly rainfall and rainy days in Marathwada region.

Study Area

Nanded is the second most populous city in Maharashtra's Marathwada division located on the bank of Godavari, having altitude 362 m above sea level. Nanded district has a physical area of 10,528 Sq. Km., which forms 3.41% of the total geographical area of Maharashtra State. The district of Nanded has between 18°15' and 19°55' North latitude and 77°7' to 78°15' east longitudes (Yannawar et al., 2013) [13] .

Fig 1: Location map of study area

Data requirement

For the study of daily rainfall data of sixteen tehsils of Nanded district was collected for a period of 30 years i.e. from 1994 to 2023.

Sources of data

Rainfall data of all sixteen tehsils of Nanded district was collected from following websites.

Methodology Mann-Kendall Method

Compare first year data point with 2 nd , 3 rd , ......,30 th year data point.

Sum of assigned values will give Mann-Kendall Statistic (S).

A very high value of Mann-Kendall statistic is an indicator of an increasing trend and a very low negative value indicated a decreasing trend. However, it is necessary to compute the probability associated with Mann-Kendall Statistic and the sample size, n, to statistically quantify the significance of the trend. The Mann-Kendall test statistic (S) is calculated using the formula that follows;

Where, Xj and Xk are the annual values in years j and k, j > k respectively, Xk represent the data point at time k. The value of sign (xj -xk) is computed as number follows:

This statistic represents the number of positive differences minus the number of negative differences for all the differences considered. For large samples (N>10), the test is conducted using a normal approximation (Z statistics) with the mean and the variance as follows, Where, n = number of years, g = number of tied groups (A tied group is a set of sample data having the same value.) tp = number of items in the tied group Calculate a normalized test statistic Z by the following equation

Where, S = p -q p = number of (+1) values and q = number of (-1) values The presence of a statistically significant trend is evaluated using the Z value. A positive value of Z indicates an upward trend and its negative value a downward trend. The statistic Z has a normal distribution. In the present study, at confidence level of 99, 95 and 90 percent the positive or negative trends is determined by the test statistic.

Sen's Slope Method

Sen's slope method was used for predicting the magnitude of hydro meteorological time series data. This method uses a linear model for the trend analysis by using a simple nonparametric procedure developed by Sen (1968) [12] . To derive an estimate of the slope Qt, the slope of all data pairs is calculated by Where, xj and xk is the data values for j and k times of a period and j>k The slope is estimated for each observation. If there are 'n' values of Xj in the time series then as many as N = n(n-1)/2 slope estimates, Qt are to be computed. The Sen's estimator of slope is the median of these N values of Qt. The N values of Qt are ranked from the smallest to the largest and the Sen's estimator is calculated as, If N is odd Median of all slope values gives Q, which is magnitude of trend. A positive value indicates increasing and negative values indicates decreasing trend of rainfall and rainy days.

Results and Discussion

Rainfall features

Taluka wise rainfall characteristics of Nanded district are shown in Table 1. Mean annual highest rainfall 1005.4 mm with a standard deviation 252.2 mm and coefficient of is 25.1% was recorded at Kinvat tehsil of the Nanded district followed by Mahur at 999.4 mm. The lowest mean annual rainfall recorded was 769.9 mm at Naigaon tehsil.

References

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