Spatio-temporal analysis of rainfall distribution and its variability in mid central table land zone of Odisha

Introduction

In the monsoonal climatic region, there is a noticeable seasonal fluctuation in climatic factors, particularly in rainfall (Rathore et al. 2013) [1] . In India, approximately 80% of the annual rainfall comes from the southwest monsoon (Lal 2003) [2] . The spatial and temporal variations in monsoonal and annual precipitation are well recognized, and this year-to-year fluctuation in monsoon rainfall significantly affects agricultural production. Among climatic factors, precipitation is one of the most unstable, showing marked spatial and temporal differences over daily, decadal, and longer periods (Juan-Carlos et al. 2009) [3] . Krishna Kumar et al. (2008) [4] examined the temporal (monthly, seasonal, and annual) rainfall trends in Kerala, India, throughout the twentieth century and observed a notable decline in rainfall during the southwest monsoon, alongside an increase during the post-monsoon period. In order to study rainfall characteristics, Chakraborty et al. (2013) [5] conducted a study on the spatial and temporal variability of rainfall within the Seonath sub-basin in Chhattisgarh State, India, over a period of 49 years (from 1960 to 2008). They employed various statistical methods. The Modified Mann-Kendall (MMK) test (a non-parametric method) and Spearman's rho test (a parametric method) were utilized to identify trends. Sen's slope method was used to determine the magnitude of the trends. Change points were identified using the CUSUM and cumulative deviations tests. For variability analysis, the Coefficient of Variation was employed. Both tests revealed a downward trend in the annual and seasonal rainfall series across the entire river basin. Mondal et al. (2002) [6] utilized the Mann-Kendall test to assess the shifting patterns of rainfall in a river basin located in the coastal area of Odisha. The objective of this study to analyse the spatial-temporal rainfall distribution and its variability in mid central table land zone of Odisha. The information will help the farmers and policy makers in planning agriculture objectively for the region.

Materials and Methods

Study Area

The study has been carried out in the Mid Central Table Land

Zone in the state of Orissa, India. It is geographically situated between 80.0°E longitude and 20.0°N latitude and covers districts of Angul, Dhenkanal, part of Cuttack and Jajpur. The zone experiences a warm and humid climate. The average annual rainfall of this zone is 1426.6 mm, of which 86.5% occurs during the southwest monsoon season. However, there is significant variation in rainfall from year to year. Mean maximum temperature is 38.7°C, and means minimum temperature is 14.0°C. The soils in the zone are alluvial, red, lateritic, and mixed red and black.TheDhenkanal District consists of eight blocks: Bhuban, Dhenkanal Sadar, Gondia, Kamakhyanagar, Hindol, Kanakadahad, Parjang, and Odapada. Similarly, the Angul district also has eight blocks: Angul, Athmallik, Banarpal, Chhendipada, Kaniha, Kishorenagar, Pallahada, and Talcher.

Rainfall & rainy days analysis

Rainfall data of thirty three year (1991 to 2023) was collected from special Relief Commissioner (SRC), Odisha for the purpose of rainfall analysis in the study area. The analysis was conducted for each block of Dhenkanal and Angul districts. Annual, seasonal, monthly rainfall as well as the probability of rainfall was computed using the software known as 'Weather cock'. This software includes 26 modules that pertain to agroclimatic factors. A day is considered as a rainy day if the rainfall is equal to or exceeds 2.5 mm, as defined by the India Meteorological Department for the Indian region.

Trend analysis

The daily rainfall data at block level were subjected to nonparametric Modified Mann-Kendall Test using 'mmky' package in 'R studio' software to detect trends in annual and seasonal rainfall. The modified Mann-Kendall test (Yue and Wang, 2004) [7] was used as it addresses the issue of serial correlation, which exists in time series data, by variance correction approach.

Results and Discussion

Annual Rainfall and rainy days

The average annual rainfall for the Mid Central Table Land zone was 1371 mm. Among the two districts, Dhenkanal district recorded the highest rainfall of 1423 mm (Table 3.1). The annual rainfall variability ranges from 15% to 30%. The average number of rainy days in this zone is 64 days. The average annual rainfall in Angul district is 1318 mm. The variability in annual rainfall across the district ranges from 15% to 30%. The highest rainfall was recorded in Pallhada block of 1791 mm, while Chhendipada had the lowest of 1137 mm. The blocks that received rainfall exceeding the district's average include Athmallik, Kishorenagar, and Pallahada. The highest number of rainy days, 73days, was observed in Pallahada, whereas Banarpal and Chhendipada blocks experienced the lowest number of rainy days, with a total of 58. The mean annual rainfall of Dhenkanal district was found to be 1423.2 mm. There is significant variability in rainfall at the block level. Hindol block recorded the highest rainfall of1649.11 mm, while Parjang received the lowest rainfall of 1246.01 mm. Variability of the district annual rainfall was 18% to 27%. Consequently, blocks with a coefficient of variation (CV) of less than 25% can be considered as highly dependable rainfall. Except Kamakhyanagar all 7 blocks that fall into this category. In contrast, Kamakhyanagar Block exhibits a CV above the threshold, suggesting significant variability in the rainfall amounts they receive across different years. Hindol Block recorded the maximum number of rainy days of 72, while Parjang had the minimum with 59 days.

Seasonal Rainfall

In Angul district, approximately 77% of the total annual precipitation comes from the South West monsoon. The average rainfall during the South West monsoon in the district is 993mm, with a standard deviation of ± 181mm. The Pallahada block received the highest rainfall (1483 mm), while the Chhendipada block experienced the lowest (817 mm) during the monsoon season. The least amount of precipitation was noted in winter season. In the post-monsoon season, the variability in rainfall ranged from 70% to 129%, whereas in the summer season, it fluctuated between 42% and 77% (Table 3.4). The average rainfall during the SW monsoon in Dhenkanal district was 1107 mm with a variation of ±269 mm. The highest recorded monsoon rainfall was in Hindol block, which received 1227 mm, while Parjang block experienced the lowest with 998 mm. Nearly 78% of the total annual rainfall took place during the SW monsoon (Table 4). Consequently, it is evident that the SW monsoon represents a significant portion of rainfall distribution compared to other seasons. The variability of rainfall during the SW monsoon ranged from 18% to 29%. The least amount of rainfall recorded was 25 mm during winter, although this season exhibited the highest variability at 137%. During the post-monsoon period, rainfall variability ranged from 71% to 93%, while it was between 58% and 84% in the summer season (Table 3.5). The coefficient of variation for seasonal rainfall, except South West monsoon, exceeds the threshold of 50% for seasonal rainfall.

Probability of annual rainfall

Analyzing rainfall probabilities provides a better scope for predicting the minimum assured rainfall, which helps in crop planning. The quantities of rainfall corresponding to various probability levels (10-90%), referred to as assured rainfall, have been calculated for each standard week using the Incomplete Gamma Distribution model. This research involved a probability analysis of annual rainfall at the block level, focusing on three probability thresholds: 90%, 75%, and 50%.

For Angul district, Expected rainfall at 90% probability level was found to be above 1000 mm in two blocks Angul and Pallahara. At 75% probability, rainfall was above 900 mm in all blocks. All blocks receive more than 1100 mm rainfall at 50% probability (Fig

References

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