This article analyzes the rainfall in Birgunj and its surrounding areas, including Kalaiya, Simara, and Amlekhgunj. These regions lie in the Chure-Bhabhar and Terai zones, stretching from north to south.
Rainfall is the primary source of surface water as well as groundwater in these areas and occurs mostly during the monsoon season, nearly 80%.
The Chure-Bhabhar region of Nepal is considered the key groundwater recharge zone for the Terai (Source). Therefore, analyzing rainfall patterns in these areas is crucial. Any shift in rainfall patterns can have significant impacts.
Data
For the analysis, 14 years of monthly data were collected from the Department of Hydrology and Meteorology for the following stations:
- Birgunj Station (0918)
- Kalaiya Station (0921)
- Simara Station (0909)
- Amlekhgunj Station (0907)
Average Yearly Rainfall
The average yearly rainfall was calculated, with missing data interpolated. The results show that Simara receives an average annual rainfall of 1,683 mm, while Birgunj records the least annual rainfall at 1,260 mm.
The data also indicate the highest standard deviation for Amlekhgunj and the lowest for Kalaiya.
Average Monthly Rainfall
The average monthly rainfall over a 14-year period showed that most rainfall occurred during the monsoon season, which lasts from June to September, accounting for 80 to 85% of the yearly total. The peak monthly rainfall occurred in July for all locations, while the other months received minimal rainfall.
| Station | % of Yearly Rainfall (in monsoon) |
| Amlekhgunj | 85 |
| Simara | 80 |
| Birgunj | 83 |
| Kalaiya | 80 |
The monthly rainfall shows a clear trend of rainfall in the monsoon season. This shows a high dependency of monsoon rain in these areas.
Average Rainfall Comparison
The figure below shows the amount of rainfall Nepal received in 2023. It indicates that the highest rainfall occurred in the northern part of central Nepal, whereas western Nepal experienced minimal rainfall.
In Birgunj and its surrounding areas, the annual rainfall ranged from 1,000 to 1,500 mm in 2023, as per the figure below.
The annual average rainfall in Nepal fluctuates, with 2023 showing a recent decline, as observed in the figure below.
Conclusion
The rainfall pattern in Birgunj and its surrounding areas is highly seasonal, with the majority of precipitation occurring during the monsoon months. This seasonal rainfall is the primary source of water for rivers and groundwater recharge in the region.
Understanding the distribution and amount of rainfall is essential for effective water resource management, especially due to the reliance on monsoon rain for direct irrigation and groundwater pumping for both irrigation and drinking purposes.
Protecting the natural landscape to enhance rainwater infiltration is critical to maintaining groundwater levels and ensuring sustainable water availability throughout the year.
Changing monsoon patterns, degradation of the Chure–Bhabhar recharge zones, and over extraction of groundwater for drinking and irrigation in Nepal’s Terai and northern India together pose a serious threat to water security in the study area.
Additional Charts
Bar charts for monthly data for stations were prepared, which shows rainfall occurring in different stations for 14 consecutive years from 2010 to 2023 AD.
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