Investigating the changing pattern of groundwater levels and rainfall in the peninsular region of Bhagalpur and Khagaria, Bihar

Groundwater is a valuable natural resource, which is generally available as fresh water, and it is used for domestic, industrial, and agricultural purposes (Mirdha et al. 2020; Ali et al. 2021). Many cities rely on the groundwater for daily uses of water (Boretti & Rosa 2019). Several agriculture-based countries depend on the groundwater for irrigation purposes. The rapid increase in population, urbanization, decreasing flora and fauna, and various other factors have affected the climate and rainfall patterns, which in turn put more stress over the groundwater and its potential in different regions across the world (Jan et al. 2007; Wakode et al. 2018). This stress is increasing day by day particularly in arid and semi-arid regions with more and more groundwater being extracted and ultimately resulting in a decline of groundwater levels (GWLs). The decreasing GWLs have drawn the attention of the world. Studies conducted by many researchers have reported the problem of declining GWLs (Patle et al. 2015;,Pathak & Dodamani 2019; Goswami & Rabha 2020; Halder et al. 2020; Joshi et al. 2021;,Vousoughi 2022;,Ghosh & Bera 2023). In the Ardabil plains of Iran, significant decrease in the GWL was reported caused by the human activity over a period of 22 years. A concern was raised about the inaccuracy of long-term GWL records and its trend analysis for understanding the groundwater dynamics at agricultural scales (Le Brocque et al. 2018). A study conducted in the United Arab Emirates by Liaqat et al. (2021) had reported a decline in GWL due to an expansion in urban and agricultural areas.

India is an agricultural country and more than 70% of the population depends on agriculture according to the Food and Agriculture Organization of the United Nations. The majority of irrigation is dependent on groundwater. In general, the decrease in rainfall results in a decrease of the available water for groundwater recharge. The GWL is declining due to the unlimited extraction and misuse of groundwater (Wakode et al. 2018). Sharma et al. (2022) conducted a study on environmental change and groundwater variability for south Bihar in the areas adjacent to the peninsular study region and reported an average increase of 0.5 °C in temperature during 1958–2019 with a deficit of surface and groundwater above 600 mm. Furthermore, monsoon rainfalls were highly unpredictable after 1990. Unscientific extraction of groundwater for domestic, agricultural, and industrial purposes by the 1.4 billion population has created an acute crisis for the Indian aquifers. Studies conducted by various researchers across different regions of India concluded that the GWL has declined (Thakur & Thomas 2011;,Patle et al. 2015;,Wakode et al. 2018;,Pathak & Dodamani 2019; Goswami & Rabha 2020; Halder et al. 2020). It may create a groundwater drought situation, which is a relatively newer concept for the Indian subcontinent (Pathak & Dodamani 2019).

Generally, groundwater is found at shallow depths between the plains of the Ganga and the Kosi rivers. The study area, the peninsular region of Bhagalpur and Khagaria districts of Bihar, is part of these plains. The agriculture is mostly dependent on groundwater in this area. Most of the farmers extract groundwater using diesel operated pumping sets, but there is no account of the number of pumps being operated. The absence of accounting for groundwater pumping leads to overexploitation, which has been identified as a prime reason for groundwater depletion across different regions (Thakur & Thomas 2011; Patle et al. 2015;,Wakode et al. 2018; Joshi et al. 2021;,Ghosh & Bera 2023). The blockwise groundwater resource assessment by the Central Ground Water Board (CGWB 2020) categorizes the entire area as ‘safe’ in groundwater resource. Based on the field visit made by the authors and the interaction with the local people of the study area, it was found that the GWL is going down year by year. Furthermore, local people reported that drinking water was available at shallow depths in all seasons a decade ago. However, during the last decade, deeper borings were required for groundwater extraction due to the failure of pre-existing shallow boring or due to the requirement of additional efforts to pump out water during the summer season, although it is available at shallow depths during the monsoon and post monsoon. This observation shows that the GWL decreases in the pre-monsoon season and is recharged during the monsoon season. Thus, it is important to study the variation of the rainfall and GWL to understand the problem of GWL depletion.

Trend analysis methods vary from simple linear regression to more enhanced parametric and non-parametric methods (Chen et al. 2007). Though, linear regression is the simplest method, but there is lack of accuracy in non-linear trend assessment of groundwater (Shamsudduha et al. 2009). Various other methods such as stochastic analysis, non-parametric Spearman's rank correlation coefficient, non-parametric time series decomposition technique, etc., have been used by researchers for trend detection of GWLs in different regions in the past (Shamsudduha et al. 2009). The non-parametric Mann–Kendall test (M–K test) has become the most popular method among researchers and is extensively used to identify trends in time series data. Mann (1945) formulated it as a non-parametric test to detect the trend. Later, Kendall (1975) introduced it as a test statistic. It compares the relative magnitude of the sample data (Gilbert 1987). The advantage of this test is that the data need not follow any particular probability distribution. Moreover, a common value less than the smallest value in the data set may be used to incorporate non-detected data (Kendall 1975). Panda et al. (2012) observed a large number of declining trends in GWLs across Gujarat (India) using the non-parametric M–K test. Thakur & Thomas (2011) determined the seasonal GWL trends using the Kendall rank correlation along with the parametric linear regression test in the Sagar district of India. Several other researchers have successfully applied this method and reported that the non-parametric approach of M–K test can be used for the trend analysis of GWLs (Shadmani et al. 2012; Vousoughi et al. 2012; Das et al. 2020; Aditya et al. 2021; Citakoglu & Minarecioglu 2021; Frollini et al. 2021).

The present study focuses on the area where GWL is declining even though it is bounded by perennial rivers along three sides. Although the study area may not have an acute groundwater crisis at present, current trends may lead to an acute shortage of groundwater in future, if timely attention is not paid toward it. Surface water has already been suffering from the crisis. Hence, there is a need to conduct a study for the analysis of rainfall and GWL in the study area, so that necessary measures can be taken up for the same. Because no such study has been carried out in particular for this study area earlier, the main objective of this study is to investigate the changing pattern of rainfall and GWLs in space and time using the spatial analysis tools of ArcGIS, graphical plots, and statistical methods such as the M–K test and Sen's slope estimator.

As per the India Meteorological Department (IMD), the climate is hot and humid during the summer season, with the maximum temperature of 41 °C at some urban locations, and very cold during the winter season, with the minimum temperature of 12 °C in diara areas. The study area receives good amount of annual rainfall ranging from 1,300 to 1,400 mm on an average and more than 80% of the annual rainfall occurs during the monsoon (June to September). However, this rainfall is not evenly distributed in space as some parts receive more and some parts receive less rainfall. The variation in rainfall leads to a situation of varying groundwater recharge (Panda et al. 2012;,Halder et al. 2020). The areas at lower elevation located near the rivers experience flood during monsoons. Agriculture is the prime occupation in this area and almost the entire irrigation is dependent on groundwater by pumping from bore wells. The decreasing level of groundwater is a reason for the failure of the bore wells as reported by the farmers and dwellers at several locations. The cost of irrigation has also increased because of the decreasing level of groundwater, and thereby, increasing the overall cost of farming. The study area has very fertile alluvial soil. The top soil is mostly clayey loam with sand deposits at around 1 m depth. Crops like wheat, maize, gram, and mustard are widely grown in this area. The agricultural lands close to the rivers have heavy silt deposits due to floods and thus make the land unsuitable for normal cropping. However, the Krishi Vigyan Kendra (KVK) in Khagaria organizes training programs (Action Plan 2018-19) to grow cucurbits such as watermelon and pointed gourd in the silty conditions of the area and farmers are practicing this.

The surface water has been adversely affected in the study area due to pollution and encroachment. Although groundwater storage in the study area has not reached a crisis situation, prompt action is required for conservation and management of this vital resource. In order to plan an appropriate action, it is required to investigate the patterns of the rainfall and GWLs in the study area.

The Central Ground Water Board (CGWB) has about 25,000 dedicated stations for monitoring GWLs across India, which are known as Hydrograph Network Stations (HNS). A total of 10 such HNS located in the study area are shown in Figure 3. The details of the CGWB HNS obtained from the India-Water Resource Information System (India-WRIS) portal of the Government of India are presented in Table 1 (www.indiawris.gov.in).

Pre-monsoon (May) and post-monsoon (November) GWLs (mbgl) from the year 1996 to 2020 for all stations obtained from the India-WRIS portal were used. The GWLs data (mbgl) obtained had many values missing for different durations at different stations. The GWLs data for pre- and post-monsoons for each year were processed and maps were prepared using the Kriging technique (Sakizadeh et al. 2019; Hasan et al. 2021). Missing data were filled using the interpolation method.

The peninsular region of Bhagalpur and Khagaria districts has been divided into three sub-parts depending upon the location of the wells, viz. eastern part, central part, and western part, to understand and investigate the changing pattern of rainfall and GWLs in space and time. The first sub-area located in the eastern part consists of the Madrauni Chowk and Naugachia observation wells, the second sub-area located in the central part contains the Bihpur and Marwa observation wells, and the third sub-area located in the western part contains the remaining observation wells, which are nearer to each other and fall under Khagaria district.

The M–K (Mann 1945; Kendall 1975; Gilbert, 1987) test for trend analysis and Sen's slope estimator (Sen 1968) to detect the slope of trend line have been used for rainfall and GWLs. Non-parametric methods are easier to use due to lesser assumptions as compared to the parametric approaches (Esterby 1996). This makes the M–K test advantageous to use for studies where some data are missing. The M–K test is a statistical assessment of a monotonic trend (decreasing or increasing) in the test variable over time. However, the trend may or may not be linear. The M–K test is an exploratory method for identification of the trends in data sets having significant changes and does not substitute visual examination of time series plots (Hirsch et al. 1982).

⁠, if N is odd and ⁠, if N is even. A positive value of  Q_i indicates an increasing trend in time series data whereas a negative value indicates a decreasing trend.

M–K and Sen's slope tests were also performed on the annual rainfall data to identify the behavior of the rainfall trend during 1996–2020. The results of M–K tests are summarized in Table 2. The probability, P-value, at 5% significance level was found to be 0.0019 along with τ-value of −0.447. Furthermore, the slope value was computed to be −25.049 from Sen's slope test. As the P-value was less than 0.05 and the τ-value was negative, it can be concluded that the annual rainfall had a significant decreasing trend over the study area. The rate of decrease in rainfall has been 25.049 mm/year during the period of study. Similar results were indicated from the time series plot along with trend line in Figure 6. The decreasing trend indicates a possible future deficit in rainfall.

From the above analysis, it is found that the annual rainfall in the study area varied in space as well as in time during the period of 25 years. During field visits, it was observed that urbanization was more rapid in the study area due to the increased connectivity and business opportunities, which might have affected the vegetation cover. The decrease in vegetation cover disturbs the water cycle, which leads to reduction in rainfall in a region. The declining trend of rainfall may also be due to the global climate change impact; however, a detailed investigation is required to establish this for the study area.

Pre-monsoon GWL varied from 1.63 to 10.67 m (bgl) over the whole study period. It can be noted that the GWL in pre-monsoon was very deep in the year 2014, whereas it was shallow in the year 1998. Further, it can be noted that the decrease in pre-monsoon GWL with time in the central and eastern parts was more than in the western part. This may be due to a decrease in the rainfall in these parts and also due to the increase in the unlimited extraction of groundwater in the pre-monsoon season for different purposes.

Post-monsoon GWL varied from 0.75 to 9.13 m (bgl). Again, it can be observed from Figure 7(b) that the GWL was deeper in the years 2010 and 2012, whereas it was shallower in the years 2013 and 2019. It can be noted that the annual rainfall was less in the year 2010 and relatively more in the year 2019, consequently, the groundwater recharge might have been lesser in the years 2010 and 2012 whereas more in the years 2013 and 2019. Further, it can be noted that the GWL in the post-monsoon season was shallower in the eastern part as compared with the other parts.

M–K and Sen's slope tests have also been carried out for the pre-monsoon and post-monsoon GWLs of each station and the results are presented in Table 3. The statistical parameters P-value, τ-value, and Sen's slope have similar meanings as described for Table 2.

The test results showed insignificant increasing trend at Madrauni Chowk in both the pre-monsoon and post-monsoon seasons with P-value greater than 0.05 and positive values of τ and Sen's slope. It means the depth to the GWL was increasing, and hence, the GWL was decreasing. The same trend was shown by the graphical trend line in Figure 8(a) for Madrauni Chowk. For Naugachia, a significant increasing trend in depth to GWL was observed in the pre-monsoon and post-monsoon seasons with a P-value less than 0.05 and positive values of τ and Sen's slope. This implies a decline in GWL in both the seasons. The depth to GWL (mbgl) had a significantly increasing trend (P < 0.05, τ: +ve) at Bihpur in the pre-monsoon season, which means that the GWL was declining in the pre-monsoon season. However, an insignificant decreasing trend (P > 0.05) was observed in the post-monsoon season depth to GWL with negative τ-value and Sen's slope. This implies the GWL was increasing. A similar trend was observed from the trend line behavior as shown in Figure 8(c) for Bihpur. At Marwa, the results indicated an insignificant decreasing trend in depth to GWL of the pre-monsoon season and a significant decreasing trend in the post-monsoon season. This implies the GWLs were increasing. A similar result was observed from graphical trend analysis in the post-monsoon season, but it differed in the pre-monsoon season. In the Khagaria district, all the six observation wells (Mohaddipur, Dewri, Gandhinagar, Jamalpur, Maheshkhunt, and Lohiya Chowk) were having similar trends as observed in their graphical trend analysis above. Dewri and Jamalpur had a increasing trend of GWL in both the seasons whereas others had the same pattern as in the graphical trend analysis.

It can be observed that the trend lines of GWL in the pre-monsoon season had a decline or a nearly constant slope value at all the well locations except at Jamalpur and Dewri. During the post-monsoon season, GWLs at all the wells had either an increasing or a nearly constant slope value of trend except at Madrauni chowk and Naugachia located in the eastern part of the study area, where it was decreasing. From the results, it was observed that the majority of stations are affected by decline in GWL in the pre-monsoon season. From the field visit experience, it was drawn that the variation in groundwater may be mainly attributed to overextraction of groundwater for irrigation as the flood irrigation method is most commonly practiced in the study area. Many pumping-set owners make their livelihood by charging farmers at per hour basis for irrigation. In order to make more money, there is a tendency to continue pumping even after the required irrigation has been done. Further, urbanization and concrete flooring of courtyards has also affected the GWLs in the majority of the parts as they hamper the recharge process. The wells situated in the south-west side of the study area are nearer to the River Ganga. These locations are to the south of the ridge line, i.e. NH 31, at varying distances, so there might be a possibility of river–aquifer interaction. Through field visits, it was observed that different parts of the study area are going through urbanization due to the developmental activities in the last decades. The urbanization in the eastern part has been more rapid. Therefore, the human activities might have increased the groundwater extraction and also affected the recharge to groundwater, which is also reflected in the post-monsoon GWLs.

The areas in south of the study area on the other bank of the River Ganga have become water scarce due to unaccounted overexploitation of groundwater and poor recharge. Therefore, it would be wiser for the authorities to consider the situation as alarming and necessary measures should be initiated for the proper use and management of groundwater so as to preserve this valuable resource.

It was observed that there was no significant effect of the monsoon rainfall on the post-monsoon GWL. This means that there was no effect on recharge from the rainfall alone, rather it might have been due to the river–aquifer interactions. It can be observed that the post-monsoon GWLs in the study area did not completely follow the monsoon rainfall trend.

Since the graphical plots did not show any concrete effect of monsoon rainfall on the post-monsoon GWL, the post-monsoon GWL and monsoon rainfall were further checked to confirm any statistical correlation. The statistical correlation coefficient values are presented in Table 4.

Table 4 reveals that there was very weak correlation between the post-monsoon GWL and monsoon rainfall during the period of study, which means that the fluctuations in the post-monsoon GWLs were not due to monsoon rainfall alone as observed from the graphical plots.

The changing land-use pattern and concreting would have increased the runoff and thereby decreased the amount of available water for groundwater recharge. Therefore, this could be a reason for the post-monsoon GWL not exactly matching the monsoon rainfall trend. Since the groundwater recharge is not influenced by monsoon rainfall alone; it can be concluded that the above results may also be due to river–aquifer interaction as the study area is bounded by rivers from three sides. Further research is required to find out more about this possible effect.

The peninsular region of Bhagalpur and Khagaria districts has been divided into three parts, viz. eastern, central, and western parts, to understand and investigate the changing pattern of rainfall and GWLs in space and time. Spatial and temporal variations of annual rainfall and the pre- and post-monsoon GWLs in the peninsular region of Bhagalpur and Khagaria districts during 1996–2020 have been analyzed using spatial analysis tools, graphical plots, and the M–K test with Sen's slope estimator. It was found that the average annual rainfall over the period of 25 years (1996–2020) has remained fair, but it has spatial and temporal variations. The western part of the study area receives lesser rainfall compared with the central and eastern parts, with higher rainfall in the area near to the Vikramshila Bridge in the eastern part. Trend analysis showed a decreasing trend of rainfall in the whole study area and the rate of decease was 25.049 mm/year during the period of study. The pre-monsoon GWL had a decreasing trend at a majority of the wells and rate of decrease was varying from 0.005 to 0.102 m/year over the whole study area whereas the post-monsoon GWL shows an increasing trend with an increasing rate varying from 0.005 to 0.083 m/year in the wells, which are located nearer to the River Ganga except Maheshkhunt. The monsoon rainfall has a slight effect on the post-monsoon GWL. Thus, it can be concluded that the rainfall and groundwater are varying spatially as well as temporally in the peninsular region with a decreasing trend in both. Based upon the findings of this study, future studies can be carried out for the groundwater potential assessment and the river–aquifer interaction.

Finally, it is recommended that appropriate measures, including awareness against overexploitation and misuse of water, training to harvest rainwater in households, encouraging adaptation to high-efficiency modern irrigation systems, and keeping a record of borings at households and agricultural lands through local governing bodies at village levels may be taken up. These measures will certainly contribute to prevent any future water stress situations.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.