Introduction

Nowadays, water resource management is an increasing concern globally due to the rapidly expanding population growth and the anthropogenic effects on the river systems and other natural systems (Crutzen and Stoermer 2000; Meybeck 2001; Varis and Vakkilainen 2001; Maybeck 2003; Zhang et al. 2008; Das 2018, 2019; Das and Pardeshi 2018a, b). Several studies have been carried out since last few decades regarding the changes of river system, attributable to climate change and human activities (Nearing et al. 2005; He et al. 2008; Ghaffari et al. 2009; Li et al. 2009; Ouyang et al. 2010). Examination of a long-term trend of water discharge and sediment load can deliver an understanding of how climate change and human actions affect the drainage system (Walling 1997; Asselman 2000). Likewise, in future, water resource management will become easier by this understanding of changes in water and sediment flux through a long-term trend analysis (Walling and Fang 2003; Kundzewicz 2004).

The river geomorphology depends on its discharge and sediment load characteristics. The geomorphic attributes of a river can be affected due to the variations in the water discharge and sediment load. Several studies have reported such changes worldwide such as the Rhone River (Petit et al. 1996; Arnaud-Fassetta 2003), Wisloka River (Wyzga 1997), Cache Creek and Stony Creek (Collins and Dunne 1990; Kondolf 1997), the Yangtze River (Yang et al. 2003) and Pearl River (Zhang et al. 2008). There is a growing body of evidence pertaining to the significant reduction in sediment supply to the ocean due to the anthropogenic entrapments in dams and reservoirs which has resulted a noticeable change in geomorphological, hydrologic and ecological function of the river basins (Walling and Fang 2003; Chakrapani 2005; Syvitski et al. 2005; Panda et al. 2011). Zhang et al. (2008) demonstrated Mann–Kendall statistics to detect the long-term hydrological and sediment load change of Pearl River over a period of 1950–2004 by analyzing data from nine stations. Their study showed that there was a significant decreasing trend in sediment load which was mainly because of the construction of several numbers of reservoirs and dams. Decrease of sediment load with subsequent increase in coastal erosion appears to have severe inferences as coastal zones perform a vital role in economic activities.

Indian rivers can be categorized into two broad systems based on the tectonic origin (Himalayan and peninsular) and show a significant difference in the sediment production. The Gangetic system produces annual average sediment of 2390 t km−2 from constantly uplifting and erodible Himalayan range while the peninsular river systems produces 200–300 t km−2 sediment (Milliman and Meade 1983). The summer monsoon rainfall predominantly influences the stream flow, sediment transport and channel morphology of the tropical monsoon rivers (Kale 2002; Panda et al. 2011; Doke et al. 2018). The Himalayan rivers are perennial due to the glacial melts while the seasonal flow of peninsular rivers leads to construction of dams and reservoirs (Agarwal and Chak 1991). Although damming the rivers able to bring ample social benefits in terms of addressing water; the hydrological, geomorphic and ecological consequences have received scientific attention in these regions (Panda et al. 2011).

Subarnarekha can be considered as a distinctive tropical river. Many researchers have tried to understand the hydrological and sediment flux characteristics of many small tropical rivers, but an understanding of these characteristics of Subarnarekha River is still unknown. Therefore, an attempt has been made in this study to assess the changes in hydrological and sediment load trend of the Subarnarekha basin during the late Anthropocene period of last 40 years.

Study area

Subarnarekha–Burhabalang river basins are located in the eastern part of peninsular India, lying in Jharkhand, Odisha and West Bengal state, encompassing an area of 24,000 km2 (Subarnarekha ~ 19,000 km2 and Burhabalang ~ 5000 km2) (India-WRIS 2012). Latitudinal and longitudinal extension of the study area is 21°15′–23°34′N and 85°08′–87°32′E (Fig. 1). The basins are bound by Chhota Nagpur plateau on the north and west. The ridges on the south are separating the Subarnarekha basin from Baitarani basin.

Fig. 1
figure 1

Geographic location of Subarnarekha basin and the hydrological stations. (Hydrological stations: M. Muri; A. Adityapur; J. Jamshedpur; G. Ghatsila; Go. Govindpur.) Notice the darker shade of the Burhabalang basin which is a relatively small adjacent river basin of Subarnarekha

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Subarnarekha and Burhabalang are the major river networks in the study area. Subarnarekha is a typical rejuvenated river having a length of about 450 km, experiencing a steady tectonic uplift (Guha and Patel 2017). All the streams within Subarnarekha basin are flow eastward and fall into the Bay of Bengal. Based on the elevation map, in the study area, elevation varies from 0 to 1172 m above sea level (Fig. 2). Prominent hills within Subarnarekha basin are located above an elevation of 450 m from mean sea level and flanked by denudation scarps situated at an elevation of 300–450 m (Guha and Patel 2017). From the geomorphic perspective, the study area can be categorized in three distinct broad physiography: the hilly region in the west from where the rivers originate, the undulating topography in the central and the coastal lowland with very low regional slope (Fig. 2).

Fig. 2
figure 2

Digital elevation model of the study area representing the altitudinal variation. The graphs indicate the longitudinal altitude and slope variation along the lines. The black line on the graph indicates elevation and the gray color represents slope

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Mainly three prominent geological formations in the study area are Archaean–Precambrian metamorphic rock, Tertiary, and Quaternary alluvium. In the upper Subarnarekha basin, major rock types are granite and quartzite and the lower section shows mainly vast deposition of sediments (Geological Quadrangle maps for Subarnarekha region, Geological Survey of India).

Climatically, the study area shows typical monsoonal characteristics where heavy rainfall occurs in monsoon months (June–Sept). During non-monsoon period rainfall is very less in the study area. The average annual rainfall in the study area ranges between 1100 and 1400 mm (Chatterjee et al. 2014).

Site description

Geomorphic characteristics of a river channel can easily be addressed through analyzing longitudinal profile of the river. Figure 3 presents the longitudinal profiles of Subarnarekha main channel and its tributaries on which hydrological stations are located. The transverse profiles on the longitudinal profile are representing the channel cross sections of each site. The Subarnarekha main channel shows higher relative steepness compared to the other two tributaries and all the hydro-observation stations are located below an elevation of 300 m.

Fig. 3
figure 3

Longitudinal profile of the Subarnarekha main channel and other tributaries where hydrological stations are located. Transverse profiles represent cross section of each site. Scale of both axes of transverse profiles is in meters. (Channel names: A. Subarnarekha River; B. Kharkai River; C. Burhabalang River.)

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Data and methodology

Hydro-geomorphic characteristics of Subarnarekha River have been assessed in this study by analyzing discharge and suspended sediment concentration data of five gauging stations. Water discharge (Qd), suspended sediment concentration (Qs), and cross-sectional data of five hydrological stations are collected from Water Resources Information System in India (India-WRIS). The detailed information regarding the hydrological stations is presented in Table 1.

Table 1 General information of all hydrological stations in Subarnarekha basin
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Initially, to understand the hydro-geomorphic condition of Subarnarekha river basin, daily discharge and sediment concentration data have been used for the calculation of monthly mean discharge and sediment load pattern, annual discharge, annual sediment load, and sediment yield for all the stations. Later, the trends of discharge and sediment load have been calculated by implementing bivariate linear regression, Mann–Kendall non-parametric tests. The homogeneity analysis has been performed to detect the change-points in the time-series data through Pettitt test. A detailed information and procedures are discussed below.

Mann–Kendall test

To determine the trend of discharge and sediment load, non-parametric test as Mann–Kendall (MK) is performed. Zhang et al. (2008) suggested that non-parametric tests are more ideal than parametric tests because of their ability to handle non-normally distributed data or missing data. The Mann–Kendall equation is as follows:

$$S = \mathop \sum \limits_{i = 1}^{n - 1} \mathop \sum \limits_{j = i + 1}^{n} \text{sgn} (X_{j} - X_{i} ),$$
(1)

where Xi and Xj are data values, n represents the length of time-series, and:

$$\text{sgn} (\theta ) = \left\{ {\begin{array}{*{20}c} { 1\quad if\quad \theta > 0} \\ { 0 \quad if\quad \theta = 0} \\ { - 1 \quad if\quad \theta < 0} \\ \end{array} } \right..$$
(2)

Mann (1945) and Kendall (1975) explained that when n ≥ 8, S is almost normally distributed with the subsequent mean and variance:

$$E(S) = 0,$$
(3)
$$V(S) = \left\{ {n\left( {n - 1} \right)\left( {2n + 5} \right) - \mathop \sum \limits_{i = 1}^{p} t_{i} i\left( {i - 1} \right)\left( {2i + 5} \right)} \right\}/18,$$
(4)

where ti is the number of ties to the extent i. The statistics S is approximately normal distributed provided that the following Z transformation is employed:

$$Z_{\text{MK}} = \left\{ {\begin{array}{*{20}c} {\frac{S - 1}{{\sqrt {{\text{Var}} \left( S \right)} }}(S > 0)} \\ {0 \left( {S = 0} \right)} \\ {\frac{S + 1}{{\sqrt {{\text{Var}} \left( S \right)} }} (S < 0)} \\ \end{array} } \right.,$$
(5)

where ZMK pursues the standard normal distribution with μ = 0 and δ = 1.

The statistics S is closely related to the Kendall’s τ is given by:

$$\tau = \frac{S}{D},$$
(6)

where

$$D = \left[ {\frac{1}{2}n\left( {n - 1} \right) - \frac{1}{2}\mathop \sum \limits_{i = 1}^{p} t_{i} i\left( {i - 1} \right)} \right]^{{\frac{1}{2}}} \left[ {\frac{1}{2}n\left( {n - 1} \right)} \right]^{{\frac{1}{2}}} .$$
(7)

The rate of change (regression slope) for the non-parametric test can be assessed by Sen’s slope estimator method. The linear slope is calculated as follows:

$$d_{\text{k}} = \frac{{X_{j} - X_{i} }}{j - i},$$
(8)

where d is the slope, \(X_{i}\) and \(X_{j}\) are values at times i and j, \((1 \le i < j \le n)\)n is number of variables.

Pettitt test

The Pettitt test is used to detect a change-point considering sequence of random variables X1, X2, …, Xt, which have a change-point at τ (Xt for t = 1, 2, …, τ, have a common distribution function F1(X) and Xt for \(t = \tau + 1\), …, T, have a common distribution function F2(X), and F1(X) ≠ F2(X)) (Pettitt 1979) (cited by Wolfe and Schechtman 1984; Zhang et al. 2008; Memarian et al. 2012). It tests the null hypothesis (H0) as the T variables follow one or more distributions that have the same location parameter (no change), against the alternative hypothesis (Ha) where a change-point exists. The non-parametric statistics are as follows:

$$K_{T} = \hbox{max} \left| {U_{t,T} } \right|,$$
(9)

where

$$U_{t,T} = \mathop \sum \limits_{i = 1}^{t} \mathop \sum \limits_{j = t + 1}^{T} \text{sgn} (X_{i} - X_{j} ).$$
(10)

The change-point of the series is located at \(K_{T}\), provided that the statistics are significant. The significance probability of \(K_{T}\) is approximated for p ≤ 0.05 with

$$p \simeq 2\exp \left( {\frac{{ - 6 K_{T}^{2} }}{{T^{3} + T^{2} }}} \right).$$
(11)

The statistical trend of the data was based on 95% confidence level. The above procedure was performed using XLSTAT software package.

Results and discussion

Monthly discharge and sediment load pattern

Based on the daily hydrological and suspended sediment data, the calculated graphs indicate that the Subarnarekha River is highly seasonal, and high water discharge can be seen during monsoon months (June–Sept) compared to other seasons (Fig. 4). As the monsoon season ends, the runoff in all the streams within study area decline significantly. Therefore, Subarnarekha is a distinctive seasonal river exhibit monsoonal characteristics.

Fig. 4
figure 4

Monthly mean water discharge and sediment load pattern for all hydrological stations

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Sediment load in Subarnarekha is very high during Aug and Sept months, which is attributable to the seasonal discharge pattern. During the dry season, in situ weathering process predominates; whereas erosional process becomes very active and yields a gigantic amount of sediments during monsoon season. A yearly yield of sediment in the Subarnarekha basin is varying between 200 and 260 t km−2 year−1 for all the stations. Sediment yield is higher in the downstream stations (Ghatsila and Govindpur) as many small tributaries join the trunk stream in between upstream stations, transporting a significant amount of sediments. Abundant sediment supply from the hillslope regions are the major sources of sediments which results remarkably high sediment yield in a river channel (Oguchi et al. 2001). Yoshikawa (1974) indicated that the sediment yield in a steep mountainous tropical river can exceed 1000 t km−2 year−1. The Brahmaputra drains more than 500 million t km−2 year−1 ranks first among the world’s river in terms of sediment yield (Tandon and Sinha 2007). However, for small watersheds the sediment yield remains much lower (Ohmori 1983; Millman and Syvitski 1992). In a tectonically active environment, where uplift rate is very high, rivers show very high stream power and produce a large volume of sediment each year (Hovius 1998). In India, the Ganga, Brahmaputra drains exceptionally high volume of sediment due to the tectonically active environment (Tandon and Sinha 2007). However, the only exception is Godavari which shows high sediment yield despite the cratonic configuration (Tandon and Sinha 2007). Compared to these large river systems, Subarnarekha and Burhabalang shows a significant sediment yield due to its mountainous erosional environment in the upper reach and the hill-slope processes, contributing a greater amount of sediment.

Water discharge (Q d) trend

Table 2 shows the results of trend analysis of discharge using linear bivariate regression and Mann–Kendall non-parametric tests for five major hydrological stations in the Subarnarekha basin for a time span of about 40 years. Discharge pattern in Subarnarekha is reasonably complicated as few stations show an increase and few others show decrease in the hydrological regime in last four decades. However, a statistically significant decrease of the Qd is found only in case of Muri station (Fig. 5). Barring this, all the other stations show insignificant trends. Adityapur and Ghatsila show a positive, but an insignificant trend of Qd, while Jamshedpur, located in the middle of these two stations shows a negative rate. The explanation of this anomaly lies in the locational settings of the Adityapur and Jamshedpur stations. Adityapur hydrological station is located at the outlet of Kharkai River, a major right-hand tributary of Subarnarekha. By contrast, Jamshedpur is situated on the mainstream of Subarnarekha about 8–10 km downstream of the confluence of Kharkai and Subarnarekha. The negligible rate of change of discharge in Adityapur (− 0.005) indicates the mainstream above the confluence of Kharkai and Subarnarekha have a very low flow regime which supports the significant declining Qd trend in Muri station. This observation clearly indicates that the main channel of Subarnarekha is a contributing lesser amount of water compared to its major upstream tributaries such as Kanchi, Karkari and Kharkai. Govindpur station on Burhabalang River shows an insignificant decreasing trend of Qd.

Table 2 Results of trend analysis of discharge (Qd) for Subarnarekha River
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Fig. 5
figure 5

Temporal variation and linear trends of average yearly discharge (Qd). *Significant at 95% confidence level

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Numerous studies have been carried out by many researchers to understand the rainfall pattern of this area since last two decades. Kumar and Jain (2011) reported that an increasing trend of rainfall is found in Subarnarekha basin during the period of 1951–2004. Few other researchers have carried out subdivision and state-wise rainfall trend analysis, where they reported a significant decreasing trend in Jharkhand region from where Subarnarekha originates (Rajeevan et al. 2006; Guhathakurta and Rajeevan 2008). All these studies suggest that climate change (rainfall variation) may have a strong influence on the discharge regime in Subarnarekha and its adjacent Burhabalang River.

Sediment load (Q s) trend

Table 3 shows the results of trend analyses of Qs for all the major hydrological stations except Muri (Muri: data unavailable). The results indicate that in the last few decades, the Qs of Subarnarekha has been decreased (Fig. 6). In case of Adityapur and Ghatshila, this trend is statistically significant. A report by Panda et al. (2011) indicates that there is a negative change in the sediment load of the tropical rivers in India (i.e., Godavari, Krishna, Mahanadi, Narmada, Tapi, Mahi), indicates the recent climate variability and its interaction with the human activities. The early report shows that almost all the peninsula rivers are experiencing a decrease in sediment load during the last few decades, while in few rivers the change is significant (Panda et al. 2011). The reason behind declining trend in sediment discharge in Subarnarekha may be due to the potential anthropogenic factors and the impact of climate change (variation of precipitation) on sediment load which (the exact reason) is very hard to recognize without additional analysis. Long-term trends of Qs of a river depends on several factors such as climate attributes, lithology and geological setting of an area, land-use practice, and water conservation projects as a dam or reservoir construction. Panda et al. (2011) advocates that for all the peninsula tropical Indian rivers, the sediment supply is significantly low during drought years. Abrupt reduction of the sediment load can be attributed to the construction of dams which can entrap up to 60–80% of the sediment load (Gupta and Chakrapani 2005; Yang et al. 2006). According to the CWC report, totally 36 dams have been constructed in last four decades in which a large dam has been built in the upstream of Subarnarekha main channel and several small dams and reservoirs are made in other tributaries. Perhaps there is a severe impact of these dams for declining sediment load in the downstream only if a significant proportion of sediment gets deposited in this dam’s reservoir.

Table 3 Results of trend analysis of sediment load (Qs) for Subarnarekha River
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Fig. 6
figure 6

Temporal variation and linear trends of average yearly sediment load (Qs). *Significant at 95% confidence level

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Change-point detection

Table 4 attributes the results of homogeneity analysis of Qd and Qs data by implementing Pettitt test to find out the abrupt changes in the time-series. Muri shows a significant abrupt downward change for Qd in the year 2000 (Fig. 7). Other hydrological stations do not exhibit any such change for Qd.

Table 4 Results of abrupt change-points of discharge (Qd) and sediment load (Qs) in Subarnarekha River
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Fig. 7
figure 7

Abrupt change in water discharge (Qd). Significance at 95% confidence level

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Sediment load at downstream of both Subarnarekha (Ghatshila) and Burhabalang (Govindpur) show statistically significant abrupt change from 1998 and 2010, respectively (Fig. 8). However, Qs at Adityapur and Jamshedpur does not show any abrupt change during last four decades. Variation of Qs in Subarnarekha downstream indicates two episodic manners of sediment discharge pattern. Most of the dams in the study area have been built during the period of 1980–1990s. Therefore, it clearly indicates the impact of dam construction on the sediment load pattern, as all the change-points are found post-dam construction period on the Subarnarekha River.

Fig. 8
figure 8

Abrupt changes in sediment load (Qs). Significance at 95% confidence level

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Double mass plot has been instigated to understand the changes in the ratio of Qs and Qd in Subarnarekha (Ghatshila) and Burhabalang River (Govindpur). In case of Ghatshila, the Qd–Qs ratio shows a steeper regression slope (0.53) after 1997 when the extreme outliers are considered; otherwise the regression slope is declining, while the slope is 0.43 before the change-point (Fig. 9). Govindpur station shows an opposite scenario, where before 2009 the regression slope is steeper (0.33). These results imply that in recent times the water discharge and sediment load ratio is increasing in the downstream of Subarnarekha River. By contrast, Burhabalang River is experiencing a decrease in the ratio of water discharge and sediment load.

Fig. 9
figure 9

The relationship between yearly discharge and sediment load for Ghatshila and Govindpur hydrological station, before and after the abrupt change

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The fourth assessment of the Intergovernmental Panel on Climate Change (IPCC) prognosticated that sea-level rise with rainfall variability may intensify the coastal ecosystem for the densely populated river basins of Asia (Cruz et al. 2007). With the rising sea level in Indian coast (Unnikrishnan and Shankar 2007), the reduction of sediment supplies in the Subarnarekha and Burhabalang rivers appears to have made the coastal regions more vulnerable to coastal erosion. Malini and Rao (2004) indicate that the adjacent second largest river system in India, Godavari River was the 9th largest sediment transporting river on a global scale. However, a change-point is found during the period of 1990–1998 due to the construction of dams and after that the sediment load has been declined significantly. Hence, all the declining rates of sediment and the increasing anthropogenic activities with climatic variability likely to cause coastal erosion due to loss of mangrove vegetation and the displacement of the human habitation in the near future (Malini and Rao 2004; Rao et al. 2008; Gamage and Smakhtin 2009; Panda et al. 2011; Wang et al. 2011).

Conclusion

A very detailed assessment of water and sediment discharge is carried out in this study, which provides a few interesting results regarding the changes of discharge and the sediment flux pattern of the Subarnarekha basin since last few decades. Bivariate linear regression, Mann–Kendall non-parametric test, and Pettitt test have been applied in the data sets of past 40 years to identify the trends and abrupt changes in the discharge and sediment load using the data provided by WRIS-India. A statistically significant decreasing trend of discharge and downward abrupt change is found in the upper reach of the Subarnarekha River (Muri station). The downstream reach of the study area shows an insignificant but increasing trend of discharge which has been impacted by precipitation variation in the past few decades. Sediment load at all the stations show decreasing trend for past few decades. Not all the changes are statistically significant, but stations located at downstream of the Subarnarekha and Burhabalang have shown a significant decreasing trend at 0.05 level of confidence. The results of this study divulge clear signals of the declines in sediment load which may be attributed to the impact of recent climate change and its interaction with anthropogenic activities in the study area. However, additional research is essential to enumerate the relative influences of different factors that regulate the sediment load and variability in Subarnarekha and Burhabalang rivers. Based on the substantial results found in this present study, the regions where sediment load is declining significantly, policy measures are essential to be commenced for the environmental flow requirements to protect the ecology and geomorphology of the densely populated regions and coastal environment.