Hydrological modeling as a tool for water resources management of the data-scarce Brahmaputra basin

A robust hydrological assessment is a challenging task in regions of limited hydro-climatological information. This level of uncertainty is further augmented in studies of flood hydrology for regions like the Brahmaputra River basin, where spatial variations of topography, land use, soil, and weather components are very high. The present study describes the development of a suitable hydrologic model for the data-scarce transboundary Brahmaputra River basin occupying an area of more than 5,42,000 km. The main objective is to provide hydrologic assessment of the Brahmaputra River basin, even at locations having hardly any historical records. The Soil and Water Assessment Tool (SWAT) model is calibrated and validated using observed discharge of three sections located on the main stem. The results show a fair strength of the statistical parameters. Moreover, the model has been found to produce a satisfactory replica of historical flows at the tributaries with a fair value of correlation (R1⁄4 0.77) at Golaghat. The results of this model would facilitate the ability of the local authorities with science-based elements to carry out decisions on the management of water resources at the main basin, and even at the sub-basin level. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wcc.2020.186 om http://iwaponline.com/jwcc/article-pdf/12/1/152/851744/jwc0120152.pdf er 2021 Pulendra Dutta (corresponding author) Arup Kumar Sarma Department of Civil Engineering, Indian Institute of Technology, Guwahati 781039, India E-mail: pulendutta@gmail.com


INTRODUCTION
Expanding over four different countries, the Brahmaputra River basin occupies an area of 5,42,450 km 2 with the outlet (23.576N: 89.442E) in Bangladesh. The Brahmaputra is one of the most highly sediment-laden rivers of the world (Goswami ). Also, this river basin is located in an area of high structural instability as a large number of earthquakes have been evidenced in the Himalayan catchment through which it flows (Lahiri & Sinha ). The Brahmaputra carries the largest amount of water and silt of all Indian rivers. However, this is probably the least exploited basin, even though it has enormous potential regarding water resources management and hydropower generation.
Hydrologic models are among the available tools used to estimate the parameters required for water resource planning and management.
There have been many case-specific studies on definite parts of the Brahmaputra basin ( (Swamee et al. ). None of these studies addressed the hydrologic assessment of the basin. However, Rao et al. () attempted a hydrologic assessment, but the entire Brahmaputra basin was not covered in their study. In another study, although the whole basin was considered by Aktar et al. (), they failed to establish whether the hydrologic model developed would be applicable for deriving outputs across the basin. Here, the model was calibrated at only one location and no consideration was paid to spatial outputs. As such, we have attempted in this study to understand the hydrology of the entire Brahmaputra River basin so as to facilitate the water resource planners with science-based information at the desired locations.
Understanding the Brahmaputra basin has always been challenging due to its complex characteristics, both hydraulically and hydrologically (Rao et al. ). Although ) of data are the major concerns for obtaining poorer simulation results. In the absence of available data, however, the missing variables may be generated (Nyeko ) and/or explicitly trained as constraints in mathematical programming (Pande et al. ). Unfortunately, these approaches often provide less satisfactory simulation results. In this study, we, therefore, have tried to incorporate the presently available data at many sources locally or globally. The Brahmaputra River basin has huge potential with regard to water resources projects, although severe hazards like flood and bank erosion that result in huge loss of life and property every year are also much pronounced. Developing a comprehensive management policy is the current priority, but requires adequate data regarding flow, sediment, soil moisture, water quality, etc. We, therefore, adopted hydrologic models as a tool in order to estimate these data for the Brahmaputra basin, although their establishment was a major challenge due to the lack of input data. Furthermore, we could not find any observational records for the majority of the tributaries and sub-tributaries that join into the main stem of the Brahmaputra River, within Indian territory in particular. Therefore, modeling these tributary basins individually is really a challenging task, which can probably only be solved through modeling practice at a comprehensive level covering the entire transboundary Brahmaputra River basin, for which certain data are at least available. The present study presents a modeling-based approach to understand the dominant processes controlling the water balance to bridge the gap of base-line knowledge of water resources in the data-scarce Brahmaputra basin. The main aim of this modeling exercise is to identify a suitable dataset for carrying out hydrologic assessment across the basin, including locations having no historical records. This study would provide information required for management of water resources across the basin including its sub-basins.

STUDY AREA
The proposed study is intended for the transboundary Brahmaputra River basin ( Figure 1). The Brahmaputra River originates in southern Tibet at an elevation of 5,300 m.
Out of its total length of 2,880 km, the Brahmaputra covers a major part of its journey in Tibet as 'Tsangpo' River (also popularly known as 'Yarlung Zangbo' in

MATERIALS AND METHODS
In this approach, the results of the SWAT model obtained by using various weather datasets are presented. Gassman et al.    Table 1.
SWAT-CUP is an interface to SWAT, and any calibration/uncertainty or sensitivity program can easily be linked to the later models. It provides a decision-making framework that incorporates a semi-automated approach (SUFI2) using calibration and incorporating sensitivity and uncertainty analysis (Arnold et al. ).
The key parameters are necessary to be identified prior to calibration (Ma et al. ), and the present study identifies nine sensitive parameters (Table 2) during the iterations of each SWAT model. As the watershed models suffer from various uncertainties in regard to model, input, and parameters, the present study has adopted an uncertainty analysis for the parameters identified through the sensitivity analysis. Table 2 shows the range of sensitive parameters to mark the uncer-

RESULTS AND DISCUSSION
The Brahmaputra is one of the largest river systems in the world and, as such, researchers have faced a huge challenge to understand the physical processes (Pahuja & Goswami ). It is, therefore, the need of the moment to develop a comprehensive water management cooperation mechanism for water sharing between the co-basin nations -

Simulation results
It is very important to use adequately correct data while establishing a hydrologic model, without which reasonably valid outputs can rarely be achieved. Figure 4 shows a scatter plot between the simulated and observed flows for the models at Pancharatna (Figure 2(b)), as an example. Similar trends were also observed at the other two outlets, namely, Bhomoraguri and Pandughat. This plot provides an idea of how the simulated flow is different from the observed flow. A perfect match between the two represents a straight 1:1 line.
It is understood from the results that MODEL_1 provides the least satisfactory output, as evidenced by the calibration statistics in Table 3 although they were carried out by using the same sensitive parameters (Table 2) as the previous ones. Among all models, MODEL_6 is found to perform the best, since it provides a very good strength of statistics for all the parameters, even during both the calibration and validation (Table 3).
This fair result is due to using a proper combination of input datasets in this model (MODEL_6), as compared to those used in the other models (MODELs_1-5). The Rfactor (0.97) value during calibration (Table 3) is nearly equal to the desired value (1.00). It indicates a very good measure of parameter uncertainty. As such, this model has only been extended for subsequent analyses.

Multi-outlet calibration/validation
The calibration at a single site is judicious for the small The time series plots are shown in Figure 6 as a continuous graph for simplicity in the presentation. Due to the nonsymmetry of available discharge data length, different calibration periods were chosen for the outlets (Figure 2(b)).
Due to its complex characteristics, a large watershed may not be expected to produce good statistics against each outlet during multi-site calibration; however, a balance among the statistical parameters is required for reasonable acceptance of a model. During this spatial analysis (Figure 6(c)), the most downstream outlet (i.e., Pancharatna) is found to conform with good statistics during calibration (R 2 ¼ 0.84; NS ¼ 0.74) and validation (P-factor ¼ 0.94; R-factor ¼ 0.91). Similarly, the model is found to produce satisfactory statistics at the other two outlets, namely, Bhomoraguri (Figure 6(a)) and Pandughat ( Figure 6(b)). However, the calibration results at Pancharatna, especially the value of R 2 (0.84) are relatively low as compared to Bhomoraguri (0.86) and Pandughat (0.91). This is probably due to different lengths of warm-up period maintained for initiation of watershed responses by the hydrologic models. The warm-up period for Pancharatna is only two years (1991)(1992) against eight years (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998) for the later two outlets. Overall, the spatial calibration/validation results of MODEL_6 may be termed as satisfactory.

Verification of model results at tributary
The present SWAT models were developed for the large Brahmaputra River basin, but whether this model built for a basin area of more than 5,42,000 km 2 would be suitable for its sub-basins occupying smaller areas, or not, needs to be evaluated. This is done through cross verification of the model results. This analysis is possible for several outputs (discharge, sediment, evapotranspiration, groundwater substances, etc.) of the SWAT model, provided historical

Model application in water resources management
The SWAT model provides location-specific hydrologic information across a river basin which may be utilized by the water resource managers for planning and designing hydraulic structures. This information belongs to the quantitative and qualitative assessment of river flows. A list of certain parameters is enumerated in Table 4 corresponding to a location (latitude: 29.15485N; longitude: 95.006979E) termed the 'Indo-China Border' (Figure 2(b)).
Here, monthly values are obtained from the outputs of the   The figures are obtained as output from the final run of the SWAT model suitably adopted in the present study, i.e., MODEL_6.