Surface runoff generated in the monsoon months in the upstream parts
of the Ganges River Basin contributes substantially to downstream
floods, while water shortages in the dry months affect agricultural
production in the basin. This paper examines the parts (sub-basins)
of the Ganges that have the potential for augmenting subsurface
storage (SSS), increase the availability of water for agriculture
and other uses, and mitigate the floods in the downstream areas. The
Soil and Water Assessment Tool (SWAT) is used to estimate
sub-basin-wise water availability. The water availability estimated
is then compared with the sub-basin-wise un-met water demand for
agriculture. Hydrological analyses revealed that five sub-basins
produced more than 10 billion cubic meters (B m
Matching water demand with supply in river basins with monsoonal climate is a major challenge. The monsoon-driven seasonal hydrology in India is often associated with floods and droughts, which affects the most vulnerable people of society (women and children, the poor and other disadvantaged social groups), and causes damage to crops and infrastructure. In these basins, upstream storage is generally the preferred solution to buffer the variability of flow and reduce floods downstream (Khan et al., 2014). Traditionally, dams are the major surface water storage structures. However, the construction of large dams requires huge investments, displaces people, submerges forests, and some of the water released is lost to non-beneficial evaporation (Pavelic et al., 2012). In contrast, underground aquifers are efficient water reservoirs with minimum evaporative losses, no displacement of people or submergence of land (Bouwer, 2000; Dillon, 2005; Ghayoumian et al., 2007).
For centuries, the utilization of water resources in the Ganges River Basin
has been severely hampered by substantial seasonal variation in river flows.
In the basin, the main source of water is the (southwest) monsoon rainfall,
and also the snowmelt and ice melt in the Himalaya during the summer season
(Sharma and de Condappa, 2013) which is about 1170 billion cubic meters
(
Extensive flooding in the Ganges River Basin, especially in the
downstream areas, occurs annually (Mishra, 1997). The major causes of
floods in the downstream areas are the shallow groundwater table and
high monsoonal rainfall in these areas, and the large surface runoff
generated in the upstream sub-basins. Previous studies (Revelle and
Lakshminarayana, 1975; Sadoff et al., 2013) indicated that, due to the
limitation of the construction of large surface reservoirs, recharging
groundwater beyond the natural level is the best way to control floods
downstream. Subsurface storage (SSS) also allows meeting water
requirements during the dry months. Popular belief is that having
large dams is the only option to meet the basin's water storage needs
(Onta, 2001). However, contrary to that, the Ganges strategic basin
assessment conducted by the World Bank (2012) found that the
sustainable use of the basin's vast groundwater aquifers can store far
greater volumes of water compared to the potential of man-made storage
in the basin, which is about 130–145
From a purely biophysical perspective, four conditions are necessary
to develop sustainable SSS solutions (that involve groundwater
recharge beyond the natural levels) to tackle water scarcity and flood
damage in the basin:
Existence of adequate un-met demand (e.g., for agriculture and
other uses) to deplete the water pumped from the aquifers in
a basin/sub-basin. Existence of adequate flows for capture during the monsoon
season. Existence of extra space underground which can be created by
pumping and depleting groundwater before the onset of the monsoon. Ability to actually capture the excess monsoon surface runoff to
recharge that additional space created – naturally (through surface
water and groundwater interactions) or artificially (through managed
aquifer recharge (MAR)).
Amarasinghe et al. (2015) examined the first condition above and
estimated un-met demand throughout the basin under two scenarios of
irrigation expansion. The main objective of this paper is to examine
the second condition above, i.e., assess the potential availability of
runoff, by conducting a hydrological analysis of the sub-basins of the
Ganges River Basin.
Many models have been developed (e.g., Eastham et al., 2010; Gosain
et al., 2011; World Bank, 2012) to study water issues in the Ganges
River Basin (Johnston and Smakhtin, 2014). However, they are not
available to the public. To overcome this restriction and provide the
research community with a working hydrological model for the Ganges
River Basin, the International Water Management Institute (IWMI) has
developed a publicly available hydrological model for the basin
(Muthuwatta et al., 2014) using the Soil and Water Assessment Tool
(SWAT) (Arnold et al., 1998). The model set up files can be downloaded
from the website
SWAT is a widely used, semi-distributed conceptual hydrological model
developed by the Agricultural Research Service of the United States
Department of Agriculture (USDA) over the last 30
SWAT simulates the local water balance of the catchment through four
storage volumes – snow, soil profile, shallow aquifer and deep
aquifer – based on the soil water balance (Eq. 1):
The model used in this study was set up using the datasets shown in
Table 1. The Ganges River Basin was delineated using 3000
Figure 1 shows the catchments delineated for SWAT, 22 major sub-basins (Table 2) in the Ganges River Basin and the area covering Nepal. The 19 main sub-basins in the Indian part selected in this paper are those considered by the Central Water Commission (CWC) of India, which is the main government agency responsible for water resources development and management in the Ganges River Basin. Since the focus of this study is to estimate water availability in the sub-basins within India, Nepal is considered as one region. Hereafter, in this paper, “sub-basins” are referred to as the 22 major areas shown in Fig. 1, while the smaller spatial units inside those 22 sub-basins and Nepal are termed “catchments”. For details of the model setup, including calibration and validation, please refer to Muthuwatta et al. (2014).
Annual time series of catchment-scale surface runoff from 1991 to 2010
were constructed by aggregating daily surface runoff simulated by
SWAT. Next, using geographic information system (GIS) techniques,
annual runoff time series were estimated for all sub-basins within the
modeled area of the Ganges River Basin. The study uses the hydrographs
of the simulated runoff (SR) to estimate the 75 % dependable
runoff (SR
The spatial and temporal distribution of the annual surface runoff is analyzed to determine the water availability in different sub-basins. Streamflow includes surface runoff and baseflow from groundwater, which can be captured by diversion or from dams. Surface runoff is part of the precipitation that is left after evapotranspiration and infiltration, which can be captured for MAR before it reaches the stream. Therefore, only the surface runoff portion was considered for augmenting SSS. Figure 2 shows the simulated catchment-scale mean annual surface runoff.
The surface runoff of catchments ranges from less than
0.1
The estimates of mean annual surface runoff at sub-basin-scale ranges
from 2.24
Ghaghara (10) sub-basin and Nepal have, by far, the largest
SR
Amarasinghe et al. (2015) estimated the un-met agricultural water
demand. Two scenarios were considered in the analysis (Table 4).
Scenario 1 assumed that all irrigable land will be irrigated in
the Scenario 2 considered all cropland to be irrigated in the
As of now, all the sub-basins in the Ganges River Basin have
substantial un-met water demand for agriculture in the dry
season. Therefore, capturing a substantial portion of the surface
runoff during the monsoon months can help close the gap between
current supply of water and demand in the dry months. Therefore, there
is potential for increasing agricultural productivity in these
sub-basins with more irrigation in the dry months. Table 4 presents
the sub-basin-wise un-met demand (Amarasinghe et al., 2015) and the
percentage of dependable runoff required to close the un-met demand.
In the sub-basins, the total un-met demands are 55.03 and
108.4
Floods are a recurrent phenomenon in some parts of the Ganges, such as
the State of Bihar, which is located in the middle part of the basin
(Fig. 4). More than 20 million people have been affected by floods in
1987, 2004 and 2007. The spatiotemporal flood inundation extent across
Bihar, revealed from the Moderate Resolution Imaging Spectroradiometer
(MODIS) satellite data, showed that the flood inundated area can be
more than 14 000
The highest flow of 142.7
As presented in Fig. 3, five sub-basins located in the upstream part
of the Ganges River Basin consistently produce higher surface runoff
compared to other sub-basins. These sub-basins are Ghaghara (10), Son
(17), Yamuna Lower (20), Ramganga (16) and Kali Sindh (13). Figure 5
shows the relationship between the outflows from four upstream
sub-basins with inflow to the State of Bihar. Coefficient of
determinant (
Flow from Ghaghara and Yamuna Lower sub-basins is approximately 30 % of the total inflow from the upstream Ganges River Basin to Bihar. The contributions from Son, Kali Sindh and Ramganga are 17, 10 and 7 %, respectively. This implies that, by capturing a portion of the upstream flows during peak runoff periods would reduce the flow to Bihar, which creates floods during the wet season.
Creating additional SSS beyond the current levels in the Ganges River
Basin can simultaneously enhance water supply for beneficial depletion
and control downstream floods. Water availability analysis conducted
and based on time series of simulated surface runoff using SWAT showed
that annual total surface runoff generated in the Ganges River Basin
is about
Further analysis revealed that the annual surface runoff from the upstream of the Ganges River Basin to the State of Bihar, a flood-prone area located downstream, is twice the amount of rainfall in the same area. Sub-basin-wise streamflow analysis in the Ganges River Basin showed that approximately 60 % of the upstream flow to Bihar comes through the Ghaghara and Yamuna Lower sub-basins. This runoff contributes to the recurrent floods in Bihar. As shown in Fig. 5, there are strong linear correlations between annual outflows from the upstream sub-basins and the inflow to the State of Bihar. This suggests that SSS upstream has the potential to control floods downstream, by capturing a portion of the surface runoff during the wet season in the upstream sub-basins.
This study only discusses the surface water availability for SSS, and further analysis is needed to ascertain the storage capacity of the aquifer and how much additional storage capacity can be created by pumping groundwater during the dry months. Further, a detailed analysis of the soil, topographic and geological characteristics is required to determine the suitable areas for groundwater recharge.
Finally, it is pertinent to understand the interactions between groundwater and surface water in the sub-basin. This requires coupling a groundwater-surface water model to run some scenarios to investigate the effect of pumping and recharging of groundwater on the hydrology of the Ramganga sub-basin.
This research study was undertaken as part of the CGIAR Research Program on Water, Land and Ecosystems (WLE) by the International Water Management Institute (IWMI), Colombo, Sri Lanka, and the National Institute of Hydrology (NIH), Roorkee, India. The authors would like to acknowledge the valuable assistance provided by staff members of IWMI's GIS, RS and Data Management (GRandD) unit, especially Salman Siddiqui (Senior Manager, GRandD unit).
An overview of the main datasets used in this study.
Names of the sub-basins.
Surface runoff of the sub-basins.
Sub-basin-wise un-met agricultural water demand and the percentage of surface runoff required to close the un-met demand.
Sub-basins and catchments of the Ganges River Basin.
Mean annual surface runoff of the 1684 catchments (1991–2010).
Sub-basin-scale annual dependable runoff (SR
Mean annual outflow (
Relationship between outflows from upstream sub-basins and inflow to the State of Bihar.