Water planning decisions are only as good as our ability to explain historical trends and make reasonable predictions of future water availability. But predicting water availability can be a challenge in rapidly growing regions, where human modifications of land and waterscapes are changing the hydrologic system. Yet, many regions of the world lack the long-term hydrologic monitoring records needed to understand past changes and predict future trends.
We investigated this “predictions under change” problem in the data-scarce Thippagondanahalli (TG Halli) catchment of the Arkavathy sub-basin in southern India. Inflows into TG Halli reservoir have declined sharply since the 1970s. The causes of the drying are poorly understood, resulting in misdirected or counter-productive management responses.
Five plausible hypotheses that could explain the decline were tested using data from field surveys and secondary sources: (1) changes in rainfall amount, seasonality and intensity; (2) increases in temperature; (3) groundwater extraction; (4) expansion of eucalyptus plantations; and (5) fragmentation of the river channel. Our results suggest that groundwater pumping, expansion of eucalyptus plantations and, to a lesser extent, channel fragmentation are much more likely to have caused the decline in surface flows in the TG Halli catchment than changing climate.
The multiple-hypothesis approach presents a systematic way to quantify the relative contributions of proximate anthropogenic and climate drivers to hydrological change. The approach not only makes a meaningful contribution to the policy debate but also helps prioritize and design future research. The approach is a first step to conducting use-inspired socio-hydrologic research in a watershed.
Freshwater has been identified as one of the gravest challenges
of the twenty-first century
Making hydrological predictions is a non-trivial problem in any context, but it is confounded by three issues encountered in rapidly changing, data-scarce regions: (i) non-stationarity arising from anthropogenic drivers, (ii) the sparse availability of historical data, and (iii) lack of original, place-based scientific research leading to oversimplified assumptions. The prediction challenges arise both from the nature of the system (point i) and researcher constraints (points ii and iii), but the net result is that water managers are forced to rely on conceptual models that poorly represent the underlying system.
The TG Halli catchment with major features. BBMP is the Greater Bangalore Municipal Corporation Boundary (data source: Survey of India toposheets at 1 : 50 000 scale; ASTER DEM imagery, maps prepared at the ATREE EcoInformatics Lab).
The mismatch between the needs of water managers and what off-the-shelf models can generate is not a sufficient reason for inaction or ad hoc decision making in regions with rapidly increasing water demand. There is an urgent need to formulate new approaches to frame and conduct hydrologic investigations in human-dominated, data-scarce situations. The conventional response would be to initiate primary data collection and to build new site-specific models from scratch. However, hydrologic data collection is expensive and takes many years. In contrast, information is often needed quickly and projects are limited by time and resource constraints.
How should hydrologists proceed in these circumstances? First, as
The Arkavathy River is located in the state of Karnataka in southern India (Fig.
The TG Halli reservoir catchment also contains an older water supply
reservoir at Hesaraghatta, as well as an estimated 617 small surface storage
structures called “tanks”. Tanks are traditional in-stream water harvesting
systems that were commonly built in southern India and Sri Lanka over the last
6 centuries to store monsoon runoff for post-monsoon irrigation
Most of the TG Halli catchment is underlain by gneissic and granitic aquifers. Highly weathered soils extend to about 20 m below grade level (b.g.l.), and form a shallow aquifer in which seasonal perched water tables can develop. Between about 20 and 60 m b.g.l. lies a fractured rock zone with considerable jointing and cracking, acting as a deeper aquifer. Groundwater yields decline beyond 60 m b.g.l., although fractures continue to be encountered down to 300 m.
From 1937 up to the 1980s, the TG Halli reservoir was a major source of water
for Bengaluru. However, inflow to the reservoir has steadily declined since
the early 1980s (Fig.
Changes in hydrology and hydrometeorology of the Arkavathy Basin,
1970–2010.
Water use in the TG Halli catchment.
The drying of flows into the TG Halli reservoir and tanks in the catchment has clear implications for the 800 000 people that live in the catchment, both in terms of current water availability and because the declining flows may be an indicator of the overall unsustainability of water use in the basin jeopardizing future populations and economic growth.
Given the urgency of the problem, several uncoordinated and often
contradictory actions have been undertaken. One reason for this is that the
causes of the inflow reductions to the TG Halli reservoir remain unclear. In
order to formulate hypotheses that could be investigated systematically, we
consulted a range of sources to understand the positions and perceptions of
different groups: one-on-one meetings with government officials, written
policy documents and reports, a comprehensive literature review
This initial review identified several policy positions that reflect
different perceptions on the drying of the river:
The Bangalore Water Supply and Sewerage Board (BWSSB), which owns and operates the
TG Halli reservoir, commissioned a study The Cauvery Neeravari Nigam Ltd. (CNNL) was made responsible by the state government
for “rejuvenating” the Arkavathy River. CNNL commissioned its own study Meanwhile, local rural development programmes have focused on constructing check dams
to recharge the shallow aquifer and ostensibly restore baseflow in the stream. A number of urban based citizen's groups have emerged with the objective of
rejuvenating the river or saving Bangalore's water bodies (see
The state Water Resources Development Organization (WRDO) has argued that
climate change, via declining rainfall and rising temperatures, is responsible for
the drying of the river. This perception was also held by most farmers we interacted
with during the water literacy meetings, many of whom favour inter-basin imports from
west-flowing rivers.
By examining the different explanations of the causes of streamflow decline
and plausible runoff generation mechanisms, we identified and investigated
Details of various data sets used.
To test these hypotheses, we collected available secondary data within and
around the Arkavathy Basin. Data were quality-checked and triangulated
against other sources and supplemented with field surveys when needed
(Table
Monthly inflow data for the TG Halli reservoir were obtained from Bangalore Water Supply and Sewerage Board (BWSSB) for the period 1937 to 2010. Additionally, daily records of inflows from 1970 onwards were obtained from the local BWSSB offices and digitized. The daily and monthly data were cross-validated and any errors were corrected.
The goal of the analysis was two-fold: (i) to determine whether the perceived
changes in hydrological drivers have occurred and (ii) whether the magnitude
of changes in the drivers could explain the magnitude of the change in flow
in the Arkavathy Basin (i.e. consistent with the observed 320 ML day
Data from four long-term rainfall gauges located in Devanahalli,
Doddaballapura, Magadi and Nelamangala towns within the TG Halli catchment
were available for analysis (see Fig.
Annual rainfall was computed over the water year (June to May). Seasonal
rainfall totals were computed in terms of pre-monsoon
(January-February-March-April-May: JFMAM), monsoon
(June-July-August-September: JJAS) and post-monsoon
(October-November-December: OND) rainfall totals. To identify changes in
rainfall depths at daily timescales, the number of days per year in which
rainfall volumes exceeded 10, 25 and 50 mm were determined for the
1934–2009 period. Trend detection was undertaken for each of the above
data sets in two ways. First, we determined whether a trend was present over the
full time series. As the data generally did not conform to the assumptions
for least-squares regression, we evaluated the trends using a non-parametric
Mann–Kendall test. Second, we evaluated whether a change in the mean values
of the meteorological parameters had occurred from the pre-1970 and post-1970
period, taking 1970 as a point after which the Arkavathy River flows
obviously declined. Where the data were normally distributed we made these
comparisons with
In the absence of detailed meteorological data in the Arkavathy Basin, we
estimated changes in the mean daily potential evaporation rate as a function
of temperature (PET) using the modified 1985 Hargreaves evapotranspiration equation
The modified Hargreaves equation relies on the diurnal temperature range to
provide a surrogate for solar radiation and is widely used to estimate
potential evaporation when only limited ground data (temperature) are
available. The resulting PET estimates are typically within 10 % or better of
those derived from lysimeter or Penman–Monteith methods, when results are
averaged over 5-day or greater time periods
Long-term groundwater level data (
We undertook two different analyses to explore whether changes in groundwater were compatible with the observed changes in surface flow. In one analysis we used a baseflow recession technique to benchmark the changes in mobile subsurface water storage that would be needed to account for the decline in annual flows and then estimated how these changes might manifest as a decline in groundwater levels. If this change in storage greatly exceeds observed well declines in the catchment, then the hypothesis that lower groundwater levels have led to streamflow reductions could be rejected. In a second analysis, we performed a baseflow separation on the daily runoff data from 1970 onwards to determine how the trends in total streamflow were reflected by changes in quick-flow and baseflow.
This methodology was applied to the monthly flow data from the Arkavathy at
TG Halli, focusing on the seasonal recessions from 1937 to 1970 (i.e. prior to
the discernible reductions in river flow). There are two major limitations to
using monthly data for this analysis. First, the estimation of the rate of
change of the flow is coarse. Second, the contribution of rainfall to runoff
events is unlikely to be negligible, even during the seasonal recession.
However, because the daily flow data were only available for the post-1970
period, the monthly analysis provides the only opportunity to evaluate the
storage–discharge relationship when the river was flowing “normally”. As
outlined in the results, the calibrated model had an exponent
We calculated the change in eucalyptus plantation area from 1973 to 2001 by comparing the mapped land uses in both years. We used two sources: a land use map provided by Karnataka State Remote Sensing Application Centre (KSRSAC) and Survey of India topographic sheets. The KSRSAC land use map was derived from Indian Remote Sensing (IRS) LISS-3 merged with PAN satellite imagery with an effective 6 m resolution. The map reported the area under eucalyptus plantations in 2001. For other years, no such maps were readily available. So we digitized 1 : 50 000 scale topographic maps prepared by the Survey of India during the 1970s (1973 to 1979), which show eucalyptus plantations on public lands.
We made three assumptions about water use by eucalyptus plantations (which
are typically unirrigated). First, the plantations could not themselves have
led to groundwater mining (as has been claimed in other parts of Karnataka
Data on the number of channel obstructions in the TG Halli catchment were
available in a report commissioned by Cauvery Neeravari Nigam Limited (CNNL)
Number and type of stream encroachments in each section TG Halli catchment source: Zoomin Tech Report to CNNL, 2011.
To validate the CNNL data, we conducted a comprehensive survey of all stream
obstructions in two milli-watersheds covering a 26 km
The volumes of typical obstructions were estimated based on stream profiles
made using a dumpy-level instrument on seven check dams. Interpolation of the
stream profiles allowed us to estimate the maximum storage volumes as ranging
between 100 and 1500 m
We then plotted the cumulative density function of the daily inflow events into the TG Halli for 15 years from 1976 to 1990 (the period before check dams and unculverted roads were constructed for which we had daily inflow records). We took all flow events less than or equal to the peak storage volume and assumed that the entire flow would be impounded. For events that generated inflows greater than the peak storage, the volume impounded was capped by the peak storage the catchment; anything higher would have overflowed. The volumes impounded were summed to estimate the total loss downstream. This calculation is likely to overestimate the fraction of daily runoff that is impounded behind check dams and unculverted roads, since the structures are unlikely to be empty at the beginning of every rain event.
Results are presented separately for each of the hypotheses.
The rise in temperature of about 0.6 to 1
From the recession analysis, the fitted
storage discharge relationship for the pre-1970 period was
As can be seen from Fig.
The area under eucalyptus plantations in 1973, as indicated by Survey of
India toposheets, was only 11 km
Change in eucalyptus area in Arkavathy Basin between 1973 and 2001.
Conversion of 93 km
Based on the assumptions about check dam and encroachment water storage, the
total loss in runoff at the basin scale attributable to channel encroachment
is on the order of 18–54 ML day
Our analysis indicates that rainfall changes or temperature increases cannot account for any significant fraction of the decline in inflows into TG Halli reservoir. The causes found to be plausible are groundwater extraction, expansion of eucalyptus plantations and to some extent increased obstructions in the stream course. Importantly, many policy approaches currently under consideration do not reflect the major underlying causes of the drying of the Arkavathy River, and in some cases (check dam construction) they are clearly counter-productive. In the future, climate change could play a critical role in exacerbating water stress, but climate stressors will only add to existing local stresses.
Although the hypotheses have been framed as independent, the mechanisms undoubtedly interact with each other, so their inter-relations should be considered in formulating a conceptual model of the catchment and in attributing the effects of each mechanism in terms of the change in river flow. For example, check dams not only impound flow, but also locally elevate recharge. Check dams may thus facilitate high levels of groundwater extraction locally. Spatial heterogeneity in water table levels and eucalyptus root zone access to saturated conditions may vary throughout the catchment, meaning that the assumption that eucalyptus plantations do not contribute to groundwater mining and reduced baseflow should be relaxed in future studies.
The analysis presented here is preliminary. Further work is needed to
understand the hydrological processes in the catchment, including the
contemporary and historical flow generation pathways and their changes. There
are, however, suggestive clues of timing that suggest a potential working
hypothesis for the flow generation mechanisms. Expansion of electricity and
installation of wells began to increase in the late 1960s – although this
period also coincided with a period of relatively high rainfall and
streamflow in the 1970s. Flow declines began to emerge in the early 1980s,
with baseflow indices and numbers of “baseflow months” plummeting in the
early 1990s, approximately at the same time that open wells went dry and
deeper boreholes become more prevalent (Fig.
Inflows continued to decline after 1992, suggesting that additional
mechanisms beyond the decline in baseflow must be considered. Possible
additional mechanisms include the conversion of the Arkavathy River into a
“losing” river, which provides a source of recharge to the local aquifers,
the continued expansion of eucalyptus plantations and increasing
implementation of management techniques that prevent surface runoff from
leaving farm fields and increasing obstruction of the stream channels. Based
on these observations, further research targeting runoff generation
mechanisms, establishing the pathways for surface–groundwater connections,
evaluating the effect of land use on water balance and estimating groundwater
extraction rates has now been initiated in the catchment (see
Finally, from a policy perspective, the fuzzy perception of the causes of
streamflow decline and the lack of coordination between agencies have
resulted in contradictory policies. The range of policy responses observed
reflect both different
The TG Halli catchment case study shows that humans can play a
significant role in altering the hydrology of watersheds
Third, the hypotheses themselves are derived not just from the academic literature but also from perceptions of all stakeholders in the debate. This ensures the legitimacy and usefulness of the research.
This research is part of a larger study titled “Adapting to Climate Change
in Urbanizing Watersheds (ACCUWa) in India” (
We are grateful to Jayalakshmi from ATREE's EcoInformatic Lab for RS/GIS support and Sowmyashree for help with the 1973 Eucalyptus area calculation. We thank NIT Suratkal student D. N. Shilpa and ATREE Administrative Assistant H. Usha for translation and data entry support on the annual season crop reports. We thank our outreach coordinators K. Janardhan and G. Manjunath for the insights obtained from water literacy meetings. We are grateful to Sekhar Muddu and the other ACCUWa advisory committee members for input on the hydrology study. We are grateful to P. N. Ballukraya for sharing his borehole data and advice on the hydrogeology investigations.
Financial support for most of this research comes from grant no. 107086-001 from the International Development Research Centre (IDRC), Canada. In addition, S. Thompson acknowledges NSF CNIC IIA-1427761 for support of ATREE-UC Berkeley collaborations. G. Penny acknowledges support from the NSF Graduate Research Fellowship Program under grant no. DGE 1106400, the NSF and USAID Research and Innovation Fellowship Program and NSF International Research Experience for Students (OISE-1031194). Edited by: M. Sivapalan