Demystifying Arkavathy fiasco
Atmometers installed at water bodies across Arkavathy basin to measure rate of evapotranspiration
Scientists from India, US bust myths regarding river; prove low rainfall & climate change not responsible for drying up of water body
Chasing a mirage of rejuvenating Arkavathy river, the state government has put several crores of rupees at stake. Even a year after the announcement that the river would be developed on the lines of the Thames in London, there seems to be no great action in reality. However, many including the scientific community consider that the ambitious plan is nothing but hogwash, given the fact there are too many loose ends that need to be tied up before embarking on a project that would make the Arkavathy flow all over again, naturally.
The Arkavathy basin including Bengaluru city, contrary to what has been widely believed, has neither witnessed a decline in annual rainfall nor has any increase in temperature (climatic change) rendered the river dry. Yet, despite the high rainfall in the basin, downstream tanks have hardly witnessed any inflow. While one (Hesaraghatta) has dried up long back and no longer overflows, the other (TG Halli) is hardly able to supply 30 MLD to Bengaluru, far lower than the design capacity of 148 MLD. Then where does all the rainwater that's collected around Bengaluru go? What has made Hesaraghatta dry up? Why is inflow into TG Halli reservoir declining? While policy makers, activists and the general public have tended to cite 'decreasing rainfall' or 'encroachment' of river channels as the main reason, researchers led by a team at Bengaluru-based Ashoka Trust for Research in Ecology and the Environment (ATREE) along with collaborators at the University of California (Berkeley) suggest that the real reasons may lie elsewhere. The team has come up with five plausible hypotheses that could explain the decline using varied data collected from the basin and secondary sources. Their research paper, 'Why is Arkavathy River drying? A multiple-hypothesis approach in a data-scarce region' has been published in the recent edition of scientific journal Hydrology and Earth System Sciences.
The team comprising V Sriniva-san, S Thompson, K Madhyastha, G Penny, K Jeremiah and Sharachch-andra Lele not only conducted extensive experiments across the basin but painstakingly collected data from various sources to scientifically assess the cause for the decline in inflow to Arkavathy. Their study was centred on popular assumptions such as decrease in rainfall, rising temperature and changing climatic conditions, extensive use of groundwater and large-scale eucalyptus plantation in the catchment area triggering the loss of inflow into the river course.
Probing the prevailing assumptions such as decline in rainfall and evaporation due to excessive heat among others, the team studied long-term rainfall data obtained from rainfall gauges located in Devanahalli, Doddaballapur, Magadi and Nelamangala. The chosen gauges provided daily rainfall data for over 75 years (1934-2010). According to the researchers, with an average of 830 mm rainfall per year and a standard deviation of 210 mm per year, there was no significant change in rainfall pattern. "During the period, high-decadel variability in rainfall proved that the years between 1970 and 1980 were exceptionally wet. Yet, no statistically significant change in rainfall was observed," explained Lele.
Commenting on yet another popular argument that run-off water could have been evaporated due to increasing heat, the team had gone around the basin installing evaporation meters at tanks and lakes and measured evaporation rates during all seasons. "The rise in temperature too was well within the prediction calculated by experts in the past. Accordingly, the temperature rose from 0.6 degree to 1 degree centigrade over the last 100-years. During that time, the potential evapotranspiration (PET) rate too did not show any significant change along the basin. Hence, there is no evidence to support the argument that increasing temperature or climate change has triggered the potential evaporation leading to a decline in stream flow," Veena Srinivasan clarified.
When both populist theories could not be established scientifically, the researchers focussed on the fact that over exploitation of ground water and water consuming plantations such as eucalyptus plantations along the basin could have triggered the decline in inflow. "There have been studies in well-monitored basins that say that groundwater depletion can reduce base-flow contribution to stream flow and thereby reducing overall flow along the basin. As per the data obtained during our field visits, there was considerable shift from dry farming to deep borewell irrigation on a large scale. This was supported by large-scale construction of check dams along the basin helping percolation of run off water. As a result of intensive ground water pumping, the water table plummeted from 50 ft back in 1970 to 600 to 1,000 feet in recent times. Similarly, during the same period, irrigated crop land more than doubled," Lele said.
"The eucalyptus plantations that were once rare now covered almost 20 per cent of the TG Halli catchment, probably contributing to the decline. The 11-sq.km area in 1973 was extended to a whopping 104-sq.km area in 2001. This had caused a loss of 75 to 135 ML per day in the area as against the overall decline in runoff by 320 ML per day. Hence, the eucalyptus plantations and over extraction of groundwater through pumping could have triggered the drying of Arkavathy," yet another researcher explained.
The researchers collectively opined that these hypotheses were not fully understood by either the government or the common man. The team conducted several water literacy meetings at villages educating farmers about the possible cause behind the decline in inflow. "During our interactions, many farmers believed that rainfall has decreased and evidently no tank/lake was filled in the last 20-years. As authorities continue to distinguish between surface water and groundwater, they do not see any link between pumping of groundwater and decline in surface water flow. Our study shows that there is a link between the both and it is critically important to address issues of water scarcity," Lele said.
Chasing a mirage of rejuvenating Arkavathy river, the state government has put several crores of rupees at stake. Even a year after the announcement that the river would be developed on the lines of the Thames in London, there seems to be no great action in reality. However, many including the scientific community consider that the ambitious plan is nothing but hogwash, given the fact there are too many loose ends that need to be tied up before embarking on a project that would make the Arkavathy flow all over again, naturally.
The Arkavathy basin including Bengaluru city, contrary to what has been widely believed, has neither witnessed a decline in annual rainfall nor has any increase in temperature (climatic change) rendered the river dry. Yet, despite the high rainfall in the basin, downstream tanks have hardly witnessed any inflow. While one (Hesaraghatta) has dried up long back and no longer overflows, the other (TG Halli) is hardly able to supply 30 MLD to Bengaluru, far lower than the design capacity of 148 MLD. Then where does all the rainwater that's collected around Bengaluru go? What has made Hesaraghatta dry up? Why is inflow into TG Halli reservoir declining? While policy makers, activists and the general public have tended to cite 'decreasing rainfall' or 'encroachment' of river channels as the main reason, researchers led by a team at Bengaluru-based Ashoka Trust for Research in Ecology and the Environment (ATREE) along with collaborators at the University of California (Berkeley) suggest that the real reasons may lie elsewhere. The team has come up with five plausible hypotheses that could explain the decline using varied data collected from the basin and secondary sources. Their research paper, 'Why is Arkavathy River drying? A multiple-hypothesis approach in a data-scarce region' has been published in the recent edition of scientific journal Hydrology and Earth System Sciences.
The team comprising V Sriniva-san, S Thompson, K Madhyastha, G Penny, K Jeremiah and Sharachch-andra Lele not only conducted extensive experiments across the basin but painstakingly collected data from various sources to scientifically assess the cause for the decline in inflow to Arkavathy. Their study was centred on popular assumptions such as decrease in rainfall, rising temperature and changing climatic conditions, extensive use of groundwater and large-scale eucalyptus plantation in the catchment area triggering the loss of inflow into the river course.
Probing the prevailing assumptions such as decline in rainfall and evaporation due to excessive heat among others, the team studied long-term rainfall data obtained from rainfall gauges located in Devanahalli, Doddaballapur, Magadi and Nelamangala. The chosen gauges provided daily rainfall data for over 75 years (1934-2010). According to the researchers, with an average of 830 mm rainfall per year and a standard deviation of 210 mm per year, there was no significant change in rainfall pattern. "During the period, high-decadel variability in rainfall proved that the years between 1970 and 1980 were exceptionally wet. Yet, no statistically significant change in rainfall was observed," explained Lele.
Commenting on yet another popular argument that run-off water could have been evaporated due to increasing heat, the team had gone around the basin installing evaporation meters at tanks and lakes and measured evaporation rates during all seasons. "The rise in temperature too was well within the prediction calculated by experts in the past. Accordingly, the temperature rose from 0.6 degree to 1 degree centigrade over the last 100-years. During that time, the potential evapotranspiration (PET) rate too did not show any significant change along the basin. Hence, there is no evidence to support the argument that increasing temperature or climate change has triggered the potential evaporation leading to a decline in stream flow," Veena Srinivasan clarified.
When both populist theories could not be established scientifically, the researchers focussed on the fact that over exploitation of ground water and water consuming plantations such as eucalyptus plantations along the basin could have triggered the decline in inflow. "There have been studies in well-monitored basins that say that groundwater depletion can reduce base-flow contribution to stream flow and thereby reducing overall flow along the basin. As per the data obtained during our field visits, there was considerable shift from dry farming to deep borewell irrigation on a large scale. This was supported by large-scale construction of check dams along the basin helping percolation of run off water. As a result of intensive ground water pumping, the water table plummeted from 50 ft back in 1970 to 600 to 1,000 feet in recent times. Similarly, during the same period, irrigated crop land more than doubled," Lele said.
"The eucalyptus plantations that were once rare now covered almost 20 per cent of the TG Halli catchment, probably contributing to the decline. The 11-sq.km area in 1973 was extended to a whopping 104-sq.km area in 2001. This had caused a loss of 75 to 135 ML per day in the area as against the overall decline in runoff by 320 ML per day. Hence, the eucalyptus plantations and over extraction of groundwater through pumping could have triggered the drying of Arkavathy," yet another researcher explained.
The researchers collectively opined that these hypotheses were not fully understood by either the government or the common man. The team conducted several water literacy meetings at villages educating farmers about the possible cause behind the decline in inflow. "During our interactions, many farmers believed that rainfall has decreased and evidently no tank/lake was filled in the last 20-years. As authorities continue to distinguish between surface water and groundwater, they do not see any link between pumping of groundwater and decline in surface water flow. Our study shows that there is a link between the both and it is critically important to address issues of water scarcity," Lele said.
http://www.bangaloremirror.com/bangalore/others/Demystifying-Arkavathy-fiasco/articleshow/47031374.cms?
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