The impact of global warming on the hydrological cycle should be of paramount concern to all because global warming affects rainfall patterns in various ways like triggering more extreme rainfall events. Unpredictable changes in runoff make it difficult to plan infrastructure to manage water resources such as dams.
How do human disturbances affect hydrological resilience of catchments in a warming climate?
For the first time since independence, India’s urban population grew faster than its rural population from 2001 to 2011. This led to large changes in land use and land cover (LULC) in recent years. Humans have affected the hydrological cycle by using up groundwater and changing the land use patterns through various activities like deforestation.
A team of researchers from the Indian Institute of Technology at Guwahati and Indore wanted to find out how human activities such as urbanisation contribute to changes in runoff in Indian catchments and how they affect the resilience of catchments. To do that, they first classified catchments according to whether they are dominated by human activities or controlled by climate. Then they analysed which catchments were resilient to climate warming.
The team applied the “Budyko framework” to formulate an equation, which was then used to calculate the relative contributions of climate variability and human-generated stress to changes in runoff. The Budyko framework formulates a curve, called the Budyko Curve, that describes the partitioning of precipitation to stream flow and actual evapotranspiration. When rain hits the ground, it is divided into runoff and “actual evapotranspiration”—the amount of water actually transferred into the atmosphere by both evaporation and transpiration from plants. According to the framework, as the air becomes drier, the portion of precipitation that is partitioned into actual evapotranspiration increases and runoff decreases.
Quantifying resilience
The meteorological and runoff data during 1988 to 2011 was collected for 55 Indian catchments across 17 river basins comprising 13.25 percent of the landmass (except for the Indus, Ganges, and the Brahmaputra due to data unavailability). The contribution of human activities to changes in runoff was calculated by taking the difference in runoff between two time periods—1988 to 1997 and 2001 to 2011. The first time period was considered as the baseline period with low human impact whereas the second period was treated as the assessment period when urbanisation rose rapidly.
“If climatic variability has resulted in more change in runoff (more contribution in runoff change), then the catchment is termed as climatic-dominated. Similarly, more change in runoff from anthropogenic [human] stress means the catchment is anthropogenic-dominated,” explains Manish Kumar Goyal, senior author of the study and associate professor at IIT Indore.
Using the Budyko-derived equation, they constructed two indices to measure resilience to warming—dynamic deviation and modified elasticity. “Dynamic deviation simply tells us how differently the catchment has responded from what the Budyko Curve suggests in the warm period,” says Goyal.
The Budyko Curve is the relationship between the ratio of actual evapotranspiration to precipitation and the ratio of potential evapotranspiration to precipitation, explains Goyal. It is considered to be the stable equilibrium state. If there is a higher deviation from the Budyko reference curve, then the catchment is not resilient.
“Modified elasticity is the measure of the ability of a catchment to maintain the precipitation partitioning consistent with the Budyko Curve following warming,” explains Goyal. If catchments are able to maintain the partitioning process suggested by the curve when they go through the warm period, they are then elastic in nature.
Humans reducing catchment resilience
The researchers found that 35 of the total catchments studied were dominated by human activities. Out of the 55 catchments studied, 23 showed hydrological resilience of which 13 were human-dominated.
Most of the catchments that were human-dominated did not follow the Budyko Curve. Only 37 percent of the human-dominated catchments were resilient whereas 59 percent of the climate-dominated catchments had resilience attributes. Among the resilient catchments, most were in the eastern and the upper southern part of India whereas non-resilient ones were located in the western and the extreme southern part of the country.
The researchers believe that human activities such as “surface water withdrawals, sub-surface withdrawals, deforestation and urbanisation” have pushed the response of the catchments away from the expected hydrological functioning as stated by the Budyko Curve, reducing their resilience.
Goyal cautions that although the study considers climatic variability and human-induced stress as factors that are independent of each other, in the real world, they are dependent on each other.
Sharachchandra Lele, a distinguished fellow at the Ashoka Trust For Research in Ecology and the Environment in Bengaluru, says that within the Budyko framework this study is rigorous and interesting. But since “resilience is a social construct, and is always related to the temporal behaviour of an attribute that is socially desirable,” he says, it is unclear why staying at the Budyko Curve is desirable.
He also points out that there are there are many kinds of human-caused changes such as “urbanisation (that increases flows), groundwater overexploitation (that decreases flows) or dams that may drastically decrease (if for irrigation) or completely change the seasonal pattern (if for hydropower).” So, clubbing all human interventions into one category may not be very useful, he feels. “We need to understand what is the nature of human actions in the catchment and how they influence not just flows but all uses and users in the catchment.”
The findings highlight the need for sustainable development and proper watershed management practices, says Goyal. In the next step, the team hopes to pinpoint specific human activities to characterise how they affect the hydrology of catchments.
The paper can be accessed here.