CHAPTER 5 Effects of Inter-Basin Water Transfer and Cascading Reservoirs on Streamflow of
5.6.2 Effect of all dams
The effect of all dams and IBWT was analyzed by comparing the SWAT simulation under different scenarios. In the first scenario (All Dams), all the reservoirs are considered to be active including the water transfer from the Umiam Reservoir and thus represents the present condition.
The second scenario (IBWT-No dams) considers IBWT from the Umiam Dam but do not consider any dams in the Umtru Watershed. It is to be noted that IBWT from Umiam Dam is considered as inflow series from outside of the watershed and Umiam Dam is not depicted in the current Umtru model. The third scenario (No IBWT-No Dams) does not consider IBWT as well as dams, thus representing unaltered flow. All three scenarios were simulated from 1990 to 2015.
Figure 5.12 Effect of cascading reservoirs under different scenarios
The monthly flows under the three scenarios described above are shown in Figure 5.12. The median of discharges for the month of January differed only by 0.12 m3/s when All dams scenario (8.979 m3/s) and IBWT-No dams scenarios (9.099 m3/s) were compared while No IBWT-No Dams scenario had median value of 1.23 m3/s. Similarly, for the month of February to April the difference in median flow rates between the first two scenarios does not reach 1 m3/s. The variation in flow increases from May onwards. Highest median flow occurs in July which are 58.69 m3/s for All dams, 39.49 m3/s for IBWT-No dams and 30.02 m3/s for No IBWT-No Dams scenario. The higher discharge in case of All dams scenario may be due to the storage of water in the reservoir.
Figure 5.13 Hydrological alteration for different scenarios
Figure 5.13 shows the hydrological alterations on monthly flows calculated by IHA software. The positive alterations ranging from 0.2 to 0.4 are seen in case of Kyrdemkulai, Nongmahir and Umiam Stage IV reservoir that occurs in May and June. When all the reservoirs are considered (All Dams with IBWT) and compared with No IBWT-No Dams scenario, the positive alterations are seen in February and from July to November.
When No IBWT-No Dams was compared with All Dams without IBWT, negative alterations were highest in the month of March and August. Thus, having IBWT is beneficial in maintaining higher flow rate in the Umtru River.
Figure 5.14 Hydrological alteration of magnitude and duration of annual extreme water conditions
The parameters related to magnitude and duration of annual extreme water conditions has significant role in structuring of river channel morphology and physical habitat conditions (Figure 5.14 ). The analysis revealed positive change due to Kyrdemkulai and Nongmahir dam for 1-day, 3-day, 7-day, 30-day and 90-day maximum flow. Stage IV reservoir shows negative alterations in 7 out of 12 parameters. When All Dams with IBWT is considered, the 1-day, 3-day, 7-day, 30- day and 90-day minimum flows show positive alteration ranging from 0 to 0.5. Whereas, maximum flows show negative alteration. When all dams are considered without IBWT, the hydrological alteration tends towards negativity for the minimum flows but shows positive alteration for 30-day maximum flow.
The effect of individual and cumulative effects of all reservoirs under different scenarios are presented in Figure 5.15. It is observed that Umtru Reservoir has the highest influence on yearly median flows with change percentage of 106.8%. The next highest effect is when all dams are operated without IBWT which can change yearly median flows by 95%. The effect is lowest when all dams are operating with the IBWT in place (38.49%).
Figure 5.15 Individual and cumulative effects of reservoirs under different scenarios
Conclusions
The effect of individual reservoirs and IBWT was analysed in this chapter. The analysis revealed increase in monthly flows due to Kyrdemkulai, Nongmahir, Umiam Stage IV and Umtru Reservoirs. The individual effect of Umtru Reservoir was found to be highest among the four.
The second highest effect was due to Nongmahir Reservoir. The cumulative effect of all reservoirs could increase the yearly median flows by 38.49% as compared to unaltered flow without IBWT and reservoirs. The change in yearly median flows were 95.43% in Umtru Basin if no water was transferred from Umiam Basin. There was increase in 1-day, 3-day, 7-day, 30- day and 90-day maximum flow due to Kyrdemkulai and Nongmahir dams.
0 10 20 30 40 50 60 70 80 90 100 110 120
Kyrdemkulai Reservoir
Nongmahir Reservoir
Umiam Stage-IV Reservoir
Umtru Reservoir All Dams Without IBWT
All Dams With IBWT
Change in Yearly Median Flows (%)
Introduction
According to the IPCC (2018), climate change in the latter half of the 21st century will have a significant impact on human lives, assets and ecosystems from extreme weather events such as heat waves, extreme precipitation, drought and associated wildfires, and coastal flooding. Among these calamities, floods and droughts are the most damaging natural phenomenon that causes loss of life, degrades the economy, nature, and property worldwide. With the consistent rise in the level of greenhouse gases, the downwelling infrared radiation gets enhanced, resulting in increased global mean temperatures. It has been observed that global mean temperatures have been increasing since the pre-industrial period and 2015-2019 recorded the warmest period since records began in 1850 (World Meteorology Organization 2019). The rise in temperature enhances evaporation and increases the moisture holding capacity of the air by 7% per 1°C rise in temperature (Trenberth 2011). Increase in atmospheric moisture can cause more intense precipitation and increases the risks of floods. For instance, Lutz et al., (2014) projected an increase in runoff over upper Ganges, Brahmaputra, Salween, Indus, and Mekong basins in Asia due to increased precipitation and ice melt. Recent analysis of rainfall trends on a global scale show an increase in the frequency of extreme events in many parts of the world (Myhre et al.
2019; Papalexiou and Montanari 2019). Increasing heavy rainfall events have been associated with increased flood occurrences in India in the past few decades (Singh and Kumar 2013).
Arnell, (2004) observed that the increase in runoff due to climate change generally takes place during the high flow season and thus increase the risk of flooding. Since climate change can induce changes in the extreme meteorological events rather than the averages, the risk of droughts in addition to floods is also a major concern (Mishra and Singh 2010). In contrast to floods, all climatic zones irrespective of rainfall patterns can experience drought characterised by the