**3.1 Floods**

Floods are not only the excess flow of water in rivers during rainy season, but also the flash floods which are induced by changes in rainfall pattern and temperature rise. Such flash floods are expected more frequently in the mountainous regions as a consequence of convective activity during monsoon, collapse of natural ice or debris deposited due to landslides or sudden subsidence of glacial lake. The global warming has accelerated the melting of glaciers in Himalayas and number and size of glacial lakes have increased [5, 23]. The outburst of the glacial lake is anticipated due to moraine deposition, and flash floods play havoc to such conditions. It is disastrous for thousands of people living downstream and are capable of paralysing hydropower projects [24].

The more common riverine floods are still a disaster in many parts of the world and climate change has increased its probability. It has long-term effects on land-use, economy, and development of the effected region. Naturally, the floods are essential feature of a river life cycle and is highly required for the nutrients carried down through silt and water, which is beneficial for the crops like rice. However, the increased in the frequency and intensity of rainfall has resulted in longer flood duration and greater spatial extent. The frequent flooding damage crops, delays cropping cycles, spreads infectious diseases and damages properties and land.

The sea-level rise due to warming of atmosphere and increased frequency of storms in sea has aggravated the coastal floods making them more damaging. The low-lying lands get completely submerged everytime and the adjoining coastal lands face severe agricultural losses. The frequency and severity of coastal storms has increased, and cyclone have become more common, posing a serious threat to the coastal regions. The subsidence and retreating of delta are other consequences due to increase in silt load of the rivers in the coastal region [5, 18].

## **3.2 Droughts**

Increasing water demand and increasing water scarcity in areas due to climate change have made regions even more vulnerable to droughts [24, 25]. The reduction in groundwater recharge is more prominent due to altered flow and changed rainfall patterns. The discharge in the rivers which are snow-fed will face huge changes in pattern as the peak season discharge will be smoothed throughout the year and extended in the early part of the year. Hence during the peak summer season less water will be available, along with increased rate of evaporation, for irrigation and hydropower projects [26]. This problem will increase as the temperatures increases, owing to enhanced evapotranspiration, increased water demand by crops and greater risk of drought stress, siltation of reservoirs and reducing storage capacities of the reservoirs [27].

#### **3.3 Groundwater**

Groundwater is the major source of freshwater, estimating up to one third of the total freshwater withdrawals, and is crucial for agriculture, industry as well as domestic requirement [28]. The groundwater also sustains the baseflow of surface waters such as rivers, lakes and wetlands and help them to survive during low or no rainfall period. On the other hand, the water table is maintained by percolation of rainwater and leakage from surface water such as lakes, streams and wetlands.

Hence, the groundwater recharge is highly dependent upon climate, land cover, terrain and geology of the region. The amount of precipitation and evapotranspiration is decided by the climate and vegetation cover, and the water percolation in the soil also depends upon the type of soil and underlying rocks [29]. It is the surplus water after run-off and evaporation, which enters the repository of ground water. At higher altitudes, the spatial distribution of snow and ice is largely affected by climate change, less accumulation of snow and earlier melting. In winter season this effect causes more precipitation and increased frequency of rainfall plus snow events. The overall impact is that the recharge regime of the groundwater is changed in this era of water scarcity and global warming, and it does not get replenished with sufficient water [30, 31].

Groundwater storage is the prime source of freshwater which has been stored since decades and is one of the cleanest sources of water. The reserves of groundwater act as buffer for the surface water supplies and provide a sustainable flow in surface water bodies under climate variabilities, however, there is a limit to their natural resilience due to withdrawal exceeding the recharge rate [32]. The recharge is in areas which allow inflow by infiltration of excess rainwater and surface water bodies, also includes irrigation practices in agriculture, water seepage of wastewater or storage water. Natural outflows include discharges in springs, wetlands and lagoons, most prominently extraction through borewells. The balance between inflow and outflow in groundwater determines the maintenance of the water table. The sustainability of groundwater depends upon the balance between its recharge and discharge; climate change would affect this balance in case of increasing aridity and overexploitation, decreasing the recharge rate. The lesser and more intensive rainfall may increase the water table of an area, even higher than their previous level, and damage the crop and property. It will also deplete the soil moisture, cause soil compaction, soil erosion and will further reduce the infiltration capacity and groundwater recharge.

Another important consequence is saline water intrusion in coastal aquifers and reduced flow in delta region. Many coastal countries are at the risk of climaterelated groundwater contamination [1]. As the sea-level rise, the aquifers and wells are affected by saltwater intrusion and contaminate the drinking water source [33]. Extensive pumping of groundwater due to increase water demand in warm temperatures, accelerates the pollution of aquifers through lateral movements of saltwater or contaminants. The geogenic contaminants including arsenic, uranium, fluoride [34, 35] can be hazardous for living beings. Agricultural additives, fertilisers and pesticides also infiltrate to the groundwater and make it toxic. Domestic wastewater and other industrial effluents are increasingly reaching the groundwater and changing its quality. This will further deteriorate the situation as it will lead to excessive pumping from tube wells and water withdrawal from upstream rivers. This phenomenon of sea-level rise, depleting aquifers, increased water demand will accelerate the groundwater pollution and is already happening in many parts of the world [36, 37].
