3.1.1. Acceleration of evaporation

As mentioned above, the major process in the water cycle is the process of evaporation. In this phase, water in the liquid state is transformed into vapor or gas (Graham et al. 2010). The transformation occurs when some molecules in a water mass have attained sufficient kinetic energy to eject themselves from the water’s surface.

There is a multitude of factors that affect the evaporation rate in the water cycle; they include humidity, temperature, wind speed, and, most importantly, solar radiation (Britannica 2020). Simulations based on one of about 20 coupled models (ocean–atmosphere–land models) estimated that global mean evaporation will experience an increase of about 5.2% by 2050, which is a clear sign of the acceleration of the water cycle (Wetherald, Manabe 2002). Figure 1 represents the changes in meteorological parameters that have occurred in the past few decades around the globe.

Fig. 1.

(a) Global and Hemispheric Monthly Means and Zonal Annual Means based on Land-Ocean Temperature Index or L-OTI (Data source: Lenssen et al. 2019; GISTEMP Team 2023); (b) Water vapor pressures e(T) and e(Td) (Data source: Koutsoyiannis 2020); (c) Specific humidity at 850 hPa and 300 hPa (Data source: Koutsoyiannis 2020); (d) Water vapor amount (Data source: Koutsoyiannis 2020).

3.1.2. Acceleration of precipitation

With the help of precipitation, some amount of water is returned to the atmosphere by evaporation, some is evaporated from the plants by transpiration, some percolates into the soil or ground via infiltration (which may later percolate into streams as groundwater runoff), and the remaining proportion flows directly as surface runoff into the sea (Graham et al. 2010). However, disturbances of all these phenomena related to precipitation are occurring due to the intensification of the water cycle. In this regard, recent modeling studies suggest that due to increased humidity, precipitation would increase by about 3.4% per degree Kelvin. That amount is less than is suggested by the Clausius–Clapyeron relation model that indicates precipitation would increase by about 6.5% because evaporation would be energy-limited as the evapotranspiration rate increases with the rising temperature (Huntington 2005). For instance, Figure 2 shows the precipitation trend (by USEPA) in the contiguous 48 states of the USA from 1960 to 2020, revealing that these states have been experiencing precipitation increase and hydrological cycle acceleration throughout these years. Moreover, based on climatic changes, changes in the precipitation over small islands have also been estimated (Fig. 3).

Fig. 2.

Precipitation in the contiguous 48 States of USA (1960-2020). Data source: USA-EPA (2021b).

Fig. 3.

Projected change in precipitation on small islands (%). Data source: Bates et al. (2008).

Based on the midrange calculation, rainfall rates were 43.096 kg m^-2 s^-1 in January 2000, increasing to 43.902 kg m^-2 s^-1 in January 2010 (hence representing the changes in the hydrological cycle). The opposite trend was shown for July; midrange values were 42.724 kg m^-2 s^-1 (2000), 42.543 kg m^-2 s^-1 (2010), and 42.05 kg m^-2 s^-1 (2020). These patterns demonstrate that the hydrological cycle has been experiencing changes throughout the world, with either less rainfall and more evaporation or more rainfall and less evaporation. In 2020 (during January), in contrast to 2000 and 2010, the high rainfall rates in areas of Asia near India expanded. Moreover, as compared to 2000 and 2010, the South American region receiving a high rainfall rate expanded in July 2020.

3.3.1. Increase in water availability due to acceleration of the hydrological cycle

As mentioned previously, hydrological changes (including acceleration of the hydrological cycle) can have positive impacts in terms of water availability. For instance, intensification of precipitation leads to an upsurge in annual runoff, which further increases renewable water resources (if the water is captured and stored in the waterbodies) (Magrin et al. 2005). In regions of the world with adequate infrastructure, this stored water can be beneficial for irrigation. Southern South America is an example, where the flooded area has increased, and crop yields have improved in the Pampas region of Argentina. The increase in flooded land has also provided new opportunities for fishing in the area (Magrin et al. 2005).

3.3.2. Decrease in water availability due to acceleration of the hydrological cycle

Overall, the climate-related projections indicate that the more prevalent impact of intensification of the water cycle will be adverse in the context of decreasing water availability (Bates et al. 2008). For instance, a study conducted in the Llobregat River Basin in Catalonia, northeastern Spain, evaluated the relationships between climate change scenarios and impacts on water availability in the region. Climate change scenarios and hydro-logical modeling were combined to estimate water availability in the future. AEMET (The Agencia Estatal de Meteorologia) and SMC (The Meteorological Service of Catalonia), the Spanish weather agencies, provided the data for climate projections. The results based on AEMET climate scenarios indicated that there would be a decrease of 10% in the annual volume of water available in the future. The results obtained based on SMC scenarios were even more daunting because they revealed the possibility of a 40% decrease in the annual volume of water available in the future due to climate changes (Versini et al. 2016). Figure 6 depicts those factors of the hydrological cycle’s acceleration that adversely influence water availability over land.

Fig. 6.

Factors of the hydrological cycle’s acceleration that can cause water unavailability.

3.3.3. Frequent floods

The acceleration of the water cycle has the potential to increase floods. In the 21st century, it is projected that the events of heavy precipitation or intense rainfalls will become more frequent in most of the regions around the globe. This dynamic will eventually result in frequent events of flash flooding (Bates et al. 2008). Moreover, the results of a study based on multi-model analysis projected an upsurge in the risk of very wet winters in central and northern Europe. The reason for this is the intensification of precipitation. For example, it is projected that boreal winter precipitation is prone to increase about five- to seven-fold with likely winter flood hazards. It is also projected that the wet monsoon season will be prolonged and more intense in Asian regions (Palmer, Räisänen 2002). For instance, according to climate models, along with a 2°C rise in global temperature, the flooded area of Bangladesh will increase by 23-29% (Mirza 2003). It is also projected that a traditionally 100-year flood will return more frequently (every 2-5 years) (Zhang et al. 2003). These scenarios of frequent flash floods are mainly negative for those regions of the world that lack the proper infrastructure for water storage. Therefore, such regions are expected to face declining water availability despite experiencing frequent precipitation. Intense floods and runoff often do not fill or recharge water reservoirs, and ultimately the discharge mixes with oceanic water, making it unavailable for use – another negative influence on water availability (Bates et al. 2008). It is estimated that 913 floods have occurred in the world just in the span of 2000-2018, and about 300 million people have been affected by 21st-century floods (Cavallito 2021).

Furthermore, it is crucial to understand flood frequency analysis, because flood frequency indicates the probability of floods occurring at a regional scale in both gauged and ungauged catchments. Flood frequency analysis also helps to assess the risks associated with floods, and to plan appropriate flood mitigation measures. For instance, some studies have used statistical methods for the estimation of flood frequency based on historical data (Acharya, Joshi 2020; Samantaray, Sahoo 2020). These studies provide a clear understanding of flood dynamics in diverse geographical regions. Hence, they provide clarity regarding the intensity and variability of flood incidences from small to larger scales.

Figure 7 represents the statistics given by the US-EPA about the one-day extreme/heavy precipitation events in the contiguous 48 states of the USA from 1960 to 2020. First of all this index shows that 2000-2010 and 2010-2020 decades had prominent difference in terms of precipitation. Secondly, it represents that 2015 was the year when the precipitation in USA was maximum as compared to the rest of the included years.

Fig. 7.

Extreme One-Day Precipitation Events in the Contiguous 48 States of USA (1960-2020). Data source: USA-EPA (2021a).

From the analysis of the flood events from 1973 to 2002, it was found that the proportion of floods among all disasters was 33%. During this time frame, most of the floods (40%) occurred in Asian countries. Moreover, within Asia, most of the flood events occurred in South Asia (Fig. 8) (Dutta, Herath 2004).

Fig. 8.

(a) Flood percentages in the world. (b) Flood percentages in different regions of Asia, Data source: Dutta, Herath (2004).

3.3.4. Disturbance of water quality

Increases in precipitation intensity are expected to cause many forms of water pollution. These include the amplification of nutrients, sediments, salt, dissolved organic carbon, pesticides, pathogens, and thermal pollution. There will be concomitant increases in algal blooms and fungal content in water (Bates et al. 2008). The increased precipitation has the potential to elevate turbidity levels in surface-water sources. For example, heavy precipitation events raised turbidity levels up to a factor of 100 in some of the main reservoirs of New York City, compromising the city’s water resources (Miller, Yates 2006). The quality of water is also degraded by flood water because the flowing flood water (having sediments and contaminants) gets mixed with the clean water of existing reservoirs. Additionally, with increased evapotranspiration, shallow groundwater is prone to salinization in semi-arid and arid areas. Plus, due to decreasing streamflow in semi-arid areas, there is a strong probability of increases in the salinity of rivers and estuaries. For instance, in the headwaters of Australia’s Murray-Darling Basin, salinity levels are projected to rise by 13-19% by 2050 (Pittock 2003). Increasing salinity will be accompanied by increasing concentrations of pollutants, such as those mentioned previously, making the water unsuitable for human uses, ultimately decreasing the availability of clean and safe water (Fig. 9) (Bates et al. 2008).

Fig. 9.

Flow chart depicting the process of water contamination.

3.3.5. Extensive droughts

On one hand, intense evaporation causes heavy precipitation and floods in some regions of the world. On the other hand, this rapid evaporation makes some regions dry and causes prolonged droughts (Bates et al. 2008). Based on a single-model study of the frequency of drought around the globe, it is projected through various models that the frequency and duration of droughts will increase in the coming decades in many regions of the world. In this regard, the SRES A2 scenario has projected that by the decade of 2090, there will be an increase of 10-30-fold in land experiencing extreme drought, and a 2-fold increase in global drought frequency and mean duration of drought (Burke et al. 2006). It is also projected that by the 2070s, a 100-year drought under today’s conditions will return more frequently (10 years) in various countries, including France, Spain, Portugal, Poland, and Turkey (Lehner et al. 2005). These statistics make it clear that areas that are already facing or will face prolonged drought conditions will ultimately face the issue of lacking available water (Bates et al. 2008). Figure 10 represents the statistical analysis by the US-EPA of drought in the contiguous 48 states of the USA from 1960 to 2020.

Fig. 10.

Average Drought Conditions in the Contiguous 48 States of USA (1960-2020). Data source: US-EPA (2021c).

Prominent floods have been occurring in Central Europe, South Asia, East Asia, Africa, Oceania and South America (You et al. 2016; Ansah et al. 2020; Chhetri et al. 2020; Mu et al. 2020; Nanditha et al. 2023). Meanwhile, extensive droughts have been taking place in various regions around the world, including Central Asia, South Asia, North America, Oceania, South America, and Africa (Schwalm et al. 2012; Nicholson 2014; Yuan et al. 2018; Garreaud et al. 2020; Tan et al. 2022). Moreover, water contamination, including reduction of dissolved oxygen(DO), rise in temperature, and increase in phosphorus, pathogen contamination, etc., exacerbates water scarcity in the water resources of various regions of the world (Li et al. 2015; Carpenter et al. 2018; Shih et al. 2021; Zhu et al. 2021). The impacts of hydrological cycle acceleration on water availability have been observed and analyzed by many researchers around the world; instances of which are mentioned in Table 1.

3.3.6. Disturbances in groundwater reservoirs

Groundwater is almost 30% of global freshwater, which means that it has a large share of the water usable for human beings. Moreover, the most accessible groundwater is found within 1 km of the surface of the Earth. Groundwater availability and quality decrease below this depth due to high concentrations of minerals (Brands et al. 2016). A very valuable attribute of groundwater aquifers is that they can store water without evaporation losses and in stable condition. Moreover, as water percolates through the ground and soil, the groundwater becomes de-contaminated (Turral et al. 2011). Almost 24% of the world’s groundwater aquifers are already over-exploited, indicating that any further disturbance in groundwater zones will significantly decrease fresh water’s availability (Gleeson et al. 2012).

Disturbance and acceleration in the hydrological cycle have adverse impacts on the depth of the groundwater table and recharge rates (Fig. 11). Because many groundwater reservoirs are recharged from precipitation and surface water flow, any change in these processes eventually affects groundwater (Bates et al. 2008). One of the main reasons for the adverse impact on the amount of groundwater is that rainfall has become more flashy (intense) and less predictable in many areas of the world (Brands et al. 2016). Therefore, according to the aforementioned phenomenon of precipitation increase resulting from acceleration of the water cycle, the upsurge in the intensification of rainfall affects the infiltration capacity of the soil, which eventually decreases the groundwater recharge (Bates et al. 2008). However, this is mostly happening in humid areas. In arid and semi-arid regions of the world, on the other hand, more intense rainfall causes fast infiltration and percolation through the ground before evaporating. Plus, alluvial aquifers are mostly recharged by the overflows of floods. Overall, it can be observed that the acceleration of the water cycle that affects groundwater reservoirs in any form ultimately disturbs the very important resources of fresh water present on Earth. Hence, any influence on groundwater influences overall water availability (Bates et al. 2008).

Fig. 11.

Groundwater vulnerability to climate change. Data source: Turral et al. (2011).