Groundwater and Climate Change

As basic concepts of groundwater have been briefly introduced in the last post, exactly how is groundwater related to climate change and how can climate change influence the entire groundwater system?

Groundwater is estimated to supply 36%, 42% and 27% of the water utilised for domestic, agricultural and industrial purposes respectively. 

An earlier broad-based overview of the topic was built to examine substantial recent advances and I have summarised the examples of groundwater and climate change around the world elaborated by Taylor et al. (2012): 

Figure 3.1: Simplified version of a global groundwater resources map, highlighting the locations of regional aquifers systems

Part 1: Influence of Climate on Groundwater Systems

(1)  Direct: replenishment by recharge

 A global overview of groundwater depletion provided by Wada, Y. et al. (2010) has applied a global hydrological model to assess groundwater recharge and subtract estimates of groundwater abstraction, where groundwater depletion is defined as the rate of groundwater abstraction in excess of natural recharge rate, and spatial variability in modelled recharge is primarily linked to the distribution of global precipitation. Groundwater recharge is strongly affected by climate variability including climate extremes such as floods and droughts. These are often related with modes of climate variability, for instance, the El Niño/Southern Oscillation (ENSO) at multiyear timescales as well as the Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation (AMO) at longer timescales. 
In the table below, I have combined and extracted some specific examples of direct impacts that climate has exerted on groundwater worldwide from Leblanc et al. (2009), Taylor et al. (2012), Taylor et al. (2009) and (Scanlon, B. R. et al., 2005), which I have found representative.

Location	Incident
Recent multi-annual Millennium Drought in Australia	Groundwater storage in the Murray–Darling basin declined substantially and continuously by ~100 ± 35 km3 from 2000 to 2007 in response to a sharp reduction in recharge (Leblanc et al., 2009)
In tropical Africa	In tropical Africa, heavy rainfall has been found to contribute disproportionately to recharge observed in borehole hydrographs (Taylor et al., 2012)
In southwest United States during ENSO years	Incidences of greater (x2.5) winter precipitation give rise to enhanced evapotranspiration from desert bloom that largely consumes the water surplus (Scanlon, B. R. et al., 2005)

Table 3.1

Figure 3.2: Conceptual representation of key interactions between groundwater and climate

(2)  Human Activity and Indirect: changes in groundwater use

In the modern era, land use change (LUC) might influence more on terrestrial hydrology than climate change owing to the expansion of rain-fed and irrigated agriculture. Taylor believed that ‘Increased recharge may not only degrade groundwater quality through the mobilization of salinity in soil profiles but also flush natural contaminants such as arsenic from groundwater systems’.

Some examples around the world have shown both pros and cons of indirect impacts caused by human intervention including irrigation:

Time	Location	Incident
Later half of the 20th century	West African Sahel	LUC from Savannah to cropland àSurface runoff↑ àGroundwater recharge↑ àGroundwater storage↑
Early 20th century	SE Australia and SW United States	LUC from natural ecosystems to rain-fed cropland àGroundwater recharge↑ àGroundwater storage↑ àGroundwater quality↓ (as mobilization of salinity accumulated in unsaturated soil profiles)
21st century	Semi-arid and arid environments in North China Plain, NW India, the US High Plains; humid environments in Brazil and Bangladesh	Irrigation àDepletion of groundwater storage

Table 3.2

(3) Palaeohydrological evidence:

Time	Evidence	Proved Statement
Thousands of years ago	Most of groundwater flowing in large sedimentary aquifers of the central U.S., Australia, southern Africa and North Africa was recharged by precipitation	Long-term responses of groundwater to climate forcing
Before and occasionally during Late Pleistocene glaciation, possibly Early Holocene	Stable isotopes of oxygen and hydrogen & concentrations of noble gases	The recharge occurred under cooler climates (≥5°C cooler)
Current state	Current groundwater recharge rates are responsible for at most a tiny fraction of total groundwater storage, fossil aquifers are strongly dominated rather than recharge-flux dominated	Lifespan of fossil aquifers is determined by the rate of groundwater abstraction relative to exploitable storage

Table 3.3

Part 2: Influence of Groundwater on Climate System

Groundwater interacts with and impacts climate system through soil moisture owing to irrigation and global sea level rise (SLR).

(1)  Groundwater with soil moisture

Water and energy budgets can be influenced by transforming areas from moisture-limited to energy-limited evapotranspiration (ET) through irrigation.
(2) Groundwater with sea level rise
For the last 70 years in the 20th century sea level rise has been estimated as 1.8 mm yr^-1. I personally agree with the speculation that the ascending sea level might have triggered the movement of fresh-saline-water interfaces inland.

Groundwater abstraction is also worth-mentioning. Climate change has been going on for about 150 years due to both natural and human factors, and at least eight urban areas worldwide have suffered from land subsidence resulting from groundwater abstraction and pumping. Decades of ground water extraction witnessed Tokyo sink two metres before the practice was stopped, and parts of Jakarta, Ho Chi Minh City, Bangkok and numerous other coastal urban settlements would descend below sea level unless action was taken. 

Figure 3.3: Comparison of land subsidence in coastal megacities around the world

Most of land subsidence occurring in the coastal megacities globally is due to groundwater abstraction and majority of the groundwater is pumped for as drinking water and for domestic use. These cities are usually situated on thick clayey delta deposits.

We were discussing about sea level rise just now, why am I talking about land subsidence? In fact, sea level rise and land subsidence are the two halves of the same problem! 

Figure 3.4: Sea level rise and land subsidence

Many scientists argue that ‘subsiding land is a bigger immediate problem for the world's coastal cities than sea level rise’, as ground is sinking about 10TIMES FASTER than the water is rising, resulting in a more severe inundation of flood water in those coastal megacities.

I agree with Gilles Erkens that sea level rise and land subsidence are both contributing to larger and longer floods, bigger inundation depth of floods. Moreover, global groundwater depletion derived from both flux-based method and volume-based method suggest that such depletion lead to SLR of 34% or 0.57 ± 0.09 mm yr^-1 versus 23% or 0.4 ± 0.1 mm yr^-1 (Table 3.4).

Table 3.4: Estimates of global- and continental-scale groundwater depletion

Figure 3.5: Flood in Bangkok 2011

ondering all the above, I cannot help thinking, although groundwater depletion worldwide contributes to sea level rise in the same regions, would the amount of water increase via sea level rise be capable and sufficient enough to compensate that of water decline through groundwater depletion? It is still questionable whether these two would be in balance, and is definitely more complicated than a simple plus and minus math problem. Therefore, I will leave this question open and should you hold any unique understandings towards groundwater depletion and land subsidence, you are more than welcome to comment below!

To wrap up, groundwater and climate change interact with each other a lot, in both favourable and unfavourable ways. Human activity, not negligibly, does manipulate groundwater management. In the next post, I will provide a case study of that in Tokyo Japan based on personal research experience there last summer and find out what we could learn from it so as to apply for a potentially more reasonable and appropriate management of global groundwater resources.