Australian researchers develop model to estimate groundwater recharge more accurately

Cities like Bengaluru, Delhi, Chennai, Mumbai are reeling under severe groundwater crisis for not managing their underground water resource. The situation is similar in other urban areas. Recently, Bengaluru announced the use of artificial intelligence to monitor their groundwater sources. The need of the hour is understanding the groundwater recharge rate, so that the cities can plan for the management of their groundwater.

A study published in the Hydrology and Earth System Sciences journal of European Geoscience Union (EGU) in April 2024 talked about the estimation of groundwater recharge at a country level. The study was led by Stephen Lee from the Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, Australia. 

The study has used 200,000 groundwater chloride measurements to estimate groundwater recharge rates using chloride mass balance (CMB) method across Australia. CMB is used to quantify the recharge rates for groundwater wells in a dry environment using the chloride content of both precipitation and groundwater. Researchers clearly indicate that the factors which influence the groundwater recharge rate are both climate and vegetation related. The climate-related variable include the rainfall distribution, evapo-transpiration and, whereas, the vegetation-related factors include the health and density of the vegetation. 

Soil properties and the geographic variation also influence the rate of groundwater recharge but can be ranked as low influencers, according to the researchers. The study established that it is very important for any modelling of groundwater recharge rates to consider the change in land-use pattern. At times, the land-use pattern due to high rate of urbanisation may increase or decrease the groundwater recharge rate. Bengaluru is a glaring example where the areas of recharge like the green spaces and waterbodies reduced at the expense of built up area growth to 93 per cent in 2020 from from 8 per cent in 1973. The city is facing acute groundwater crisis, with the groundwater levels declining every year. 

Compared to previous studies that also estimated the groundwater recharge rate, the new research used “improved chloride deposition map with enhanced data and spatial coverage” which talked about more precision, the authors of the report claimed. 

The study analysed the proportion and mean recharge in different categories of aridity in Australia. The proportion of recharge was maximum for the semi-arid category (58.33 per cent) and the mean recharge was maximum for the humid category with 203.4 millimetres per year. 

The output of this study was a gridded map for groundwater recharge. This will help to estimate the recharge rare where data are scarce or absent, said the researchers, adding: 

Our study produced long-term recharge maps of the Australian continent. While Australian recharge maps have been produced previously, this is the first time that a model of such scale has been developed from recharge estimates derived from only a single recharge estimation technique. 

“Furthermore, by providing the Python code, point estimates and gridded map, we facilitate a transparent and reproducible workflow that enables the broader community to utilize our methodology or further improve the approach,” the authors of the report explained. The chloride data used as a basis of estimation of groundwater recharge can also be used any other place in the world, they added.

Nandakumaran P, former chairman, Central Ground Water Board (CGWB), explained that few studies attempting to estimate groundwater recharge rates using CMB have been taken up in India. Studies carried out up as part of the groundwater recharge project in Gujarat by CGWB with international collaboration is one of such example. 

Similar studies have been carried out by individual researchers in parts of Rajasthan and Jharkhand as well. However, analyses of gridded chloride deposition, runoff and precipitation datasets, of this scale mentioned in the study have so far been attempted in the country so far. 

In India, groundwater recharge rates for resource assessment on a regional scale are most commonly computed using the Water Table Fluctuation method, which estimates groundwater recharge by studying changes in water levels in monitoring wells. It works on the assumption that an increase in the level of water table in shallow wells is due to water being added to the aquifer from above.

Plunging groundwater levels is quite evident with the escalating rate of urbanisation. To plan for the groundwater recharge at the city level, the first step should be estimation of the recharge rates. This means that the city authorities should explore different scientific methods to accurately measure the groundwater recharge. Once this is done, then only can a city develop a plan for managing groundwater, focusing recharge techniques at places where the rate of recharge is maximum.