A farmer from Rajasthan with some of her harvested wheat
In 2022, India will surpass China as the most populous nation, according to a new study on world population projections released by the UN. This rapidly rising population is not only increasing the demand for food supply, but also for the water required to grow it.
India is already the world's largest consumer of groundwater, with the USA following in second place. However, India withdraws more than double the amount of groundwater extracted in the USA. In northwestern India (Rajasthan, Punjab, Haryana, and Delhi) alone, 17.7 cubic kilometers of groundwater storage is estimated to be lost every year [1]. To put this in perspective, if we loaded all this water into train cars, the length of the train would wrap around the earth almost 3 times! This rapid depletion of groundwater resources and rising food demands in India raise an important question pertaining to how much water can be extracted sustainably without impacting food production.
Sustainable water use is defined as "the ability to use water in sufficient quantities and quality from the local to the global scale to meet the needs of humans and ecosystems for the present and the future to sustain life, and to protect humans from the damages brought about by natural and human-caused disasters that affect sustaining life" [2]. Holy cow...sounds ambitious, right? So, if this is what we need to aspire to to achieve sustainable groundwater use, how do we proceed? Well, we need a way to measure it and monitor progress.
Establishing a water balance is challenging every where, but particularly tricky in India. A lack of irrigation metering to measure how much water is extracted is one issue, but also the methods available to measure how much groundwater is replenished is difficult. Many of the established methods to measure groundwater replenishment (or "groundwater recharge") can be cost prohibitive and logistically challenging. This is largely the case because two-thirds of India is underlain by fractured hard rock. This means that water travels through erratic fractures and fissures in the rock that are hard to predict.
One of the most widely used approaches to measuring groundwater recharge is called the Water Table Fluctuation method. The major appeal in using this method is its simplicity. Groundwater recharge is estimated using the Water Table Fluctuation method by measuring the change in groundwater depth before and after the monsoon rains (this can be done in open wells very easily by dropping a disk on a rope into the well and measuring the length of the rope) and multiplying it by a factor called specific yield. Specific yield is a dimensionless factor that characterizes how much water a rock/sediment type can hold in an aquifer. In more simple terms, if we pretended that a half saturated sponge was an aquifer, we could calculate how much water was in the wet portion of the sponge by multiplying the thickness of the wet part of the sponge (change in water table height) by the fraction of the sponge was empty hole space (the specific yield). Easy, right? The only problem with this approach is that we don't always know what the specific yield is. Especially in groundwater stored in hard rock aquifers because the fractures are not consistent throughout.
Last week, a paper I wrote with my fellow colleagues, was published in the Hydrogeology Journal to explore which methods are best for estimating groundwater recharge in fractured hard rock aquifers. In this paper we compared the Water Table Fluctuation method with the Chloride Mass Balance method. One of the major benefits of using the Chloride Mass Balance method is that it is a chemical approach that compares the chloride content in groundwater to that in rainwater. This is possible because chloride dissolves in water and mimics the flow of water. This is a huge bonus for fractured hard rock aquifers, since the amount of chloride you measure in a well would essentially integrate the pathway of water recharged into that well. The major assumption with this method is that chloride acts conservatively in nature and that when water is evaporated away the chloride is left behind. This method works well in groundwater aquifers that are annually replenished by the monsoon.
When comparing the two methods, our analysis demonstrates that recharge rates estimated with the Water Table Fluctuation method were difficult to accurately measure at each and every well. This is because specific yield was not homogenous across a watershed. Specific yield is difficult to measure, especially at the well scale. For example, the specific yield estimated for fractured hard-rock using a global model is based on one value - 2%. If this value is used in the Water Table Fluctuation method, but in reality the specific yield was 1% at a particular well then the amount of groundwater recharge would be overestimated. This is problematic when we are using this method to estimate groundwater renewal, which often is used as an upper limit to how much water can be sustainably withdrawn for consumption.
There are certainly many challenges with both these methods, but being aware of their limitations is a right step forward.
References
[1] Rodell, M., Velicogna, I., & Famiglietti, J. S. (2009). Satellite-based estimates of groundwater depletion in India. Nature, 460(7258), 999–1002. http://doi.org/10.1038/nature08238
[2] Mays, L. W. (2013). Groundwater Resources Sustainability: Past, Present, and Future. Water Resources Management, 27(13), 4409–4424. http://doi.org/10.1007/s11269-013-0436-7