Researchers have developed a new technique that could help monitor groundwater changes more accurately.
In this context, scientists from Potsdam and the United States have made a comparison between gravity field data obtained from the GRACE and GRACE-Follow On satellite missions and other measuring techniques.
The researchers analyzed the seasonal water storage in nearly 250 river basins spread in Asia, the water regime of which is controlled by monsoon. The outcomes made it possible to scale down the large-scale GRACE data to smaller areas. The scientists have published their findings in the Earth and Planetary Science Letters journal.
A better understanding of the storage of underground water is of existential significance for both agriculture and the drinking water supply in several areas. Such reservoirs are refilled by seeping water and precipitation, which consequently feed lakes and rivers and enable rivers to flow in dry seasons.
But measurements are complicated as it is hard to examine the earth, and therefore, one needs to depend on either point values only—through wells and boreholes—or on calculations made from runoff and precipitation data.
Another technique has been used for quantifying groundwater changes since 2002; through the GRACE satellite missions (between 2002 and 2017) and GRACE-Follow On (since 2018), changes in the quantity of water both in and on the earth can be established based on its gravity field signal.
However, this technique also has its drawbacks. Firstly, the changes in mass quantified by the GRACE-FO satellites do not reveal anything about the depth at which the mass is situated—for example, is the level of rivers falling? Do lakes empty at the surface? Or does water drain from deeper layers?
Secondly, the GRACE-FO satellites offer information for relatively large regions of several tens of thousands of square kilometers. At present, the gravity field data cannot be solved more accurately.
In the latest study, scientists from the German Research Centre for Geosciences and a collaborator from the United States are demonstrating how different techniques can be ingeniously integrated to achieve dependable groundwater data even in the case of small river basins.
The researchers investigated monsoon rainfall data and seasonal water storage in nearly 250 river basins spread in Asia. The size of the separate regions changes from one thousand to one million square kilometers. The study covers most of Asia.
The water balance on earth is defined by three key variables—evaporation, surface runoff, and precipitation. The difference between these variables flows out or goes into numerous reservoirs, for example, the groundwater.
Time series of quantifying stations on rivers, or hydrographs, following constant precipitation, indicate normal falling curves (the supposed recession curves), which mirror the clearing of water reservoirs. Fluctuations in groundwater can be predicted from such curves.
Another technique involved the comparison of runoff and precipitation values by the time delay of the runoff; the short-lived intermediate storage leads to a supposed P-Q hysteresis. Here, Q stands for runoff and P indicates precipitation. The size or area of the hysteresis loop can be employed as a measure for the intermediate storage.
At present, the study published in the Earth and Planetary Science Letters journal reveals that the gravity field data of GRACE missions and the P-Q hysteresis are firmly correlated.
As per the study, these two measures reflect seasonal groundwater variations quite well. Consequently, this implies that a combination of GRACE gravity field data and runoff and precipitation data can also be utilized to record groundwater in catchment regions that measure only around 1000 km2 in size.
Schmidt, A. H., et al. (2020) Multiple measures of monsoon-controlled water storage in Asia. Earth and Planetary Science Letters. doi.org/10.1016/j.epsl.2020.116415.