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Old Groundwater Keeps Plants Fresh During Drought in the Eastern Sierra Nevada

Plants rely on rainfall and groundwater to stay alive, with the relative proportion of each varying according to different factors such as rainfall seasonality and rooting depth. In periods of drought, plants must increase their dependency on groundwater; however, not all groundwater is the same. Some groundwater systems are recharged by recent precipitation, while others continue to transmit water from storms that happened several decades or even centuries in the past. In a 2021 paper published in Environmental Research Letters, Zach Meyers and his colleagues sought to answer an important question: do plants nourished by old groundwater do better than those dependent on young groundwater? They hypothesized that the former would do better during periods of drought because the old groundwater source would be less affected by the current climatic conditions and, thus, would vary less and be more consistent. From 2011-2017 California experienced its worst 6-year drought on record, providing the authors with a golden opportunity to test this idea in spring-fed areas along the eastern slope of the Sierra Nevada.

Google Earth image of two springs (yellow pins) in the study area.

The authors used Landsat imagery to gauge the health of the vegetation by measuring the 'greenness' of the leaves. To estimate the age of the groundwater, they measured tritium and chlorine-36 isotope concentrations. Because these isotopes are formed and injected into the atmosphere during nuclear explosions, they can be used as a marker for the 1950's and 1960's, a period of time when nuclear testing was at a peak. Therefore, groundwater mainly recharged by storms that happened before this nuclear fallout will have low concentrations of these isotopes, whereas groundwater recharged by more recent storms will have higher concentrations.

A 1951 nuclear bomb test, near Las Vegas, creating and propelling tritium and chlorine-36 isotopes into the atmosphere. (Time Life Pictures/National Archives/Getty Images)

After comparing the health of the vegetation and the age of the spring water, the authors found that the plants in areas near springs fed by old groundwater were not affected by the drought conditions. In contrast, the health of the plants reliant on springs fed by young groundwater steadily worsened as the drought progressed. These results are nicely illustrated in the plots below (their Figure 3).

On both plots, the x-axis is 'Time' and the y-axis, NDVI, is a measure of plant health determined from the Landsat imagery. The data for this plot are from an old-groundwater area: the flat line shows that the health of the plants did not change throughout the drought. 
 
On the other hand, the data from a young-groundwater area (below) show a decline in plant health as the drought progressed.


One of the interesting insights from this study is that the resistance of water-limited ecosystems to drought is dependent on the ability of the groundwater system to buffer against short-term climatic perturbations.

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