The Irish Meteorological Service Online
Felt a bit heavy this winter? It's partly due to water retention.
Where does the all the rain go? Some of it is returned to the atmosphere though evaporation, some will run into rivers to be recycled into the oceans but some will penetrate into soils or percolate deeper into underground aquifers to stay with us a bit longer.
Figure 1: A diagram of the hydrologic cycle. Gravity fluctuations correlate with variations in the density of the land surface below. These variations can be used to track water movement. Source: NASA Goddard Space Flight Center.
The extra mass spread over and under the land following a wet season provides an additional gravitational pull on all masses on the surface. It's a subtle, very gentle, additional tug that makes us all weigh slightly more in a wet season compared to a dry season. The effect is small but measuring these small shifts in the distribution of masses around the Earth underpins the Gravity Recovery and Climate Experiment (GRACE) satellite mission.
Figure 2: Illustration of the two GRACE spacecraft. Source: NASA Goddard Space Flight Center.
Launched in 2002, GRACE consists of two platforms approximately 220 kilometres apart but capable of detecting changes in that distance as fine as the thickness of a human hair. As the satellites orbit Earth any changes in local gravity will alter the separation; as the first satellite passes over a gravity anomaly it speeds up very slightly while the second one is initially unaffected until it follows the path of the first. This information can be used to measure changes in gravity, which can be linked to currents in the ocean, runoff and ground water storage, melting ice sheets or glaciers, and changes wrought by earthquakes.
For example, before the year 2000 the Earth's spin axis was drifting towards Canada but since then it has been heading south towards the UK at a rate of about 17 centimetres per year. Using data from GRACE, scientists have suggested that changes in terrestrial water storage, including the melting of the Greenland ice sheet, may be the cause.
Figure 3: Monthly mass anomaly over Ireland, expressed in centimetres of equivalent water thickness. Based on average values extracted over Irish land grid cells from the scaled GRCTellus Land Data.Global data are available at http://grace.jpl.nasa.gov supported by the NASA MEaSUREs Program.
The seasonal changes in gravity over Ireland (presented as equivalent water depth), measured by GRACE are shown in Figure 3; the changes reflect terrestrial water storage anomalies (in aquifers, river basins, etc.). Note the spike in December 2015 when we had record rainfall amounts. In general the GRACE data for Ireland show some correlation with the Met Éireann monthly rainfall totals but the agreement is not perfect because rainfall, and the runoff and storage of water in underground aquifers, will have different timescales.
How much extra would the average Irish citizen have weighed this winter following the exceptionally heavy rain and flooding we experienced in December 2015? It's a tricky question to answer as GRACE measures the changes at the satellite position, not at ground level. A rough calculation* suggests that the extra water beneath our feet increased the weight of a person by ... a fraction of a milligram. This tiny amount would be completely masked by the daily fluctuations in body weight associated with food intake and exercise. Of course, body weight is more substantially and directly influenced by weather and climate, in response to changes in humidity and temperature. But that's a story for another day.
* Assuming a large disk of water ~5 centimetres in thickness, lying beneath Ireland.
For more information about GRACE see:
For more information on the movement of the Earth’s pole see:
For more information on the monthly mass changes over land, from which data the graph for Ireland (Figure 2) was produced, see:
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