The Improved Hydrological Gravity Model for Moxa Observatory, Germany
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The gravity variations observed by the superconducting gravimeter (SG) CD-034 at Moxa Geodynamic Observatory/Germany were compared with the GRACE results some years ago. The combination of a local hydrological model of a catchment area with a 3D-gravimetric model had been applied successfully for correcting the SG record of Moxa which is especially necessary due to the strong topography nearest to the SG location. Now, the models have been corrected and improved considerably by inserting several details in the very near surrounding. Mainly these are: the observatory building is inserted with the roof covered by a soil layer above the gravity sensor where humidity is varying, snow is placed on top of the roof and on topography (steep slope), and ground water is taken into account, additionally. The result is that the comparison of the corrected gravity residuals with gravity variations of the satellite mission GRACE, now using RL5 data, shows higher agreement, not only in amplitude but also the formerly apparent phase shift is obviously not realistic. The agreement between terrestrial gravity variations (SG) and the GRACE data is improved considerably which is discussed widely.
KeywordsSuperconducting gravimeter hydrology 3D gravity modeling local effects
Hydrological effects in gravity observations have been a subject of discussion for several decades, since the precision and repeatability is in the range of µGal. Repeated gravity campaigns, time lapse gravity, as well as time series are affected. One of the first reporting this effect, especially of soil moisture, was Bonatz (1967). Lambert and Beaumont (1977) found effects of ground water variations when seeking for tectonic gravity signals. Elstner et al. (1978) already used soil moisture measurements for estimation and correction of the effect of some 5 µGal (50 nm/s2). While at times, precipitation data had been used for statistical estimations, Mäkinen and Tattari (1988) predicted separately soil moisture and ground water effects applying the Bouguer plate model and could verify the calculated effect with repeated gravity observations. Over years, also statistical methods have been investigated, e.g. Braitenberg (1999) found in tiltmeter and extensometer data that the hydrological induced signal showed time variable correlations, which were not significant over longer periods or masked by other effects. Especially regarding repeated campaigns, same seasons are chosen for measurements to reduce the effect of hydrological impact. Hydrological effects can be amplified where strong topography occurs near the stations, for example at volcanoes (Jentzsch et al. 2004).
At the latest, general knowledge concerning seasonal changes of continental hydrology in gravity is widely spread since the GRACE project. Amongst others, this gives information about weak regions in continental hydrology models, e.g. with the ‘nulltest’ in arid regions (Hinderer et al. 2016). On the other hand, comparisons with terrestrial gravity observations from superconducting gravimeters (SG) show local effects at some stations. While Crossley et al. (2014) favor the Bouguer plate model for reduction of local hydrological effect where necessary at stations of superconducting gravimeters, in contrast, Weise et al. (2009, 2012) found that this holds only for flat and homogeneous surroundings like at Strasbourg station. But where strong topography leads to considerable effects physical modeling is needed, e.g. for stations like Moxa and Vienna. Naujoks et al. (2010) developed a successful correction model of local hydrological effect first combining hydrological and 3D-gravimetrical modeling for the SG station Moxa. After reduction the agreement of the SG time series with the result from GRACE for this region had been approved considerably (Weise et al. 2009, 2012).
During the last 3 years. the time series of the existing hydrological model were extended in period, and the combined hydrological–gravimetrical modeling could be clearly enhanced by inserting, especially some important details into the gravimetric model.
2 Hydrology Around Moxa Observatory
The hydrological situation around the Geodynamic Observatory Moxa is characterized by the fact that precipitation and topography cause mass movements above and below the sensor-level of the SG CD-34 (see Fig. 2 in Naujoks et al. 2010). In addition, different flow-paths are active for water transport from the hills to the valley and into the small creek Silberleite.
3 Hydrological Gravity Modeling
The hydrological modeling is based on the conceptual model J2000 (Krause 2001; Eisner 2009) and exemplarily shown for three horizons in Fig. 1a. The HRUs are defined according to different soil parameters, layers, permeability, drain information, and slope, among others. The variations of water content in the HRUs are modeled which cause density changes, due to three main storages: surface water (DPS), mid pore storage (MPS), and large pore storage (LPS), (s. Fig. 1a). Input data are temperature, precipitation, humidity, wind velocity, and radiation which together result in the input-storages defined by snowmelt (SM), precipitation (P), and throughfall (TF)/interception. The modeling provides the surface runoff (SQ), subsurface flow (SSQ), evapotranspiration (ET), and flow into groundwater (PERC). The main result of the model is the time dependent amount of stored water content (water column) in different water storages (layers) for all HRUs as time series. The final comparison of modeled vs. observed runoff gives an estimation of the quality of the modeling.
snow is stored on the roof of the observatory and on the ground above the SG (Fig. 3),
with respect to the plastic cover soil humidity in the roof coverage is reduced to 30% (not zero),
building acts as a shield which leads to reduced variation of humidity in clefts beneath the observatory and SG, estimated with 10%,
topography next to the SG has been improved (especially steep slope),
geometry next to SG has been adapted more specific, including pillar height, thickness of basement, height of rooms, cover layer in the roof,
constant thickness of units next to the SG is set for exact conversion of water storage to density change,
slow ground water has been included (up to 30 mm WC seasonal). The equivalent density change has been included in the gravimetric model partly in the soil layer and partly in the deeper layer. The impact in the seasonal amplitude of gravity change at the SG site is about 20–30%. The apparent phase shift against the satellite data in former results vanished after including the slow ground water.
The results of the hydrological modeling are inserted into the gravimetric model by continuously changing densities dependent on the water content. In time steps of 1 h the gravity-effect due to water mass changes in the upper model units is calculated for the location of the SG resulting in the time series of the hydrological induced gravity effects for correction of the local hydrological effect in the SG recording.
4 Results: SG-Residuals and Comparison with GRACE
We compare the SG residuals after reduction of local hydrological effect (red in Fig. 5) with gravity variations from the satellite mission GRACE for Moxa area which are computed according to Abe et al. (2012) using monthly sets (RL5) from JPL (Jet Propulsion Laboratory, Pasadena/California) and the spherical harmonic (SH) approach to the degree/order 120 (download from GFZ Information System and Data Center, ISDC, http://isdc.gfz-potsdam.de/, see Abe et al. 2012). The deformation effect is added. The GRACE data are Gauss filtered with 1000 km radius, and additionally DDK1 anisotropic filter from Kusche (2007) was applied, both exemplarily as their suitability was demonstrated in Weise et al. (2012).
Modeling of the first start of simulated gravity decrease is basically agreeing in time, amplitude and in course of process. The subsequent gravity increase due to relaxation is too slow and delayed in the original modeled version (pink in Fig. 7). The main effective process in the model seems to be evaporation. The flow downhill should be driven by quick ground water. The orange model version shows that the flow of the ‘quick ground water’ downhill, which is not sufficiently fast in the model, could be increased, also by dividing the HRU at the slope. In the red model version the storage capacity of the quick ground water has been reduced, in the orange version, the amount of quick ground water is increased. Both experiments show that the dynamic is not sufficient in the model. Simulating the full dynamic of flow in the complex situation in the slope through weathered and fissured rock remains a big challenge. Still, there remains a lack of knowledge about the real flow path in the underground and its characteristics. One point is also the steepness of the slope where the realistic ground water modeling seems to be problematic.
5 Discussion and Conclusion
The main improvements against Naujoks et al. (2010) of modeling the local hydrological effect at Moxa observatory which is mainly caused by the near and strong topography are part of the 3D-gravimetric model: Snow on and humidity in the cover layer of the observatory roof above the SG, reduced storage changes beneath the observatory and the SG due to the building acting as a shield, improved geometry and topography nearest to the gravimeter sensor. Especially including the ground water which had been neglected before, lead to higher effects in the modeling and finally to more realistic amplitude in gravity compared to GRACE. With these improvements the agreement with the GRACE time series is considerably higher, not only in amplitude but also in phase which seemed to be a problem before. This enhancement is mainly regarding long-term gravity changes in the range of months and probably weeks. This result also supports the position, that terrestrial gravity mainly ‘sees’ the same temporal variation as satellites as GRACE do.
As discussed in Crossley et al. (2014) a certain part of the local effect which is representative for a larger region is included in the GRACE data and should, in best case, remain in or should be added to the terrestrial gravity record after subtraction of a local effect. This will be small for the area around Moxa observatory due to a thin soil layer on the rocks of “Thüringer Schiefergebirge”. We estimate, that the part subtracted from below the sensor is <5% of the whole modelled effect, and thus perhaps within the uncertainty. Up to now, it is not solved how to estimate a realistic value for the smooth effect representing a wider area and to be added after reduction of the local effect due to topography and an underground location of the SG. On the other hand, if we would leave out the part from below the SG in the reduction model we would neglect the fact that also the building is working as a shield. This point remains an open question in this work.
In the higher frequency range of days to hours the comparison of the hydrological effect from the model combination with the observed gravity time series from the SG shows on the one hand a certain improvement but on the other hand also the obviously limited possibilities of the modeling. Flow processes of high dynamics within the deeper underground and the slope next to and under the observatory seem to remain a big challenge to be modeled in a realistic way.
The investigations were carried out in the frame of the project JA-542-24-1, funded by the German Research Foundation (DFG), which is gratefully acknowledged. We also thank our project partners Daniel Hagedorn (PTB, Braunschweig) and Ronny Stolz (IPHT, Jena) and the colleagues from the Arbeitskreis Geodäsie/Geophysik for the fruitful discussions about the reduction of hydrological effects in SG recordings. Further, we thank Maiko Abe, GFZ Potsdam, for calculating the gravity variations of GRACE for Moxa location with all her expertise. To keep the Geodynamic Observatory Moxa running, the superconducting gravimeter SG-CD034 as well as all the components of weather and hydrology recordings, was guaranteed by Wernfrid Kühnel and Matthias Meininger; many thanks for doing this job in highest quality. We appreciate valuable suggestions improving the manuscript from two reviewers, one anonymous and N. Florsch.
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