Measuring the effect of local water storage changes on in situ gravity observations: Case study of the Geodetic Observatory Wettzell, Germany

Abstract

[1] Local water storage changes (WSC) are a key component of many hydrological issues, but their quantification is associated with a high level of uncertainty. High precision in situ gravity measurements are influenced by these WSC. This study evaluates the influence of local WSC (estimated using hydrological techniques) on gravity observations at the Geodetic Observatory Wettzell, Germany. WSC are comprehensively measured in all relevant storage components, namely groundwater, saprolite, soil, topsoil, and snow storage, and their gravity response is calculated. Total local WSC are derived, and uncertainties are assessed. With the exception of snow, all storage components have gravity responses of the same order of magnitude and are therefore relevant for gravity observations. The comparison of the total gravity response of local WSC to the gravity residuals obtained from a superconducting gravimeter shows similarities in both short-term and seasonal dynamics. A large proportion of the gravity residuals can be explained by local WSC. The results demonstrate the limitations of measuring total local WSC using hydrological methods and the potential use of in situ temporal gravity measurements for this purpose. Nevertheless, due to their integrative nature, gravity data must be interpreted with great care in hydrological studies.Citation: Creutzfeldt, B., A. Güntner, H. Thoss, B. Merz, and H. Wziontek (2010), Measuring the effect of local water storage changes on in situ gravity observations: Case study of the Geodetic Observatory Wettzell, Germany, Water Resour. Res., 46, W08531,

Gravity changes associated with variations in local land-water distributions: Observations and hydrological modeling at Isawa Fan, northern Japan

“…Researchers have carried out electrical resistivity surveys (Van Camp et al, 2006), lysimeter measurements (Creutzfeldt et al, 2010a), and borehole-core sampling (Creutzfeldt et al, 2010b), to understand, in detail, depthdependent water distributions below the ground surface; however, such depth profile data is not available for all gravity observation sites, mainly because of financial limitations. A different approach is to realize that the bulk of the water mass distribution occurs in the uppermost ground layers, because soil pores and free spaces at the ground surface can preserve significant amounts of water in the form of soil water and snow (e.g., Sato et al, 2006).…”

Relación de la variación del almacenamiento de agua local y el gravímetro superconductor en el Observatorio Geodésico TIGO, Concepción, Chile

“…Hasan et al (2008) calibra conjuntamente un modelo de humedad del suelo y un modelo de almacenamiento en pendiente de Boussinesq, para la escorrentía de aguas subterráneas y la señal de gravedad in situ, respectivamente. Creutzfeldt et al (2010), centran su atención en un pequeño radio de influencia cercano al gravímetro donde las variables hidrológicas y su efecto sobre la gravedad son más pronunciadas en la profundidad que en el área. Debido a esto resuelven las variables de profundidad en un enfoque 1D centrándose separadamente en cada componente de almacenamiento y calculan la respuesta gravimétrica de la variación del almacenamiento y las comparan con los datos del SG.…”

The benefits of gravimeter observations for modelling water storage changes at the field scale

“…Topography determines the distribution of hydrological masses in space and influences the relationship of WSC and gravity response. For the Geodetic Observatory Wettzell, for example, distributing the infinitely extended plate along the topography, a water mass change of 1 m causes a gravity change of 52 µGal (Creutzfeldt et al, 2008). Hence, the effect of WSC on gravity measurements depends on the topography around the gravity sensor and is also a function of the vertical distribution of mass change below the sensor.…”

“…Different studies focus on the interpretation of the gravity signal by single storage components (e.g., surface water (Lampitelli and Francis, 2010;Bonatz and Sperling, 1995), snow (Breili and Pettersen, 2009), soil moisture (Van Camp et al, 2006), or groundwater (Takemoto et al, 2002;Harnisch and Harnisch, 2006)), or by estimation of different subsurface properties (e.g., porosity (Jacob et al, 2009), fractures (Hokkanen et al, 2007), block content (Van Camp et al, 2006) or specific yield (Pool and Eychaner, 1995)). The unambiguous identification of the exact source of the gravimeter signal is difficult or even impossible if no additional information is available implying that the estimation of single parameters on the storages or properties is associated with a high uncertainty (Pool, 2008;Creutzfeldt et al, 2010a). Blainey et al (2007), for example, pointed out that the estimation of hydraulic conductivity and specific yield by gravity data alone was likely to be unacceptably inaccurate and imprecise.…”