IGCP 565 Workshop 3: Separating Hydrological and Tectonic Signals in Geodetic Observations Reno, Nevada, October 11-13, 2010

Session Descriptions

P1: Hydrology, Tectonics, and Geodesy: Status and Challenges

Co-Chairs: Jay Famiglietti (University of California, Irvine), David Lavallee (University of Delft)

This session will have a number of invited lectures giving general overviews on hydrology, tectonics, and geodesy with focus on the key scientific challenges as they relate to monitoring mass movements in the water cycle and the solid Earth.

P2: Transient tectonic and hydrological signals

Co-chairs: Matthew Rodell (NASA), Jessica Murray (USGS)

In this session presentations will describe the nature and characteristics of transient signals caused by tectonic and hydrologic processes that may be recorded with geodetic techniques. The goal is to provide an overview of the variety of such signals and the settings in which they occur, as well as the similarities among tectonic and hydrologic signals and features that may be used to distinguish among them. Considering typical time scales captured by highly accurate geodetic techniques, the term transient encompasses any signal with significant non-linear components on time scales from sub-daily to decades. Tectonic examples include aseismic fault slip events, postseismic motion, and volcanic deformation. Hydrologic transient signals include the effects of natural seasonal to interannual variability in the water cycle.

P3: Case Studies: The South West of the U.S. and East Africa

Co-chairs: Norman Miller (UCB), Elias Lewi (Addis Ababa University), Eric Callais (PUD)

One of the two focal geographical regions is the Southwest of the U.S., where significant changes are taking place in hydrology and where tectonic processes contribute secular and transient signals to geodetic observations. The goal of this session is to give an overview of the relevant processes, to assess the state of knowledge, to discuss the observing system, and to characterize the observed or expected signals.

The recent western US droughts have significantly lowered groundwater levels, impacting water resource availability. Regions of overdraft have also resulted in subsidence, especially where clay soils have dried down. Application of GRACE and GPS has emerged as new tools for monitoring such terrestrial water storage (TWS) change, however the scale of application remains on order 150,000 km2. Ground truth via well data has helped to provide validations of geodetic derived changes in the TWS. Removal of TWS terms, such as the soil moisture, snow water equivalent, and surface water reservoirs requires additional data based on a combination of observations and simulations. Land surface models calculate soil moisture and snow water equivalent and can be evaluated with additional remote sensed data (e.g. National Operational Hydrologic Remote Sensing Center).This session seeks to provide a forum for discussing and advancing new and advanced approaches for improving ground based observations with remote sensing to develop high resolution estimates of change in TWS and geodetic response.

The second focal geographical region is East Africa, where significant hydrological and tectonic signals are superimposed. The goal of the session is to identify the relevant processes, to introduce on-going studies, to given an overview of existing and planned observation infrastructure, and to discuss the science challenges.

B1: Forward modeling of tectonic and hydrological signals

Co-Chairs: William Hammond (UNR), Jean-Paul Boy (University of Strasbourg)

This breakout session will discuss the current state of models for the prediction of geodetic signals resulting from tectonic and hydrological processes. We will evaluate the state of knowledge of the physics behind the Earth surface deformations and gravity changes on local to global spatial scales and various time scales. Examples of tectonic models include those describing earthquake deformation, postseismic transients, interseismic fault slip, creep, plate motions, etc.. Hydrologic models can include solid earth response to the motion of fluids, atmosphere, cryosphere, pumping, etc. We will evaluate the uncertainty in the sources and identify science challenges in improving the models and reducing the uncertainties. The session will be roughly 50% presentations and 50% facilitated discussion intended to gather input on the status and roles of models in interpreting geodetic observations.

B2: Separation of signals on micro to river basin scales

Co-Conveners: Michel Van Camp (Royal Observatory of Belgium), Charles Meertens (UNAVCO), Shimon Wdowinski (RSMAS, University of Miami), Ty Ferre (University of Arizona)

Geodetic quantities integrate many phenomena, and it remains a challenge to separate the contribution from the different sources, in our case tectonic and hydrological signals. This can only be achieved by two different ways: (1) if the two sources have deeply different space/time signature, they can be separated using numerical processing methods, (2) if it is possible to determine one of the sources by any other observation/modelling techniques, it can be subtracted to isolate the other one.

The increasing accuracies of space geodetic techniques enable the detection of small surface changes over large areas. Apparent geodetic changes are the result of tectonic, hydrologic, atmospheric and other processes. Therefore, understanding the significance of the measured signal can be challenging and requires a multidisciplinary effort.

In particular, land-based gravity measurements are very sensitive to local water storage changes (WSC). Consequently, at the local scale, in many areas, the hydrological signal dominates the measured gravimetric signal, after the classical correction for tidal, atmospheric and polar motion effects. The gravimetric signature of the WSC depends on very local geologic and climatic conditions, e.g. rock porosity, vegetation, evaporation, and runoff rates; this signature can be more important than any tectonic effects and may induce large time correlated noise into the data.

On the other hand, observable ground deformations are usually associated with larger scale WSC. Nevertheless, in some particular conditions, tilts and deformations can be observed at the local scale, for example in the vicinity of hydrothermal and volcanic systems or on locations undergoing anthropogenic uplift/subsidence due to mass withdrawal/injection or surface loading from lakes.

This session will consider geodetic signals at spatial scales from several meters and up to river basin scales (> tens of kilometers) resulting from tectonic and hydrological processes. We will discuss the fingerprints of tectonic and hydrological signals at these scales in geodetic observations.

At river-basin scales we will consider basins located within active plate boundary regions, where tectonic processes form the large-scale landforms, surface and hydrological processes shape the landscape, and subsurface water storage changes on seasonal and longer periods. These larger-scale basins are located along active mountain belts, such as the Ganges Basins, which drains the southern Himalayas. Smaller- and intermediate-scale basins are located within transcurrent and divergent plate boundaries, in which their segmented nature gives rise to variable scale landforms.

The session will include observations from several basins in Asia, the western U.S., the eastern Mediterranean, and possibly other regions where both tectonic and hydrological signals affect geodetic observations. We will discuss the level to which these signals can be separated as well as uncertainties in the separation. Emphasis will be placed on the causes of uncertainties and methods to reduce them. Finally, we will evaluate how anthropogenic impacts on hydrological processes can act as a source of interpretation error and, simultaneously, offers an opportunity for environmental monitoring.

B3: Separating plate tectonics, postglacial rebound and contemporary global change signals

Co-Chairs: Bert Vermeersen (Technical University Delft), Holger Steffen (University Calgary)

Geodetic observations such as GPS measurements or the GRACE satellite mission contain signals of different sources. For example, GRACE observes variations in the gravity field that result from the integral effect of mass variations in the atmosphere, hydrosphere and geosphere. On regional to global scales, processes associated with plate tectonics, postglacial rebound, and, recently, increased contemporary mass changes due to melting of glaciers and ice sheets and large-scale changes in land water storage are producing overlapping and correlated geodetic fingerprints that are hard to separate. Thus, a key issue is the separation of the various signal parts and, to get the signal of interest, the reduction of unwanted observed quantities by applying dedicated models. This Breakout Session is expected to focus on the separation of signals from large-scale surface loading and tectonic process in geodetic observations such as GPS, VLBI, tide gauges, satellite altimetry and terrestrial and satellite gravity measurements. We invite papers that review the current status in modeling and extracting these signals from different geodetic observations and that discuss the main obstacles for separating the fingerprint.

B4: Improving the observation system and consistency of fingerprints in surface displacements, gravity and rotation

Co-Chairs: Xiaoping Wu (JPL), Peter Clarke (University of Newcastle), Gerald Bawden (USGS)

Mass transports in the global water cycle, and related or unrelated deformation of the solid Earth, should produce "fingerprints" in Earth's gravity field, rotation and surface displacement that are consistently related to each other through the mass transported in both systems. This session will explore the consistency of predictions of fingerprints and their match with observations including Earth's rotation, gravity field, surface deformation and sea level. It will also discuss the confounding factors that in addition affect these observations on seasonal, decadal and longer timescales. We aim to identify the sources of potential inconsistencies between the various observation types and with theory, and to propose a research "roadmap" leading to their eventual resolution. We plan a mixture of invited and contributed presentations, and facilitated discussion. We will also consider the geodetic observation system and aim to identify gaps and insufficiencies that impact the separation of tectonic and hydrological signals in geodetic observations on all scales from local to global.