Transient Hydrological Effects from Surface Loading and Pumping in the Western U.S. Observed with the Plate Boundary ObservatoryMeertens, C. 1; Boursa, A. 1; Herring, T. 2; Jackson, M. 1; van Dam, T. 3; Wahr, J. 4 The Plate Boundary Observatory (PBO) GPS network of over 1,200 continuously operating GPS stations in the W. U.S. is providing accurate and spatially coherent vertical signals that can be interpreted in terms of hydrological loading and poroelastic effects from both natural and anthropogenic changes in water storage. Data for this analysis are the precise coordinate time series produced on a daily basis by PBO Analysis Centers at New Mexico Institute of Mining and Technology and at Central Washington University and combined by the Analysis Center Coordinator at the Massachusetts Institute of Technology. These products, as well as derived velocity solutions, are made freely available from the UNAVCO Data Center in Boulder. Post-processed air pressure loading corrections are produced by the University of Luxembourg Analysis of interannual trends and annual variations in the time series are determined using the analysis software of Langbein, 2008. Spatial variations in the amplitude and phase of the annual vertical component of motion allow for identification of anthropogenic effects due to water pumping, irrigation, and reservoir lake variations, and of outliers due to instrumental or other local site effects. Vertical annual signals of 8-10 mm peak-to-peak amplitude are evident at stations in the mountains of northern and central California and the Pacific Northwest. The peak annual uplift is in October and is correlated to hydrological loading effects. Mountainous areas appear to be responding elastically to the load of the water contained in surface soil, fractures, and snow. This vertical elastic hydrologic loading contribution can be modeled using the Global Land Data Assimilation System (GLDAS) and the NOAA ?Leaky Bucket? hydrologic models. In contrast to mountain stations that are installed principally in bedrock, stations in the valleys of California are installed in sediments. Observations from these stations show greater spatial variability ranging from almost no detectable annual signal to very large, 20-30 mm, vertical amplitudes that reach a maximum in March. Vertical signals in the valleys are the result of poroelastic effects induced by groundwater variations caused by pumping for irrigation or other purposes and are highest when groundwater is at maximum recharge level. Interannual trends in the vertical time series typically show rates of 1-3 mm/yr across the western U.S. In areas of groundwater pumping the rates can be considerably greater approaching several cm/yr showing subsidence as pumping exceeds annual recharge over a multi-year time period. In the mountainous areas where hydrologic loading is evident in the annual signals, interannual trends show uplift of 1-3 mm/yr possibly due to regional drought and decreased overall water volumes that result in less load and vertical uplift. Overall, these results illustrate the potential of using GPS data to constrain hydrological models. In return, accurate hydrologic loading models will be needed to better measure and detect vertical tectonic motions at the mm-level. |