Recent changes in terrestrial water storage in the Upper Nile Basin: an evaluation of commonly used gridded GRACE products
Mohammad Shamsudduha1,2, Richard G. Taylor2, Darren Jones3, Laurent Longuevergne4, Michael Owor5, and Callist Tindimugaya61Institute for Risk and Disaster Reduction, Universi ty College London, UK 2Department of Geography, University College London, UK 3Centre for Geography, Environment and Society, University of Exeter, UK 4CNRS – UMR 6118 Géosciences Rennes, Université de Rennes 1, France 5Department of Geology & Petroleum Studies, Makerere University, Uganda 6Directorate of Water Resources Management, Ministry of Water & Environment, Uganda
Received: 14 Mar 2017 – Accepted for review: 19 Mar 2017 – Discussion started: 21 Mar 2017
Abstract. GRACE (Gravity Recovery and Climate Experiment) satellite data monitor large-scale changes in total terrestrial water storage (ΔTWS) providing an invaluable tool where in situ observations are limited. Substantial uncertainty remains, however, in the amplitude of GRACE gravity signals and the disaggregation of ΔTWS into individual terrestrial water stores (e.g. groundwater storage). Here, we test the phase and amplitude of GRACE ΔTWS signals from 5 commonly-used gridded products (i.e., NASA's GRCTellus: CSR, JPL GFZ; JPL-Mascons; GRGS GRACE) using in situ data and modelled soil-moisture from the Global Land Data Assimilation System (GLDAS). The focus of this analysis is a large and accurately observed reduction in ΔTWS of 75 km3 from 2004 to 2006 in Lake Victoria in the Upper Nile Basin. We reveal substantial variability in current GRACE products to quantify the reduction of ΔTWS in Lake Victoria that ranges from 68 km3 (GRGS) to 50 km3 and 26 km3 for JPL-Mascons and GRCTellus, respectively. Representation of the phase in ΔTWS in the Upper Nile Basin by GRACE products varies but is generally robust with GRGS, JPL-Mascons and GRCTellus (ensemble mean of CSR, JPL and GFZ time-series data) explaining 91 %, 85 %, and 77 % of the variance, respectively, in in-situ ΔTWS. Resolution of changes in groundwater storage (ΔGWS) from GRACE ΔTWS is greatly constrained by both uncertainty in modelled changes in soil-moisture storage (ΔSMS) and the low annual amplitudes in ΔGWS (e.g., 3.5 to 4.4 cm) observed in deeply weathered crystalline rocks underlying the Upper Nile Basin. Our study highlights the substantial uncertainty in the amplitude of ΔTWS that can result from different data-processing strategies in commonly used, gridded GRACE products.
Shamsudduha, M., Taylor, R. G., Jones, D., Longuevergne, L., Owor, M., and Tindimugaya, C.: Recent changes in terrestrial water storage in the Upper Nile Basin: an evaluation of commonly used gridded GRACE products, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2017-146, in review, 2017.