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Global-scale modelling of groundwater vulnerability taking into account groundwater availability and use

Congress: 2008
Author(s): Karin Berkhoff, Petra Döll
Institute of Physical Geography J. W. Goethe University Frankfurt am Main PO Box 11 19 32 60054 Frankfurt am Main, Germany Tel. +49 69 798-40 157 Fax +49 69 798-40 347 Email Berkhoff@em.uni- frankfurt.de

Keyword(s): global hydrological modeling, groundwater recharge, groundwater use
AbstractIntroduction Global freshwater resources in general are influenced heavily by global change. As a part of the freshwater resources, groundwater is a key source of water for society, but there is currently no approach for quantitatively including both groundwater resources and use in global assessments. Objective Three aspects have to be considered when evaluating groundwater vulnerability: 1. Amount of the groundwater resources 2. Groundwater use 3. Further restrictions for the groundwater resource (e.g. depletion by pesticide exposure) Therefore, the aim of the study is to combine groundwater recharge modelling and downscaled statistical information on groundwater withdrawals to estimate global groundwater vulnerability. Further restrictions for the groundwater resource will be considered. Methods Groundwater recharge is computed by the WaterGAP Global Hydrology Model (Döll et al. 2003). WaterGAP currently simulates groundwater recharge at a spatial resolution of 0.5° globally. Calculations are performed with daily temporal resolution. WaterGAP uses a heuristic approach of partitioning total runoff into surface runoff and groundwater recharge taking into account global data sets of slope characteristics, soil texture, hydrogeology, and the occurrence of permafrost and glaciers. In the study, the algorithm will be further improved based on river discharge measurements (base flow estimates) and independent smaller-scale information on groundwater recharge. Modelling of groundwater recharge is generally found to be more difficult in semi-arid and arid regions than in humid regions due to the small values of the variables used for estimating recharge there. Döll et al. (2005) found out that WaterGAP generally overestimates recharge values below 10 mm/yr. By tuning groundwater recharge as calculated by WGHM against independent local-scale estimates of groundwater recharge in semi-arid and arid regions, they were able to reduce the discrepancies between model results and the independent estimates in a globally homogenous way. The publication of a global synthesis of the findings from 140 recharge study areas in semi-arid and arid regions by Scanlon et al. (2006) allows to elaborate more on the approach described above. Freshwater withdrawals already calculated by the WaterGAP model for the sectors irrigation, household/manufacturing and thermal power plants are split up in surface water withdrawals and groundwater withdrawals. The estimation of groundwater withdrawals is based on statistical as well as on demographic data. Data collection on groundwater withdrawals is performed particularly with regard to groundwater use information on the scale of sub-national units. Collected data are downscaled to the 0.5° grid taking into account groundwater resources and hydrogeology. Results Beginning of the year 2008, first results will be available. They cover global-scale groundwater recharge as well as sectoral groundwater withdrawals. Thus, an analysis of groundwater resources for the whole of the 20th century will be available as result from the study. Conclusion The described approach supports an improved understanding of the groundwater resources of the 20th century. The groundwater study is embedded in the WATCH (Water and Climate Change) project, commissioned by the European Commission under the Sixth Framework Programme. The objective of WATCH is to analyse the overall vulnerability of global water resources related to the main societal and economic sectors. Döll, P. and M. Flörke (2005). Global-scale estimation of diffuse groundwater recharge. Frankfurt Hydrology Paper 03. Frankfurt am Main, Germany, Institute of Physical Geography, Frankfurt University. Döll, P., F. Kaspar and B. Lehner (2003). A global hydrological model for deriving water availability indicators: model tuning and validation. Journal of Hydrology 270(1-2), 105-134. Scanlon, B. R., K. E. Keese, A. L. Flint, L. E. Flint, C. B. Gaye, W. M. Edmunds and I. Simmers (2006). Global synthesis of groundwater recharge in semiarid and arid regions. Hydrological Processes 20(15), 3335-3370.
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