Improved approaches to model low flows and droughts with the global water model WaterGAP

Seasonal recurring low flows in rivers and sporadically occurring droughts are the most crucial limiting factors for the satisfaction of human and environmental water demands. Changing climate and growing human water demand affect the intensity, duration and frequency of low flows and droughts and thus the water availability worldwide. Water scarcity is a problem in numerous countries yet, particularly in regions with highly variable climate, and will probably become even more severe in the future. Whereas a large number of local-scale studies on low flows and droughts and related indicators have been published, continental or global assessments are still rare. However large scale studies are particularly essential for decision makers in order to prevent a further aggravation of water stress problems in many parts of the world.

The global water model WaterGAP 2 was developed to assess and predict water availability worldwide. It comprises a hydrological model that computes time series of surface runoff, groundwater recharge and river discharge, taking into account agricultural, industrial and domestic water use. With a monthly temporal resolution and a spatial resolution of 0.5 degrees WaterGAP is supposed to be applicable for large scale low flow and drought assessments. In several studies it proved an overall well model performance. Nevertheless in specific regions and under certain conditions the seasonality and amount of discharge are captured less satisfactorily.

The major focus of the project is to enhance the global water model WaterGAP in order to ensure an improved assessment of low flows and droughts on a global scale. The project work is divided into four steps:

(1) Several parts of the WaterGAP model approach are revised and supplemented or replaced where necessary to improve the model performance. The following critical regions and weak points in the model structure, identified in previous studies, are considered in particular:

  • Basins in semi-arid and arid regions - oversimplification or neglect of certain processes, e.g. river channel losses or evaporation from small ephemeral ponds

  • Basins dominated by reservoirs, lakes or wetlands - inadequate water balance simulation of stagnant water bodies

  • Generally - unrealistic constant river flow velocity (currently one meter per second globally).

(2) Adequate low flow and drought indicators, applicable on the global scale, are identified based on literature review and on experiences gained by first model runs.

(3) The new modeling approaches are evaluated. Model results are checked for plausibility, compared to observed data and to already published findings.

(4) The improved WaterGAP model and the newly identified indicators are applied to assess low flows and droughts globally.

Keywords: low flows, droughts, water availability, water scarcity, global hydrologic modeling, climate change

Participants: Martin Hunger,PetraDöll

Cooperating institutions: Center for Environmental Systems Research,University of Kassel,Germany

Duration: since 2004 (ongoing)

Funding entity: state funds

Contact: hydrology@em.uni-frankfurt.de