Consistent assessment of global green, blue and virtual water fluxes in the context of food production: regional stresses and worldwide teleconnections
Final Report Abstract
This project provided comprehensive and spatially explicit analyses of the past, present and potential future water flows in global irrigated and rainfed agriculture, the results of which have been published in, or are under review for, almost a dozen peer-reviewed scientific journals. As a precondition for these studies, two macro-hydrological models - the Lund- Potsdam-Jena managed Land model (LPJmL) and ihe Global Crop Water Model (GCWM) - were developed from eariier model versions toward state-of-the-art global agro-hydrological models with unique features. In addition, a new landmart^ dataset of crop areas at 5" resolution (MIRCA 2000) was created, which for the first time consistently combines inigated and rainfed areas at an unprecedented (monthly) temporal resolution. Both models were used to assess the total water consumption (evapotranspiration) and the water consumption per unit of yield (virtual water content) of the worid's major crops in a novel way, in that the contributions of blue water (stemming from rivers, surface water bodies and groundwater) and green water (precipitation stored in the soil) were thoroughly differentiated. A key result is that the majority of global agricultural water use, about 90%, is constituted by green water, which quantitatively underpins the need to consider the green water resource in any freshwater assessments. The specific impact of irrigation on global crop production was quantified for the first time, and a key finding was that present irrigation an increase in global cereal production by about 20% is achieved. Given the outstanding importance of green water and the demonstrated fact that food production is presently water-limited over many areas, the project also investigated the potential to increase global food production of optimised green-blue water management ("vapour shift" from unproductive soil evaporation to productive plant transpiration, and "harvesting" of runoff from cropland used for supplemental irrigation in dry periods). It was found that such management could substantially increase crop production in many regions and globally by up to about 20% or even more. Adverse effects of climate change are suggested to be partially offset by the fertilisation effects of rising atmospheric CO2 concentration, but we found that irrespective of the actual degree of the (rather uncertain) CO2 effect there will be not enough water on present cropland, even if effectively used, to produce the food for a higher world population of 9-10 billion by the mid of this century. The project also made a significant contribution to the scientific debate about what drivers have caused the variations and trends in global discharge (blue water flows) over the past century. While earlier studies suggested a major influence of the rising atmospheric CO2 content (which increased global discharge through reductions in transpiration), we found that although this effect gained more importance recently it was superimposed by a negative impact of rising temperature, and that both effects were of minor importance compared to precipitation fluctuations. We also identified land use changes as the second-most relevant factor having caused past changes in global river discharge. These results were made publicly visible through a press release and some related interviews and (online) newspaper articles. Uncertainties in blue, green and virtual water flows as stemming from differences in precipitation datasets and differences in model designs were quantified in this and other studies within the project. In sum, this project advanced significantly the knowledge of key processes in the global water system and particulariy in agriculture. The here developed models and datasets provide a first-rate basis for further systematic intercomparisons of global hydrological and crop models, with the goal of deriving a more complete picture of global freshwater uses and limitations and the inherent uncertainties of such assessments.
Publications
- Advances and visions in large-scale hydrological modelling: Findings from the 11th Workshop on Large-scale Hydrological Modelling. Adv. Geosci. 18, 51-61
Döll. P., Berkhoff. K., Bormann. H., Fohrer. N., Gerten, D., Hagemann, S., Krol. M.
- 2006: Irrigation in Africa, Europe and Latin America. Update of the Digital Global map of Irrigation Areas to Version 4. Frankfurt Hydrology Paper 05. University of Frankfurt am Main
Siebert, S., Hoogeveen, J., Frenken, K.
- 2007: Development of functional irrigation types for improved global crop modelling. PIK Report 104, 91 pp
Rohwer, J., Gerten, D., Lucht, W.
- 2007: Irrigation water use - a global perspective. In: Lozán, J. L., Graßl, H., Hupfer, P., Menzel, L., Schönwiese, C.-D. (eds.): Global Change: Enough Water for all? Universität Hamburg / GEO, 104-107
Siebert, S., Döll, P.
- 2008: Agricultural green and blue water consumption and its influence on the global water system. Water Resour. Res. 44, W09405
Rost, S., Gerten, D., Bondeau, A., Lucht, W., Rohwer, J., Schaphoff, S.
(See online at https://doi.org/10.1029/2007WR006331) - 2008: Causes of change in 20th century global river discharge. Geophys. Res. Lett. 35, L20405
Gerten, D., Rost, S., von Bloh, W., Lucht, W.
(See online at https://doi.org/10.1029/2008GL035258) - 2008: Global data set of monthly growing areas of 26 irrigated crops. Frankfurt Hydrology Paper 06. Institute of Physical Geography. University of Frankfurt, Frankfurt am Main, Germany. 400 pp
Portmann. F., Siebert. S., Bauer, C., Döll, P.
- 2008: Human alterations of the terrestrial water cycle through land management. Adv. Geosci. 18. 43-50
Rost, S., Gerten, D., Heyder, U.
- 2008: Klimawandel: Folgen für die weltweite Verfügbarkeit von Süßwasservorräten und Trinkwasser. In: FEI (Hrsg.): Zukunftsfähige Wasserversorgung: Von der lokalen zur globalen Herausforderung. 22. Trinkwasserkolloquium am 14. Febmar 2008. Oldenbourg, 1-10
Gerten, D.
- 2008: The Global Crop Water Model (GCWM): Documentation and first results for inigated crops. Frankfurt Hydrology Paper 07. Institute of Physical Geography. University of Frankfurt, Frankfurt am Main, Germany, 42 pp
Siebert, S., Döll, P
