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- Water quality 56
- Hydrology 22
- Irrigation 19
- Politics and government 18
- Groundwater 16
- Technology 13
- Economics 11
- Agriculture 10
- Watersheds 9
- Aquifers 8
- Bureaucracy 8
- Demography 8
- Water-supply 8
- Dams 7
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- Resource allocation 7
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- Climate 5