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- Irrigation 124
- Agriculture 68
- Bureaucracy 30
- Building 28
- Hydrology 26
- Economics 25
- Politics and government 24
- Dams 20
- Groundwater 20
- Watersheds 18
- Drainage 15
- Technology 14
- Canals 13
- Water quality 13
- Water-power 13
- Finance 12
- Reclamation of land 12
- Water-supply 11
- Climate 10
- Demography 10