Agence France-Presse: From South Asia and to the Middle East, from Australia to California, rivers and aquifers that cross boundaries have become potent sources of friction. Farmers squabble with city dwellers over irrigation rights while countries in river basins complain about pollution or water theft from upstream, as their neighbours build dams to siphon off flow from the watershed. “Conflicts about water can occur at all scales,” the UN warned ahead of the World Water Forum, which winds up in …