Sorry, the title went ahead before I'd finished fixing it. Charging water exports is all you can really do because charging agriculture for irrigation is actually charging those who are already making a big impact in reducing pollution. Instead of stripping money from farmers you should be giving them money to plant riparian strips along rivers. Research has shown that irrigated farms cause less pollution due to the live plants utilising all of the nitrates that would otherwise leach into the water table. If you want to stop agricultural pollution then you should help us, not penalise us. Also saying that farmers don't pay for water is absolutely stupid. The amount of jobs they create by installing irrigation systems is huge. The community cash flow is huge. Their power bills are enormous.