Rethink irrigation mechanics? (Update 5)

The irrigation-blocking dams/floodgates/levees intruduced in U5 caused a new discussion on "exploits". This could be the starting point to rethink the current irrigation mechanic:

1. A single block of water provides the same irrigation radius as a big river. Instead each block of water could only partially irrigate surrounding tiles (e.g. 100% in radius 1, decreasing to 10% in radius 15). Therefore 100% irrigation for tiles would only be reached, if they are either close to a water source or have several water blocks around.

2. Currently afaik water only vanishes through evaporation. Instead also seepage could be calculated for every earth or otherwise permeable block in touch with water. This would cause small amounts of water (e.g. single blocks) to vanish quite fast, while lakes or canals would survive longer.

3. Based on 2., irrigation towers could again become more efficient than water outlets. Also this could better explain, how irrigation goes around non-permeable blocks.