This guide delves into what I've uncovered about the effects of canals on land fertility based on v64.

• The larger the canal, the wider its area of effect of the fertility boost, capping out at 15 tiles
  
• The boost drops of by 3% per tile away from the canal
  
• A 5x5 canal hits the sweet spot, with larger sizes seeing diminishing returns
  
• Maps have a limit on water tiles, digging canals reduces it, "Remove water" on rivers/lakes replenish it
  
• Aim to maximize land use without surpassing the 100% fertility cap
  
• Place canals next to lower-fertility tiles for the best results

Note: The drop off from tile to tile isn't always exactly 3%, sometimes it's 4%. I've tested on a few maps and not quite yet sure what causes the difference.

The following table shows the impact of a given canal to the base fertility of a tile. A few callouts

• The outer tiles of a 5x5, 6x6, and 7x7 canal have a smaller initial bonus compared to the inner tiles
  
• That does not impact the overall width range of the AOE fertility bonus. On the 6x6 example, 30% bonus is repeated for tiles 6 and 7 in the outer rows.
  
• The difference between 6x6 and 7x7 is how many inner tiles get the max bonus. There's a little + icon in the detail images indicating which tiles get the largest bonus.
  
• The boost isn't uniform, and I'm not sure why. In the 6x6 examples, the tiles to the top and right have the little + icons and get a 3% additional boost. The tiles to the left and bottom of the canal don't get this extra boost. This does not affect the overall range of the fertility boost, just the immediate impact to the tiles adjacent to the canal.

Though 5x5 canals are typically ideal, remember that tile fertility has a 100% cap, so any boosted fertility beyond that is wasted. Generally avoid placing a canal in tiles with a high base fertility above 90%.

Aim for the smallest canal that nudges you towards the 100% fertility threshold. There are several benefits

• not wasting high fertility land on unnecessary canals
  
• not wasting building land (road/workshop/house) on unnecessary canals
  
• conserving the water table

You can support a 6 tile wide farm with a 4x4 canal when the base fertility is at least 80%. A 5x5 canal in the same scenario can support up to a 10x wide farm. If you only plan to build a 5 tile wide farm, anything beyond a 4x4 is overkill.

The tables below shows how wide a farm should be given a canal size and base fertility percentage.

Targeting 95%+ overall fertility

Targeting 90%+ overall fertility

Place canals next to the lowest base fertility land since the immediate tiles next to a canal receive the highest fertility boost, which tapers off as you move away.

For example, with base fertility values ranging 60-85 (let to right), positioning a 4x4 canal to the left of the 60% fertility tiles results in an average fertility of 97%, while placing it to the right of the 85% tiles drops the average to 92.3%.


Each map has a "water-table" limit, indicating how many tiles can be converted to water-filled canals. As you dig, this value diminishes. However, by using the "Remove water" command near existing water bodies, you can replenish the water table. A strategy could be to add canals in high-fertility zones (70% and up) and remove water where base fertility is less (below 50%).

To see the value on your map, look at the "water-table" value when you select dig canal.