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11. Subsurface Drainage Pros and Cons
Subsurface drainage reduces surface runoff by allowing water to infiltrate into the soil rather than flowing over the land. Draining the excess water from the soil profile provides the necessary aeration needed for proper crop root development (Figure 11). Drainage promotes deep root development and prevents the crop roots from drowning in too much water.
Drainage helps the soil dry sooner, which provides timely field operations while reducing soil compaction. Earlier drying of the soil allows earlier seed germination because the soil warms faster in spring. Crop yield is less variable from year to year with drainage. The overall impact of subsurface drainage is a healthier, more productive soil with more stable crop yields (Table 1).
Drainage improves soil aeration, thereby increasing biological activity, which leads to better soil structure by increasing soil aggregation. In contrast, an undrained field with prolonged waterlogged condition degrades the soil structure by breaking down soil aggregates.
When compared to an undrained field, drainage improves nitrogen use efficiency, leading to less nitrogen fertilizer input (Mass et al. 2022). Also, drainage reduces nitrous oxide (N2O) emissions (Kumar et al., 2014), which is about 300 times more potent than carbon dioxide (CO2).
A disadvantage of drainage is that it provides less opportunity for groundwater recharge due to reduced deep percolation, which may be important in locations where the aquifer is in decline.
Furthermore, when soil aeration is increased with water removal, oxidation of soil organic matter increases, thereby accelerating the loss of soil organic matter. Thus, it is important to combine drainage with regenerative agriculture (conservation tillage, cover crops, and diverse crop rotations) to build soil carbon.
Table 1. Pros and Cons of Subsurface Drainage in Humid Regions.
|Provides timely field operations and trafficability||Excess phosphorus transport|
|Reduces year-to-year crop yield variability||Excess nitrate transport|
|Reduces nitrous oxide (N2O) emissions||Less groundwater recharge|
|Reduces N fertilizer input by increasing N use efficiency||Accelerates loss of soil organic matter|
|Increases soil aeration|
|Improves soil structure|
|Decreases surface runoff|
|Increases crop yield|