When is preferential flow most commonly a concern in coarser soils?

Preferential flow is most commonly a concern in coarser soils when those soils are at or above field capacity. At this moisture level, the larger pore spaces in sandy or gravelly soils allow water to move quickly through them, leading to the formation of preferential flow paths. This rapid movement can bypass the finer soil material where more water would usually be retained, which increases the risk of contaminants reaching groundwater or surface water without being filtered.

Moreover, when soils are at field capacity, they are fully saturated after excess gravitational water has drained away, creating optimal conditions for this kind of rapid water movement. Under these conditions, water can flow through preferential pathways formed by soil structure, root channels, or earthworm activity, making it particularly significant in managing water and nutrient movement in agricultural contexts.

Coarser soils tend to have a higher hydraulic conductivity, leading to quicker drainage and heightened concerns over preferential flow mainly when saturated or at field capacity. During dry conditions or when heavy rainfall occurs without saturation reaching field capacity, the dynamics of water movement differ and the risk of rapid flow is not as pronounced.