Effects of large macropores on soil evaporation in salt marshes

The occurrence of macropores in salt marsh sediments is a natural and ubiquitous phenomenon. Although they are widely assumed to affect pore-water flow in salt marshes significantly, the mechanisms involved and their extent are not well understood. We conducted laboratory experiments and numerical simulations to examine the effect of macropores on soil evaporation. Soil columns packed with either sand or clay and with or without macropores were set up with watertables in the columns set at different levels. A high potential evaporation rate was induced by infrared light and a fan. The results showed that in the soil with a low saturated hydraulic conductivity (and thus a low water transport capacity), the macropore behaved as a preferential flow path for groundwater to recharge the surrounding soil during evaporation. The evaporated water originated largely from the macropore rather than the soil matrix, maintaining a high evaporation rate in comparison with a homogeneous soil. This effect was more pronounced for sediments with lower hydraulic conductivities and shallower watertables. These results improve our understanding of water flow and evaporation in salt marshes with continuous macropores between the soil surface and groundwater.