Before ponding During ponding After ponding

Figure 5. Conceptual model describing the process of soil swelling because of air entrapment. a) before ponding: free water and air movements throughout the profile (bio-opened system); b) during ponding: soil swelling by air trapped within water table perched over the impervious Btk horizon (air-occluded system); c) after ponding: soil shrinkage after perched water and water table decreases, and rapid air escape to the atmosphere.

We can then conclude that air entrapment due to water table rises from depth and surface ponding was the main factor determining the swelling of soils. These results explain the structural behaviour of seasonally flooded soils and their recovery from soil deterioration cause by grazing in general and trampling in particular.

During flooding the tough natric horizon separates two kind of water qualities: the salty groundwater below it and the rainwater above it (Lavado and Taboada, 1988; Taboada et al., 2001). The process of ponding can be then regarded as a huge perched water table, which accumulates over the impervious natric horizon during water excess periods. This actually impedes the arrival of salty groundwater to soil surface. So, rather than being a disturbance, ponding with rainwater is a key factor preserving soil physical properties in this lowland.

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