Water movement and soil swelling in a dry, cracked Vertisol

Vertisols are widely used for irrigated rice in the Senegal River Basin, where farmers are increasingly reporting problems with soil salinization. Water movement at the onset of the growing season, when soils are dry and cracked, may have a large impact on soil salinity and water economy. Water movement and soil swelling processes were studied in a dry, cracked Vertisol in the Senegal River Valley. Surface irrigation and simulated rainfall (intensity 88 mm h(-1)) on cracked 2.25 m(2) plots (crack width 0.01-0.02 m; crack depth 0.3 m) resulted in crack closure within 4.5 h, starting at the soil surface. Soil swelling was separated into two components: (1) swelling of the 0.01-0.02 m border zone of a soil island (soil mass distinctively separated by cracks), and (2) swelling of the rest of the soil island as a whole. Soil swelling was heterogeneous, with a very rapid expansion of the bolder zone. At the moment of crack closure, the relative contributions of these two components to crack closure were respectively 80-90% and 10-20%; 24 h later these percentages changed slightly only. Maximum bulk linear shrinkage determined at the field level was only 7%. Results indicate the importance of rapid, local, heterogeneous swelling processes to water flow into cracked Vertisol, resulting in limited bypass flow. Use of bulk shrinkage curves and assumptions on isotropy of shrinkage and swelling processes when modelling water and solute flow in cracked Vertisols should therefore be done with great care. (C) 1997 Elsevier Science B.V.