Soil chemical aspects of water management: modeling topsoil water, salt and sodicity dynamics

In large areas worldwide, both in semi-arid and humid climates, the availability of good quality water for primary production in agriculture or natural vegetations is limited. Whereas soil and groundwater pollution are nowadays general problems, much larger areas have to deal with the hazards of soil and groundwater salinity. For soil types that are susceptible to physical degradation in view of their swelling and shrinking behaviour, also the salinity associated hazard of soil sodicity needs to be anticipated. Sodicity related soil physical degradation may be quite irreversible and is a major candidate for preventive rather than curative measures. The process of salt displacement has received much attention, and spatial variability has been given explicit attention by Dagan and Bresler (1979). Temporal variability of the boundary conditions has been investigated only recently, when both primary and secondary salinity were studied with so-called minimalist ecohydrological modelling (Suweis et al., 2010, Shah et al., 2011, Vervoort and Van der Zee, 2012). This work emphasized the atmospheric forcing of erratic rainfall on water and salt dynamics in the rootzone layer, either envisioned as a Poisson process, or with seasonal periodicity. Taking such erratic rainfall as well as spatial variability of the depth of the groundwater level into account, an impression can be obtained of the importance of different sources of variability. For our parameterization, we find that both spatial and temporal variability were responsible for the variation in root zone salinity, C, but that root zone sodicity, expressed with the Exchangeable Sodium Percentage, ESP, depends predominantly on spatial variability. The difference is understandable from the buffering mechanisms involved with C and ESP.