Understanding the spatial and temporal variation of soil salinity forms a crucial part for developing appropriate management strategies to control and prevent its spread. In order to understand salinization and its causes, use of rapid, efficient and reliable methods to monitor this process are essential. Soil salinity monitoring is thus described by Metternicht and Zinck (2003) as identifying places where salt accumulate first, and then detect its temporal and spatial distribution to track its changes and anticipate further expansion. In that respect remote sensing technique plays an important role, but it is more useful for surface observation as it lacks capabilities to extract information from the third dimension (depth) of 3-D soil body. Then modeling becomes a fundamental technique to overcome the remote sensing constraints related to soil depth by complimentary use of these methods. Peng Xu and Yaping Shao (2002)clearly describe the process of salinization to be closely related surface-soil and groundwater hydrological processes. This stems from the fact that movement of water in the landscape is mainly responsible for the transportation of salts. From that perspective three main regions of interest can be considered in modeling salinization, namely:
The vertical exchange of salts between the groundwater system and unsaturated zone;
The accumulation of salt in the vegetation root (vadose) zone; and
The horizontal transportation of salts through groundwater movement, surface runoff and stream flow.
It is thus apparent that modelling salinization poses difficulties and challenges, due to the complexity of the hydrological processes, as well as soil properties and their variability. These modeling difficulties are further aggravated by external forces such as the atmosphere and human activities which also influence the soil and hydrological process. Therefore interactions between the atmosphere, the land surface and groundwater system including human activities need to be carefully considered to better model salinization process.
SaltMod Model
SaltMod is a computer program designed for the prediction of the salinity of the soil moisture, ground water and drainage water, the depth of water table, and the drain discharge in irrigated lands. It considers different geo-hydrologic conditions and varying water management options, and several cropping rotation schedules. In terms of water management options it also includes irrigation by ground water, subsurface drainage water from pipes drains, wells and ditches[26].
The program is designed for simplistic operation to promote use by technicians, engineers and project manager[26]. Contrary to other computer models that use short term time steps, and require complex daily data of hydrologic phenomena and soil characteristics that can vary greatly over short spatial intervals, SaltMod uses simple input data that are generally available, or can be estimated with reasonable accuracy, or can be measured with relative ease[26]. It uses long term time steps to predict salinity based on general trends rather than exact predictions. It also takes into account farmers responses regarding water logging, soil salinity, water scarcity and over pumping from the aquifer.
This computer program was designed and developed at the International Institute for Land Reclamation and Improvement (ILRI), Wageningen by R.J. Oosterbaan and Isabel Pedroso de Lima. The model is being improved upon by its developers and as such the present model is version 1.3 which is extension of earlier version. Further, a combination of SaltMod and a ground water flow model is being pursued which is believed to provide more flexible in the description of the depth of the water table. A provisional version of the combined model is now available under the name Sahysmod (Spatial agrohydro- salinity model), Oosterbaan, R.J. et. al. (2002) .
The SaltMod model is based on three component systems, viz. water balance (hydrological) model, salt balance model and seasonal agronomic aspects. Therefore the model would require input data that is related to agricultural aspects, hydrological data, and soils characteristics. The general principles and assumptions of the model as given by Oosterbaan R.J. et. al. (2002) are discussed in the subsequent sections.
References
Metternicht, G.I. and J.A. Zinck, Remote sensing of soil salinity: potentials and constraints. Remote Sensing of Environment, 2003. 85(1): p. 1-20.
Xu, P. and Y. Shao, A salt-transport model within a land-surface scheme for studies of salinisationin irrigated areas. Environmental Modelling & Software, 2002. 17(1): p. 39-49.
Oosterbaan, R.J., SALTMOD: Description of Principles, User Manual, and Examples of Application. 2002: Wageningen, The Netherlands.
Development of Methodologies for
Land Degradation Assessment Applied to
Land Use Planning in Thailand
Metternicht, G.I. and J.A. Zinck, Remote sensing of soil salinity: potentials and constraints. Remote Sensing of Environment, 2003. 85(1): p. 1-20.