Remote Sensing

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Monitoring of the spatial and temporal variations in soil salinity can benefit from remote sensed data. Commonly, most methods use satellite based data. Several methods to analyze satellite data exist, described below.


Crop response to soil salinity

In agriculture, salinity affects the production of crops negatively. Thus, in areas where soil salinity is known to be a problem, vegetation can be monitored to deduct the area affected by excess salts. A standard approach to monitoring vegetation is through the indicator called "normalized difference vegetation index" (NDVI). Using this indicator and linking it to soil salinity needs several assumptions to be met. The most important assumption is that salinity is the only reason for shifting vegetation patterns. This is an assumption that is in most cases incorrect, since the availability of water usually has a larger immediate effect on crop production. However, water availability and increasing soil salinity are usually related as well.

Soil salinity and declining water resources

Secondary salinity, or salinity that is introduced or increased through anthropogenic changes in water management, usually occurs in areas where water demand (potential evapotranspiration) is larger than water supply (rainfall). This is the reason to introduce irrigation, thus balancing the water supply with the required water demand to allow agricultural crops to be cultivated. Introducing a new water source introduces new volumes of salt in an area, as well as a transport medium (water) to mobilize existing salts. If sufficient natural or man-made drainage is present, and enough water is used to allow salts to "leach" through the soil, no salt accumulation affecting crop production will occur. However, when insufficient drainage is present, salts will accumulate, sometimes exuberated by the formation of shallow groundwater (water table less than 3 meter below the soil surface).

When the original volumes of introduced irrigation water decline (which occurs in most irrigated areas due to expansion of water demand in a watershed, either through increased population pressure, expanding irrigation areas upstream in the watershed, or through shifting weather patterns reducing water availability) while shallow groundwater remains, an increase of soil salinity in the root zone of crops is expected. Thus, monitoring shifting cropping patterns in areas with known salinity problems can be linked to increasing salinization.

Note that the remote sensing analysis of vegetation is only one of the inputs in the analysis of soil salinity, since knowledge on groundwater levels, irrigation water application depths, the functionality of natural or man-made drainage systems and water salinity are other factors that improve the accuracy of the determination of soil salinization.