531result of surface drainage may be sufficient to ‘reclaim’ salt-affected land when the
interaction between the two problems is removed.
Mapping such salt-affected land onto aerial photography is a common form of
ground truthing for the methods mentioned below. However, it is subject to operator
bias because it is subjective. This technique can be used to map the changes in
extent of known areas.
5.1.3 Groundwater Level and Salinity Measurements
For dryland salinity caused by high groundwater levels, the ultimate test of salinity
is the presence of groundwater at a depth that would allow saline water to rise by
capillarity into the root zone of plants or to the soil surface. For clayey soils this may
be 2 or 3 m but for sandy soils it may be less than a metre. Drilling observation
bores, even shallow ones, is expensive and it only provides a point assessment so
this is not common for mapping large areas. It is, however, a very good way of
assessing changes over time, enabling the processes to be understood and engaging
farmers in the assessment of extent and risk, and therefore comprises a good moni-
toring tool.
5.1.4 Electrical Conductivity Using Geophysical Instruments
When detailed maps are required of both the extent and intensity of dryland-salinity
for an area it is possible to use geophysical equipment to measure the apparent con-
ductivity of the soil to depths ranging from 0.2 to more than 200 m (George et al.
1998 ; Farifteh et al. 2006 ). Airborne and ground based electromagnetic techniques
generally work well in acid to neutral soils but ground-based systems may over
estimate salinity in sodic and alkaline soils. For example, results from shallow
EM-systems become increasingly confounded by the presence of lime – an insolu-
ble salt that has little or no adverse effect on crop growth. As well as the salinity of
the soil, readings are affected by the amount and type of clay and the soil needs to
be wet enough to allow an electrical flow to be induced by the measurement equip-
ment. Changes in salinity over time can also be recorded by well-calibrated electro-
magnetic induction equipment.
5.1.5 Airborne Remote Sensing
Multispectral or hyperspectral sensors can be used to measure the amount of indi-
vidual salt in soils (sodium, chloride, sulphate, etc.) with the spatial resolution
depending on the type of instrument and the height at which the imagery is acquired
(Howari 2003 ). Often these instruments provide a lot of detailed information (e.g.
soil properties, mineralisation) but their expense is only justified in research areas
or where land is very valuable.
Salinity in Dryland Agricultural Systems: Challenges and Opportunities