312
A.M.J. Meijerink
(e.g. band 6 of TM) or aircraft thetmal imagery. In cold climates, places with
groundwater emerging from some depth will be watmer than the surroundings and
in summer it is the other way around. However, thetmal imagery is difficult to
interpret and therefore results are usually compared with those of multispectral
classifications in the optical domain. A fairly good relation between the two was
found by Peters and StuUtmatl (1989) in The Netherlands and by Bobba et a!.
(1992) in an area in Canada. However, vegetation may confound the situation.
High transpiration results in relatively low temperatures (in fact, emissivity) but
this is not necessarily restricted to discharge areas.
Groundwater leakage and upwelling in coastal seas can be detected on thetmal
imagery, if sufficient thetmal contrasts exist at sea surface between upwelling
groundwater with low density and surrounding sea water with higher density. Such
conditions may exist when the aquifer is good and when groundwater is under
pressure, rests with sufficient thickness on the salt water interface, and can escape
through localized pathways, such as faults, fractures, karst openings.
14.3 Aspects of water budgets
The above stages lead to a regional hydrogeologic overview. Based on this, areas
with groundwater potential can be identified for further study as well as evaluation
of the groundwater vulnerability to pollution. Numerical groundwater models may
be used if sufficient data is available, whereby remotely sensed data plays a role in
assessing upper boundary conditions, in particular groundwater drafts for irrigation
and recharge.
14.3.1 Groundwater irrigation drafts
In many parts of the world, records for irrigation drafts are lacking and for the
development of a groundwater model, both steady state and transient, this
infotmation is required. The procedure involves two steps, first the detetmination
of the acreage involved, followed by a calculation of the crop evapotranspiration.
The size of the irrigated areas can be determined accurately on multispectral
satellite data if the right time is selected. There should be appreciable contrast
between green irrigated crops and non-irrigated cover.
A Notmalized Vegetation Difference Index (NDVI) image is generally sufficient
to discriminate the two during the dry season, Fig.14.5.
All fields with crops irrigated by groundwater in this granitic area are shown by
a white tone, resulting from separating (slicing) the high NDVI values from the
remainder on the histogram, which showed two separated populations (irrigated
and non-irrigated) in this case.
In more complex cases, supervised multispectral classification may be required
for the determination of the areas irrigated by groundwater. Irrigation by diversion
of water from local rivers, mixed with irrigation from groundwater confounds the
situation (as is the case in the area shown by Colour Plate 14.A) and usually field
work is required for the separation of the two.
A.M.J. Meijerink
(e.g. band 6 of TM) or aircraft thetmal imagery. In cold climates, places with
groundwater emerging from some depth will be watmer than the surroundings and
in summer it is the other way around. However, thetmal imagery is difficult to
interpret and therefore results are usually compared with those of multispectral
classifications in the optical domain. A fairly good relation between the two was
found by Peters and StuUtmatl (1989) in The Netherlands and by Bobba et a!.
(1992) in an area in Canada. However, vegetation may confound the situation.
High transpiration results in relatively low temperatures (in fact, emissivity) but
this is not necessarily restricted to discharge areas.
Groundwater leakage and upwelling in coastal seas can be detected on thetmal
imagery, if sufficient thetmal contrasts exist at sea surface between upwelling
groundwater with low density and surrounding sea water with higher density. Such
conditions may exist when the aquifer is good and when groundwater is under
pressure, rests with sufficient thickness on the salt water interface, and can escape
through localized pathways, such as faults, fractures, karst openings.
14.3 Aspects of water budgets
The above stages lead to a regional hydrogeologic overview. Based on this, areas
with groundwater potential can be identified for further study as well as evaluation
of the groundwater vulnerability to pollution. Numerical groundwater models may
be used if sufficient data is available, whereby remotely sensed data plays a role in
assessing upper boundary conditions, in particular groundwater drafts for irrigation
and recharge.
14.3.1 Groundwater irrigation drafts
In many parts of the world, records for irrigation drafts are lacking and for the
development of a groundwater model, both steady state and transient, this
infotmation is required. The procedure involves two steps, first the detetmination
of the acreage involved, followed by a calculation of the crop evapotranspiration.
The size of the irrigated areas can be determined accurately on multispectral
satellite data if the right time is selected. There should be appreciable contrast
between green irrigated crops and non-irrigated cover.
A Notmalized Vegetation Difference Index (NDVI) image is generally sufficient
to discriminate the two during the dry season, Fig.14.5.
All fields with crops irrigated by groundwater in this granitic area are shown by
a white tone, resulting from separating (slicing) the high NDVI values from the
remainder on the histogram, which showed two separated populations (irrigated
and non-irrigated) in this case.
In more complex cases, supervised multispectral classification may be required
for the determination of the areas irrigated by groundwater. Irrigation by diversion
of water from local rivers, mixed with irrigation from groundwater confounds the
situation (as is the case in the area shown by Colour Plate 14.A) and usually field
work is required for the separation of the two.