WET LAND AND SALT CRUST FORMATION BASED ON LANDSAT TM DATA
Alavi Panah, S.K. *
* Assistant Professor, Iran Desert Research Center, University of Tehran, Tehran , Iran.
E-mail: salavipa@chamran.ut. ac.ir
Key words: wet land, salt crust, TM data, false color composite
Abstract
In this study, in order to study the causes of wet land and salt crust formation in Central Iranian Deserts, different sources of data
such as remotely sensed data, geologic geomorphologic and topographic maps were used. The result of visual interpretation and
comparison between the false color composites in the study period led us to classify the TM images in the meaningful way. The
visual and supervised classification revealed some changes in land cover types in the study area. Based on the obtained results we
concluded the extension of cultivated lands which are irrigated by flood irrigation system and water erosion have the most important
role in wet land and salt crust formation. We also concluded that remotely sensed data including TM reflective and TM thermal
bands and geographic information system are useful tools to study the effect of microtopography and multitemporal analysis.
1. INTRODUCTION
Wet land is a term applied to ecosystems dominated by water in
their formation, development and ecological functions
(Williams, 1993). Hydrophytic vegetation, hydric soils and wet
land hydrology are of the three basic elements for identifying
wet lands. Precipitation, topography, soil permeability and plant
cover may influence the wetness of an area. Causes of wet land
formation are many and often complex. Human has accelerated
or slowed these processes in a number of ways, sometimes
dramatically. The places where wet land conditions occur in
desert and where the water table lies at shallow depth, the
capillary rise play an important role in the accumulation of salt
on the surface. With evaporation of water from the surface, salt
concentration increases until crystals of the halite begins to
precipitate at its saturation point. The crust that grows from
evaporation of saline ground water is common in the Iranian
Desert. The change of dry land to wet land and then wet land to
salt crust have been reported in some places in the Iranian
deserts and playas (Alavi Panah, 1997).
Of the many land surface parameters that can be derived from
remotely sensed data, surface moisture is one of the least well
characterized. Existing studies (Carlson et al, 1992) have
produced moisture estimates from remotely sensed inputs using
coincident field measurement of soil and foliar moisture.
A soil line is given by the pixels that contain bare soil and dry
vegetation (Jasinki and Eagleson, 1989). For vegetation index
(and soil line) noise is introduced by variations in soil
parameters (moisture content, soil color and organic matter and
atmospheric influences). Therefore a good understanding of wet
land and soil with different moisture condition in relation to
other conditions of soil color and texture and structure may be
necessary.
There are three types spectral indices of soil moisture: those
employing the absolute in one or more bands; those that
examine the contrast between the visible and near infrared
bands to the middle infrared; and those which contrast
information at mid-infrared wavelengths only (Musick and
Pelletier, 1988). It has long been established that reflectances at
mid-infrared wavelengths respond primarily to water content
(Verbyla, 1993). Verbyla (1993) contents that MIR wavelength
are more useful under an extreme range of conditions, but fore
less divergent situation, complication may arise as a result of
changes in soil texture, plant canopy, and train conditions.
2. STUDY AREAS
2.1. Meybod depression: Our field work was carried out on
salt crust regions in the Ardakan area. Among the places where
salt crust has been developed, Meybod depression was
considered for a more detailed interpretation of salt crust
formation using standard FCC,s of MSS and TM images. Based
on the main explanatory report of the soil survey, the main
characteristics of soil surface horizon (layer) are: medium soil
surface texture, which overlaid on clay loam soil. The water
table depth is about 1.2 m and a hardpan is formed at a depth of
about 3 m. In the lowest part of the depression, the salt crust
with dark color was developed. The depth to moisture is
typically only a few centimeters below the dry salt crust, due to
recharge by acquifers.
2.2. Lut desert: This area located between Yardang and sand
dune regions which is characterized by an extremely arid
climate, with excessive summer heat, winter temperature below
freezing point. The Yardang and sand dune are rare and striking
feature. Lut desert includes great diversity of hydroaeolian
process with a very interesting pattern of landforms. Yardang
which is a wind-abraded ridges of cohesive materials is one of
the most striking geomorphologic feature in the Iranian deserts.
Although generally, the Yardangs have been formed by wind
erosion, an alternative explanation for aligned topography in
Lut involves collaborative wind and stream action. Yardangs
are parallel ridges of lacustrine sediments between wind-
scoured furrows aligned with the dominant wind. Wind and
water as exogenetic processes, work landscapes, such as
tectonic and volcanic activity. Deserts have experienced great
temporal and spatial variability of these endogenetic process in
Iran, the Lut Desert are apparently tectonically unstable area
(Cooke, 1993).
—510—
tay KA aS RE. cR a EE
b CD WO, mS
thz
the
