Remote sensing of desert boundaries: Challenges, Concepts and Methods

Hot deserts cover between 19 and 25 million square kilometers of the Earth land surface. Climate change together with impacts from anthropogenic activities which intensify with the growing world population are claimed to cause desertification. One interpretation of desertification processes concern shifting of the deserts’ boundaries into semi-arid zones. However, “ the delimitation of desert areas is difficult, particularly the location of the outer boundaries.... desert boundaries are often considered as shifting zones of transition rather than lines clearly demarcated by climate or by abrupt changes in species  or associations. Transitional boundaries may result from human impact or from decadal climatic fluctuations.” (Laity, 2008). In broad terms, desert boundary bounds  terrain which may not sustain natural vegetation growth. When considering herbaceous plants, such bound shifts locally in response to small amounts of rainfall. Shrubs’ extents are bounded by sub-surface water accumulation which is affected by hydrology, topography and yearly rainfall fluctuations. Drought tolerant  dwarf-shrubs may spread quickly into bare soils during rainy years and sustain there during long draughts. These growth forms’ and their  mixed patterns responses to periodic, seasonal and annual precipitation fluctuations are thus complex and highly vary in space and time. Adding to this complexity anthropogenic impacts, such as from grazing, wood cutting and fire, make the search for desert boundaries a challenging  task. Remote sensing offers  data and tools for  the search for such boundaries across wide regions. Landsat TM is instrumental for this purpose with its continuous coverage since 1985 at moderate resolution.  Seasonal / phenological vegetation cover fractions or NDVI allow  for differentiating between growth-form patterns and their transitions. The following four conceptual methods were developed utilizing multi-date Landsat TM imagery for discovering such transition zones across  desert fringes:

• Extreme rainfall conditions:
  • Total (all growth forms) green cover at extremely high winter rainfall year: reveals the boundary between the maximal extents of vegetation and the areas which has very low rainfall  or cannot support vegetation growth.
  • Shrubs cover at years of extremely low rainfall: reveals the boundary of sustainable and resilient shrubs.
• Vegetation drying rates at the beginning of the dry season (spring) indicate differences in soil chemical and physical properties . Transitions from clay soils and those of high organic matter to lithosols and rocky surfaces can be clearly detected.
• Winter trends of green vegetation change as a function of the  progression of rainfall accumulation may reveal transition from phrygana to Mediterranean shrublands. 
• Imagery spatial erosion and dilation of patchy vegetation patterns may allow differentiation  between  areas according to their draught  recovery potential: high for dense and large patches and low for small and sparse shrubs.

The conference presentation will demonstrate the results of applying these methods for years of average, high and low rainfall across  a Mediterranean to arid gradient  in the south-eastern side of the Mediterranean basin