Skip to main content

Monitoring and mapping techniques

The assessment (extent) of desertification involves monitoring and mapping on various spatial and temporal scales. As direct monitoring/mapping of desertification is rather complicated, in most cases desertification indicators (see 5.2) are assessed. Depending on the (spatial) scale that needs to be monitored, different techniques are used. The Land Degradation in Drylands (LADA, see Appendix II) project aims to combine traditional and scientific knowledge to assess degradation severity and extent using a variety of techniques to measure environmental indicators, from local to national and international scales (Van Lynden and Kuhlmann, 2002). Van Lynden and Kuhlmann (2002) propose a combination of methods, including field monitoring, remote sensing, agricultural productivity change, expert opinion and land user perspectives. These techniques are briefly reviewed and discussed here. Modelling desertification (indicators) is covered in Chapter 6.

An overview of EU funded research into the monitoring and mapping of Mediterranean desertification can be found in Drake and Vafeidis (2004).

Field monitoring

Field surveys are still important and used in virtually all studies. The general disadvantages of field studies are the often high costs (due to instrumentation and personnel) and mostly small (local) scale. Advantages, however, include the very many parameters and processes that can be assessed, and although often biophysical characteristics of e.g. soil, landscape and climate are assessed, this approach is certainly not restricted to them. Human factors such as population dynamics, living standards etc. can be included (Van Lynden and Kuhlmann, 2002).

Indicators that are often assessed in field studies include rainfall characteristics, using rainfall gauges; vegetation status (e.g. vegetation cover, LAI, biomass etc); soil characteristics (e.g. soil moisture, aggregate stability, organic matter content); (ground)water salinization and landscape characteristics (mainly soil erosion features).

Methods that are required for the measurement of these parameters include (geo)statistics for soil sampling, a variety of measurement techniques for the assessment of erosion ranging from point, to plot and small catchment scales, laboratory analysis for soil properties etc. Many of these methods are also based, at least in their choice of where and when to measure, on expert opinion (see 5.3.2) and/or local knowledge.

Expert opinion

Qualitative assessment of degradation in the case of expert opinion is based on the perception by experts of the intensity of the degradation process (degree) and the impact on agricultural suitability, biotic function of decline in productivity (Van Lynden and Kuhlmann, 2002). An expert in this context is a scientist who has specific knowledge and experience in a certain field of work and specific geographical area (Van Lynden and Kuhlmann, 2002). Some degree of expert opinion, in any phase of a specific research or research project, is almost always applied. In some projects, expert opinion is explicitly named as a method of assessment. As has been said in the introduction, the GLASOD estimate of the extent of degraded drylands was based on expert opinion, eliciting information about the type, extent, degree, rate and cause of soil degradation over the last 50 years from over 250 soil scientists and environmental experts in 21 regions of the world (Oldeman et al., 1990; UNEP, 1997). By its nature, it is a qualitative and potentially subjective assessment (ISRIC, 2003). It is difficult to replicate; even if the same experts can be used, their perceptions of degradation may have changed unpredictably (van Lynden and Kuhlmann, 2002). An ongoing project that works with expert opinion is WOCAT (World Overview of Conservation Approaches and Technologies, see Appendix II; WOCAT, 2007). The WOCAT map method will also be used in DESIRE in combination with a method based on Remote Sensing (GLADA), see below.

Land user perspectives

It is now widely recognized that the views and interests of the land user as one of the most important stakeholders in the fate of the land is essential in assessing degradation and rehabilitation or prevention (Van Lynden and Kuhlmann, 2002; Geeson, pers. comm.., 2007). Land users often have the best local knowledge of land degradation and influencing factors. A disadvantage for the actual assessment phase might be the bias of the land user and his or her dependency on the outcome. However, the neglect of the land users' perception of (degradation) problems is perhaps one of the gravest omissions to date in land degradation and conservation research (Critchley, 2000). Above this, major advantages include more realistic measurements of actual field level processes, the assessment uses the integrated view of the ultimate client (i.e. the farmer or landowner) and results provide a far more practical view of the types of interventions that might be accepted by land users (Stocking and Murnaghan, 2001).

Remote sensing

The availability of remotely sensed data is increasing with the development of RS techniques and satellites. Pinet et al. (2006) give a summary of the theoretical background of Earth surface spectroscopy. Lantieri (2003) presents an exhaustive overview of remote sensing tools available today, including information on resolution, spectral bands, revisit capacity, swath, price levels, catalogues access and websites. The most common and cost effective remote sensing data used are high resolution (HR) and in particular Landsat TM (Lantieri, 2003). Radar images can also be used in cloudy areas - which in general has less relevance in dryland areas - but with a much lower performance than optical data (Lantieri, 2003). The remotely sensed data do not correspond directly with the information needed and must be interpreted to derive soil and vegetation parameters (Hill et al., 1995). For example, reflection data need to be converted to properties relevant for the soil erosion process, requiring detailed fieldwork to establish relations to be used for the conversion (Lacaze et al., 1996). On the other hand, the advantages of remote sensing are that large areas can be covered at relatively low cost, with a high temporal frequency. Applications of remote sensing for drylands include land cover, including vegetation types; land form and landscape; vegetation activity and growth; rainfall and related droughts; soil types and state (moisture, level of erosion); indicators based on climate and ecological modelling. It is possible to map directly land degradation features from remote sensing images, especially using HR or very high resolution (VHR) data. These features include (Lantieri, 2003):

  • wind erosion patterns, in particular over large areas;
  • salinization patterns in field crops of large irrigated schemes;
  • overgrazing features, shown by low cover grasslands around animal paths for example;
  • sedimentation of lakes or rivers and consequently upstream soil erosion;
  • soil water erosion patterns, but only when of great size and over large areas (gullies);
  • areas already burnt or areas subject to wildfire.

Under the GEF/UNEP/FAO project Land Degradation in Drylands (LADA), ISRIC uses Remotely Sensed NDVI data to assess changes in Net Primary Productivity (NPP) and Rain Use Efficiency (RUE) as proxy indicator for land degradation or improvement. This method (GLADA) will also be applied to DESIRE study sites.

In should be kept in mind, however, that field checking is important to characterize better the degradation types. Above this, not all land degradation features can be seen on satellite imagery, for example, sheet erosion, rills, fuelwood depletion, loss of soil fertility are not visible on RS data.

Projects that have focussed on monitoring desertification with the use of remote sensing techniques include ASMODE, CAMELEO, DESURVEY, DEMON I and II and DESERTSTOP (see Appendix II)