The assessment of land degradation is greatly hindered by serious weaknesses in our knowledge of the current situation (Pagiola, 1999; Branca, 2001). According to some analysts, land degradation is a major threat to food security, it has negated many of the productivity improvements of the past, and it is getting worse (Pimentel et al., 1995; UNEP, 1999; Bremen, Groot and van Keulen, 2001). Others believe that the seriousness of the situation has been overestimated at the global and local level (Crosson, 1997; Scherr, 1999; Lindert, 2000; Mazzucato and Niemeijer, 2001).

...Despite these improvements in techniques of assessment, a number of serious difficulties remain in using them for perspective analysis. They are still heavily based on expert judgement, for entirely justified reasons. There is no clear consensus as to the area of degraded land, even at the national level. In India, for example, estimates by different public authorities vary from 53 to 239 million ha (Katyal et al., 1997). Land degradation is very variable over small areas, e.g. as a consequence of differences in soil type, topography, crop type and management practice, so impacts are highly site specific. They can also be time specific: soil erosion impacts can vary in the short term because of interannual differences in rainfall, with no yield reductions in high rainfall years but appreciable losses in dry years (Moyo, 1998). Some forms of degradation are not readily visible, for example, soil compaction, acidification and reduced biological activity. Lack of data and analytical tools for measuring such differences prevents or limits estimation of their impact on productivity, and makes scaling up to the national or regional level problematic. There are no internationally agreed criteria or procedures for estimating the severity of degradation and most surveys do not make reliable assessments. Few if any countries make systematic assessments at regular intervals that permit estimation of rates of change. Finally, major changes in socio-economic conditions, improved market opportunities, infrastructure and technology over the medium to long term can induce farmers to overcome degradation (Tiffen, Mortimore and Gichuki, 1994).