Assessment of dryland condition using spatial anomalies of vegetation index values

A landscape's long-term capacity to retain, utilize and recycle local resources is an objective basis for assessing its ecological functionality or condition. In subtropical and tropical drylands, where plant growth is moisture-limited for much of the time, land condition is reflected in the local water balance. The ratio of long-term actual evapotranspiration and precipitation (E-a/P) is proposed as an objective indicator of dryland condition. A spatial modelling framework is developed for the quantification of E-a/P over large areas using remotely sensed vegetation density patterns. Model parameters are defined by two particular situations: (i) non-vegetated sites, where E-a/P depends on the long-term runoff coefficient of bare soil surfaces (RCbs, and (ii) non-degraded sites with a vegetation density close to the potential value for which E-a/P congruent to 1.0. Specht's evaporative coefficient is used as an independent variable for the prediction of the potential vegetation density, whereas RCbs is estimated with the curve number method. The performance of the method was evaluated in a 900 km(2) area in south-east Spain, where predicted land condition was found to be in good agreement with qualitative field observations on the nature and intensity of land degradation processes.