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Abstract
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≅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 km2 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.
Acknowledgements
The research for this paper was originally carried out as part of the MEDALUS (Mediterranean Desertification and Land Use) collaborative research project and a post‐doctoral fellowship, provided by the Commission of the European Communities (contract number EV5V‐CT‐94‐5242) to M. M. Boer. Further support was received from the LADAMER project (Land Degradation Assessment in Mediterranean Europe). MEDALUS and LADAMER were funded by the European Union, respectively under its Environment Programme (contract EV5V‐CT92‐0128), and its Global Monitoring for Environment and Security Action Plan (contract EVK2‐2002‐0599), the support being gratefully acknowledged. We thank colleagues at EEZA in Almería, Spain, and former colleagues (of M.M.B.) at CSIRO in Alice Springs, Australia, for their helpful comments and suggestions on earlier versions of the manuscript.