Erosión y desertificación.-Contribution to the edaphic component definition in the desertification susceptibility index

Abstract

ABSTRACT Many of the biophysical processes involved in the scope of desertification depend on the hydric characteristics of the soils that impact on vegetation cover. To protect soils against desertification, it is necessary to understand how some of these characteristics (such as water storage) interact in a complex and integrated chain of degradation processes. Several works have been developed to contribute to the definition of a Index of Desertification Susceptibility (DSI) expressed as a function of several components, climatic, edaphic, vegetative and slope. However, the various built-in edaphic components already defined, leave aside the water retention in soil. Furthermore, these components only focus on the characteristics of the uppermost surface soil layer (A-layer). In fact, desertification is simultaneously cause and consequence of the depleted soil water retention with a positive feedback on the plant life and on the hydrological cycle. This work intends to respond to the question of assessing if the B-layer exerts a significant influence in the definition of the edaphic component of the DSI. This may reflect the influence of the B-layer on the soil resilience to external factors. An experimental study has been performed on several profiles (n = 50) of representative soil units at Mértola, Southern Portugal (a region classified as having high DSI). Soil columns, have been delimited having at the upper and lower boundaries respectively the soil surface and the C-layer. The total volume (VT) of the Soil Available Water Content (AWC) was calculated as the sum of the elementary volumes (in the case, VA and VB) stored in each layer of the prospected soil column. Furthermore, volumetric ratios VA/VT and VB/VT have been determined. A possible existing empirical relationship between the ratios VA/VT and VB/VT, was investigated aiming to establish the relative importance of each term to the total volume VT. The results reveal a clear linear trend between VA/VT and VB/VT suggesting that the B-layer assumes the greater importance in terms of the holding water capacity of soil. It was found that except for soils constituted only by the A-layer, or when this layer is deeper than 45 cm, the relative weight of the B-layer is preponderant. For the most representative soil units of the study area, the referred relationship is persistent and is dependent on the layer thickness. To conclude, the foregoing relationship allowed identifying the soil units with greater desertification susceptibility through their inability to store sufficient water to maintain vegetation. It also allowed one to identify soil units whose B-layer assumes the greater importance in this soil function, and therefore should be take into account in defining the edaphic component of DSI. Thus, it is understood that the results of the present exercise have contributed to a better understanding of desertification processes, allowing to outlining strategies of action and implementing technologies for soil and water conservation, more appropriate to each situation. A more extended and detailed study will have to be done in order to more effectively contribute to upscale the results to the regional level.