Hydrologic connectivity and degradation thresholds in semiarid landscapes

The vegetation of dryland landscapes is particularly sensitive to climatic and/or anthropogenic pressures. Land degradation can cause simultaneous structural and functional changes in dryland ecosystems such as increased runoff, increased landscape, and soil resource heterogeneity, decreased secondary production and physiognomic changes. The amount and spatial arrangement of vegetation cover are closely linked to the process of resource redistribution. Disturbances, such as overgrazing and harvesting activities, can disrupt the original spatial structure of vegetation. Vegetation removal can generate increased overland flow and promote leak out of natural resources from the ecosystem by increasing landscape hydrologic connectivity. Thus, changes in vegetation cover can have profound effects on structural hydrologic connectivity and functionality of the landscape. In order to study the ecohydrologic responses of drylands and the effects of land degradation on these ecosystems, a combined structural-functional approach is needed. Thus, the main objective of this research is to investigate the structural and functional responses of dryland ecosystems to land degradation due to anthropogenic disturbances. Due to the wide variety of patterns they display, mulga landscapes provide an ideal setting for case studies aimed at investigating the spatial organization of dryland vegetation. This study was carried out in four study sites within two regions located on the central portions of Mulga (Acacia aneura) range distribution and Mulga lands bioregion in eastern Australia (Northern Territory, Lake Mere, Wallen and Croxdale) along a broad precipitation gradient (200- 500 mm/yr). These study sites can be considered as representative examples of Australian grazing rangelands. At each study site, several plots of the identical area but with varying degrees of vegetation disturbances were selected. For each study site, different datasets including daily rainfall and, high-resolution remote sensing products for DEM and vegetation index (NDVI) estimation during the period of 2001- 2013 were acquired to conduct the study. At first, the effects of changes in vegetation cover on the structural hydrologic connectivity due to land degradation were investigated. Then, the effects of these changes on the functional response of Mulga landscapes were estimated.