- Title
- A global eco-hydro-geomorphic analysis in aspect-driven semiarid ecosystems
- Creator
- Kumari, Nikul
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- In semiarid regions, a small variation in the solar insolation has a profound impact on the topography via the feedback mechanism among the hydrology, vegetation, sediment transport, and landscape evolution. The emergence of aspect-controlled vegetation patterns caused by solar insolation in semiarid ecosystems leads to differentiation in available soil moisture and vegetation characteristics on opposing hillslope aspects. Previous studies have shown that in mid- to high-latitudes where available soil moisture is a limiting factor for vegetation growth, polar-facing slopes (PFS) develop denser vegetation cover than equatorial-facing slopes (EFS). However, these studies have been limited by observations at the field or hillslope scale and do not reflect during the entire year. Some recent findings have challenged this common paradigm and have shown that there may occur a seasonal reversal of vegetation greenness across opposing hillslopes in semiarid regions. In addition, transitions in vegetation patterns in semiarid landscapes can influence the dominant erosion processes on opposing hillslopes and result in hillslope asymmetry. This research aimed to explore different eco-hydro-geomorphic processes in the semiarid ecosystems, especially with aspect control. This research started by studying 60 catchments from 23 different locations from all continents, except Antarctica, to identify the vegetation greenness patterns across opposing hillslopes. The normalised differences vegetation index (NDVI) was used as a proxy for the vegetation greenness in catchments. A total of 18 years of mean monthly NDVI (2000–2017) data across PFS and EFS in all selected catchments were compared and analysed. Three different vegetation greenness patterns were observed: (1) PFS were always greener than EFS; (2) PFS were greener in summer, but EFS were greener in winter; and (3) there were almost negligible differences in vegetation greenness on opposing hillslopes. An overwhelming 70% of the total catchments showed higher vegetation greenness on EFS than PFS during the winter. These seasonal reversals in the vegetation greenness pattern on opposing hillslopes were associated with transitions from water‐ to energy‐limited conditions during wet winters. In addition, such contrasting behaviour in vegetation greenness was attributed to the precipitation timing and different growth responses of vegetation types on opposing slopes. These findings provide a novel approach towards understanding the vegetation pattern across opposing hillslopes in aspect-controlled semiarid ecosystems. Further, these differences in the vegetation greenness can provide variations in the erosion mechanisms on PFS and EFS. Over long timescales, this erodibility difference induced by the coevolution of vegetation and landforms is responsible for the emergence of hillslope asymmetry. The hillslope asymmetry index (HAI), which was developed to measure the magnitude of this asymmetry, is calculated as the ratio of the median slope angles of PFS and EFS. Although previous studies have documented hillslope asymmetry values for individual sites, no analysis has investigated the relationships of HAI with geographical, geomorphic, and climatic factors globally. Here, these relationships were investigated using a combined approach of observed data analysis and numerical modelling. The digital elevation model data (to compute HAI) and existing data on vegetation and climate were used for 75 different catchments worldwide, where aspect-controlled vegetation has been reported in the literature. The Channel-Hillslope Integrated Landscape Development landscape evolution model was used to understand the relationship of different climatic, geographical, ecohydrological, and geomorphic factors with hillslope asymmetry via a modelling approach. The majority of catchments showed a positive HAI, i.e. PFS were steeper than EFS, and approximately 13% of catchments showed a negative HAI. Further, the findings from the modelling approach suggest that uplift (function of elevation and slope), aspect-controlled hillslope diffusion, different cloudiness levels (i.e. percentage of diffused solar radiation) and latitude and precipitation played key roles in controlling hillslope asymmetry in the modelled landscapes. Thereafter, understanding of topographic asymmetries was extrapolated over young landscapes whose time of origin is known and has a minimum effect from non-eco-hydro-geomorphic mechanisms such as initial faulting and structural defaults. Cinder cones are one of the best examples of such young landscapes due to their small basin size, and uniform lithology and climate across the topography. We studied ~200 cinder cones in the San Francisco Volcanic Field, Springerville Volcanic Field, and Medicine Lake Volcanic Field from the United States, and Goran Heights in Israel to understand the ecohydrological and geomorphic processes of these cinder cones chronosequences. The HAI and mean vegetation cover of the PFS and EFS were estimated for the cinder cones and were related to several geographical, ecohydrological, and geomorphic factors to understand the major factors influencing HAI. PFS were found to be consistently steeper than EFS within the population of the older cinder cones with age greater than 1500 ky, whereas ~50% of the cinder cones showed negative HAI, showing that EFS were steeper than PFS for the younger cinder cones. A period between 1000 ky and 1500 ky was long enough to generate steeper poleward-facing aspects in the investigated cinder cones. These results improve understanding of the main factors contributing to hillslope asymmetry globally and have important implications for the analysis and modelling of coevolving landscapes.
- Subject
- vegetation; hillslope asymmetry; solar radiation; semiarid ecosystems; cinder cones; landscape evolution; ecohydrology; geomorphology
- Identifier
- http://hdl.handle.net/1959.13/1504308
- Identifier
- uon:55492
- Rights
- This thesis is currently under embargo and will be available from 30.06.2025, Copyright 2021 Nikul Kumari
- Language
- eng
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