Associate Professor Patricia Saco

Copyright © 2019 The Modelling and Simulation Society of Australia and New Zealand Inc. All rights reserved. Contrasts in insolation lead to the development of aspect-controlled ecosystems characterized by heterogeneity in vegetation type and density in semi-arid ecosystems. The aspect-controlled solar radiation creates variation in the type and amount of vegetation across the two opposite facings of the hillslopes. In the Southern Hemisphere (SH), the north-facing slopes (NFS) have an abundance of paleotropical xeric biota, whereas the south-facing slopes (SFS) have higher densities of mesic temperate species. The reverse patterns are mostly observed in the Northern Hemisphere (NH). In the SH, SFS are dominated by the evergreen sclerophyllous woodland, while open scrub vegetation with spiny shrubs, sub-shrubs, and small trees exist on the NFS. This general vegetation pattern creates differences in erosion control and resistance on different slopes, and thus the underlying landscapes evolve differently. Although many previous studies have focused on aspect-controlled vegetation growth in the NH, there have been limited studies in the SH, especially in Australia. Remote sensing provides one of the best options to capture the long-term biomass changes over the large spatial and temporal coverage. The normalized difference vegetation index (NDVI) is based on the relationship between the reflectance of the red and near-infrared bands of multispectral sensors, and it can be used due to its computational simplicity and easy accessibility. In this study, we considered two catchments, Mount Wilson, South Australia and Risdon Hills in Tasmania to study the long-term spatial and temporal variation in NDVI at these catchments. Both sites are unaffected or minimally affected from anthropogenic activities upon visual inspection through Google EarthTM, in addition to reviewing both sites from the literature. We also explored how the precipitation and potential evapotranspiration patterns at these sites affect the vegetation growth during the year. In this study, we extracted NDVI values derived from Landsat 5, 7, and 8 (obtained from Google Earth Engine) for a 18-year period (2000-2017) for both catchments. Thereafter, we used 30-m SRTM DEM to calculate the aspect and slope datasets for two locations. With the aspect data classified, the vegetation index NDVI is computed for each slope, NSF and SFS. We compared and contrasted the inter-annual variability in NDVI at the two sites to capture the temporal variation in NDVI. We have also introduced NDVIdiff as the difference between NDVI at NFS to SFS, where NDVIdiff > 0 states that NDVI is higher on NFS than SFS and vice-versa. The spatial NDVI is extracted for the summer and winter months, November and June, respectively, to see the seasonal NDVI at each catchment. The results show that the Mount Wilson site (~35°S) has higher NDVI values than the Risdon Hill site throughout the year though receiving similar annual precipitation. It is observed that the Mount Wilson site shows approximately similar NDVI on NFS and SFS in the austral summer period. However, in the winter season when seasonal total precipitation exceeds total PET demand, the NDVI on NFS is comparatively higher than on SFS, which is attributed to differences in vegetation phenology on opposing hillslopes and relatively more incoming solar radiation on NFS than SFS. On the other hand, the site at Risdon Hills (~42°S) has relatively lower range of NDVI at both NFS and SFS, and NDVI at NFS and SFS does not vary noticeably. Further, the spatial NDVI patterns at both locations also illustrate similar behaviour, following the temporal patterns at both locations.

Copyright © 2019 The Modelling and Simulation Society of Australia and New Zealand Inc. All rights reserved. Soil moisture in semi-arid areas plays a critical role as it regulates numerous ecohydrologic processes in land surface hydrology, subsurface hydrology, and vegetation dynamics. Studies on soil moisture distribution and dynamics currently rely on data obtained using three types of approaches: In situ (generally point-scale) measurements, remotely sensed observations, and modelling approaches. The spatial variability of soil moisture plays a vital role in the estimation of land surface fluxes (evapotranspiration (ET) and runoff) due to the non-linear relationship between soil moisture and the associated physical processes. Understanding this variability is essential for the optimal management of water resources and ecosystem sustainability. Although a considerable amount of work has been done on the subject, the ability to understand and characterize the mechanisms that determine the distribution patterns of soil water content still remains a challenge at the centre of hydrological research, especially for ungauged catchments. It is necessary to understand the spatial variability of soil moisture and its influencing factors, which will provide a basis to improve our understanding of hydrological, biogeochemical processes, and lateral and subsurface flow processes. The effects of several factors that control soil moisture variability (SMV) in semi-arid landscapes (microclimate, vegetation, topography, soil depth, soil texture, etc.) have been documented in previous work. However, the control of latitude on SMV under different environmental conditions still remains poorly understood. Latitude significantly affects the availability of water and energy as the global distribution of solar radiation varies from the equator to higher latitudes. Latitude has a dominant control on the availability of water because of the varying amount of solar radiation on north-facing slopes (NFS) and south-facing slopes (SFS), which influences soil moisture variations. This study focusses on evaluating and comparing the effect of latitude on SMV, and its control on soil moisture patterns. To this end, we use a modelling framework to capture the joint effects of aspect and latitude on SMV. We used the Bucket Grassland Model (BGM), equipped with a vegetation dynamics component, to analyse soil moisture patterns and variability at various latitudes (45°N, 34°N, and 15°N). The main objective of this study is to investigate changes in soil moisture patterns at various latitudes and differences in SMV on the different aspects for a synthetic domain. We conducted different simulations as a sensitivity analysis (at various latitudes) using BGM to study the effect of aspect-related soil moisture variations in a semi-arid landscape. The latitudinal patterns of modeled soil moisture are analysed, and distinct variations are identified in the SMV. The results show that water stress varies with aspect and are affected by latitude, which in turn affect the SMV. Further, they show that SMV increases moving towards higher latitudes. Also, aspect-related soil moisture differences are enhanced at higher latitudes. Therefore, it is not possible to characterize soil moisture variations or model surface hydrological processes at the catchment scale, without explicitly accounting for aspect, particularly in ecosystems where the aspect has a dominant effect.

Copyright © 2019 The Modelling and Simulation Society of Australia and New Zealand Inc. All rights reserved. Pacific Islands are one of the world hotspots for climate change, with sea level rise (SLR) and increases in tropical cyclones (TC) activity posing a serious threat to coastal areas and ecosystems. Precipitation and extreme sea level events associated with TC generate floods that cause damage to agriculture, home and businesses and also produce considerable amounts of sediment that end up in the adjacent coastal areas. Our study focuses on coastal wetlands that receive sediments from the Dreketi River catchment on the northern coast of Vanua Levu, Fiji which are likely to be heavily affected by climate change. Recent studies have identified this area of the coast as a storm tide high-risk zone, and also that the Dreketi River catchment contributes most of the sediment to the adjacent Great Sea Reef (GSR) or Cakaulevu. The purpose of this work is to identify the impact of TC on the annual sediment yield through a physically-based hydro-sedimentological model. To address this, the period from 1970 to 2017 was simulated daily with SWAT, obtaining flow and sediment discharges at the outlet of Dreketi River catchment. For the same period, the cyclones within a radius of 600 Km of the barycentre of the catchment were analysed using the Southwest Pacific Enhanced Archive of Tropical Cyclones (SPEArTC). Two types of analysis were performed. The first one focused on the meteorological data, and the aim was to relate the maximum rainfall in the catchment with TC. The second one was based on the results of the hydro-sedimentological model assessing two aspects; i) which percentage of the annual sediment budget can be explained by TC, and ii) in how many cases the maximum annual sediment yield is due to a TC. Regarding the meteorological data, three meteorological stations were analysed with focus on the maximum daily rainfall. It was found that a TC caused the extreme values in each station in 10, 13 and 15 out of 45 years, respectively. However, the modelling results showed that on average 14% of the total annual sediment yield is related to TC and that TC caused the maximum annual sediment discharge in 19 out of 45 years (42%). These results indicate that even though TCs could not always generate the highest daily value during a year, due to the duration of the event and its intensity they have a significant impact on the annual sediment budget.