A New Framework for Modelling and Monitoring the Conversion of Cultivated Land to Built-up Land Based on a Hierarchical Hidden Semi-Markov Model Using Satellite Image Time Series

Large amounts of farmland loss caused by urban expansion has been a severe global environmental problem. Therefore, monitoring urban encroachment upon farmland is a global issue. In this study, we propose a novel framework for modelling and monitoring the conversion of cultivated land to built-up land using a satellite image time series (SITS). The land-cover change process is modelled by a two-level hierarchical hidden semi-Markov model, which is composed of two Markov chains with hierarchical relationships. The upper chain represents annual land-cover dynamics, and the lower chain encodes the vegetation phenological patterns of each land-cover type. This kind of architecture enables us to represent the multilevel semantic information of SITS at different time scales. Specifically, intra-annual series reflect phenological differences and inter-annual series reflect land-cover dynamics. In this way, we can take advantage of the temporal information contained in the entire time series as well as the prior knowledge of land cover conversion to identify where and when changes occur. As a case study, we applied the proposed method for mapping annual, long-term urban-induced farmland loss from Moderate Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) time series in the Jing-Jin-Tang district, China from 2001 to 2010. The accuracy assessment showed that the proposed method was accurate for detecting conversions from cultivated land to built-up land, with the overall accuracy of 97.72% in the spatial domain and the temporal accuracy of 74.60%. The experimental results demonstrated the superiority of the proposed method in comparison with other state-of-the-art algorithms. In addition, the spatial-temporal patterns of urban expansion revealed in this study are consistent with the findings of previous studies, which also confirms the effectiveness of the proposed method

Anthropogenic and climatic controls on surface water across the contiguous United States

Anthropogenic activities and climatic processes heavily influence surface water resources by causing their progressive depletion, which in turn affects both societies and the environment. Therefore, there is an urgent need to understand the contribution of human and climatic dynamics on the variation of surface water availability. Here, this investigation is performed on the contiguous United States (CONUS) using remotely-sensed data. Three anthropogenic (i.e., urban area, population, and irrigation) and two climatic factors (i.e., precipitation and temperature) were selected as potential drivers of changes in surface water extent and the overlap between the increase or decrease in these drivers and the variation of surface water was examined. Most of the river basins experienced a surface water gain due to precipitation increase (eastern CONUS), and a reduction of irrigated land (western CONUS). River basins of the arid southwestern region and some river basins of the northeastern area encountered a surface water loss, essentially induced by population growth, along with a precipitation deficit and a general expansion of irrigated land. To further inspect the role of population growth and urbanization on surface water loss, the spatial interaction between human settlements and surface water depletion was examined by evaluating the frequency of surface water loss as a function of distance from urban areas. The decline of the observed frequency was successfully reproduced with an exponential distance-decay model, proving that surface water losses are more concentrated in the proximity of cities. Climatic conditions influenced this pattern, with more widely distributed losses in arid regions compared to temperate and continental areas. The results presented in this Thesis provide an improved understanding of the effects of anthropogenic and climatic dynamics on surface water availability, which could be integrated in the definition of sustainable strategies for urbanization, water management, and surface water restoration