Abstract
The Tibetan Plateau (TP), known as “The Roof of World”, has expansive alpine grasslands and is a hotspot for climate change studies. However, cropland expansion and increasing anthropogenic activities have been poorly documented, let alone the effects of agricultural activities on food security and environmental change in the TP. The existing cropland mapping products do not depict the spatiotemporal characteristics of the TP due to low accuracies and inconsistent cropland distribution, which is affected by complicated topography and impedes our understanding of cropland expansion and its associated environmental impacts. One of the biggest challenges of cropland mapping in the TP is the diverse crop phenology across a wide range of elevations. To decrease the classification errors due to elevational differences in crop phenology, we developed two pixel- and phenology-based algorithms to map croplands using Landsat imagery and the Google Earth Engine platform along the Brahmaputra River and its two tributaries (BRTT) in the Tibet Autonomous Region, also known as the granary of TP, in 2015–2019. Our first phenology-based cropland mapping algorithm (PCM1) used different thresholds of land surface water index (LSWI) by considering varied crop phenology along different elevations. The second algorithm (PCM2) further offsets the phenological discrepancy along elevational gradients by considering the length and peak of the growing season. We found that PCM2 had a higher accuracy with fewer images compared with PCM1. The number of images for PCM2 was 279 less than PCM1, and the Matthews correlation coefficient for PCM2 was 0.036 higher than PCM1. We also found that the cropland area in BRTT was estimated to be 1979 ± 52 km2 in the late 2010s. Croplands were mainly distributed in the BRTT basins with elevations of 3800–4000 m asl. Our phenology-based methods were effective for mapping croplands in mountainous areas. The spatially explicit information on cropland area and distribution in the TP aid future research into the effects of cropland expansion on food security and environmental change in the TP.
Highlights
Agricultural activities in the Tibetan Plateau (TP) are uncommon and only comprise less than 1% of the land cover, they play an important role in regional food security given the strong dependence of people’s livelihoods on local grains
From the temporal profile of normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and land surface water index (LSWI) at three randomly selected sites with different vegetation types in the study area, we found that croplands had a different phenology profile than the other vegetation types
Our study indicated that the mapping of croplands using Landsat data with 30 m resolution and the phenology-based approach in the alpine regions with complicated landscapes and fragmentized topography is feasible and accurate
Summary
Agricultural activities in the Tibetan Plateau (TP) are uncommon and only comprise less than 1% of the land cover, they play an important role in regional food security given the strong dependence of people’s livelihoods on local grains. The Tibetan Plateau has experienced significant warming in the past few decades, affecting cultivation suitability at higher elevations through temperature stress, water availability, and other environmental conditions [1,2]. With rapid population growth and climate warming, agriculture has expanded significantly both horizontally and vertically in the past few decades [3]. This expansion has come with ecological consequences due to Tibet’s fragile environment, including desertification, degradation, and water supply loss [1,4,5,6,7,8]. Croplands in the TP have been overlooked and their spatial distributions are unclear in the existing land cover and land use products due to the complicated topography and fragmented landscape on the plateau [10]
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