The impact of irrigated agriculture on landslide activity: A spatio-temporal analysis in Heifangtai, China

The Heifangtai loess terrace in Gansu Province (China) suffered from frequent irrigation-induced landslides. In the past 50 years, the perennial channel irrigation has resulted in 210 slope failures within a small area of 9 km². The landslide activity is particularly linked with the cultivation of vegetables, which need more frequent channel irrigation compared to other crops. In order to reveal the long-term relationship between the recurrent landslides and irrigation intensity variations induced by land use changes, we used remotely sensed images from different sources, covering a period of 59 years, to map land use changes as well as to create a landslide inventory. Based on field surveys, samples of land use and vegetation phenology were studied, which were used in classification using a random forest classifier based on synergy multi-temporal Normalized Difference Vegetation Index (NDVI) for characterizing features during a crop cycle. A landslide inventory was created by visual interpretation of images and comparison of landslide maps from literature. Based on the land use map and crop irrigation frequency, the irrigation intensity of all the crop types was calculated. For studying the interaction of horizontal and vertical water infiltration over long periods, kernel density estimation (KDE) was applied to identify irrigation hotspot areas, which were used to correlate with slope failure hotspots. Furthermore, temporal and spatial relationship was analyzed among the landslide density, land use, and irrigation intensity. The results of this study showed that irrigation intensity varied with changes in land use as well as agriculture technology, which was later correlated with landslide sensitivity. Most of the landslides in Heifangtai (90 %) were loess landslides, which were retrogressive with a small average area and the recurrence was intensified after vegetable cultivation; bedrock landslide occurred a little more frequently before vegetable cultivation initiated and has large average area. This study revealed that the hotspots of recurrent loess landslides were near the hotspot irrigation areas and local groundwater table dome, which were in the concave edges with groundwater seepage in the loess layer. This study is also the first to evaluate the annual irrigation volume and the time for recent Groundwater Table (GWT) distribution, and implies that irrigation water management in arid farming areas was effective and viable for landslide preventing.