Effect of Land Use Change on Gully Erosion Density in the Black Soil Region of Northeast China From 1965 to 2015: A Case Study of the Kedong County

Kedong County is typical of the black soil region of northeast China in being highly susceptible to accelerated soil erosion by gullying. Using data sourced from Corona satellite imagery for 1965, SPOT5 for 2005 and GF-1 for 2015, the spatial distribution of gullies in the research area was mapped. Land use data for 1965, 2005, and 2015 were obtained from the topographic map of 1954, and from Landsat images for 2005 and 2015. Over the last 50 years, the extent of gully erosion in the study area has increased markedly, most notably on cultivated land, while gully density rose from 2,756.16 m2/km2 to 14,294.19 m2/km2. Cultivating land on slopes, especially on slopes greater than ∼4°, may rapidly aggravate gully erosion. The greatest increases in gully density occurred in situations when cultivated land and other/degraded land were transformed, which gully erosion density increased by 49,526.69 m2/km2. Other/degraded land is the most vulnerable land in the study area, with the highest gully erosion density. In these cases, gully density initially increases and, although the “Grain for Green” project has been implemented, gully erosion density has not always declined in the recent past.

Gully erosion in Northeast China -- a case study on history, erosion rates and causes

Mollisols are of major importance for food security worldwide but are increasingly degraded by soil erosion. Mollisols in Northeast China have been converted into agricultural use only recently, but gullies are widely distributed and gully erosion history, rates and causes remained unclear. We chose a study typical village to estimate initiation years and development rates of the gully systems from 1968 to 2018 by using aerial and satellite imagery. The outlet fan deposits of a large gully system were dated by Caesium-137 (137Cs) and artefacts. To verify the results, we collected information from local farmers. Gully volumes were measured by structure-from-motion technique using photos taken from an unmanned aerial vehicle. Our results showed that gully systems had already appeared on the steep slopes and along unpaved roads in 1968 and had become more complex by 2018 despite terracing and afforestation. Based on gully morphology and 137Cs, gully erosion was estimated to have started in the 1950s to 1960s when the original grassland and forest were completely converted into arable land. From 1968 to 2018, the gully density increased from 1.2 to 2.3 km km-2. The gully heads retreated at speeds from 1.5 to 2.5 m yr-1, and the soil loss from gully erosion ranged from 25.7 to 44.7 Mg yr-1 ha-1. These data demonstrate the severity of gully erosion in study region and underline the importance of appropriate countermeasures, such as maintenance of abandoned terraces under reforested land and better design and construction of roads within the arable land.

Mapping Permanent Gullies in an Agricultural Area Using Satellite Images: Efficacy of Machine Learning Algorithms

Gullies are responsible for detaching massive volumes of productive soil, dissecting natural landscape and causing damages to infrastructure. Despite existing research, the gravity of the gully erosion problem underscores the urgent need for accurate mapping of gullies, a first but essential step toward sustainable management of soil resources. This study aims to obtain the spatial distribution of gullies through comparing various classifiers: k-dimensional tree K-Nearest Neighbor (k-d tree KNN), Minimum Distance (MD), Maximum Likelihood (ML), and Random Forest (RF). Results indicated that all the classifiers, with the exception of ML, achieved an overall accuracy (OA) of at least 0.85. RF had the highest OA (0.94), although it was outperformed in gully identification by MD (0% commission), but the omission error was 20% (MD). Accordingly, RF was considered as the best algorithm, having 13% error in both adding (commission) and omitting pixels as gullies. Thus, RF ensured a reliable outcome to map the spatial distribution of gullies. RF-derived gully density map reflected the agricultural areas most exposed to gully erosion. Our approach of using satellite imagery has certain limitations, and can be used only in arid or semiarid regions where gullies are not covered by dense vegetation as the vegetation biases the extracted gullies. The approach also provides a solution to the lack of laser scanned data, especially in the context of the study area, providing better accuracy and wider application possibilities.

Distribution characteristics of gullies with slope gradient in Northeast China

<p>Twenty selected watersheds were divided into five small watershed sets according to location in Liaoning Province (LN), China. Watersheds and slopes were extracted from a 1:50,000 DEM, and gully data for each watershed were obtained by remote sensing interpretation. The gullies and associated slopes within the small watersheds were identified, and the distributions of gully density, proportion of dissected land, and gully length-width ratio in each small LN watershed and in the five small watershed sets were obtained. The correlations between the small watershed sets and the gully distributions throughout LN demonstrate regional distribution differences, and the watershed area has a great influence on both the area and length of gullies. Regional differences are present in the gully density and the proportion of dissected land in the small watersheds. The distribution of gullies with respect to slope depends on both the gully parameters and the proportion of terrain in the different slope grade ranges. The distribution results for the five small watershed sets are similar to those from a census of the Liaoning-Around Bohai mountainous and hilly sub-region. The gully density and proportion of dissected land in LN showed a single-peak curve with respect to slope, with slope thresholds of 8° and 5°, respectively. The constructed distribution equation has a high degree of fit. The comprehensive distributions of gully density, proportion of dissected land, and length-width ratio with slope indicate that gully erosion in LN is serious within the slope range of 3~8°.</p>

Process-oriented gully density modelling at the continental scale of Africa: first insights

<p>Gully erosion is an important land degradation process, threatening soil and water resources worldwide. However, contrary to sheet and rill erosion, our ability to simulate and predict gully erosion remains limited, especially at the continental scale. Nevertheless, such models are essential for the development of suitable land management strategies, but also to better quantify the role of gully erosion in continental sediment budgets. We aim to bridge this gap by developing a first spatially explicit and process-oriented model that simulates average gully erosion rates at the continental scale of Africa.</p><p>We are currently developing a spatially explicit model that (i) allows to simulate the spatial patterns of gully density at high resolution (30-90 m); (ii) is based on the physical principles that control the gully erosion process; (iii) uses GIS and data sources that are available at the continental scale. Our model structure is based on the threshold-dependent character of the gully-initiation process where a proxy of flow shear stress is weighted against a proxy of local shear resistance at the pixel scale. To calibrate and validate this model, we make use of an extensive database of 44 000 gully heads mapped over 1680 sites that are randomly distributed across Africa. The exact location of all gully heads was manually mapped by trained experts, using high resolution optical imagery available in Google Earth. This allows to extract very detailed information at the level of the gully head, such as the local slope and the area draining to the gully head. Based on these variables, we simulate indices for peak runoff (based on the Curve Number method), the shear stress of the concentrated runoff and the critical shear stress of the soil. The combination of these indices reflects the process leading to gully initiation and therefore provides an accurate indication of the susceptibility of that location to gully initiation.</p><p>Preliminary results indicate that it is feasible to model gully head locations and densities using this process-oriented approach. However, important trade-offs exist between an accurate description of the (threshold-dependent) gully initiation process and the uncertainties on the GIS data used to describe this process. One important issue is the resolution of the digital elevation model (DEM) used to extract local slopes (S) and to delineate contributing areas (A).  Comparing S- and A-values obtained from 30m SRTM-data with those obtained from higher resolution DEMs (5-12m) showed that SRTM data allows to obtain reasonable proxies of S and A but that uncertainties can be significant and correction factors are needed to avoid biases.</p><p>Overall, our results indicate that modelling gully densities using a process-oriented and spatially explicit approach has (conceptual and pragmatic) advantages as compared to a purely empirical ‘black-box’ modelling approach and offers opportunities to better quantify this important land degradation process at the global scale.</p>

Spatial variation of gully density and morphology and their controlling factors at a regional scale: a case study for the Chinese Loess Plateau

<p>Gully erosion is a major cause of land degradation in many regions worldwide. Recent research shows that the challenges posed by gully erosion are likely to further increase as a result of climate change and increasing land use pressure. Nonetheless, our understanding of this process remains limited in many ways. While numerous studies have focused on the occurrence and morphology of gullies at local (catchment) scale, relatively little research has explored their spatial variations at regional to continental scales. As a result, the factors controlling the density, size and morphology of gullies at such scales remain poorly understood. This is especially the case for the role of climate/weather conditions. Here we aim to advance our understanding on this topic by studying gully densities and gully morphology in the Chinese Loess Plateau (CLP), a region severely affected by gully erosion. <br>We selected five representative catchments in the CLP that are relatively similar in size (7-30 km²), topographic context, soil characteristics and land use but represent a large gradient in rainfall conditions. We mapped 2511 gullies in these catchments, using Pleiades-1B (panchromatic resolution at 0.5 m) and WorldView-3 images (panchromatic resolution at 0.31 m). For each of the gullies, we calculated a range of morphological parameters including the gully length, width, surface area, length-width ratio and shape index. Next, we explored to what extent differences in gully density and morphology are correlated to contrasts in rainfall and other environmental factors.<br>Overall, the gullies showed large variations in gully length (2.1-308 m, average 38.1 m), width (1.3-87 m, average 11.5 m) and density (0-4.8 km/km², average 2.3 km/km²). Gully densities showed a negative correlation with rainfall amounts. This is likely partly attributable to feedbacks between rainfall amounts and vegetation cover. However, also contrasts in rainfall intensity and regime likely play an important role. Also variations in gully width appear strongly correlated with rainfall patterns (with more humid catchments resulting in overall wider gullies). Surprisingly, gully lengths (a first indicator of gully headcut retreat) showed no clear correlation with rainfall patterns. Overall, our results indicate that contrasts in rainfall regime are crucial to understand gully erosion dynamics at regional to continental scales. This is true for their initiation but also for their subsequent expansion (and especially gully widening). These findings have important implications for the development of models aiming to predict gully erosion at regional to continental scales.</p>

THE STUDY OF WATERSHED TOPOGRAPHY CHARACTERISTICS IN VAKHSH RIVER BASED ON ZY3-DSM

The Vakhsh River is one of the major rivers in Tajikistan. The quantitative analysis of watershed topography and developmental characteristics in Vakhsh River catchment can reflect the morphological characteristics of the region, which is of great significance for revealing the quantitative relationship between the hydrological and the geomorphological process. In this paper, the D8 algorithm and the spatial analysis method were used to extract the river networks, the catchment boundary profile lines and the longitudinal valley profile lines of the four major tributaries in the Vakhsh River from the ZY3-DSM of 10 meters resolution. On this basis, five quantitative indices including the frequency of wave, amplitude of wave, gully density, the longitudinal slope and roundness rate were used to analyze the watershed landform and its development degree. According to the experimental results, the catchment have a high surface complexity and a mature landform. Yovonsu river catchment which is in the downstream of Vakhsh River is oval and has low terrain complexity with large frequency and small amplitude. Among the midstream and upstream, the Mukson River has developed into geriatric terrain that is the most mature and has the highest surface complex, while the Obikhingon River and the Kizilsu River have developed into a stable maternal terrain. In terms of topography, the boundary elevation of the Obikhingon is basically in accordance with the normal distribution, while the Kizilsu and the Muksu show a peak state with elevations of 4,000–5,000 m and 5,000–5,500 m, respectively.