Rudi Hessel

Soil erosion simulations of land use scenarios for a small Loess Plateau catchment

Several land use scenarios were developed for the 3.5 km2 Danangou catchment on the Chinese Loess Plateau. These scenarios consist of four groups of three scenarios each: one group is based on the present land use distribution, the other three (alternative land uses) on a redistribution of land use according to slope gradient, soil type, slope aspect and land form. For example, cropland areas are restricted to slope gradients smaller than 25%, 20% and 15%. All groups consist of one scenario that only uses present or alternative land use and two scenarios that apply simple forms of biological conservation practices (mulching, improved fallow) and mechanical conservation practices (contour ridges). The LISEM soil erosion model was used to simulate the effects of these different scenarios on erosion. To do this, a calibrated LISEM-data set for a real storm was used. The results show that the predicted erosion rates for woodland/shrubland are much lower than those for other land uses. According to the simulations, runoff and erosion decrease by about 10% if conservation measures are applied to the present land use, while the predicted decrease for the alternative land uses is much larger at between 40% and 60%. This large decrease can be explained by the fact that one of the main differences in the alternative land uses is that they have much more woodland/shrubland. Differences in predicted erosion rates between the different alternative land uses are caused by conversion of cropland to orchard/cash tree. The present study is one of the first attempts to use soil erosion modelling as a tool for optimising land use and management strategies to reduce runoff and erosion rates on the Chinese Loess Plateau. More research will be needed to validate obtained simulation results with actual field measurements.

Modelling gully erosion for a small catchment on the Chinese Loess Plateau

The rolling hills region of the Chinese Loess Plateau is one of the areas with the highest erosion rates on earth. A striking feature of this area is the occurrence of many large, permanent gullies. A 3.5-km2 catchment was selected to study the processes of erosion and to adapt the storm-based Limburg Soil Erosion Model (LISEM) to the conditions prevailing on the Loess Plateau. Part of this work consisted of mapping and measuring the largest gully headcuts. The amount of loose soil material beneath the headcuts was also estimated. Observations suggest that gully headcuts are relatively stable (i.e., do not migrate rapidly), but that gullies can nevertheless produce significant amounts of sediment during overland flow events. Erosion of headcuts occurs mainly by soil falls in between storms. The loose soil material produced by these soil falls accumulates on the gully bottom. As the LISEM simulates storm erosion, the development of gullies over time can be ignored, and only the amount of material produced by them during runoff events needs to be studied. A digital elevation model (DEM) was used to estimate the position of existing gully heads by applying an adapted form of the Montgomery and Dietrich [Science 255 (1992) 826] index. Using the assumption that headcuts are vertical, it is possible to calculate headcut height from the slope angle map. A simple stability model, which assumes soil falls on gully headcuts to be a function of soil moisture content and headcut height, was applied. This daily-based model can then be used to simulate the accumulation of loose soil material below the headcut. The results show that while the DEM is not accurate enough to allow the detection of individual headcuts, this method can be used to produce a reasonable estimate of the amount of loose soil material available. A map showing the amount of loose soil material accumulated can then serve as input for a storm-based erosion model such as LISEM.

Calibration of the LISEM model for a small loess plateau catchment

The Limburg Soil Erosion Model (LISEM) soil erosion model was calibrated for a 2-km2 catchment on the Chinese Loess Plateau. The most important calibration factors were saturated conductivity and Mannings n. Calibration on catchment discharge was done by using the discharge peak (timing and discharge) followed by an adjustment for the total discharge to obtain the correct amount of sediment output. The results showed that LISEM can be successfully calibrated for a Loess Plateau catchment, and that small runoff events need to be calibrated separately from large runoff events. A separate calibration might even be needed for each event. The model performance was also evaluated using catchment wide spatially distributed data on rill erosion. Rill erosion intensity was mapped in the field and compared to spatial patterns of erosion predicted by LISEM. The simulated erosion patterns do show some resemblance with mapped erosion patterns in a general sense but they are very different in detail. The cause for this can be found in the extremely steep slopes and abrupt slope changes in the catchment. Some of the process descriptions in LISEM are not intended for such an environment, while the grid based kinematic wave routing cannot cope with the abrupt changes in flow conditions. The effects of this are amplified by inaccuracies in the input data and the DEM. For topographically complex catchments it will be very difficult to obtain data that are good enough for an accurate simulation of erosion patterns. This limits the use of a model such as LISEM as a predictive tool for future events. Simulation of different land use scenarios is less problematic, if a known event is used for all scenario simulations.

Hydrological triggering conditions of landslides in varved clays in the French Alps

Abstract In this paper the meteorological and hydrological conditions are analyzed which trigger shallow and deeper landslides in glacio-lacustrine deposits (varved or laminated clays) in the French Alps. The hydrological system of these landslides consists of a colluvial cover which supplies water into the fissures of the underlying varved clays. From these fissures water can infiltrate more or less horizontally into the silt layers of the varved clays. A combined reservoirs model was used to simulate the water fluctuations in the colluvial cover and the fissures. Both the water level in the fissures and the residence time of water in the fissures are mainly controlled by the amount of water storage in the colluvial cover. Simulations over the last 25 years show that almost each year the fissures are completely filled with water for several months a year. Infiltration experiments in the field show that infiltration into the varved clays occurs mainly by horizontal infiltration into the silt laminae. Calculated infiltration rates from these fissures into the silt layers show that the mean yearly residence time of water in the fissures is not sufficient to fully saturate the clay mass each year. It is therefore concluded that the triggering of the landslide movements is mainly controlled by the development of positive pore water pressures in the fissure system and that the rise of pore water pressures induced by the matrix system of the varved clays only plays a minor role. The calculations also show that drainage of the colluvial cover is a very efficient measure to stabilize the deeper landslides.

Soil conditions in a small catchment on the Loess Plateau in China

Abstract This article presents the results from soil surveys carried out within the framework of a soil conservation research project with several components: soil erosion modelling, land evaluation and participatory planning. The study area was a small catchment (3.5 km2), ranging in altitude between 1085 and 1370 m, in the vast Loess Plateau area in northern China. It is continuously affected by soil erosion by water, to a certain extent due to the unprotected slopes resulting from cultivation of subsistence food by the land users. The information on variability of soil properties in the study catchment was required as a basis for development of land use scenarios opting for ecological and economical sustainable production for the land users in the future. Soil profiles to 1-m depth were described, using FAO guidelines, at 17 sites along two transects covering the different facets of the morphology: hilltop, hillslope, valley bottom. Samples were collected from each horizon in the soil profiles for laboratory determination of physical and chemical properties. It was found that the nutrient status of the soils was poor, with ranges in contents of: sand=6.5–30.0%, silt=53.5–74.5%, clay=11.1–29.0%, organic matter=0.11–1.32%, available P=0.1–11 ppm, available N=5–56 ppm. The extremely high pH values, ranging between 8.7 and 9.3, effectively restricted the availability of nutrients. The soils have a high available water storage capacity so that, during years with a good supply of rain water, fair yields of crops can be achieved if the soil is fertilised. However, during dry years, when the soil dries out, the soils are vulnerable to drought and the crops fail. A divide at approximately 1200 to 1225 m above sea level was found between stratified and unstratified loessial soils. Above this altitude, the soils were yellowish, homogeneous silt loams with no or few restrictions for tillage, root penetration or water percolation. Below this altitude, layers that were more reddish (due to soil processes during climatically wetter periods), slightly cemented or higher in clay content, although still having a silt loam texture, were found in the soil profiles. These layers could cause problems at tillage, and could give rise to some restrictions to root penetration and water percolation, thereby increasing the erosion risk. A partitioning into characteristic soil types is proposed, to be used in the other parts of the project: erosion modelling, land evaluation and participatory planning.

Discharge and sediment measurements at the outlet of a watershed on the Loess plateau of China

A dam and weir system was constructed to measure the discharge of water and sediment from a selected small catchment on the Loess plateau in China. The aim of the system described here was to collect data on discharge and sediment content during occasional summer storms. These data can be used for calibrating and validating the LISEM erosion model. A V-notch weir was selected since it can measure a wide range of discharges. The measurement structure was equipped with an ultrasonic sensor to measure the water level. The system automatically switched on as soon as rain had been detected and the water level data were stored when a certain level threshold was surpassed. As a back-up system a flow meter was used, while a local farmer had also been hired to manually record water level during events. Sediment content of the runoff was determined on samples taken by an automatic sampler and a local farmer took additional samples. The system operated from April 1998 until September 2000. In this period, six events occurred and data could be collected during five of those. The data were corrected to make them useful for comparison with erosion simulation results. The collected data show that runoff only occurs during high-intensity rainstorms that produce more than about 11 mm of rain.

Modelling water flow and sediment processes in a small gully system on the Loess Plateau in China

Abstract A single gully system was selected in a small agricultural watershed on the Loess Plateau of China with the objective of measuring and then simulating water and sediment transport and defining alternative land uses to reduce discharge and soil loss. The gully had a total length of about 40 m and was about 30 m wide. The watershed feeding the gully occupied about 1950 m 2 (including the gully itself). The gully bottom had a slope of about 32–40°, whereas the gully walls were from 40° to over 60°. Soil water content and water and sediment discharge were measured automatically. The physically based hydrological and soil erosion model LISEM was used to calculate water and sediment discharge. Calibration has been carried out for one event, by adjusting the saturated conductivity values. Validation was performed for two additional runoff events. Calibration results show reasonably comparable hydrographs between measured and calculated discharge. LISEM could be calibrated on the hydrograph quite satisfactorily using the saturated conductivity as a calibration factor. Also, validation of LISEM for two other runoff events showed reasonably good results. The calibration on total soil loss also shows good results. For the validation runs, these results are poor, probably due to limitations of the measurement equipment and the incapability of the model to simulate small events. Scenario analyses showed that forest, as an alternative land use for the gully bottom, will result in significantly lower water and sediment. This strongly supports the implementation of the reforestation policy recently suggested and activated by the central government of the P.R. China.

Evaluation and Selection of Indicators for Land Degradation and Desertification Monitoring: Methodological Approach

An approach to derive relationships for defining land degradation and desertification risk and developing appropriate tools for assessing the effectiveness of the various land management practices using indicators is presented in the present paper. In order to investigate which indicators are most effective in assessing the level of desertification risk, a total of 70 candidate indicators was selected providing information for the biophysical environment, socio-economic conditions, and land management characteristics. The indicators were defined in 1,672 field sites located in 17 study areas in the Mediterranean region, Eastern Europe, Latin America, Africa, and Asia. Based on an existing geo-referenced database, classes were designated for each indicator and a sensitivity score to desertification was assigned to each class based on existing research. The obtained data were analyzed for the various processes of land degradation at farm level. The derived methodology was assessed using independent indicators, such as the measured soil erosion rate, and the organic matter content of the soil. Based on regression analyses, the collected indicator set can be reduced to a number of effective indicators ranging from 8 to 17 in the various processes of land degradation. Among the most important indicators identified as affecting land degradation and desertification risk were rain seasonality, slope gradient, plant cover, rate of land abandonment, land-use intensity, and the level of policy implementation.

Evaluation and selection of indicators for land degradation and desertification monitoring: types of degradation, causes, and implications for management

Indicator-based approaches are often used to monitor land degradation and desertification from the global to the very local scale. However, there is still little agreement on which indicators may best reflect both status and trends of these phenomena. In this study, various processes of land degradation and desertification have been analyzed in 17 study sites around the world using a wide set of biophysical and socioeconomic indicators. The database described earlier in this issue by Kosmas and others (Environ Manage, 2013) for defining desertification risk was further analyzed to define the most important indicators related to the following degradation processes: water erosion in various land uses, tillage erosion, soil salinization, water stress, forest fires, and overgrazing. A correlation analysis was applied to the selected indicators in order to identify the most important variables contributing to each land degradation process. The analysis indicates that the most important indicators are: (i) rain seasonality affecting water erosion, water stress, and forest fires, (ii) slope gradient affecting water erosion, tillage erosion and water stress, and (iii) water scarcity soil salinization, water stress, and forest fires. Implementation of existing regulations or policies concerned with resources development and environmental sustainability was identified as the most important indicator of land protection.

Using contour lines to generate digital elevation models for steep slope areas: a case study of the Loess Plateau in North China

In soil erosion models digital elevation models (DEMs) play an important role. Most interpolations from contour lines fail to address multi-value cells (MVCs) problems and therefore find it difficult to deal with steep slopes. These interpolation methods randomly assign a single value for MVCs and use this value in interpolation for nearby cells. This approach is very inaccurate. The frequency of MVCs and the problem caused by them was investigated, and the errors created by using the random values for MVCs was analyzed in this paper. A special treatment for MVCs and practical solution to create more accurate interpolation cell values in DEM building was developed. The new approach involves storing additional information of contour lines that go through the MVCs, such as maximum height and minimum height values, and the cardinal orientation relationship of the contour lines. A special MVC filter kernel was developed to decide the appropriate elevation value for interpolation. The computation method is based on raster data. An area on the Loess Plateau in North China was selected as an example to demonstrate the problems of the previously common used approach and to show results of the new method.