J. Stolte

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.

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.

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.

Effects of crust and cracks on simulated catchment discharge and soil loss

Sealing, crusting and cracking of crusts of the soil surface has been observed in many parts of the world in areas with sandy, silty and loamy soils. Sealing and crust formation occurs under the influence of rain storm and drying weather. With prolonged drying, surface crusts might crack, leading to complex situations with respect to infiltration and runoff generation. Cracking of crusted loamy soils appears to be a general process. This study aims to measure the hydraulic properties of fully crusted and cracked-crusted areas and to evaluate the effects of these measurements on catchment discharge and soil loss in a loess region of the Netherlands, using the LISEM soil erosion model. Samples with minimum infiltration rates (fully crusted) and with maximum infiltration rates (cracked-crusted surfaces) were taken from fields with bare soil or winter wheat and their soil hydraulic functions were measured. The results of these measurements were used as input in the LISEM soil erosion model. Simulations of discharge and soil loss were done for each of these two land-uses and for two rain events. Additionally, simulated discharge and soil loss under actual recorded land-use were calculated. In all cases, soils with no surface cracks produced higher figures for discharge and soil loss than those where 10% of the surface crust was cracked. For a good interpretation of the results for soil loss, the spatial distribution of cracked-crusted areas and fully crusted areas has to be investigated in detail. To deal with cracked-crusted and fully crusted areas in simulation modelling, care has to be taken to accurately measure the soil physical functions representing the maximum and minimum infiltration rates. An assignment of these functions to calculation grids has to be made. As the LISEM model is capable of assigning different soil physical functions to each calculation grid, an improved prediction of the soil physical behaviour of the catchment can be simulated.

Intensive water content and discharge measurement system in a hillslope gully in China

Abstract An automatic monitoring system was constructed to intensively study the water–profile–runoff interactions during rain events, of a hillslope gully. The system was installed on the Loess plateau in the Shaanxi province in northern China. The primary goal was to obtain a better understanding of soil erosion processes in this district to aid in better management. To accomplish this, infiltration and runoff data for running, calibrating and validating the LISEM erosion model was needed. The system presented here consisted of 29 Time Domain Reflectometry (TDR) water content sensors that monitored the water content variation in time. Sensors were installed in different subsystems throughout the gully geometry such as in the cropland surrounding the gully, in some gully slopes and in the gully floor. At the outflow point of the gully, an H-flume was installed in order to measure the actual discharge of the gully system and to measure directly the sediment concentration in the discharge. The monitoring system was programmed in this way so that more measurements were recorded when a rainstorm occurred. Installation took place in April 1998 and it was used until September 2000. In 1998, five major rain events were recorded, which generated runoff and erosion, in 1999 and 2000 only one. Cropland measurements showed high water contents and high infiltration rates all through the measuring period and also showed a strong reaction to precipitation. The sidewalls of the gully showed much lower water contents where only very shallow parts showed some infiltration during rain showers, while steep parts of the gully, such as pipes and vertical walls, showed an extremely low water content and practically no water content variation with time. Simulation results confirmed the suspicion that these areas are an important source for Hortanian overland flow within the gully system. Model simulations further showed that most of the sediment being eroded from the gully system was coming from rills and smaller gullies being cut into the main gully bottom. Saturation excess overland flow proved to be dependent on the characteristic of the rain event as well as on the water content of the soil at the beginning of the event.

Evaluation of vertical and lateral flow through agricultural loessial hillslopes using a two-dimensional computer simulation model

On four hill-slopes in the loess region of the Netherlands pressure heads were monitored during rain events with time intervals of five minutes. Water flow through these hill-slopes during erosive rain events in summer and winter was simulated two-dimensionally. These simulations showed that vertical flow is dominant during rain events, whereas lateral water transport is of minor importance. The average lateral water movement varied between 1.6 and 4.7% of the total water displacement. Therefore, it was decided to incorporate a one-dimensional water flow module into the soil erosion and hydrological model LISEM.

MEASURING AND MODELLING OF SOIL WATER DYNAMICS AND RUNOFF GENERATION IN AN AGRICULTURAL LOESSIAL HILLSLOPE

Surface run-off may be generated when rainfall intensity exceeds infiltration capacity, or when the soil profile is saturated. Both types of overland flow may occur in hilly agricultural loess regions. It was shown with pressure head and run-off measurements that Hortonian overland flow occurs during summer rain events. A two-dimensional water flow model could simulate pressure head changes and run-off. Simulated potential run-off for the transect studied was three times as high. This indicates effects of surface ponding and the probable location of this particular transect in a region with high run-off production.

Case studies on water and nutrient management

For the development ofthe Limburg Soil Erosion Model (LISEM), a validated hillslope hydrology module was needed. Therefore, hydraulic heads were monitored on four hillslopes in the southern part of The Netherlands during the years 1992 and 1993. The hydraulic heads were recorded automatically , using stand-alone measuring devices. Simultaneously, rainfall amounts and runoff were recorded as well. Observed changes in hydraulic head and runoff were simulated using the SWMS_2D computer program. Measured and simulated results are compared and correlated well.