Quantification of soil erosion in the Alps : measurement and modeling

Alpine regions have a high potential for soil erosion associated to extreme climatic and topographic conditions. Because of aggressive development in the recent past, environmental damage enhanced by acid deposition, global warming and development pressure of land use soil erosion in alpine areas has been an increasing concern to local, national and European policy makers (Francis, 1986; Schreurs, 2007; Yelpatyevskiy and Arzhanova, 1988). Numerous studies on soil erosion and erosion modeling were realized in lowlands or low mountain ranges. However, little is known about on- and off- site effects of soil erosion by water and snow melt in alpine terrain and the problem of quantification of these phenomena remains more or less unsolved. In mountain systems, difficulties in accessibility and data acquisition are paired with a high degree in small scale heterogeneity.
Thus, the main objective of this work was to evaluate appropriate soil erosion measurement procedures to use on alpine environments. Furthermore, the WEPP model (Water erosion prediction Project) and the USLE (Universal Soil Loss Equation) were run at the investigation sites. The aim was to assess both erosion prediction models on alpine environments since no suitable alpine model exists, so far.
Erosion measurements were done at three land use types with three replicates each. Land use types were hayfields (hf), pastures with dwarf shrubs (paw) and pastures without dwarf shrubs (pawo). These land use types represent the dominant land use types at the south facing slope in the Urseren Valley, were the sheet erosion takes place. The investigation sites are situated at an elevation of 1600-1800 meters a.s.l. The measurements to determine soil loss were done with sediment traps on plot measurements, sediment cups (point measurements) and with two Cs-137 based detection methods (point measurements). Cs-137 which is a common tracer for soil erosion in lowlands was adapted for application in high alpine environments. Cs-137 provides information about the spatial distribution and the extent of soil erosion in the investigation area. Soil erosion rates with Cesium-137 integrate the erosion since 1986, when Cesium-137 was released from the Chernobyl accident. A NaI in-situ spectrometer was calibrated for Cs-137 determination at steep mountain slopes. Calibration was done by comparing Cs-137 activities measured with GeLi detector in the laboratory and with NaI in-situ spectrometry at the same site. A close correlation between the two methods proved the validity of the in-situ measurements of the NaI detector system. Maximum monthly erosion rates during the vegetation periods 2007 and 2008 based on the sediment traps were 123 kg ha-1 for pasture without dwarf shrubs whereas minimum monthly erosion rates were obtained for pasture with dwarf shrubs and hayfields with 1 kg ha-1. Sediment cups turned out to be a useful tool for point measurements. Additionally, cups can also be applied for soil erosion measurement during winter time. The measurements integrate over the whole wintertime, since slopes are not accessible. However, when the cups are filled, no quantitative statement can be done. Cs-137 based measurements based on in-situ detection lead to a maximum annual erosion rate since 1986 of 36 t ha-1 a-1 for hayfield hf1 and a minimum annual erosion rate of 8 t ha-1 a-1 for pasture with dwarf shrubs paw2. Pastures without dwarf shrubs have a mean annual erosion rate of 22 t ha-1 a-1 (s.d. 20%). Erosion values based on laboratory analyses with a GeLi detector were similar to erosion values from in-situ Cesium-137 measurements. R2 of both measurement methods for all sites is 0.94. However, laboratory analyses need a soil sampling in the field. Since the alpine environment is very heterogeneous, especially on pasture sites, an extensive soil sampling is necessary to capture the full heterogeneity of erosion. But collecting big amounts of soil samples in the field does not seem adequate for sensitive mountain soils seriously affected by soil erosion.
Cesium-137 based erosion rates were compared with erosion rates predicted by the Universal Soil Loss Equation (USLE). The comparison was done in order to evaluate if the USLE is a useful tool for erosion prediction in steep mountainous grassland systems. Erosion rates based on the USLE are in the same order of magnitude compared to Cs-137 based results for the land use type pasture with dwarf (predicted and measured erosion rates are between 4 and 12 t ha-1 a-1). However, erosion amounts on hayfields and pasture without dwarf shrubs are underestimated by the USLE compared to Cs-137 based erosion rates. We assume that the underestimation is due to winter processes that cause soil erosion on sites without dwarf shrubs (e.g. snow gliding). The winter processes are not considered by the USLE. Dwarf shrubs may possibly prevent damage of soil erosion through winter processes.
In addition to the USLE we tested the WEPP model (Water Erosion Prediction Project) to describe the soil erosion in the Urseren Valley as it seems to be one of the most promising models for steep mountainous environments. Crucial model parameters were determined in the field (slope, plant species, fractional vegetation cover, initial saturation level), by laboratory analyses (grain size, organic matter) or taken from the WEPP manual (soil erodibility, effective hydraulic conductivity, cation exchange capacity). Erosion rates were measured with sediment traps during the vegetation period between June 2006 and November 2007. Long-term soil erosion rates were estimated by measuring Cs-137 redistribution as described above. In addition to the erosion rates, soil moisture and surface flow was measured during the vegetation period in the field and compared to model outputs. Short-term erosion rate simulations for the vegetation period in 2007 are in agreement with measured erosion rates (predicted and measured erosion rates are between 0 and 0.4 kg ha-1 mo-1 for hf3, between 0 and 3.4 kg ha-1 mo-1 for pawo2 and between 0 and 1.1 kg ha-1 mo-1 for paw2). However, simulated soil moisture is up to two times higher than measured field data. Furthermore, simulated soil moisture is increasing during spring time while measured soil moisture is decreasing during the same time and surface flow is not simulated correctly. Snow cover melting is simulated too late compared to field observations and thus water from snowmelt is available until summer time in 2007. We assume that these differences lead to the general underestimation of erosion rates for long-term rate erosion predictions for all three land use types. Thus, the WEPP model could be a useful tool for alpine regions during the vegetation period to assess the influence of different land use conditions but should be applied carefully during winter time and on snow covered regions. Generally, neither WEPP nor USLE contain avalanches and snow gliding processes. The Cs-137 based measurement rates point out that winter processes seems to be important for high erosion rates during longer time periods.
Our study demonstrates the need of soil conservation strategies in alpine regions since erosion rates are much higher than previously reported. Furthermore, results of the WEPP model are only comparable during the vegetation period with measured data on respective slopes. Also, the accuracy of USLE results is not satisfactory on the affected sites. Thus, a first attempt was done to create an alpine factor for the USLE based on the measured data. Hence, existing models have to be adapted to alpine regions or new soil erosion models have to be designed for steep mountainous slopes.