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<p>The Universal Soil Loss Equation (USLE) was initially developed to support the implementation of conservation measures to minimize soil loss by water erosion, i.e. sheet and rill erosion, on a local scale and in the context of agricultural land use. The approach was refined over decades and became the Revised Universal Soil Loss Equation (RUSLE). &#160;At the same time the scope of applications has grown considerably. Due to its rather simple structure and relatively low demand for input data, it has been used for the assessment of soil loss from water erosion for ever growing spatial entities, i.e. regional scale catchments or whole countries. Recently this has been applied on a global scale in order to identify global hotspots of soil erosion. This coherent approach for a global comparison is most welcome against the background of the large number of country-specific assessments which are rather difficult to compare.</p> <p>However, there are two issues of concern. First, one needs to remember that RUSLE-derived soil loss assessments do not account for gully erosion, which might not be linearly scaled with sheet and rill erosion. Furthermore, information on the uncertainties of RUSLE based erosion assessments are not frequently reported. This especially concerns the impact of specific combinations of varying unique input data on the results.</p> <p>In this contribution we compare in high spatial resolution the results of two RUSLE-based soil loss by water erosion assessments conducted for six 100 by 100 km large study sites in South Africa. The first assessment was conducted about two decades ago and was based on the then available data covering the whole of South Africa. The second assessment is a revision, which includes the latest input data for rainfall erosivity, soil erodibility, the topographic factor as well as land cover and management. When compared, the results of the current soil loss estimates are an order of magnitude lower than the previous estimates. Does this difference represent a temporal trend or just the inherent uncertainties reflecting different input data and slightly different data processing? This is the question discussed in this contribution.</p>
Abstract. The Greenland Ice Sheet (GrIS) responds rapidly to the present climate, therefore, its response to the predicted future warming is of concern. To learn more about this, decoding its behavior during past periods of warmer than present climate is important. However, due to the scarcity of marine studies reconstructing ice sheet conditions on the Northeast Greenland shelf and adjacent fjords including the position of the ice sheet over marine regions, the timing of the deglaciation, and its connection to forcing factors including the Holocene Thermal Maximum (HTM) on NE Greenland remain poorly constrained. This paper aims to use bathymetric data and the analysis of sediment gravity cores to enhance our understanding of ice dynamics of the GrIS near the southern outlet of the Northeast Greenland Ice Stream (NGIS), as well as give insight into the timing of deglaciation and provide a palaeoenvironmental reconstruction of southwestern Dove Bugt and Bessel Fjord since the Last Glacial Maximum (LGM). The swath bathymetry data displayed in this study is the first time the bathymetry for Bessel Fjord has become available. North–south oriented glacial lineations, and the absence of pronounced moraines in southwest Dove Bugt, an inner continental shelf embayment (trough), suggests the southwards and offshore flow of the southern branch of the NGIS, Storstrømmen. Sedimentological data suggests that an ice body, theorized to be the NGIS, may have retreated from the region slightly before ~11.2 ka BP (in the Preboreal period). The seabed morphology of Bessel Fjord, a fjord terminating in southern Dove Bugt, includes numerous basins, separated by thresholds. The position of basin thresholds, which include some recessional moraines, suggest that the GrIS had undergone multiple halts or readvances during deglaciation. A minimum age of 7.2 ka BP is proposed for the retreat of ice to or west of its present-day position in the Bessel Fjord catchment area. This suggests that the GrIS retreated from the marine realm in early Holocene, around the time of the onset of the Holocene Thermal Maximum in this region, a period when the mean July temperature according to Bennike et al., (2008) was at least 2–3 °C higher than at present, and remained at or west of this onshore position for the remainder of the Holocene. The transition from predominantly mud to muddy sand layers in a mid-fjord core at ~4 ka BP may be the result of increased sediment input from nearby and growing ice caps. This shift may suggest that in late Holocene (Meghalayan), a period characterized by a temperature drop to modern values, ice caps in Bessel Fjord fluctuated with greater sensitivity to climatic conditions than the NE sector of the GrIS.
<p>Mapping reservoir siltation is an often-used method for assessing sediment yield and soil erosion from catchments. An advantage of this approach is that measurements can potentially provide mean values that represent timeframes of several decades and thus overcome the bias induced by climate fluctuations, especially in semi-arid and arid regions. Furthermore, reservoir siltation mapping can be performed repeatedly, and thus repeated sediment yield trends over time can be derived. There are several studies that report sediment yield estimates based on reservoir siltation surveys, however, information on the uncertainties involved in these measurements is not frequently reported.</p><p>In October 2019 and March 2020, we conducted reservoir siltation surveys of eight mid-size (~ 10 mio m&#179; water storage capacity), filled and dried-out reservoirs in South Africa. The water-filled reservoirs were surveyed using single beam, single frequency echosounders mounted to a boat. The dried-out reservoirs were surveyed using differential GNSS and a Terrestrial Laser-Scanner (TLS) with a scanning range of up to 1 km deployed at multiple scanning positions.</p><p>In this contribution we present survey results, report on the issues encountered during the surveys and the uncertainties observed in the results. For the water-filled reservoirs we derived depth measurement uncertainties from the survey leg intersection points. Here, the mean measurement error is in the order of 0.1 m (p= 0.05). When this uncertainty of the volume estimation is applied to the water storage capacity of the dams, the resulting uncertainties are inthe order of a few percent, only. However, if this volume estimation uncertainty refers to the volume of the sediment at the bottom of the reservoirs, the relative error is can be in the order of a few ten percent. From this we conclude, that depending on the sediment inflow, it may take several decades before a repeated survey can establish a meaningful trend in sediment yield from the catchment beyond the measurement uncertainties involved.</p>
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