Erosion and its spatial distribution in three agricultural headwater catchments were assessed in the border of the volcanic plateau in Southern Brazil. We analyzed terrain, hydrological processes and land use influence to provide a comprehensive assessment of the catchments' sensitivity to erosion. MethodsTopographic attributes were acquired from a digital elevation map, WaterSed model was parametrized to simulate runoff, diffuse erosion and sediment yield, and sediment source contributions were estimated using sediment fingerprinting based on near-infrared spectroscopy. Results 2According to the modeled results, areas covered by crop fields, grasslands and those adjacent to the drainage network are the most sensitive to erosion. Short distances from the source to the river network and the occurrence of high magnitude rainfall events (80 mm) promoted increases in connectivity for runoff/sediment transfer. Erosion simulations show that areas of low infiltration, as unpaved roads, were important runoff generators during lower volume rainfall events (25 mm). Sediment fingerprinting provided satisfactory results to quantify the contributions of unpaved roads to sediment (~39%).Topsoil and stream channels were also significant sediment sources for the set of analyzed samples, corresponding to average contributions of 38 and 23%, respectively. ConclusionAreas sharing geomorphological similarities did not lead to similar sediment contributions. Vegetation cover controlled erosion in topographically sensitive areas.Unpaved roads provide a significant sediment source, followed by topsoil and stream channels. The complementary results provide useful insights to better coordinate planning environmental conservation strategies in these fragile landscapes.

To truly understand the hydrologic and erosive processes that occur at the catchment scale regarding land use and soil management changes, intensive monitoring is required over a long period. Variables such as precipitation, flow rate and suspended sediment concentration are the fundamentals needed to estimate sediment yield (SY). However,

Watersheds used for water supply get special attention from managers, as the quantity and quality of supplied water depends on them. Water is an indispensable element of life and a fundamental resource for many human activities. The situation of these watersheds is aggravated by conflicts of land use that end up intensifying the process of erosion and sedimentation of the water reservoirs. In addition, diagnostics that use environmental indicators are important tools for environmental planning. The present study made use of these indicators to evaluate the natural soil susceptibility to erosion in the catchment area of the DNOS water reservoir, in Santa Maria, south of Brazil. Using multi-thematic maps and geoprocessing tools (GIS), we obtained a vulnerability mapping, represented by indexes. This map is an important and skillful tool for territorial planning, for aiding environmental management and conservation of water resources.

<p>Erosion processes are accelerated by the presence of unpaved roads in catchments with shallow soils and steep slopes, favoring overland flow and sediment connectivity between hillslopes and the river network. Soil erosion modeling studies conducted at the catchment scale focus primarily on the hydrological behavior of cultivated hillslopes. Few studies address unpaved roads and suitable practices to limit their impact on hydro-sedimentary transfers in a catchment system. This study simulates soil conservation measures on unpaved roads and hillslopes and their effect on the hydrological and erosive dynamics in a small order catchment. The rainfall events were monitored at Lajeado Ferreira’s creek, in Arvorezinha, Southern Brazil (1.2 km²). The catchment is characterized by shallow soils, steep slopes, intense agricultural activity and sediment yields (SY) of around 150 t km<sup>-2 </sup>y<sup>-1</sup>. Unpaved roads cover about 3% of the catchment surface area and supply 36% of the annual average SY. The Limburg Soil Erosion Model (LISEM) was used to simulate the roads’ impact on soil erosion. Eight rainfall events, monitored from 2014 to 2017, were calibrated. Rainfall ranged from 9 to 97 mm, total runoff volume (Q) varied from 1462 to 60765 m³ and SY from 0.6 to 81 tons. These events represent different precipitation and land use conditions, so that the road’s effects on the hydrological dynamics of this catchment may be investigated. Then, modeling scenarios consisting of three levels of intervention to mitigate sediment supply were tested.  The low intervention (LI) level was based on cost-efficient practices, applied to the road only through the installation of rockfill and energy-deflecting small reservoirs. The medium intervention (MI) included the sowing of grass for gutter protection on the road area and filter strips were installed near the drainage channels. For the high intervention scenario (HI), additional grass strips were installed on hillslopes. Their impact was evaluated by comparing the hydrosedimentological variables Q (m³), peak flow (L s<sup>-1</sup>) and SY (tons), modelled at the catchment’s outlet. Rockfill and energy-deflecting small reservoirs were not enough for reducing peak flow (Qp), on average. Indeed, the main proportion of overland flow originates from other landscape components, such as hillslopes. Under the MI and HI scenarios, Qp decreased by 2 and 46%, respectively. The LI and MI scenarios led to an average Q reduction of 12%, compared to 53% under scenario HI. For one event, HI promoted a reduction of 92% of calculated Q, representing 15,693 m³. HI also showed the most positive effects on limiting SY. It becomes evident that hillslope interventions are necessary, as they allow increasing infiltration, reducing both runoff volume and stream power when the flow reaches the roads. For rainfall events of higher magnitude, it was observed that HI was responsible for reducing Qp between 9 and 25%, while during smaller events, this reduction reached 61 to 93%. This indicates the importance of managing roads in order to reduce runoff energy and concentration, but also to take measures on hillslopes to limit overland flow and erosion inputs, as well as to delay peak Q.</p>

<p>The conversion of the natural grasslands of the Pampa biome (Southern Brazil) into cropland may lead to an increase in soil erosion rates and sediment delivery to the rivers. Grasslands represent a significant sink of carbon, and according to the literature, 59% of the soil organic carbon (SOC) is lost when pastures are converted into cropland. It makes soils even more vulnerable to water and land degradation. This study aims to evaluate the impact of land use change on the river sediment composition by calculating the sediment contribution of each potential sediment source using organic matter composition, ultra-violet and visible (UV-VIS) spectra derived parameters and fallout radionuclide activities, as potential tracers in a sediment fingerprinting approach. The study site (Ibirapuitã river basin – 5,942 km²) is located in the Pampa biome, Southern Brazil, were sandy and shallow soils predominate, occupied mainly by native grasslands that are gradually being converted to cropfields, especially soybean. Potential sediment sources were sampled, which include croplands (n=36), native grasslands (n=31), unpaved roads (n=31) and subsurface sources (channel banks (n=18) and gullies (n=16)). Samples were taken from the soil surface layer of croplands and grasslands, as well as from the top layer of exposed sites of gullies, channel banks and unpaved roads. Samples were oven dried (50 °C), gently disaggregated and dry sieved to 63 mm to avoid particle size effects prior to further analysis. Suspended sediment samples were collected using time integrated samplers deployed in the bottom of the river, and during rainfall runoff events at the outlet of the catchment. Organic matter parameters (total organic carbon - TOC, total nitrogen - TN, δ<sup>13</sup>C and δ<sup>15</sup>N) were measured using a continuous flow isotope ratio mass spectrometry (EA-IRMS). Diffuse reflectance spectra in the UV-VIS wavelengths was measured using a Cary 5000 UV-VIS-NIR spectrophotometer, and 33 parameters were derived from the spectra. Fallout radionuclide (<sup>137</sup>Cs and <sup>210</sup>Pb<sub>xs</sub>) activities were measured by gamma spectrometry using low-background high-purity germanium detectors. Tracers were selected following a three step procedure, including: (i) a conservative range test, (ii) a Kruskal–Wallis H-test, and (iii) a linear discriminant function analysis. The selected tracers were introduced into a mass balance mixing model to estimate the source contributions to in-stream sediment by minimizing the sum of square residuals. TOC and TN show significant differences between cropland and native grassland, while the isotopes δ<sup>13</sup>C and δ<sup>15</sup>N, presented a lower discrimination potential. TOC and UV-VIS derived parameters did not present a good discriminant potential when they were tested in isolation, although they increased the source discrimination when combined with organic matter parameters. Fallout radionuclides have a good discriminant potential between surface and subsurface sources, but also between native grasslands and croplands. Croplands are the main sediment source in the Ibirapuitã river catchment (36%), followed by the native grasslands (33%). However, the area occupied by croplands is approximately eight times smaller, demonstrating that erosion processes have been intensified by the conversion of native grasslands into croplands and/or croplands are better connected to the river network.</p>

<p>Although sediment yield reflects a catchment’s erosive processes, material transfer from hillslopes to rivers depends on a series of phenomena occurring on variable and continuous range of scales. Physically based, distributed models can be used to evaluate erosion’s spatial variability within a catchment and to identify hotspots. Sediment fingerprinting allows source type discrimination based on sediment and soil properties. The analysis of these dynamic systems could be coupled by addressing hillslope processes with modeling, while fingerprinting enlightens the connection between them and the drainage network. We aimed to evaluate the erosive susceptibility and its spatial distribution in three environmentally fragile paired headwater catchments, nested within Guarda Mor catchment, located in the border of the volcanic plateau in southern Brazil. This catchment is characterized by intense agricultural use, diverse geology, and complex terrain. WATERSED model was used as a dynamic method to evaluate the spatial distribution of hydrologic and erosive fragility during rainfall events. WATERSED was parameterized for modeling surface runoff volume, sediment yield and interrill erosion, based on monitored data from a zero-order no-till catchment and literature data. Modeling results were analyzed for each land use. For fingerprinting, two sediment sampling strategies and source groupings were considered. One considered spatial sources, and the endmembers were the sub catchments, the other considered land use source types within each sub catchment. Deposited bed sediment samples were collected at the outlets of each sub catchment and the main outlet. Soil source samples were collected in crop fields, grasslands, stream channels, forests, and unpaved roads. Crop fields and grasslands compose the source type topsoil. Samples were analyzed by near-infrared spectroscopy. Artificial mixtures were made to calibrate the prediction models. Fifteen Support Vector Machine (SVM) models were built and independently trained. Modeled erosion indicates that the steepest areas and those near the drainage network can be the most susceptible to erosion and runoff. The spatial distribution of runoff-prone areas shows the connectivity from upper segments of these catchments increases with higher magnitude events. In fingerprinting, calibration results’ predictors show good performance by the models, validation results vary from poor to good. SVM models for unpaved roads and forest had the best validation performance. For sourcing tributaries, results and poor validation statistical results indicate the need to use different tracers, and to consider unsampled sources associated to soil and geological differences found downstream from the sub catchment’s outlets. As for the sub catchments, there is a variation among the main sediment sources and a significantly constant contribution from unpaved roads in all of them. Other important sources are topsoil and stream channels, while forests did not show significant contribution. These methodologies were useful in seeking a more holistic process understanding, as physical processes were addressed and later integrated with the resulting sediment yield. Despite the results are modelled, the complementation of their insights indicates that there is a possibility for validating the sediment fingerprinting technique once modelling is validated by monitored and measured data.</p>