How Do Ground Litter and Canopy Regulate Surface Runoff?—A Paired-Plot Investigation after 80 Years of Broadleaf Forest Regeneration

Relatively minimal attention has been given to the hydrology of natural broadleaf forests compared to conifer plantations in Japan. We investigated the impacts of ground litter removal and forest clearing on surface runoff using the paired runoff plot approach. Plot A (7.4 m2) was maintained as a control while plot B (8.1 m2) was manipulated. Surface runoff was measured by a tipping-bucket recorder, and rainfall by a tipping-bucket rain gauge. From May 2016 to July 2019, 20, 54, and 42 runoff events were recorded in the no-treatment (NT), litter removed before clearcutting (LRBC), and after clearcutting (AC) phases, respectively. Surface runoff increased 4× when moving from the NT to LRBC phase, and 4.4× when moving from the LRBC to AC phase. Antecedent precipitation index (API11) had a significant influence on surface runoff in the LRBC phase but not in the NT and AC phases. Surface runoff in the AC phase was high regardless of API11. The rainfall required for initiating surface runoff is 38% and 56% less when moving from the NT to LRBC, and LRBC to AC phases, respectively. Ground litter and canopy function to reduce surface runoff in regenerated broadleaf forests.

Effects of Slope Gradient on Runoff and Sediment Yield on Machine-Induced Compacted Soil in Temperate Forests

There has been a severely negative impact on soil water resources in temperate forests caused by the introduction of the type of heavy machinery in the forestry sector used for forest harvesting operations. These soil disturbances increase the raindrop impact on bare mineral soil, decrease infiltration rate, detach soil particles, and enhance surface flow. According to several studies, the role of slope gradient influence on runoff and soil loss continues to be an issue, and therefore more study is needed in both laboratory simulations and field experiments. It is important to define and understand what the impacts of slope gradient in harvesting practices are, so as to develop guidelines for forest managers. More knowledge on the key factors that cause surface runoff and soil loss is important in order to limit any negative results from timber harvesting operations performed on hilly terrains in mountainous forests. A field setting using a runoff plot 2 m2 in size was installed to individualize the effects of different levels of slope gradient (i.e., 5, 10, 15, 20, 25, 30, 35, and 40%) on the surface runoff, runoff coefficient, and sediment yield on the skid trails under natural rainfall conditions. Runoff and sediment yield were measured with 46 rainfall events which occurred during the first year after machine traffic from 17 July 2015 to 11 July 2016 under natural conditions. According to Pearson correlation, runoff (r = 0.51), runoff coefficient (r = 0.55), and sediment yield (r = 0.51) were significantly correlated with slope gradient. Results show that runoff increased from 2.45 to 6.43 mm as slope gradient increased from 5 to 25%, reaching to the critical point of 25% for slope. Also, further increasing the slope gradient from 25 to 40% led to a gradual decrease of the runoff from 6.43 to 4.62 mm. Runoff coefficient was significantly higher under the plot with a slope gradient of 25% by 0.265, whereas runoff coefficient was lowest under the plot with a slope gradient of 5%. Results show that sediment yield increased by increasing the slope gradient of plot ranging 5% to 30%, reaching to the critical point of 30%, and then decreased as the slope gradient increased from 35% to 40%. Runoff plot with a slope gradient of 30% (4.08 g m−2) ≈ plot length of 25% (3.91 g m−2) had a significantly higher sediment yield, whereas sediment yield was lowest under the plot with a slope gradient of 5% and 10%. A regression analysis of rainfall and runoff showed that runoff responses to rainfall for plots with different slope gradients were linearly and significantly increased. According to the current results, log skidding operations should be planned in the skid trails with a slope gradient lower than the 25 to 30% to suppress the negative effect of skidding operations on runoff and sediment yield.

Soil organic carbon and nutrients losses form the sloping land in the scenario of water erosion in the south of Rwanda

<p><strong>Abstract:</strong>Rwanda is located in the plateau of the central-eastern Africa nearby the equator of the Earth, known as ’The Land of a Thousand Hills’, and covers the part of the region of the Upper Nile. The sloping lands are ubiquitous across Rwanda, and the sloping farmlands account for more than 70 per cent of the sloping land resources. The soil and water losses are very severe on the sloping lands, especially on the sloping farmlands due to the farming activities and soil water erosion induced by the erosive rainfall events. Therefore, the soil erosion and soil organic carbon (SOC) and nutrient losses and the resultant soil deterioration and crop yield decline on the sloping farmlands in this country have attracted the widespread concerns. It is necessary to understand severity of the SOC and nutrient losses on the sloping farmland due soil erosion in term of launching the countermeasure to control the losses. The investigation on the SOC and nutrient losses in the sloping farmlands and the rainfall was carried out on the runoff plot with 20m long, 5m wide and gradient of 12°in Rubona, Huye District, south province of Rwanda. The cropping rotation of soybean-maize-groundnut was practiced on the plot during the monitor on soil losses from the plot. The contents of constituents of soils lost from the plot decreased in the order: SOC> total potassium (TK)>total nitrogen (TN)>total phosphorus (TP). The loss intensities of SOC from the plot varied from 383.0 kg/hm<sup>2</sup> to 1680.9 kg/hm<sup>2</sup> in the period from 2011 to 2013, 259.4 kg/hm<sup>2</sup> to 1138.5 kg/hm<sup>2</sup> for TK, 41.2 kg/hm<sup>2</sup> to 180.8 kg/hm<sup>2</sup> for TN, 9.2 kg/hm<sup>2</sup> to 40.2 kg/hm<sup>2</sup> for TP. The loss intensities of SOC, TK, TN and TP were 1262.3 kg/hm<sup>2</sup>, 99.0 kg/hm<sup>2</sup>, 99.4 kg/hm<sup>2</sup>, 35.4 kg/hm<sup>2</sup> in 2017, and 3786.8 kg/hm<sup>2</sup>, 2970.0 kg/hm<sup>2</sup>, 298.1 kg/hm<sup>2</sup> and 106.3 kg/hm<sup>2</sup> in 2018, respectively. The loss intensities of SOC and nutrients varied significantly over the years. It can be seen that the amounts of erosive rainfall have the crucial impacts on loss intensities of SOC and nutrients through analyzing the relation between loss intensities and erosive rainfall. The relations between loss intensities of SOC and nutrients and mounts of erosive rainfall can be described by exponential function. Compared with the loss intensities of SOC and nutrients on the runoff plot, the loss intensities were much less on the plots with the corresponding soil and water conservation measures such as terracing and plant hedges. Therefore, the measures of anti-erosion should be adopted on the sloping farmlands in an effort to reduce SOC and nutrient losses and keep the sustainable soil productivity in Rwanda. </p><p><strong>Keywords:</strong> SOC; nutrient; sloping farmland, Rwanda</p>

Soil erosion monitoring at small scales: Using close range photogrammetry and laser scanning to evaluate initial sediment delivery

<p>Soil erosion represents one of the most significant environmental problems of the 21<sup>st</sup> century with severe impacts on terrestrial ecosystems. Traditionally, soil losses by water are determined by runoff plots in situ. Micro-scale devices (<1 m length) are commonly used to monitor soil erosion rates in comparative field studies. This is especially the case in ecological-pedological experiments, investigating e.g. the effect of plant characteristics on erosion processes. The small plot size allows to focus precisely on interrill processes with the smallest possible set of confounding factors and a high number of replications. However, the runoff plot method is labour- and time-intensive, sediment handling can be challenging and the measurement accuracy varies importantly with the applied control of the measurement setup.</p><p>To optimize the acquisition of small-scale erosion data from splash and interrill processes, digital methods become more and more of interest. Therefore, we compared the use of photogrammetry with a) terrestrial and b) airborne (UAV) single lens reflex (SLR) cameras as well as c) a terrestrial laser scanner (Leica Scanstation P40) to determine event-based initial erosion rates. Rainfall simulations with the Tübingen rainfall simulator and micro-scale runoff plots (0.4 m × 0.4 m) were conducted on two substrates: a Hortic Anthrosol and sieved sand (0.10-0.45 mm). Runoff plots were exposed to rainfall events with an intensity of 60 mm h<sup>-1</sup>. The measurements were repeated 5 times per substrate for each method and images of the runoff plot surfaces were captured before and after every event. The overlapping SLR images were processed in Agisoft PhotoScan (Structure from Motion - SfM) to process digital surface models (DSMs) with sub-millimetre resolution (a + b). Laser scans were processed with Leica Cyclone and ESRI ArcGIS (c). We assessed the volume of detached sediment by calculating the differences between multi-temporal DSMs or point clouds. After every rainfall simulation, the discharged sediment was weighed to derive the ground-truth for validation.</p><p>The results showed that photogrammetry with digital cameras as well as the use of laser scanners are principally suitable methods to create small-scale 3D point clouds and to map topography differences necessary for initial erosion rate calculation. The processing with common software systems, however, proves to be challenging and high precision is required for recording in the field. All methods overestimated the erosion rates with differences to the weighed sediment delivery from 14 to 45 %. The accuracy was higher for uniform sand than for the Anthrosol treatment. The SfM approach with digital cameras derived better results than the laser scanner used in this study. The terrestrial use of cameras was superior to the airborne use in this small-scale setup, because of the necessary flight altitude to avoid air turbulences on the soil surface. Further development of the measuring techniques and their precise application in the field as well as adapted software processing are still needed. Nevertheless, the methods tested show promising possibilities even for small-scale erosion measurements. Ideas and further suggestions on improvements will be presented at the EGU 2020.</p>

Effects of Tillage Methods and Vegetation Coverage on Reducing Runoff and Soil Erosion

To study the characteristics of runoff and soil erosion of natural rainfall conditions, five standard runoff plots were set up in our experiment, and different tillage methods and vegetation coverage types were set up. The 58-month monthly precipitation data and 44-month runoff plot observation data from 2013 to 2017 were analysed. The results show that: 1) The monthly precipitation fluctuates significantly, ranging from 13mm to 683.5mm. The precipitation is unevenly distributed over the year. The largest average monthly precipitation is in June and the smallest is in January. Rainfall is mainly concentrated in the spring and summer. The precipitation from March to June accounts for 58.0% of the annual rainfall. 2) There is a positive correlation between runoff depth and precipitation in each runoff plot (R2= 0. 5101~0. 6676, Sig.<0.01); 3) There is also a positive correlation between soil loss and precipitation (R2=0. 424~0. 558, Sig.<0.01); 4) The amount of soil loss and the runoff depth increase with increasing rainfall. The runoff plot without any vegetation cover or farming measures increase the most. While the one with horizontal steps and shrubs, or a combination of arbor and grass increase the slowest, indicating that they have the best effect of reducing runoff and soil loss.