Agricultural Hillslopes Research Articles

Empirical and physical modelling of soil erosion in agricultural hillslopes

Abstract Soil erosion is a complex and highly heterogeneous process with a wide range of environmental and economic impacts. Its estimation is particularly challenging and modelling is typically used for erosion estimation over large areas. The aim of this study was to compare the two leading empirical and physical erosion estimation models, i.e. the Revised Universal Soil Loss Equation (RUSLE) and the Water Erosion Prediction Project (WEPP). The models were calibrated and validated using data collected from field experiments conducted in agricultural lands of Mexico. The simulated rainfall experiments involved measuring erosion from field plots subjected to four tillage systems (No crop, Conventional tillage, Conventional tillage + residues, and Handspike) under two antecedent soil moisture conditions (dry and wet). Different calibration approaches based on the factors K and C for RUSLE, and interrill erodibility and hydraulic conductivity in WEPP were tested. The best-performing methods in RUSLE involved measuring the K factor and adopting the recommended C factor by the National Forestry Commission of Mexico. In WEPP, the best results were obtained when interrill erodibility was estimated from experimental measurements. Overall, RUSLE outperformed WEPP in most of the treatments except for CT under WAMC.

The Loss of Soil Parent Material: Detecting and Measuring the Erosion of Saprolite

Soil parent material is a fundamental natural resource for the generation of new soils. Through weathering processes, soil parent materials provide many of the basic building blocks for soils and have a significant bearing on the physico-chemical makeup of the soil profile. Parent materials are critical for governing the stock, quality, and functionality of the soil they form. Most research on soil parent materials to date has aimed to establish and measure the processes by which soil is generated from them. Comparatively little work has been performed to assess the rates at which soil parent materials erode if they are exposed at the land surface. This is despite the threat that the erosion of soil parent materials poses to the process of soil formation and the loss of the essential ecosystem services those soils would have provided. A salient but unanswered question is whether the erosion of soil parent materials, when exposed at the land surface, outpaces the rates at which soils form from them. This study represents one of the first to detect and measure the loss of soil parent material. We applied Uncrewed Aerial Vehicle Structure-From-Motion (UAV-SfM) photogrammetry to detect, map, and quantify the erosion rates of an exposed saprolitic (i.e., weathered bedrock) surface on an agricultural hillslope in Brazil. We then utilized a global inventory of soil formation to compare these erosion rates with the rates at which soils form in equivalent lithologies and climatic contexts. We found that the measured saprolite erosion rates were between 14 and 3766 times faster than those of soil formation in similar climatic and lithological conditions. While these findings demonstrate that saprolite erosion can inhibit soil formation, our observations of above-ground vegetation on the exposed saprolitic surface suggests that weathered bedrock has the potential to sustain some biomass production even in the absence of traditional soils. This opens up a new avenue of enquiry within soil science: to what extent can saprolite and, by extension, soil parent materials deliver soil ecosystem services?

Infiltration-Based Variability of Soil Erodibility Parameters Evaluated with the Jet Erosion Test

Soil erosion by water on agricultural hillslopes leads to numerous environmental problems including reservoir sedimentation, loss of agricultural land, declines in drinking water quality, and requires deep understanding of underlying physical processes for better mitigation. It is imperative to accurately predict soil erosion caused by overland flow processes so that soil conservation efforts can be undertaken proactively before large-scale sedimentation problems arise. Soil detachment is often described by the excess shear stress equation that contains two physical soil erodibility parameters, erodibility coefficient, and critical shear stress. These parameters are normally assumed to be constant but can change across varying soil texture classes as well as during surface runoff events due to changes in soil cohesion and potential dependency on soil moisture content. These changes may significantly affect soil erosion rates at the field and watershed scale. In this study, the erodibility parameters of three soil types (sandy loam, clay loam, and silty clay loam) were analyzed using a laboratory mini-Jet Erosion Test (JET) to determine the effect of soil sample infiltration and moisture condition. Results from the experiments depicted a dynamic relationship between the soil erodibility parameters and amount of infiltrated mass of water. Data analysis displayed that for soils of different texture critical shear stress exhibited local minimum with higher values for very dry and saturated soils, while erodibility coefficient tended to increase with the increase of mass of soil water. Utilizing these dynamic soil erodibility parameters did not result in a significant difference in soil erosion rates when compared to using the averaged soil erodibility parameters taken from the experiment but the range of potential erosion rates increases with the increase of applied sheer stress to soil surface. The erosion rates with the experiment-based coefficients were found to be higher than with the baseline WEPP-based coefficients. These results highlight the importance of evaluating the effect of intrastorm dependent factors during surface runoff events, such as antecedent soil moisture content, time to peak from the start of runoff, soil cohesion, etc., on soil erodibility parameters to accurately calculate erosion rates, especially for initially dry soils or during earlier stages of surface runoff when critical shear stresses were highly affected. Further assessment of such factors with JET or other laboratory and field tests is recommended.

Assessing the Effectiveness of Alternative Tile Intakes on Agricultural Hillslopes

Existing surface inlets behind terraces and water and sediment control basins (WASCoBs) were replaced with alternative tile intakes (ATIs) in agricultural fields of southeast Iowa. These ATIs consisted of a buried column of gravel atop woodchips. Computational, experimental, and field methods were used to design and evaluate the ATIs’ capacity to reduce sediment and nutrient export. Single-storm simulations using the Watershed Erosion Prediction Project (WEPP) provided boundary conditions for permeameter experiments that yielded a hydraulic conductivity for the layered gravel–woodchip configuration of 4.59 cm/s ± 0.36 cm/s. Additionally, a proportional amount of sediment was retained in the permeameter (42%) compared to the amount that settled on the permeameter surface (58%). Event monitoring of field-installed ATIs during three growing seasons measured a sediment trapping efficiency of 86 ± 12% that led to deposition rates of 5.44 ± 3.77 cm/yr, quantified with 210Pb profiles. Percent reduction values were 43% for nitrate and 17% for ortho-phosphate. Finally, long-term continuous-storm modeling using the WEPP suggested that these ATIs could withstand at least 75 25-year events before clogging. Modeling using the Agricultural Conservation Planning Framework suggested watershed-scale load reductions of 1.6% for NO3 and 1.4% for total P for ATIs draining 6.8% of the modeled watershed. Using ATIs in conjunction with WASCOBs and terraces, or as standalone practices, can be a cost-effective means for keeping sediment and nutrients in the landscape.

A validation of WEPP water quality routines in uniform and nonuniform agricultural hillslopes

Current watershed-scale, nonpoint source pollution models do not represent the processes and impacts of agricultural best management practices on water quality with sufficient detail. A Water Erosion Prediction Project-Water Quality (WEPP-WQ) model was recently developed which is capable of simulating nonpoint source pollutant transport in nonuniform hillslope conditions such as those with BMPs. However, WEPP-WQ has not been validated for these conditions, and prior validation work only evaluated calibrated performance rather than uncalibrated performance, with the latter being most relevant to model applications. This study evaluated uncalibrated and calibrated model performance in two plot-scale, artificial rainfall studies. 179 observations were compared to corresponding WEPP-WQ simulations of runoff, sediment yield, and soluble and particulate nutrient forms for both nitrogen and phosphorus. Uncalibrated validation results were mixed for the different field conditions, model configurations, and prediction variables. Nash-Sutcliffe Efficiencies for uncalibrated simulations of uniform conditions were generally greater than 0.6 except for soluble nitrogen predictions which were poor. Simulations of nonuniform conditions were generally ‘unsatisfactory’ except for runoff predictions which were quite good (NSE = 0.78). Performance was improved substantially for almost all endpoints with calibration. Some exceptions to this occurred because the objective function for calibration was based on log-space differences so as to more equally-weight calibration of unsaturated conditions that tend to produce lesser runoff volumes and sediment yields. Calibrated results for both uniform and nonuniform conditions were generally ‘satisfactory’ or ‘good’ according to widely accepted model performance criteria.

Calibration and validation of hillslope runoff and soil loss outputs from the Water Erosion Prediction Project model in Minnesota agricultural watersheds

AbstractThere is growing interest in studying the impact of alternative agricultural management practices on runoff and soil loss under future climate change scenarios. In order to address this interest, it is important to demonstrate that runoff and soil loss can be accurately simulated under existing climates based on comparisons between modeled and experimental results. This study calibrates and validates the Water Erosion Prediction Project (WEPP) model to quantify the accuracy of predicting growing season runoff and soil erosion in agricultural hillslopes based on comparisons with experimental data from five Minnesota hydrologic unit code 12 watersheds. In order to accurately predict runoff and soil erosion in each watershed, the baseline effective hydraulic conductivity (Kbe), interrill and rill erodibility (EIR and ER), and monthly precipitation standard deviations (Pstdev) were calibrated in WEPP using observed runoff and total suspended solids data from five Minnesota Discovery Farms field sites. Before calibration, Nash–Sutcliffe model efficiency (NSE) and percent bias (PBIAS) values for predicted versus measured monthly average total runoff (Ravg‐T), runoff ratios (RRT), and total soil loss were generally not in acceptable ranges. After calibration, the NSE values showed very good fits between measured and predicted monthly Ravg‐T (0.64–0.98), RRT (0.66–0.93), and soil loss (0.58–0.80). PBIAS values were also within acceptable ranges for Ravg‐T and RRT (±25%) and soil loss (±55%), except for RRT at site BE1. NSE and PBIAS values during validation were within acceptable ranges, except for RRT at site BE1. These findings suggest that the WEPP hillslopes calibrated in this study are sufficiently robust to accurately predict monthly runoff and soil erosion in Minnesota agricultural fields during the growing season.

Soil–Water Dynamics Investigation at Agricultural Hillslope with High-Precision Weighing Lysimeters and Soil–Water Collection Systems

A quantitative understanding of actual evapotranspiration (ETa) and soil–water dynamics in a hillslope agroecosystem is vital for sustainable water resource management and soil conservation; however, the complexity of processes and conditions involving lateral subsurface flow (LSF) can be a limiting factor in the full comprehension of hillslope soil–water dynamics. The research was carried out at SUPREHILL CZO located on a hillslope agroecosystem (vineyard) over a period of two years (2021–2022) by combining soil characterization and field hydrological measurements, including weighing lysimeters, sensor measurements, and LSF collection system measurements. Lysimeters were placed on the hilltop and the footslope, both having a dynamic controlled bottom boundary, which corresponded to field pressure head measurements, to mimic field soil–water dynamics. Water balance components between the two positions on the slope were compared with the goal of identifying differences that might reveal hydrologically driven differences due to LSF paths across the hillslope. The usually considered limitations of these lysimeters, or the borders preventing LSF through the domain, acted as an aid within this installation setup, as the lack of LSF was compensated for through the pumping system at the footslope. The findings from lysimeters were compared with LSF collection system measurements. Weighing lysimeter data indicated that LSF controlled ETa rates. The results suggest that the onset of LSF contributes to the spatial crop productivity distribution in hillslopes. The present approach may be useful for investigating the impact of LSF on water balance components for similar hillslope sites and crops or other soil surface covers.

Using Dye and Bromide Tracers to Identify Preferential Water Flow in Agricultural Hillslope Soil under Controlled Conditions

Processes in hillslope soils present a particular challenge for agricultural production and soil management due to their hydropedological specifics and high soil erosion risk. Soil heterogeneities can cause preferential and/or lateral flow on the entire hillslope resulting in the off-site movement of water, fertilizers and chemicals used in crop production. A study was conducted under controlled conditions in a laboratory with undisturbed soil cores (250 cm3), which were used to estimate the soil hydraulic properties (SHP) using HYPROP and WP4C devices, while undisturbed soil columns (diameter = 16 cm, length = 25 cm) were used for the evaluation of preferential flow pathways using potassium bromide and Brilliant Blue. Samples were excavated in triplicate from the hilltop, backslope and footslope regions within the inter-rows of a vineyard from a critical zone observatory, SUPREHILL, in Croatia in Dystric Luvic Stagnosol. The aim of this study was to determine if the erosion-affected hillslope position affected the physical, chemical and hydraulic properties of soil and to identify water flow and possible preferential flow using dye and bromide tracers. The results of the sensor measurements and estimated SHPs were in agreement, showing a faster leaching of the irrigated rainwater in the footslope column. The tracer experiments showed variability even in the columns taken from the same position on the hillslope, which can be linked to plant roots and soil fauna activity. Altogether, the results showed a deeper loose layer at the footslope as a consequence of the soil erosion, which then resulted in higher hydraulic conductivity and the leached mass of the bromide due to better soil structure and pore connectivity. Thus, due to significant differences in the leached mass of bromide, this research should be later expanded in field experiments to reveal the impact of surface runoff, subsurface preferential and lateral flow on a larger scale.

A WEPP-Water Quality model for simulating nonpoint source pollutants in nonuniform agricultural hillslopes: Model development and sensitivity

The Water Erosion Prediction Project (WEPP) model code was modified extensively to support the simulation of nonpoint source (NPS) pollutant sourcing and transport in nonuniform hillslopes based on NPS science from the Soil and Water Assessment Tool (SWAT). This was accomplished utilizing WEPP's overland flow element (OFE) in place of SWAT's hydrologic response unit (HRU) construct which enabled more physically plausible routing within a hillslope. In addition, several improvements to the NPS code base were implemented. These include: free-source format, modern-Fortran conventions, minor enhancements to NPS model science, and code refactoring. This manuscript documents all model development activities, presents a comparison of relevant WEPP and WEPP-WQ code bases, and performs a local sensitivity analysis of the final model code for the most important input parameters and processes. Sensitivity results indicated that the model performed as expected according to its design and provided important insights for potential subsequent validation studies.

The role of colloids and other fractions in the below-ground delivery of phosphorus from agricultural hillslopes to streams

Colloids can be important for facilitated transfer of phosphorus (P) to groundwater (GW) and contribute to elevated P concentrations later delivered to surface water. To assess the role of colloidal P and other P fractions in delivery processes via below-ground pathways, this study investigated the influence of catchment and flow event characteristics on particulate (>450 nm), medium-sized colloidal (200–450 nm) and fine (<200 nm) P fractions in two agricultural hillslopes (TG, TA). Total and dissolved P fractions and their derivatives were also monitored. Samples in both stream and GW were taken weekly during baseflow conditions and every 2 h during storm conditions. Higher frequency monitoring of streamflow was also conducted to delineate hydrological flowpaths and determine P loads and hysteresis processes. Results indicated that during baseflow fine P was dominant in the streams (80 to 100 % of total P) and in shallow GW in TA (83 to 96 %) whereas in TG shallow GW was dominated by PP (55 to 96 %) possibly due to colloidal Fe-P complexes. Similarly, in TG shallow GW was dominated by PP (79 to 81 %) during high flow events. During a larger flow event (within the period of land fertilization) the quickflow pathway (24 % of total flow) delivered 3.2 g ha−1 of PP which was dominant in the stream (44 to 68 %). A smaller flow event (within the period of prohibited land fertilization) facilitated delivery of P via deeper baseflow pathways (87 % of total flow) as fine reactive P (1.3 g ha−1), also dominant in the stream (73 to 78 %). The research indicated a very limited presence of medium-sized colloidal P but a large presence of fine P that may contribute to elevating P concentrations above environmental thresholds. Further work should constrain the controlling factors for colloidal P presence/absence and also on the extent and speciation of coarser and finer fractions in the hillslope to stream continuum.

Pathways and composition of dissolved organic carbon in a small agricultural catchment during base flow conditions

The amount and composition of organic carbon are major controls on water quality and ecological processes in streams. In this study we explored the fate of the quantity of dissolved organic carbon (DOC) and the composition of dissolved organic matter (DOM) in an agricultural hillslope - stream network system. We conducted our study in the 66 ha HOAL (Hydrological Open Air Laboratory) in Lower Austria. We measured DOC of the soil eluates from different land use units, water samples from the stream and from seven tributaries, and estimated DOM components by fluorescent spectrophotometry and PARAFAC analyses. Soil DOC shows the highest concentrations in summer, but DOC concentrations in the tributaries are lower in summer than in winter by between 19% and 31%. DOM composition of the soil eluate differs between land use units. The forest site exhibits the largest fractions of humic-like fluorophores and less labile DOM. DOM composition in the tributaries is, in addition to DOC, controlled by soil moisture. We estimated the DOC import from the tributaries into the stream as 125 kg during base flow conditions in the period February to December 2017 and the instream DOC production as 38 kg, considering mass balance and exchange with groundwater. Six out of seven DOM components have a positive net production along the stream, only aliphatic DOM with low molecular weight is consumed (65 % of its input). These findings suggest that agricultural land use increases DOC input into streams and alters their DOM quality. Instream processes modify DOM quality over short distances.

Quantifying the impact of no‐till on sediment yield in southern Brazil at the hillslope and catchment scales

AbstractNo‐till (NT) is a soil management system designed to protect soil resources from water erosion and provide numerous benefits compared to conventional tillage through the increase of organic matter inputs into the soil. However, NT in isolation is not sufficient to control erosion processes caused by an excessive production of surface runoff. This study evaluated soil losses on agricultural hillslopes under no‐till characterized by contrasted water, soil, and crop management conditions. To this end, water and soil losses were monitored between 2014 and 2018 at two scales, including four macroplots (0.6 ha; 27 events) and two paired zero‐order catchments (2.4 ha; 63 events). The resulting dataset covered a wide range of rainfall conditions that occurred in contrasted soil, crop, and runoff management conditions. Hyetographs, hydrographs, and sedigraphs were constructed, and these data were used to evaluate the impact of management on sediment yields, including that of terraces, scarification, and phytomass on sediment yield. The installation of terraces reduced sediment yield by 58.7%, mainly through surface runoff control. Crop management including an increased phytomass input efficiently controlled soil losses (63%), although it did not reduce runoff volume and peak flow. In contrast, scarification had no impact on runoff and soil losses. The current research demonstrated the need to combine the installation of terraces and leaving a high amount of phytomass on the soil to control surface runoff and erosion and reduce sediment yield. The current research therefore reinforces the relevance of the monitoring strategy conducted at the scale of macroplots and zero‐order catchments to evaluate the impact of contrasted water, soil, and crop management methods and select the most effective conservation agriculture practices.

Estimation of the annual rainfall erosivity index based on hourly rainfall data in a tropical region

The annual mean rainfall erosivity (R) indicates the potential soil loss caused by the precipitation and runoff and is used to predict the soil loss from agricultural hillslopes. R is calculated from rainfall stations with continuously recording rainfall databases. However, many short-term real-time rainfall databases that also relate to the rainfall intensity are not readily available around Taiwan, with the hourly rainfall data being predominantly available. The annual mean rainfall erosivity calculated by the 10-min rainfall data accumulation converted to the 30-min rainfall data (R&lt;sub&gt;10_30&lt;/sub&gt;) can be estimated using the annual mean rainfall erosivity calculated by the 10-min rainfall data accumulation convert to the hourly rainfall data (R&lt;sub&gt;10_60&lt;/sub&gt;) that are calculated from the kinetic energy calculated by the 10-min rainfall data accumulation converted to the hourly rainfall data (E&lt;sub&gt;60j&lt;/sub&gt;). The maximum 60-min rainfall intensity calculated by the 10-min rainfall data accumulation converted to the hourly rainfall data (I&lt;sub&gt;60j&lt;/sub&gt;) has been established in rainfall stations throughout southern Taiwan. The 10-min rainfall data set consists of 15 221 storm events from 2002 to 2017 monitored by 51 rainfall stations located in the tropical regions in Taiwan. According to the results of this study, the average conversion factors of the kinetic energy (1.04), rainfall erosivity (1.47), and annual mean rainfall erosivity (1.30) could be estimated based on the 10-min rainfall data.

Soil conservation in an abandoned agricultural rain-fed land through inoculation of cyanobacteria

Newly abandoned rain-fed lands, due to soil quality decline, are the main sources for soil erosion in semi-dry lands agricultural hill-slopes. However, adopting compatible environmentally friend measures have not been adequately implemented. Following various resultful laboratory studies, in this paper, we assessed the effect of field inoculation of cyanobacteria in an abandoned rain-fed land on controlling the natural rainfall-induced soil loss and improving the biological soil crust (BSC) quality mixture of two cyanobacteria (viz. Nostoc sp. and Oscilatoria sp.). The cyanobacteria were inoculated on 22.1 m × 1.83 m-sized plots located on the study hill-slope. The experiment was carried out for a rainfall period of 6 months under natural conditions. Measurements of 7 erosive rainfalls showed a considerable soil loss (i.e., 0.88 ± 0.10 to 9.79 ± 1.60 g m−2) from the un-inoculated plots (control), while cyanobacteria decreased the soil loss by 36 to 57% in comparison with those recorded for the control. Soil losses in the inoculated plots were 2.39 ± 0.24, 4.8 ± 0.58, 3.32 ± 0.33, 1.49 ± 0.12, 0.87 ± 0.12, 0.52 ± 0.05, and 0.56 ± 0.08 g m−2, from the first to last rainstorms, respectively. Our results, therefore, showed a poor linear relationship between soil loss (from both treatments) and rainfall properties. Cyanobacteria also improved organic carbon and nitrogen contents as two important BSC quality indicators to the tunes of 32 and 56%, respectively, in comparison to the control, during the study period. In winter, when environmental conditions were inappropriate for growing vegetation, the effect of cyanobacteria on soil loss reduction was found to be more tangible in comparison with spring. On the whole, our field soil inoculation under uncontrolled conditions, as pioneering research, facilitated to promising results for large-scale use of cyanobacteria to rapidly restore abandoned rain-fed lands.

Erosion and hydrological response in no-tillage subjected to crop rotation intensification in southern Brazil

No-tillage (NT) is a practice that has been used for >30 years and is used as the main soil conservation strategy in South America. However, recent studies have demonstrated soil erosion problems in agricultural fields in southern Brazil related to soybean monoculture, low phytomass production by crops and an absence of runoff control. This study describes the erosion and hydrological response of chisel ploughing and crop rotation intensification in a NT traditionally managed without crop rotation on a Nitisol on an agricultural hillslope in southern Brazil. The water and soil losses were monitored in natural hillslope (Macroplots) approximately 90 m in length and 6000 m2 in area and with a rectilinear plane curvature and a slope of 9%. The treatments tested were as follows: i) NT without crop rotation (NTnocr), ii) chisel ploughing in NT without crop rotation (CH + NTnocr), iii) NT with crop rotation (NTcr), and iv) chisel ploughing in NT with crop rotation (CH + NTcr). Monitoring was performed in the period from July 2015 to June 2016, during which 9 significant rainfall events occurred at different intensities and accumulated volumes. The monitoring results indicated that NTnocr is insufficient to control soil erosion, even during low-intensity events. The crop rotation intensification in NT with increased phytomass production by crops reduced runoff only by 18% and soil losses by up to 84%. Care in relation to chisel ploughing was evidenced by a 2.4-fold increase in NTcr soil losses associated with soil mobilization due to chiseling and high-intensity events. In conclusion, the results revealed that the isolated adoption of NT is insufficient to control runoff, even under crop rotation intensification. This result clearly indicates that complementary conservation practices are necessary to control runoff and all its associated processes, from soil erosion and nutrient and pesticide mobilization to water resources, to improve the definition of conservation agriculture.

How effective is information on soil-landscape units for determining spatio-temporal variability of near-surface soil moisture?

In the last decades, a growing interest in fostering advanced interdisciplinary studies is leading to the establishment of observatories in pilot catchments for long-term monitoring of hydrological variables and fluxes. Nevertheless prior to sensor network installation, this investment necessitates preliminary surveys on key-variables such as near-surface soil moisture in order to prevent risks of erronously distributing sensors by missing sufficient spatial information for understanding hydrological processes within the landatmosphere interactions. The availability of maps describing areas with similar morphological, topographical, soil, and vegetation characteristics enable preliminary surveys to be organized for capturing spatio-temporal variability of soil moisture as best as possible. The soil-landscape classification can be considered as an interesting approach for grouping mapping units with similar hydrological behavior. Therefore, we assume the soil-landscape units as hydrotopes or hydrological similar units. Six transects were established along two hillsides of the Upper Alento River catchment (southern Italy) which is a proper candidate to become a Critical Zone Observatory. In this paper we use a soil-landscape map to infer spatial and temporal dynamics of soil moisture measured along these transects, whereas quantitative analyses were obtained by using multivariate techniques. The effectiveness of available information on soil-landscape mapping units is evaluated with respect to different observed patterns of soil moisture: wetter- and drier-than average observation points belong to agricultural and forested hillslopes, respectively. Soil texture and topographical controlling factors, especially clay content and slope gradient, are found to explain approximately 70% of the observed spatial variations in soil moisture along the forested hillslopes. The spatial structure explained by the environmental controlling factors decreases to 45% in the cases of the agricultural hillslopes mainly due to perturbations induced by grazing and tillage practices.

Long-term agricultural management and erosion change soil organic matter chemistry and association with minerals

The interaction of soil organic matter (SOM) and minerals is a critical mechanism for retaining SOM in soil and protecting soil fertility and long-term agricultural sustainability. The chemical speciation of carbon (C) and nitrogen (N) in mineral-associated SOM can be sensitive to both anthropogenic management practices and landscape positions, but these two aspects are rarely examined in tandem. Here we examined the effects of long-term (>100 years) agricultural management and erosion on mineral-associated SOM along grassland and agricultural hillslope transect. The mineral-associated SOM was obtained using particle size and density fractionation approaches. Chemical speciation of C and N in mineral-associated SOM was characterized using micro X-ray absorption near-edge fine structure (XANES) spectroscopy. The extent of SOM coverage and contribution of iron oxyhydroxides (Fe oxides) to the total specific mineral surface area (SSA) were determined using the BET–N2 adsorption method of soil samples under three conditions: untreated, SOM removal, and Fe oxides removal. The amount of SSA covered by SOM (SSASOM-covered) was lower by 61% and 37% in cultivated eroding and depositional topsoils, respectively, compared with the corresponding grassland. Depositional soils had higher SSASOM-covered than eroding positions. In the cultivated hillslopes, aromatic and phenolic C species were more abundant in depositional soils than in the eroding topsoils, indicating that deposition and burial of eroded or in-situ plant-derived phenolic C protected them from further transformation. Our results, therefore, highlight the importance of anthropogenic activities in the interaction of SOM and minerals, including C speciation changes, which may exert a considerable influence on SOM retention in soils.

Impact of broad-based terraces on water and sediment losses in no-till (paired zero-order) catchments in southern Brazil

Soil degradation is a widespread problem and currently one of the biggest challenges in modern-day agriculture. The partial adoption of conservation agriculture, such as no-till management, does not provide adequate erosion control, and the hydrology dynamics on hillslopes under no-till management remain poorly quantified. This study examines the hydrology of agricultural hillslopes under no-till management, with and without terraces in southern Brazil. Water and soil losses were measured in two paired, zero-order catchments (2.4 ha) under no-till cultivation, with and without broad-based retention terraces. Rainfall, surface runoff, and suspended sediment concentrations were monitored during major rainfall events. Analysis of hydrographs and sedigraphs was used to derive the peak flow, runoff duration, and sediment yield values and the hysteresis between surface runoff and the suspended sediment concentration during different seasons. The results show higher soil and water losses in the catchment without terraces. Terracing reduced peak flow rates by 79%, sediment yield from 0.44 to 0.16 t ha−1, and the total surface runoff from 3943 (126 mm) to 855 m3 (36 mm) during 31 events over 16 months. The no-till system without terraces was unable to adequately control surface runoff and soil erosion. Surface runoff and sediment yield were higher under no-till without terraces than under no-till with terraces. The difference in terms of surface runoff volume and sediment yield indicates an important difference in the hydrology and soil erosion in the catchment without terraces, which is represented by high-surface-runoff coefficient values observed during the rainfall-runoff events. The short lag time and steep rising limb of the hydrographs indicate high-surface-runoff responsivity to rainfall in no-till without terraces.

Potential of time‐lapse photography for identifying saturation area dynamics on agricultural hillslopes

AbstractMapping saturation areas during rainfall events is important for understanding the dynamics of overland flow. In this study, we evaluate the potential of high temporal resolution time‐lapse photography for mapping the dynamics of saturation areas (i.e., areas where water is visually ponding on the surface) on the hillslope scale during natural rainfall. We take 1 image per minute over a 100 × 15 m2 depression area on an agricultural field in the Hydrological Open Air Laboratory, Austria. The images are georectified and classified by an automated procedure, using grey intensity as a threshold to identify saturation area. The optimum threshold T is obtained by comparing saturation areas from the automated analysis with the manual analysis of 149 images. T is found to be highly correlated with an image brightness characteristic defined as the greyscale image histogram mode M (Pearson correlation r = 0.91). We estimate T as T = M + C where C is a calibration parameter assumed to be constant during each event. The automated procedure estimates the total saturation area close to the manual analysis with mean normalized root mean square error of 9% and 21% if C is calibrated for each event and taken constant for all events, respectively. The spatial patterns of saturation are estimated with a geometric mean accuracy index of 94% as compared to the manual analysis of the same photos. The patterns are tested against field observations for one date as a preliminary demonstration, which yields a root mean square error of the shortest distance between the measured boundary points and the automatically classified boundary as 23 cm. The usefulness of the patterns is illustrated by exploring run‐off generation processes of an example event. Overall, the proposed classification method based on grey intensity is found to process images with highly varying brightnesses well. It is more efficient than the manual tracing for a large number of images, which allows the exploration of surface flow processes at high temporal resolution.

Soil water storage prediction at high space–time resolution along an agricultural hillslope

Soil water storage (SWS) information at high space–time resolution is critical for understanding numerous hydrological, biological and chemical processes. However, obtaining such information is time- and cost-intensive due to the strong variability of SWS. We hypothesized that SWS information could be predicted accurately at high space–time resolution and low cost using the temporal stability (TS) concept. The water contents of different soil layers down to 1.0m in depth were measured along a 3.1ha slope from July 2013 to July 2015 at 4 locations using automatic measurement systems and at 103 locations manually. These values were multiplied by depth to convert them into SWS (0–0.2, 0.2–0.5, 0.5–1.0, and 0–1.0m). The spatial patterns of SWS were temporally stable. The SWS values were predicted at high space–time resolution by combining high-space and low-time resolution data and high-time and low-space resolution data using two methods. The first method (M1) was based on the most temporally stable locations (MTSLs) among four auto-measured locations. The second method (M2) identified the MTSLs from 107 locations including 103 manually measured locations of the four soil layers. The MTSLs of M2 were assigned high-time resolution data based on the relationships between the MTSLs of M1 and M2 at each soil layer. Once the MTSL and temporal stability relationship (TSR) of these two methods were identified, the SWS data for one auto-measured location (A4) were sufficient to predict the spatially averaged and spatially distributed SWS for the slope at any time. Although the predictive errors for M1 were generally acceptable, M2 was more accurate than M1 in most of the cases studied. The estimation errors for M2 were all less than 10% and were generally less than 5%. Among the four investigated soil layers, M2 outperformed M1 for the 0–0.2 and 0–1.0m soil layers, and the two methods yielded comparable results for the 0.5–1.0m soil layer. Meanwhile, although M1 slightly outperformed M2 for the 0.2–0.5m soil layer, both performed well. This method for predicting SWS at high accuracy and low cost could improve the prediction accuracy of early drought warnings and agricultural water resources management.