Forested Catchments Research Articles

Flowpath influence on stream acid events in tropical urban streams in Singapore

AbstractWe investigated processes contributing to periodic acidification events in headwater streams of Nee Soon Forest Catchment (NSFC) in Singapore by monitoring hydrochemical changes in response to rainfall inputs. Stream chemistry response to most rainfall events was characterized by decreases in pH from means ranging from 5.1 to 5.3 to below 4.8–5.0 and corresponding increases in specific electrical conductivity from baseflow values of 15–30 to 50–80 μS cm−1, indicative of low‐total dissolved solids in stormflow. The decreases in pH in the streams, which are typically acidic year‐round, were related to occasional highly acidic rainfall inputs (pH ~4.05) and likely the flushing of organic acids into the stream by shallow subsurface flow interacting with surface litter and/or organic‐rich soil horizons. The interaction of rainwater runoff with organic matter in the soil matrix possibly alters the chemical composition of stormflow, influencing pH. Decomposition of instream organic matter also reduces stream water pH. Leaching experiments revealed that the overland flow passing through organic matter and A horizon material has the potential to lower stream water pH by approximately a half unit or more, in part, by flushing nitrates that were produced by microbial decomposition of organic matter and/or precipitated sulphur that enterred the forest by wet or dry atmospheric deposition. The observed periodic acid events are a natural phenomenon in the stream system in this urban environment because of naturally acidic rainfall and granitic soils with low buffering potential. However, acid events are likely amplified in frequency and magnitude by anthropogenic pollution emissions of sulphur and nitrogen species (e.g., SO2and NOx) from local and regional sources that lower rainfall pH. Although, acid runoff events are typically short‐lived (<12–24 h), further longitudinal monitoring and experimental studies are needed to investigate the long‐term implications on sensitive taxa in the NSFC streams. Finally, understanding the flow pathways of stormflow water in the nested system was critical for deciphering the mechanisms driving stream acid events at the site.

Event-based quickflow simulation with OpenLISEM in a burned Mediterranean forest catchment

Recently burnt areas typically reveal strong to extreme hydrological responses, as a consequence of loss of protective soil cover and heating-induced changes in topsoil properties. Soil water repellency (SWR) has frequently been referred to as one of the explanatory variables for fire-enhanced surface runoff generation but this has been poorly demonstrated, especially at the catchment scale. This study employs a process-based modelling approach to better understand the relevance of SWR in the hydrological response of a small, entirely burnt catchment in central Portugal, in particular by comparing hydrological events under contrasting initial conditions of dry vs wet soils. The OpenLISEM model was applied to a selection of 16 major rainfall runoff events that occurred during the first 2 post-fire years. The automatic calibration procedure resulted in good model performance, but it worsened for validation events. Furthermore, uncertainty analysis revealed an elevated sensitivity of OpenLISEM to event-specific conditions, especially for predicting the events’ total and peak flows. Also, predicted spatial patterns in runoff poorly agreed with the runoff observed in microplots. Model performance improved when events were separated by dry and wet initial moisture conditions, particularly for wet conditions, suggesting the role of variables other than initial soil moisture.

Budget of Plant Litter and Litter Carbon in the Subalpine Forest Streams

Investigations on the budget of plant litter and litter carbon in forest streams can provide a key scientific basis for understanding the biogeochemical linkages of terrestrial–aquatic ecosystems and managing forest catchments. To understand the biogeochemical linkages among mountain forests, riparian vegetation, and aquatic ecosystems, the changes in litter input and output from the subalpine streams with stream characteristics and critical periods were investigated in an ecologically important subalpine coniferous forest catchment in the upper reaches of the Yangtze River. The annual litter input to the stream was 20.14 g m−2 and ranged from 2.47 to 103.13 g m−2 for 15 streams during the one-year investigation. Simultaneously, the litter carbon input to the stream was 8.61 mg m−2 and ranged from 0.11 to 40.57 mg m−2. Meanwhile, the annual litter output varied from 0.02 to 22.30 g m−2, and the average value was 0.56 g m−2. Correspondingly, the litter carbon output varied from 0.01 to 1.51 mg m−2, and the average value was 0.16 mg m−2. Furthermore, the average ratio of litter carbon input to output was 270.01. The maximum and minimum values were observed in the late growing season and the snowmelt season, respectively. Additionally, seasonal variations in temperature, together with the stream length, dominated the input of litter and litter carbon to the stream, while the precipitation, temperature, water level, and sediment depth largely determined their output. Briefly, the seasonal dynamics of litter and litter carbon were dominated by stream characteristics and precipitation as well as temperature patterns.

Macroinvertebrate community composition and richness along extreme gradients: The role of local, catchment, and climatic variables in Patagonian headwater streams

Abstract We investigated how interacting multi‐scale environmental variables affect taxonomic composition and functional feeding groups of macroinvertebrates in headwater streams of western Patagonia in southern Chile (45–47°S), a region characterised by extreme climatic gradients, intact forested catchments, and recent intense land use change. The opportunity of establishing biological/functional diversity patterns for near‐reference conditions of headwaters streams is paramount. We used multi‐dimensional scaling ordination and boosted regression tree analysis to evaluate the link between reach, catchment, spatial, and regional climatic variables and macroinvertebrate community structure. The high diversity of predators in low productivity streams, together with genera previously unreported for this latitudinal range underscore the unique characteristics and very limited understanding of Patagonian freshwater stream communities. Two main groups of environmental variables were linked to both taxonomic and functional feeding group diversity. The strongly associated variables were related to forest cover (and inversely alpine zone) at the catchment scale, together with stream velocity at the local scale. The second group reflected local indicators of productivity and weathering inputs, associated with the regional east–west precipitation gradient. We present a conceptual model for the catchment scale effects on invertebrate functional groups in forested headwater streams. While the results highlight potentially singular characteristics of the study region, they also suggest a macro‐ecological generalisation of how global change variables, operating on mountain catchments, may differentially affect stream ecosystems as a consequence of the variations in the forest/alpine gradient, across both deciduous and evergreen temperate biomes.

Crustal response to heavy rains in Southwest Japan 2017-2020

Weather fronts stationary above Southwest Japan often bring disastrous heavy rains in early summer. Here we study four such episodes in each of four summers between 2017 and 2020, and investigate transient lithospheric subsidence caused by rainwater loads using the daily coordinates of a dense network of continuous GNSS stations. After applying a network filter to remove common mode errors, we isolated subsidence signals of 1-2 centimeters in flooded regions. Such subsidence recovered mostly in a day as rainwater drained rapidly to nearby ocean promoted by large topographic slopes. Spatiotemporal correlation between subsidence and precipitation was weak due possibly to rapid post-precipitation migration of rainwater. However, a strong correlation was found between subsidence and rain spatially integrated over the entire Southwest Japan, i.e., bulk subsidence of ∼0.1 km3 (equivalent to the uniform subsidence ∼0.6 mm) occurred for every 1 Gt rainwater per day. This linearity breaks down for rains exceeding ∼10 Gt/day as rainwater possibly exceeds the water-holding capacity of forest catchments.

Small-scale topography explains patterns and dynamics of dissolved organic carbon exports from the riparian zone of a temperate, forested catchment

Abstract. Export of dissolved organic carbon (DOC) from riparian zones (RZs) is an important component of temperate catchment carbon budgets, but export mechanisms are still poorly understood. Here we show that DOC export is predominantly controlled by the microtopography of the RZ (lateral variability) and by riparian groundwater level dynamics (temporal variability). From February 2017 until July 2019 we studied topography, DOC quality and water fluxes and pathways in the RZ of a small forested catchment and the receiving stream in central Germany. The chemical classification of the riparian groundwater and surface water samples (n=66) by Fourier transform ion cyclotron resonance mass spectrometry revealed a cluster of plant-derived, aromatic and oxygen-rich DOC with high concentrations (DOCI) and a cluster of microbially processed, saturated and heteroatom-enriched DOC with lower concentrations (DOCII). The two DOC clusters were connected to locations with distinctly different values of the high-resolution topographic wetness index (TWIHR; at 1 m resolution) within the study area. Numerical water flow modeling using the integrated surface–subsurface model HydroGeoSphere revealed that surface runoff from high-TWIHR zones associated with the DOCI cluster (DOCI source zones) dominated overall discharge generation and therefore DOC export. Although corresponding to only 15 % of the area in the studied RZ, the DOCI source zones contributed 1.5 times the DOC export of the remaining 85 % of the area associated with DOCII source zones. Accordingly, DOC quality in stream water sampled under five event flow conditions (n=73) was closely reflecting the DOCI quality. Our results suggest that DOC export by surface runoff along dynamically evolving surface flow networks can play a dominant role for DOC exports from RZs with overall low topographic relief and should consequently be considered in catchment-scale DOC export models. We propose that proxies of spatial heterogeneity such as the TWIHR can help to delineate the most active source zones and provide a mechanistic basis for improved model conceptualization of DOC exports.

Impacts of Soil Properties, Topography, and Environmental Features on Soil Water Holding Capacities (SWHCs) and Their Interrelationships

Soil water holding capacities (SWHCs) are among the most important factors for understanding the water cycle in forested catchments because they control available plant water that supports evapotranspiration. The direct determination of SWHCs, however, is time consuming and expensive, so many pedotransfer functions (PTFs) and digital soil mapping (DSM) models have been developed for predicting SWHCs. Thus, it is important to select the correct soil properties, topographies, and environmental features when developing a prediction model, as well as to understand the interrelationships among variables. In this study, we collected soil samples at 971 forest sites and developed PTF and DSM models for predicting three kinds of SWHCs: saturated water content (θS) and water content at pF1.8 and pF2.7 (θ1.8 and θ2.7). Important explanatory variables for SWHC prediction were selected from two variable importance analyses. Correlation matrix and sensitivity analysis based on the developed models showed that, as the matric suction changed, the soil physical and chemical properties that influence the SWHCs changed, i.e., soil structure rather than soil particle distribution at θS, coarse soil particles at θ1.8, and finer soil particle at θ2.7. In addition, organic matter had a considerable influence on all SWHCs. Among the topographic features, elevation was the most influential, and it was closely related to the geological variability of bedrock and soil properties. Aspect was highly related to vegetation, confirming that it was an important variable for DSM modeling. Information about important variables and their interrelationship can be used to strengthen PTFs and DSM models for future research.

Fluvial sediment export from pristine forested headwater catchments in the Congo Basin

Studies on sediment export from tropical forest watersheds are scarce. Of the assessments that do exist, most are of larger rivers or are model-based and lack validation with measured data. Understanding the mechanisms of sediment export dynamics in forested headwaters is important for assessing downstream effects and as a baseline for net impacts of land-use change. To that end, we quantified annual total suspended sediment (TSS) yields in forested headwater catchments of two major forest types in central Africa (tropical lowland forest and subtropical Miombo woodland) and analyzed turbidity-discharge hysteresis over one hydrological year. We measured TSS yields of 0.24 ± 0.09 t ha−1 yr−1 in the Miombo woodland and 0.25 ± 0.05 t ha−1 yr−1 in the lowland forest catchment. The Miombo woodland experienced similar TSS yields as the lowland forest despite a shorter, five-month, rainy season and lower annual precipitation. In the Miombo forest, sparser vegetation cover, seasonal fires that remove understory vegetation and high rainfall intensity during the rainy season therefore resulted in similar TSS yields. As a result of these differences in vegetation and rainfall, approximately 68% of TSS was exported during storm events in the Miombo woodland and 30% in the lowland forest. Both sites showed mainly clockwise hysteresis (positive hysteresis index) patterns of sediment export. In the Miombo woodland, the hysteresis index (i.e., the magnitude and direction of hysteresis) increased with the ongoing rainy season, indicating source limitation already after one month of rain. In the lowland forest, the predominant clockwise hysteresis was more likely caused by the increasing contribution of baseflow during the falling limb of an event, whereas during the rising limb there is a quick flushing of surface material available in the forest. These findings based on hysteresis analysis were further supported by C:N ratio and δ13C analyses of particulate organic matter (POM). POM C:N ratios increased and δ13C signatures decreased with increased discharge in the lowland forest, indicating the mobilization of topsoil sediments during rain events. In contrast, the Miombo exhibited no shifts in C:N ratios nor in the δ13C signature. Despite the pristine nature of these forests and their assumed negligible sediment yields, our results demonstrate that erosion is a significant loss process in tropical forests and call for future research to examine its role in forest functioning and biogeochemical cycling.

Land use influences stream bacterial communities in lowland tropical watersheds

Land use is known to affect water quality yet the impact it has on aquatic microbial communities in tropical systems is poorly understood. We used 16S metabarcoding to assess the impact of land use on bacterial communities in the water column of four streams in central Panama. Each stream was influenced by a common Neotropical land use: mature forest, secondary forest, silvopasture and traditional cattle pasture. Bacterial community diversity and composition were significantly influenced by nearby land uses. Streams bordered by forests had higher phylogenetic diversity (Faith’s PD) and similar community structure (based on weighted UniFrac distance), whereas the stream surrounded by traditional cattle pasture had lower diversity and unique bacterial communities. The silvopasture stream showed strong seasonal shifts, with communities similar to forested catchments during the wet seasons and cattle pasture during dry seasons. We demonstrate that natural forest regrowth and targeted management, such as maintaining and restoring riparian corridors, benefit stream-water microbiomes in tropical landscapes and can provide a rapid and efficient approach to balancing agricultural activities and water quality protection.

Forest defoliator outbreaks alter nutrient cycling in northern waters

Insect defoliators alter biogeochemical cycles from land into receiving waters by consuming terrestrial biomass and releasing biolabile frass. Here, we related insect outbreaks to water chemistry across 12 boreal lake catchments over 32-years. We report, on average, 27% lower dissolved organic carbon (DOC) and 112% higher dissolved inorganic nitrogen (DIN) concentrations in lake waters when defoliators covered entire catchments and reduced leaf area. DOC reductions reached 32% when deciduous stands dominated. Within-year changes in DOC from insect outbreaks exceeded 86% of between-year trends across a larger dataset of 266 boreal and north temperate lakes from 1990 to 2016. Similarly, within-year increases in DIN from insect outbreaks exceeded local, between-year changes in DIN by 12-times, on average. As insect defoliator outbreaks occur at least every 5 years across a wider 439,661 km2 boreal ecozone of Ontario, we suggest they are an underappreciated driver of biogeochemical cycles in forest catchments of this region.

Effects of forest growth in different vegetation communities on forest catchment water balance

Forest ecosystems are critical for adjusting the dynamic balance of the hydrological cycle. This balance is affected by vegetation community types, phenology, and forest density. Previous long-term catchment-scale model studies have focused on changes in forest areas while ignoring the above factors. Since the 1980s, climate change caused by increases in atmospheric CO2 levels has enhanced forest growth. Moreover, amendments to forest management policies, including intermediate cuttings caused by economic factors, have yielded unprecedented changes in forest ecosystems. In this study, we designed a methodology and created a credible model using the Soil and Water Assessment Tool (SWAT) that can precisely reflect water balance variations caused by different ecosystem situations during long-term changes in forest density. We focused on the Yamato River catchment in Western Japan, which includes three planted forests and one primeval forest, each markedly different with respect to vegetation community composition and management policy. In the process, we examined the ratio of coniferous vegetation and broad-leaved vegetation in different forest areas, used remote sensing methods to quantify the maximum and minimum leaf area index (LAI) of each forest region over 40 years, and calibrated the model by comparing the LAI growth curve, evapotranspiration, and streamflow with observed data. Moreover, we separated the decadal canopy evaporation, transpiration, and soil evaporation from the SWAT output results. We found that (1) forest evapotranspiration has increased in recent decades because of the above reasons; (2) in young or well-managed forests, the forest water balance may have changed significantly with forest growth. For long-term studies, it is necessary to distinguish the growth characteristics of different forests during different periods, and a detailed definition of a mixed forest is required. The forest parameters and growth characteristics are critical for understanding forest ecosystems and cannot be ignored at catchment-scale.

Impacts of forest cover on surface runoff quality in small catchments

Forest cover influences not only the amount of surface runoff, but also its quality. The concentrations of chemicals in surface runoff differ between forest catchments and non-forest catchments (agricultural areas). The authors investigated the chemical compositions of surface runoff in two small neighboring catchments (forest, non-forest), by analyzing and summarizing data over a period of 26 years from 1986 to 2012. During this period, the stock and absorption area of forest stands increased, air quality improved, the agricultural landscape was partly regenerated, and global climate change became apparent. The authors observed differences in surface runoff between forest- and non-forest catchments. However, these differences were not mainly caused by the influence of the forest cover, but by changes in agricultural land management. Since 2006, agricultural land has been managed without the use of artificial fertilizers, which reduced the contents of pollutants in surface runoff from the non-forest catchment. The existence of the forest as such excludes or noticeably eliminates the use of fertilizers and chemical substances that affect water quality.

Spatial and Temporal Variability in Concentration‐Discharge Relationships at the Event Scale

AbstractThe analysis of concentration‐discharge (C‐Q) relationships from low‐frequency observations is commonly used to assess solute sources, mobilization, and reactive transport processes at the catchment scale. High‐frequency concentration measurements are increasingly available and offer additional insights into event‐scale export dynamics. However, only few studies have integrated inter‐annual and event‐scale C‐Q relationships. Here, we analyze high‐frequency measurements of specific conductance (EC), nitrate (NO3‐N) concentrations and spectral absorbance at 254 nm (SAC254, as a proxy for dissolved organic carbon) over a two year period for four neighboring catchments in Germany ranging from more pristine forested to agriculturally managed settings. We apply an integrated method that adds a hysteresis term to the established power law C‐Q model so that concentration intercept, C‐Q slope and hysteresis can be characterized simultaneously. We found that inter‐event variability in C‐Q hysteresis and slope were most pronounced for SAC254 in all catchments and for NO3‐N in forested catchments. SAC254 and NO3‐N event responses in the smallest forested catchment were closely coupled and explainable by antecedent conditions that hint to a common near‐stream source. In contrast, the event‐scale C‐Q patterns of EC in all catchments and of NO3‐N in the agricultural catchment without buffer zones around streams were less variable and similar to the inter‐annual C‐Q relationship indicating a homogeneity of mobilization processes over time. Event‐scale C‐Q analysis thus added key insights into catchment functioning whenever the inter‐annual C‐Q relationship contrasted with event‐scale responses. Analyzing long‐term and event‐scale behavior in one coherent framework helps to disentangle these scattered C‐Q patterns.

Effects of Hillslope Trenching on Surface Water Infiltration in Subalpine Forested Catchments

Concerns over freshwater scarcity for agriculture, ecosystems, and human consumption are driving the construction of infiltration trenches in many mountain protected areas. This study examines the effectiveness of infiltration trenches in a subalpine forested catchment in central Mexico, where public and private organizations have been constructing trenches for ~60 years. We rely on empirical data to develop rainfall-runoff models for two scenarios: a baseline (no trenches) and a trenched scenario. Field measurements of infiltration capacities in forested and trenched soils (n = 56) and two years of meteorological data are integrated into a semi-distributed runoff model of 28 trenched sub-catchments. Sensitivity analysis and hydrographs are used to evaluate differences in total runoff and infiltration between the two scenarios. Multiple logistic regression is used to evaluate the effects of environmental and management variables on the likelihood of runoff response and trench overtopping. The findings show that soil infiltration capacity and rainfall intensity are primary drivers of runoff and trench overtopping. However, trenches provided only a 1.2% increase in total infiltration over the two-year period. This marginal benefit is discussed in relation to the potential adverse environmental impacts of trench construction. Overall, our study finds that as a means of runoff harvesting in these forested catchments, trenches provide negligible infiltration benefits. As a result, this study cautions against further construction of infiltration trenches in forested catchments without careful ex ante assessment of rainfall-runoff relationships. The results of this study have important implications for forest water management in Mexico and elsewhere, where similar earthworks are employed to enhance runoff harvesting and surface water infiltration.

Carbon, Nutrients and Methylmercury in Water from Small Catchments Affected by Various Forest Management Operations

Forest management activities in boreal and hemiboreal environments have been found to increase the concentration of carbon, nutrients, and methylmercury (MeHg) in runoff water, thus contributing to environmental quality issues. We evaluated carbon, nutrient, and MeHg concentrations in water at eight small, forested catchments on organic soils in Latvia, subject to ditch cleaning and beaver dam removal. These management-induced disturbances were classified into a major, minor, or no disturbance classes. The concentrations of dissolved organic carbon and total nitrogen were elevated in disturbed catchments (both major and minor) compared to the catchments with no disturbance. The concentrations of MeHg in the water displayed a clear seasonal variation with higher concentrations in spring and summer, but there were no significant differences in MeHg concentrations between catchments with major, minor, and no disturbances. However, the higher concentrations of SO42− in the disturbed catchments compared to those undisturbed may promote MeHg formation if the conditions become more reduced further downstream. While most former studies of forest management effects on water quality have focused on forest harvest, our research contributes to the currently rather scarce pool of data on the impact of less-studied management operations, such as ditch cleaning and beaver dam removal, on carbon, nutrient, and MeHg concentrations in runoff water.

Controls of Organic Carbon and Nutrient Export from Unmanaged and Managed Boreal Forested Catchments

Understanding the anthropogenic and natural factors that affect runoff water quality is essential for proper planning of water protection and forest management, particularly in the changing climate. We measured water quality and runoff from 10 unmanaged and 20 managed forested headwater catchments (7–12,149 ha) located in Finland. We used linear mixed effect models to test whether the differences in total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) export and concentrations observed can be explained by catchment characteristics, land use, forest management, soil fertility, tree volume and hydrometeorological variables. Results show that much of variation in TOC, TN and TP concentrations and export was explained by drainage, temperature sum, peatland percentage and the proportion of arable area in the catchment. These models explained 45–63% of variation in concentrations and exports. Mean annual TOC export in unmanaged catchments was 56.4 ± 9.6 kg ha−1 a−1, while in managed it was 79.3 ± 3.3 kg ha−1 a−1. Same values for TN export were 1.43 ± 0.2 kg ha−1 a−1 and 2.31 ± 0.2 kg ha−1 a−1, while TP export was 0.053 ± 0.009 kg ha−1 a−1 and 0.095 ± 0.008 kg ha−1 a−1 for unmanaged and managed, respectively. Corresponding values for concentrations were: TOC 17.7 ± 2.1 mg L−1 and 28.7 ± 1.6 mg L−1, for TN 420 ± 45 µg L−1 and 825 ± 51 µg L−1 and TP 15.3 ± 2.3 µg L−1 and 35.6 ± 3.3 µg L−1. Overall concentrations and exports were significantly higher in managed than in unmanaged catchments. Long term temperature sum had an increasing effect on all concentrations and exports, indicating that climate warming may set new challenges to controlling nutrient loads from catchment areas.

Spatial variations in radiocesium deposition and litter–soil distribution in a mountainous forest catchment affected by the Fukushima nuclear accident

The Fukushima Dai-ichi Nuclear Power Plant accident caused serious 137Cs contamination in mountainous forest areas. To understand the spatial variation in soil 137Cs inventory in complex mountainous topography and the influencing factors, a whole-area investigation of 137Cs deposition in a broad-leaved forest catchment of a mountain stream was conducted using grid sampling. Across the catchment, organic and surface mineral soil layers were collected at 42 locations in 2013 and 6 locations in 2015. Cesium-137 deposition on the forest floor exhibited high spatial heterogeneity and altitude-dependent distribution over the catchment. The 137Cs retention ratio in the organic layer, determined as the inventory in the organic layer divided by the soil (organic and mineral soil layers) inventory, ranged from 6% to 82% in 2013, and the coefficient of variation was 0.6. The 137Cs retention ratios had positive correlations with the material inventory in the organic layer and the elevation. The 137Cs retention ratios in the organic layer were less than 20% in 2015, even at the locations where the retention ratio was higher than 55% in 2013. Although there was spatial variation in the migration speed, 137Cs migration from the organic layer to mineral soil was almost completed within 4 y of the deposition, suggesting a decrease in 137Cs circulation within the forest ecosystem. This study also examined a relationship between the 137Cs inventory and the air dose rate to assess the potential of using the air dose rate to estimate soil 137Cs inventory. Soil 137Cs inventories and air dose rates were highly positively correlated, indicating that measurement of air dose rate can provide an easier and quicker alternative to measurement of soil 137Cs inventory in forest ecosystems.

Biochemical inhibition of acid phosphatase activity in two mountain spruce forest soils

The product inhibition of the 4-methylumbelliferyl phosphate (MUB-P) decay rate, a measure of potential acid phosphatase activity, has not been considered in most of the published kinetic studies. The aim of this study was to determine the type and strength of the product inhibition in order to better define reaction conditions at which the acid phosphatase activity assay produces unbiased results. The MUB-P decay rate was measured in the forest floor and topsoil organic horizons of two spruce forest catchments at different initial MUB-P and P-PO43− concentrations. The type and strength of the inhibition was analyzed by non-linear regression. We found that MUB-P decay was competitively inhibited by the P-PO43− but also by dissolved organic P. By using estimated kinetic parameters, we calculated an underestimation of potential acid phosphatase activity by up to ~ 20% at concentrations of added MUB-P as high as 300 µmol g−1 dry weight and an incubation time one hour due to background inorganic and organic P concentrations in range of units of µmol per gram of dry soil. We discuss approaches that can be used to minimize the underestimation and show that the previously defined recommendations might not be always sufficient to avoid the bias. Therefore, we recommend to analyze progress curves at a wide range of initial MUB-P concentrations using non-linear methods when the enzyme assay is optimized for a given soil. When changes in inorganic and/or organic P concentrations are expected, we further recommend measuring background inorganic and organic P concentrations.

Rosalia: an experimental research site to study hydrological processes in a forest catchment

Abstract. Experimental watersheds have a long tradition as research sites in hydrology and have been used since the late nineteenth and early twentieth centuries. The University of Natural Resources and Life Sciences Vienna (BOKU) recently extended its experimental research forest site “Rosalia” with an area of 950 ha towards the creation of a full ecological-hydrological experimental watershed. The overall objective is to implement a multi-scale, multi-disciplinary observation system that facilitates the study of water, energy and solute transport processes in the soil–plant–atmosphere continuum. This article describes the characteristics of the site and the monitoring network and its instrumentation that has been installed since 2015, as well as the datasets. The network includes four discharge gauging stations and seven rain gauges along with observations of air and water temperature, relative humidity, and electrical conductivity. In four profiles, soil water content and temperature are recorded at different depths. In addition, since 2018, nitrate, TOC and turbidity have been monitored at one gauging station. In 2019, a programme to collect isotopic data in precipitation and discharge was initiated. All data collected since 2015, including, in total, 56 high-resolution time series (with 10 min sampling intervals), are provided to the scientific community on a publicly accessible repository. The datasets are available at https://doi.org/10.5281/zenodo.3997140 (Fürst et al., 2020).

Testing the impacts of wildfire on hydrological and sediment response using the OpenLISEM model. Part 1: Calibration and evaluation for a burned Mediterranean forest catchment

Models are typically applied to estimate the potential adverse effects of fire on land degradation and water resources and the potential benefits of post-wildfire rehabilitation treatments. However, few modeling studies have been conducted for meso-scale catchments, and only a fraction of these studies include transport and deposition of eroded material within the catchment or represent spatial erosion patterns. This study presents an application and evaluation of the OpenLISEM physically-based and spatially-distributed hydrological and soil erosion model for a burned Mediterranean meso-scale catchment (18.5 km2) in a data-scarce environment, using a robust parameterization and calibration procedure: (1) integrating satellite imagery and the topographic wetness index to support model parameterization; (2) event-based automated calibration using the Model-Independent Parameter Estimation and Uncertainty Analysis and parameters ensemble for before and after the fire; (3) a jack-knife cross-validation for model evaluation. The study shows that this procedure used in OpenLISEM provides reasonable results for pre- and post-wildfire catchment discharge and sediment transport (r2 and NSE > 0.5; absolute PBIAS < 25% for discharge and 55% for sediment transport). This may serve the needs of model applications in data-scarce burned areas. The results also provide recommended model parameters for burned areas with high severity such as random roughness (rr) = 2.41 cm, and slope manning’s n (n) = 0.038; or post-wildfire to pre-wildfire ratios of model parameters such as saturated hydraulic conductivity (Ks) = 0.98×, channel manning’s n (chn) = 0.44×, grain size (d50) = 0.61 × . The simulation results indicate that wildfire did not lead to significantly enhanced hydrological responses and soil erosion at the catchment outlet, partly explained by the spatial patterns of soil erosion. For both pre- and post-wildfire conditions, higher soil erosion was simulated in areas located far from the catchment outlet. The wildfire led to enhanced hillslope erosion, mostly in the upper part of the catchment, providing ample opportunities for transported sediment to deposit before reaching the outlet.