Subtropical Catchment Research Articles

Sediment source tracing in a Brazilian subtropical catchment using diffuse reflectance: Effect of spectral ranges, pre-processing techniques, and multivariate model

ABSTRACT Agriculture intensification in Southern Brazil’s subtropical regions combined with the frequent occurrence of erosive rainfall has rendered the area a global water erosion hotspot. In this scenario, understanding and regulating erosion processes at the river catchment scale is critical for mitigating soil and water resource degradation. Traditional methods for tracing sediment sources are expensive and time-consuming and justify the development of alternative approaches. Therefore, in this study, we employed diffuse reflectance spectroscopy analyses in the ultraviolet-visible (UV-VIS), near-infrared (NIR), and mid-infrared (MIR) ranges, combined with multivariate models and spectral pre-processing techniques to estimate sediment source contributions in a homogeneous subtropical catchment (Conceição River, 804 km²). Soil samples (n = 181) were collected to characterize the four potential sediment sources, including: cropland (n = 78), stream bank (n = 36), unpaved road (n = 40) and pasture (n = 27). Moreover, 44 sediment samples were collected, including suspended sediment (n = 8), fine sediment deposited on the riverbed (n = 15), and suspended sediment samples collected in the water column during storm events (n = 21). Vector machine (SVM) model outperformed the others, with better accuracy and reliability. While UV-VIS spectra proved less effective due to soil homogeneity across the catchment, NIR and MIR spectra provided valuable information for discriminating sediment sources. Furthermore, reducing the number of potential sources (from four to three or two) improved model predictions, especially when distinguishing between surface sources (cropland and pasture) and subsurface sources (unpaved roads and stream banks). The study’s findings shed light on the power of efficient and cost-effective alternative methods for assessing sediment sources, which are vital for promoting effective erosion control and sustainable land management in similar regions.

The limited effect of deforestation on stabilized subsoil organic carbon in a subtropical catchment

Abstract. Predicting the quantity of soil organic carbon (SOC) requires understanding how different factors control the amount of SOC. Land use has a major influence on the function of the soil as a carbon sink, as shown by substantial organic carbon (OC) losses from the soil upon deforestation. However, predicting the degree to which land use change affects the OC content in soils and the depth down to which this occurs requires context-specific information related to, for example, climate, geochemistry, and land use history. In this study, 266 samples from forests and agricultural fields were collected from 94 soil profiles down to 300 cm depth in a subtropical region (Arvorezinha, southern Brazil) to study the impact of land use on the amount of stabilized OC along the soil profile. We found that the stabilized OC content was not affected by land use below a depth of 90 cm. Along the soil profile, the amount of stabilized OC was predominantly controlled by land use and depth in addition to the silt and clay content and aluminium ion concentrations. Below 100 cm, none of the soil profiles reached a concentration of stabilized OC above 50 % of the stabilized OC saturation point (i.e. the maximum OC concentration that can physically be stabilized in these soils). Based on these results, we argue that it is unlikely that deeper soil layers can serve as an OC sink over a timescale relevant to global climate change due to the limited OC input in these deeper layers. Furthermore, we found that the soil weathering degree was not a relevant control on the amount of stabilized OC in our profiles because of the high weathering degree of the studied soils. It is therefore suggested that, while the soil weathering degree might be an effective controlling factor of OC stabilization over a large spatial scale, it is not an informative measure for this process at regional and local scales (with similar climate, bedrock, and weathering history) in highly weathered soils.

Efficiency of global precipitation datasets in tropical and subtropical catchments revealed by large sampling hydrological modelling

Satellite-based and reanalysis precipitation products are widely adopted as complementary information to in situ measurements for estimating river discharge using hydrological modelling. However, there is still a notable research gap in the literature associated with assessing the accuracy of satellite-based or reanalysis products in different tropical and sub-tropical catchments at large-sampling hydrological modelling with sensitivity analysis. We investigated the accuracy of precipitation, hydrological model performance and parameter sensitivity related to seven precipitation data sets based on satellite and reanalysis products, i.e., CHIRPS, TRMM, GLDAS, IMERG, MERRA-2, PERSIANN-CDR, and ERA5 over 714 contrasting tropical and subtropical catchments located in Brazil. We used the Génie Rural Journalier 4 (GR4J) hydrological model to simulate the hydrological processes of the different catchments with two approaches for the calibration: using measured ground-based precipitation data (approach I) or using each individual satellite/reanalysis precipitation products (approach II) to calibrate the models. The results showed that the precipitation products tend to overestimate precipitation, with the exception of ERA5 and MERRA-2. CHIRPS is the only product that produces unbiased precipitation estimates for most catchments. The model calibration using each precipitation product individually improved the hydrological model performance. CHIRPS, IMERG, and MERRA-2 showed good accuracy in terms of both, precipitation estimation and hydrological simulation performance in the calibration period. In the validation period, the best products in terms of KGE were CHIRPS, IMERG and TRMM (KGE > 0.64). The errors in precipitation products are better compensated via hydrological modelling in wet regions. The model parameter sensitivity varies according to precipitation input, climate, and catchment aridity. Overall, all seven products exhibited their worst hydrological performance in arid regions. This study helps to improve our understanding of the catchment response in tropical and subtropical regions while also providing key insights into the reliability of satellite/reanalysis rainfall products for streamflow simulation. This study is valuable in hydrometeorological applications, climate change assessment, water resources and disaster management, especially in regions with a relatively sparse density of precipitation stations.

Sensitivity of streamflow and nutrient loads to changes in leaf area index and soil organic carbon in a sub-tropical catchment subject to climate change

Study regionThe Upper Nerang River Catchment located in South-East Queensland (SEQ), Australia. Study focusThe Soil Water Assessment Tool (SWAT) model was applied to Upper Nerang River Catchment to test the sensitivity of catchment runoff and water quality to changes in Leaf Area Index (LAI) and Soil (topsoil) Organic Carbon (SOC) associated with climate change. These parameters are normally considered to be stationary in hydrological models, potentially constraining the accuracy of these models for catchment simulation under future climate scenarios. New hydrological insights for the regionLAI decreased by 0.4% to 16.9% and SOC by 5.9 to 11.6%, depending on land cover, future period, and emission scenario. Climate change scenarios showed an increase in extreme flows compared with a baseline (recent historical) case and were characterised by reduced return period and greater magnitude of discharge. Changes in LAI and SOC have the potential to mitigate these changes and may reduce loads of SS, TN and TP under future climate change scenarios. The findings of this study indicate that incorporating changes in LAI and SOC into hydrological models is important for modelling the catchment response to climate change. Furthermore, LAI and SOC could be managed to mitigate impacts of climate change on streamflow and losses of SS, TN and TP from catchment soils to receiving waters.

Prominent Role of Sulfate Reduction in Considerable Sulfur Retention in a Subtropical Soil

AbstractThe sulfur biogeochemical cycle controls the sulfur dynamics in the soil. In contrast to almost all deposited sulfur leaching out in temperate catchments, approximately 80% of the deposited S is retained in the catchments in subtropical regions. However, the mechanisms for sulfur retention were unclear, hindering the understanding of potential threats of legacy sulfur with environmental changes. Here, we demonstrated that sulfate reduction (as sulfide fixed in soil) was a prominent yet overlooked mechanism for sulfur retention in addition to the widely recognized sulfate adsorption in soil, based on a study on soil sulfur storage and stable isotope signatures within entire soil profiles in a typical subtropical catchment in China. Using a dual‐isotopic model, the sulfate reduction flux was further determined to be 30% of the S deposition. Due to a lot of deposited sulfur fixed via sulfate reduction, the release of soil legacy sulfur would be less in response to decreasing sulfur deposition compared to the projections only considering adsorption. However, the remobilization of large amounts of reduced S should be regarded as a threat to the environment under climate change in the future.

Impacts of climate change on nutrient and sediment loads from a subtropical catchment

Climate change is predicted to significantly alter hydrological cycles across the world, affecting runoff, streamflow, and pollutant loads from diffuse sources. The objectives of this study were to examine the impacts of climate change on streamflow, total nitrogen (TN), total phosphorus (TP), and total suspended sediment (TSS) loads in the subtropical Logan-Albert catchment, Queensland, Australia. We calibrated the Soil Water Assessment Tool (SWAT) against event monitoring data in the Logan and Albert rivers, respectively. Hydrological and water quality effects of an ensemble of 11 dynamically downscaled high-resolution climate models were assessed with SWAT under high (Representative Concentration Pathway 8.5 – RCP8.5) and intermediate (RCP4.5) emission scenarios. Streamflow decreased most in winter and spring and decreased least in summer. This followed the predicted seasonal changes for precipitation, although decreases tended to be amplified due to increasing evaporative loss. TSS, TN, and TP loads showed a similar pattern to streamflow, with the largest decreases predicted for the dry season under RCP8.5 by the 2080s. Annual TSS load decreased by 34.3 and 54.2%, TN load decreased by 29.8 and 30.5%, and TP load by 24.9 and 4.4% for the Logan and Albert sites, respectively. The results of this study indicate that for subtropical river-estuary systems, climate warming may lead to lower streamflow and contaminant loads, reduced flushing, and greater relative importance of point source loads in urbanising catchments.

Enhanced debris flow activities in the subtropical mountainous catchment of South China during early Marine Isotope Stage 3

Geomorphic processes and their responses to climatic change in subtropical mountainous catchments are not yet well understood. In this study, a unique sequence of alluvial sediments (>33 m thick) from the Nanxu karst valley, Guilin, South China, was investigated to characterize the nature of erosion–deposition processes in humid, subtropical catchments and decipher any climatic constraints on the earth surface dynamics and landscape evolution. Erosional outcrops show that the Nanxu valley fill sequence mainly comprises m-scale sheets of matrix-supported gravel (a debris flow facies), which, in the distal part of the valley, are intercalated with beds of fine-grained sediments (facies of horizontally interbedded sand and mud or massive mud). The fine-grained sediments were likely deposited in a subaqueous environment, which was probably formed when the karst sinkhole was contemporarily blocked by a large volume of debris flows. Optically stimulated luminescence (OSL) as well as radiocarbon dating constrained a chorology framework of ca. 62–42 ka for the valley fill sequence. Comparisons of the sedimentation phase with local and regional climate proxy records revealed that enhanced debris flow activities and sediment aggradation occurred under generally warm and wet but unstable climatic conditions during early Marine Isotope Stage 3. Under these climatic conditions, there was likely a subtropical landscape with high vegetation cover in the catchment at the time of sedimentation, as supported by our palynological data. Thus, it is suspected that enhanced debris flow activities were triggered by the recurrence of extreme monsoonal rainfall events. Since ∼ 42 ka, the stream-channel process dominated the valley, and the valley fill was deeply dissected, which might be related to changes in the overall intensity and internal variability of monsoonal rainfall over a period. These findings identify the role of high-intensity monsoonal precipitation in controlling geomorphological activities in humid, subtropical landscapes of South China.

A slurry approach to identify nutrient critical source areas from subtropical catchment erosion

Targeting catchment nutrient critical source areas (CSAs) (areas contributing most of the nutrients in a catchment) is an efficient way to prioritize remediation sites for reducing nutrient runoff to waterways. We tested if the soil slurry approach – with particle sizes and sediment concentrations representative of those in streams during high rainfall events – can be used to identify potential CSAs within individual land use types, examine fire impacts, and identify the contribution of leaf litter in topsoil to nutrient export in subtropical catchments. We first confirmed the slurry approach met the prerequisite to identify CSAs with relatively higher nutrient contribution (not absolute load estimation) by comparing the slurry sampling with stream nutrient monitoring data. We validated that: 1) differences in slurry total nitrogen to phosphorus mass ratios from different land uses were consistent with stream monitoring data; and 2) our estimated nutrient export contribution from agricultural land, via the slurry approach, was comparable to that derived from monitoring data. Additionally, we found nutrient concentrations in slurries differed across soil types and management practice within individual land uses, correlating with nutrient concentrations in fine particles. These results indicate the slurry approach can be used to identify potential small-scale CSAs. Slurry results from burnt soils were also comparable to other studies showing increased levels of dissolved nutrient loss and higher nitrogen than phosphorus loss, than non-burnt soils. The slurry method also showed the contribution of leaf litter to slurry nutrient concentrations from topsoil was greater for dissolved nutrients than particulate nutrients, indicating different forms of nutrients need to be considered for impacts of vegetation. Our study reveals that the slurry method can be used to identify potential small-scale CSAs within the same land use from erosion and can account for impacts of vegetation and bushfires, providing timely information to guide catchment restoration actions.

Hydrological response of a headwater catchment in Southeast Brazil—Threshold patterns of stormflow response

AbstractHeadwater catchments are essential water sources for sustaining low flow in downstream rivers. The montane region of Serra da Mantiqueira in Southeast Brazil is the largest water supplier for the densely populated area of São Paulo, where we investigated the hydrological response of a subtropical catchment and four of its sub‐catchments. We discussed the relationships between rainfall, streamflow, baseflow, stormflow, soil moisture, and water table depth using high‐frequency field observations over 4 years. Baseflow accounted for most of the streamflow during the dry season and for half of streamflow in the wet season. In contrast, the variations in soil moisture were dampened, with increases in the early wet season preceding the increase in streamflow and decreases in the late wet season following the decrease in streamflow. The mean runoff coefficient and baseflow index varied from 23% to 37% and from 62% to 75% across the catchments, respectively. At the event scale, all catchments had a pronounced threshold behaviour, where stormflow increased sharply when event precipitation reached ≃10 mm, soil moisture reached ≃51% and water table depth decreased to ≃135 cm. The event stormflow coefficient was highly variable, with a maximum of 25%. The temporally integrated stormflow coefficient was low for conditions below a soil moisture threshold (≃3%–10%) and high above a soil moisture threshold (≃8%–15%). Our estimates of flows and threshold behaviours were consistent across the various study catchments, improving our understanding of runoff generation at a catchment scale. This information should help improve the management of water resources focused on water and food security in a context of climate change.

Increasing areas of aquaculture ponds and reservoirs reshape runoff coefficients: evidence from a subtropical catchment, China.

Detention pond is a key storm water management measure employed both to attenuate surface runoff and to regulate depression storage, yet the effects of aquaculture ponds and reservoirs on runoff coefficient are not well quantified in a subtropical humid monsoon climate zone, China. Here, a set of six subcatchments ranging in size from 0.7 hm2 to 10,000 hm2 were evaluated over the 2011-2015 period. (i) The annual average runoff coefficient differed with different subcatchments due to the spatial heterogeneity of landscape patterns, while the event-based runoff coefficient under the same catchment showed a decreasing trend with increasing rainfall intensity. (ii) The annual average and event-based runoff coefficients initially increased and then decreased with an increase in the area ratio of aquaculture ponds and reservoirs. The critical area ratio of aquaculture ponds and reservoirs for the maximum runoff coefficient in annual, light, and moderate rainfall intensity was about 4%; but this value would be transferred forward to the position of < 4% under the intensity of heavy rain, rainstorms, and heavy rainstorms. (iii) All runoff coefficients decreased with increasing forestland but increased with increasing paddy fields, and the decreasing rate was greater than the increasing rate. The trends of runoff coefficient for the annual and event-based rainfall are opposite between river development coefficient and watershed shape coefficient. The results provide underlying insights for decision-makers in aquaculture land-use planning and the sustainable utilization of water resources in the upstream and downstream systems of a catchment.

Seasonal isotopic cycles used to identify transit times and the young water fraction within the critical zone in a subtropical catchment in China

Transit times and the young water fraction (Fyw, the proportion of the water younger than a certain age threshold, τyw) can help characterize the hydrological behaviors within a catchment. Previous studies of water age have concentrated on individual catchment hydrological components, such as river water, groundwater, plant water or soil water, which hinders the dynamic linking of various hydrological components in the critical zone. In this study, the isotopic compositions of precipitation, soil water, xylem water, river water, and groundwater in a subtropical catchment in China were continuously monitored for two years. We found that the seasonal isotopic cycles of shallow soil water (0–60 cm), xylem water and river water had different attenuations and phase shifts relative to those of precipitation. There were no obvious seasonal isotopic cycles of deep soil water (60–130 cm) and groundwater. Additionally, the transit time distributions (TTDs) of soil water at the depth of 0–10 cm, xylem water and river water in the catchment varied between the wet year and normal year. With increasing depth, the mean transit times (MTTs) of soil water increased and the Fyw decreased. For xylem water and soil water, the MTTs and Fyw in the wet year were shorter and larger than those in the normal year, respectively. The river water consisted of a small proportion of fast flow components and a large proportion of slow flow components, with the young water accounting for 16% in the wet year and 21% in the normal year. Our results contribute to an improved understanding of water ages, flow paths and linkages of different hydrological components in the critical zone at the catchment scale.

Nitrogen emission and deposition budget in an agricultural catchment in subtropical central China

The study of emissions and depositions of atmospheric reactive nitrogen species (Nrs) in a region is important to uncover the sources and sinks of atmospheric Nrs in the region. In this study, atmospheric total Nrs depositions including both wet-only and dry deposition were monitored simultaneously across major land-use types in a 105 km2 catchment called Jinjing River Catchment (JRC) in subtropical central China from 2015 to 2016. Based on activity data and emission factors for the main Nrs emission sources, ammonia (NH3) and nitrogen oxides (NOx) emission inventories for the catchment were also compiled. The estimated total Nrs deposition in JRC was 35.9 kg N ha−1 yr−1, with approximately 49.7 % attributed to reduced compounds (NHx), and 40.5 % attributed to oxidized (NOy). The total Nrs emission rate in JRC was 80.4 kg N ha−1 yr−1, with 61.5 and 18.9 kg N ha−1 yr−1 from NH3 and NOx emissions, respectively. Livestock excretion and fertilization were the two main contributing emission sources for NH3, while vehicle sources contributed the bulk of NOx emissions. The net atmospheric budgets of Nrs in paddy field, forest, and tea field were +3.7, −36.1, and +23.8 kg N ha−1 yr−1, respectively. At the catchment scale, the net atmospheric budget of Nrs was +47.7 kg N ha−1 yr−1, with +43.7 kg N ha−1 yr−1 of NHx and +4.0 kg N ha−1 yr−1 of NOy, indicating that the subtropical catchment was net sources of atmospheric Nrs. Considering that excessive atmospheric Nr emissions and deposition may cause adverse effects on the environment, effects should be conducted to mitigate the Nrs emissions from agriculture and transportation, and increasing the area of forest is good for reducing the net positive budget of atmospheric Nrs in the subtropical catchments in China.

Conversion of Forest to Agriculture Increases Colored Dissolved Organic Matter in a Subtropical Catchment and Adjacent Coastal Environment

AbstractLand‐ocean dissolved organic matter (DOM) transport is a significant and changing term in global biogeochemical cycles which is increasing as a result of human perturbation, including land‐use change. Knowledge of the behavior and fate of transported DOM is lacking, particularly in the tropics and subtropics where land‐use change is occurring rapidly. We used Parallel Factor (PARAFAC) Analysis to investigate how land‐use influenced the composition of the DOM pool along a subtropical land‐use gradient (from near‐pristine broadleaf forest to agri‐urban settings) in Belize, Central America. Three humic‐like and two protein‐like components were identified, each of which was present across land uses and environments. Land‐use mapping identified a strong (R2 = 0.81) negative correlation between broadleaf forest and agri‐urban land. All PARAFAC components were positively associated with agri‐urban land‐use classes (cropland, grassland, and/or urban land), indicating that land‐use change from forested to agri‐urban exerts influence on the composition of the DOM pool. Humic‐like DOM exhibited linear accumulation with distance downstream and behaved conservatively in the coastal zone whilst protein‐like DOM exhibited nonlinear accumulation within the main river and nonconservative mixing in coastal waters, indicative of differences in reactivity. We used a hydrodynamic model to explore the potential of conservative humics to reach the region's environmentally and economically valuable coral reefs. We find that offshore corals experience short exposures (10 ± 11 days yr−1) to large (∼120%) terrigenous DOM increases, whilst nearshore corals experience prolonged exposure (113 ± 24 days yr−1) to relatively small (∼30%) terrigenous DOM increases.

Dissolved and gaseous nitrogen losses in forests controlled by soil nutrient stoichiometry

Global chronic nitrogen (N) deposition to forests can alleviate ecosystem N limitation, with potentially wide ranging consequences for biodiversity, carbon sequestration, soil and surface water quality, and greenhouse gas emissions. However, the ability to predict these consequences requires improved quantification of hard-to-measure N fluxes, particularly N gas loss and soil N retention. Here we combine a unique set of long-term catchment N budgets in the central Europe with ecosystem 15N data to reveal fundamental controls over dissolved and gaseous N fluxes in temperate forests. Stream leaching losses of dissolved N corresponded with nutrient stoichiometry of the forest floor, with stream N losses increasing as ecosystems progress towards phosphorus limitation, while soil N storage increased with oxalate extractable iron and aluminium content. Our estimates of soil gaseous losses based on 15N stocks averaged 2.5 ± 2.2 kg N ha−1 yr−1 and comprised 20% ± 14% of total N deposition. Gaseous N losses increased with forest floor N:P ratio and with dissolved N losses. Our relationship between gaseous and dissolved N losses was also able to explain previous 15N-based N loss rates measured in tropical and subtropical catchments, suggesting a generalisable response driven by nitrate (NO3 −) abundance and in which the relative importance of dissolved N over gaseous N losses tended to increase with increasing NO3 − export. Applying this relationship globally, we extrapolated current gaseous N loss flux from forests to be 8.9 Tg N yr−1, which represent 39% of current N deposition to forests worldwide.

Establishing the relationship between the integrated multidimensional landscape pattern and stream water quality in subtropical agricultural catchments

Although a landscape is a multidimensional integrated synthesis, the relationship between land use and water quality was merely studied in relation to protecting the aquatic ecological service function from the ‘landscape perspective’. This study analyzed the integrated landscape pattern by coupling land use, soil property, and topography and determined the stream water total nitrogen (TN) and total phosphorus (TP) in Chinese subtropical catchments from 2010 to 2017 to reveal and quantify the relationship between multidimensional landscape patterns and stream water N and P levels using the GIS and partial least squares regression (PLSR) techniques. The results suggested that the areal proportions of tea field and residential area in the Ultisols and on the first three slope categories (0–28.02°) and paddy field in the Ultisols and on slope category I (0–5.35°) were positively correlated with the TN and TP, while forest in the Ultisols and on slope category IV (28.02–80.30°) was negatively correlated with the TN (p ≤ 0.05). For the multidimensional landscape metrics, the Euclidean nearest-neighbor distance (ENN_MN) and interspersion and juxtaposition index (IJI) were positively correlated with the TN, while the ENN_MN was positively correlated with the TP (p ≤ 0.05). The PLSR analysis explained the variations of TN and TP levels well (R2 ≥ 0.66), in which the areal proportions of the tea field and residential area in the Ultisols and on the first three slope categories (0–28.02°) and the paddy field in the Ultisols and on slope category I (0–5.35°) were the important variables for both TN and TP; however, the areal proportions of forest in the Ultisols and on slope category IV (28.02–80.30°) and the IJL were only important to the TN. Therefore, the multidimensional landscape pattern providing specific land use, soil property, and topography can be more capable in restoring and protecting stream N and P pollution at an applicable level.

Spatial-temporal assessment of water and sediment connectivity through a modified connectivity index in a subtropical mountainous catchment

The concept of connectivity aims to understand the hydrosedimentological (water and sediments) processes that occur in the catchment and influence the water and sediment transfer at different spatial-temporal scales. Connectivity is often assessed through connectivity indexes, commonly considering only structural features of the landscape. The present study proposes the insertion of the precipitation and surface runoff characteristics as functional components to the connectivity index (IC). Its main goal is to evaluate the space-time variation of water and sediment connectivity in a subtropical catchment. To achieve this goal, two parameters, extracted from precipitation data, were inserted in the IC and resulted in the hydrosedimentological connectivity index (IHC), which concerns the runoff generation and the characteristics of the antecedent precipitation event. The IHC was tested on a catchment scale for 7 individual precipitation events. For each event, the index provided values and maps that reflected the dynamic processes of the catchment, regarding the functional and structural components of connectivity. The performance analysis of the index was qualitative, and it was carried out by means of a visual interpretation using field data on processes and sediment paths. The IHC map was coherent with the field data and it showed a satisfactory representation of the sediment transfer patterns. It was observed that the debris flow scars corresponded to the flow paths with higher IHC values and the deposition areas with the lower IHC values. In addition, the IHC values showed a positive correlation with the catchment flow values. Statistical analysis indicated a correlation between the mean IHC and the total runoff volume (+0.69) as well as between the mean IHC and the peak flow (+0.63). The IHC was able to estimate the space-time variation of water and sediment connectivity in a catchment, identifying locations of sediment transfer and deposition for precipitation events with different magnitudes. The modified method was able to bring advances to connectivity representation, however, it presented some limitations, such as the incapability to represent the sediment exhaustion in the catchment. Even so, the IHC allows its application in areas with little data and it can be a useful tool in identifying relevant sites on watershed management.

From landscape perspective to determine joint effect of land use, soil, and topography on seasonal stream water quality in subtropical agricultural catchments

Understanding the relationship between integrated landscape patterns (coupled land use, soil properties, and topography) and stream water quality in different seasons promotes appropriate landscape planning. However, this relationship is unclear. Here, water quality nitrogen (N) and phosphorus (P) levels and the integrated landscape patterns were investigated in ten Chinese subtropical catchments during 2010–2017, using the least absolute shrinkage and selection operator (LASSO) regressions method and redundancy analysis (RDA). The results suggested that stream water N and P levels were significantly higher in the fallow season than in the rice-growing season (p < 0.05). The N and P levels in the rice-growing season were elevated with the increasing area proportions of tea fields in Ultisols on the medium slope (16.06–28.02°), and larger isolation, diversity, and geometric complexity of landscape patches, but decreased with the increasing area proportions of forests in Ultisols on the steep slope (28.02–80.30°) and interspersion of landscape patches. Stream water N and P levels were more likely influenced by landscape configuration in the rainy rice-growing season, mainly due to the rapid velocities and high quantities of surface flow strengthening landscape configuration effects on the N and P migration and exchange. In the fallow season, the N and P levels were heightened with the increasing area proportions of tea fields, residential areas, and paddy fields in Ultisols on the relative flat (0–16.06°) and medium slopes, and larger isolation of landscape, and could be greatly reduced if the area proportions of forests in Ultisols on the steep slope is increased. The N and P levels were more likely determined by landscape composition in the dry fallow season, associated with the slow and poor subsurface and underground hydrological flows. Therefore, the results promoted reasonable landscape management in different seasons and suitable soil and topography conditions for improving stream water quality.

Using Bioinformatics to Quantify the Variability and Diversity of the Microbial Community Structure in Pond Ecosystems of a Subtropical Catchment

Background:In rural China, many natural water bodies and farmlands have been converted into fish farming ponds as an economic developmental strategy. There is still a limited understanding of how the diversity and structure of microbial communities change in nature and become managed fish pond ecosystems.Objective:We aimed to identify the changes of the diversity and structure of microbial community and driving mechanism in pond ecosystems.Methods:The datasets of 16S rRNA amplicon sequencing and the concentrations of N and P fractions were achieved in water samples of pond ecosystems. Bioinformatics analysis was used to analyze the diversity and structure of the microbial communities.Results:Our results indicated that the diversity and structure of the microbial communities in the natural ponds were significantly different from ones in managed fish ponds. The nutrients of N and P and water environmental factors were responsible for 46.3% and 19.5% of the changes in the structure and diversity of the microbial community, respectively.Conclusion:The N and P fractions and water environmental factors influenced the microbial community structure and diversity in pond ecosystems. Fish farming indirectly affected the microbial community by altering the contents of N and P fractions in water bodies of ponds, when a natural pond was converted into a managed fish pond.

Modelling the impact of climate change on soil erosion and sediment yield: a case study in a sub-tropical catchment, India

Climate change most likely to increase the soil erosion (SE) and sediment yield (SY) rate by changing the rainfall–runoff erosivity in many locations worldwide. Global climate model output is commonly used to appraise the impact of climate change on SE and SY. However, determining the suitable climate model is challenging for the accurate simulation of future SE and SY. Therefore, The Revised Universal Soil Loss Equation (RUSLE) coupled with Sediment Delivery Ratio (SDR) model was used to estimate the SE and SY in the humid sub-tropical catchment. We also evaluated the high-resolution regional climate models (RCMs) using five statistical bias correction methods (linear scaling, empirical quantile mapping, quantile mapping with linear transformation, quantile mapping with smoothing spline, and quantile mapping with gamma distribution) for future simulation of the SE and SY. Our result suggests that quantile mapping with gamma distribution-based bias correction method and ICHEC-EC-EARTH-RCA model best project the changes in precipitation and consequently, the rainfall–runoff erosivity factor in the catchment. It is estimated that a 3.67–11.08% increase in rainfall–runoff erosivity may result in a 3.7–11.76% increase in SE and SY in the catchment under the representative concentration pathway (RCP) 4.5 and RCP 8.5 emission scenarios. In addition to this 1.6-thousand-hectare area will be included in high (10–20 t ha−1 year−1) to very severe (> 80 t ha−1 year−1) SE intensity class during the near (2020–2049) and far future (2070–2099) period. The result also revealed that the increasing rate of SE and SY will be observed in the sub-catchment numbers 10, 18, 20, 23, 24, 27, 29 located in the middle part of the catchment under the RCP 8.5 scenario. The result may help the policymakers to strengthen the conservation strategy and more informed decisions for the management of catchment in and around the area.

Impacts of climate change on streamflow and floodplain inundation in a coastal subtropical catchment

Climate change is expected to significantly alter river hydrological regimes throughout the world, affecting water resources and the frequency of floods and droughts. The objectives of this study were to determine the impacts of climate change and sea level rise on streamflow and floodplain inundation in the subtropical Logan-Albert catchment, Australia. An ensemble of 11 high-resolution climate models forced under high (Representative Concentration Pathway 8.5 - RCP8.5) and intermediate-emission (RCP4.5) scenarios was applied. There was considerable variation from the model ensemble result in projections of major flooding events at 5 and 100-year average recurrence intervals (ARIs). The largest events (100-year ARI) tended towards an increase, whereas the smallest (5-year ARI) tended towards a decrease. Floodplain inundation from a 100-year ARI event increased in all simulations and inclusion of sea level rise resulted in increased floodplain inundation area nearly doubling by the end of the century, which has substantial implications for flood risk. Our study highlights the non-linear nature of climate change impacts on streamflow and floodplain inundation, demonstrating the need for a comprehensive assessment at the floodplain scale when informing preparedness for future flooding events.