Subtropical Watershed Research Articles

Wash effect of atmospheric trace metals wet deposition and its source characteristic in subtropical watershed in China.

In order to better understand air pollution in deve-loping regions, such as China, it is important to investigate the wet deposition behavior of atmospheric trace metals and its sources in the subtropical watershed. This paper studies the seasonal change of trace metal concentrations in precipitation and other potential sources in a typical subtropical watershed (Jiazhuhe watershed) located in the downstream of the Yangtze River of China. The results show that typical crustal elements (Al, Fe) and trace element (Zn) have high seasonal variation patterns and these elements have higher contents in precipitation as compared to other metals in Jiazhuhe watershed. In addition, there is no observed Pb in base flow in this study, and the concentration magnitudes of Al, Ba, Fe, Mn, Sr, and Zn in base flow are significantly higher than that of other metals. During different rainfall events, the dynamic export processes are also different for trace metals. The various trace metals dynamic export processes lead to an inconsistent mass first flush and a significant accumulative variance throughout the rainfall events. It is found that in this region, most of the trace metals in precipitation are from anthropogenic emission and marine aerosols brought by typhoon and monsoon.

The spatial distribution of archaeal lipids in a mesoscale subtropical watershed, Southeast China

Archaea play an important role in global carbon and nitrogen cycles. Archaeal lipids, such as isoprenoid glycerol diakyl glycerol tetraethers (iGDGTs), are important biomarkers tracing change in archaeal community structure and biogeochemical processes in the natural environments. In this research, the spatial distributions of archaeal lipids in the surface sediments of the Jiulong River (JR) and the Jiulong River estuary (JRE) were examined. GDGT-0 (containing zero cyclopentyl ring) and crenarchaeol were the most abundant iGDGTs in the JR and JRE. From the rivers to the estuary, the total iGDGTs, GDGT-0, crenarchaeol and archaeol concentrations showed significant spatial variation; in particular, GDGT-0 and archaeol in the river may be predominantly derived in situ from methanogens, whereas crenarchaeol in the estuary mainly derived in situ from Thaumarchaeota. We inferred that archaeal community was dominated by methanogens in the Jiulong River and by Thaumarchaeota in the Jiulong River estuary, which are consistent with change in archaeal community structure observed in other estuarine environments.

Changes of C and N stocks in the subtropical Dianchi lake watershed in southwest China due to LUCC

The change from forest to agricultural land in the last three decades represents a significant shift in land use in China. This study was conducted to evaluate how change from forest to greenhouse cabbage production affected soil organic carbon (SOC) and total nitrogen (TN) stocks in surface soil. Our results showed that converting forest to greenhouse vegetable production led to increases in SOC and TN concentrations and stocks, and decreases in C:N ratio in the top soil. The accumulation rates of SOC and TN in the surface soil (0-40cm) were estimated to be 4.638 Mg ha -1 yr -1 and 1.113 Mg ha -1 yr -1 respectively, over an average period of 8 years after change of forest to greenhouse vegetable production. We conclude that greenhouse vegetable production system could be an effective strategy to improve SOC and TN stocks in the subtropical Dianchi lake watershed.

Determination of instantaneous and daily net radiation from TM – Landsat 5 data in a subtropical watershed

Remote sensing makes it possible to identify the changes that occur on the surface of the Earth as a result of natural and/or man-made phenomena. Such changes impact on the net radiation at surface which in turn controls the Earth's climate. The present study aims to determine the impact of land use changes on net radiation at surface in a tropical watershed in Brazil, based on satellite images. The instantaneous net radiation (Rn,ins) (at the time of the satellite overpass) and the daily net radiation (Rn,24h) were both estimated by TM – Landsat 5 images and complementary weather data. The net radiation (Rn) estimated from remote sensing data was compared to the measurements taken from two micrometeorological towers located in the study area. Most Rn,ins values were found to be between 457.4Wm−2 and 760.0Wm−2 during the months with more intense solar radiation (February, March, and November), especially in the areas with more vegetation cover (sugarcane and eucalyptus plantations and areas with woody savanna vegetation, locally called Cerradão). The months with the highest thermal and radiative contrast (June and November) were selected to show the spatial distribution of the daily (Instantaneous) Rn, which ranged from 28.0 (420)Wm−2 to 98.0 (520)Wm−2 in June and from 83.0 (450)Wm−2 to 264.0 (800)Wm−2 in November 9. The model used to calculate Rn,24h provided values close to those taken at surface, even on days with higher cloud cover after the satellite overpass. The Mean Absolute Error (MAE), Mean Relative Error (MRE), and Root Mean Square Error (RMSE) associated with the Rn,24h computations in the sugar cane plantation were 8.3Wm−2, 8.4%, and 10.4Wm−2, respectively, confirming the applicability and accuracy of the results. The Rn patterns registered on the woody savanna throughout the year differ very much from those found in cropped areas, particularly in sugar cane plots. This may cause an impact on the watershed climate.

Sediment cores from shallow lakes preserve reliable, informative paleoenvironmental archives despite hurricane-force winds

Comparison of sediment cores collected in 1999 and 2013 from shallow Lake Harris, Florida, USA showed that the sediment maintained stratigraphic integrity despite multiple hurricanes passing through the area in 2004. Sediments less than 50 years old displayed small losses of organic matter (OM), total phosphorus (TP) and heat-extractable (HE-P) through time that are unrelated to hurricane impacts. Nevertheless, sediment that accumulated between the two core collection dates contained 4-fold more TP than was lost from the sediment over the 14-year period, indicating that sediments in Lake Harris are a net sink for water-column TP. Persistent, elevated 137Cs activity in sediments deposited since the mid-1960s indicates protracted cesium input to the lake from this subtropical watershed. There is also evidence for downward transport of 137Cs in the sediment profile between core collection dates. Prolonged export of 137Cs from the watershed and downward mobility of the radionuclide in the sediment profile diminish the utility of 137Cs as a dating tool in Lake Harris.

Evaluation of Three Satellite Precipitation Products TRMM 3B42, CMORPH, and PERSIANN over a Subtropical Watershed in China

This study conducted a comprehensive evaluation of three satellite precipitation products (TRMM (Tropical Rainfall Measuring Mission) 3B42, CMORPH (the Climate Prediction Center (CPC) Morphing algorithm), and PERSIANN (Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks)) using data from 52 rain gauge stations over the Meichuan watershed, which is a representative watershed of the Poyang Lake Basin in China. All the three products were compared and evaluated during a 9-year period at different spatial (grid and watershed) and temporal (daily, monthly, and annual) scales. The results showed that at daily scale, CMORPH had the best performance with coefficients of determination (R2) of 0.61 at grid scale and 0.74 at watershed scale. For precipitation intensities larger than or equal to 25 mm, RMSE% of CMORPH and TRMM 3B42 were less than 50%, indicating CMORPH and TRMM 3B42 might be useful for hydrological applications at daily scale. At monthly and annual temporal scales, TRMM 3B42 had the best performances, with highR2ranging from 0.93 to 0.99, and thus was deemed to be reliable and had good potential for hydrological applications at monthly and annual scales. PERSIANN had the worst performance among the three products at all cases.

Pharmaceuticals and personal care products in a mesoscale subtropical watershed and their application as sewage markers

This study comprehensively analyzed 23 classes of 51 pharmaceuticals and personal care products (PPCPs) in the two major prongs of Jiulong River and its estuary in southeast China, where the levels of the targeted PPCPs were mostly unknown. For both Jiulong River and its estuary, nine PPCPs were detected with 100% detection frequencies including two anti-inflammatory and analgesic drugs (ketoprofen and diclofenac acid), a stimulant (caffeine), a plasticizer (bisphenol A), two preservatives (methyl paraben and propyl paraben), two antimicrobials (triclosan and triclocarban) and a β-blocks (metoprolol), among which bisphenol A and caffeine accounted for more than 60% in concentrations. PPCPs generally had higher concentrations in dry season than normal season and wet season, while certain PPCPs, such as UV filters, showed higher concentrations in wet season, which were presumed to be related to their usage patterns. The concentrations of PPCPs were significantly correlated to several quality parameters of the surface water. The selected sewage markers were also used to track sewage in the studied river and the ratios of easily removed markers and conservative markers were used to identify the contribution of raw or treated sewage input. From our result, Jiulong River and its estuary were likely polluted by potential discharge of raw sewage.

Impacts of climate and land‐cover changes on water resources in a humid subtropical watershed: a case study from East Texas, USA

AbstractThis study investigates the response of water resources regarding the climate and land‐cover changes in a humid subtropical watershed during the period 1970–2009. A 0.7°C increase in temperature and a 16.3% increase in precipitation were observed. Temperature had a lower increase trend, and precipitation showed definite increasing trend compared to previous studies. The main trend of land‐cover change was conversion of vegetation and barren lands to developed and crop lands affected by human intervention, and forest and grass to bush/shrub which considered to be caused by natural climate system. Hydrologic responses to climate and land‐cover changes resulted in increases of surface run‐off (15.0%), soil water content (2.7%), evapotranspiration (20.1%) and a decrease in groundwater discharge (9.2%). We found that surface run‐off is relatively stable with precipitation, whereas groundwater discharge and soil water content are sensitive to changes in land cover, especially land cover brought about by human intervention.

Hydrosedimentology of nested subtropical watersheds with native and eucalyptus forests

Information on the effects of eucalyptus forests on hydrosedimentological processes is scarce, particularly at the catchment scale. Monitoring and mathematical modeling are efficient scientific tools used to address the lack of information for natural resource management and the representation and prediction of those processes. This study evaluates the effects of eucalyptus cultivation on hydrosedimentological processes in watersheds and to use the Limburg soil erosion model (LISEM) to represent and predict hydrological processes. The study was conducted in two forested watersheds: the main watershed (94.46 ha) and a nested sub-watershed (38.86 ha), both cultivated with eucalyptus and residual riparian native forest, located in southern Brazil. Hydrosedimentalogical monitoring was conducted from 16th February 2011 to 31st December 2012, and LISEM model calibrations were performed on the bases of six storms events. The sediment yield for 2011 was 41.6 Mg km−2 and 38.5 Mg km−2 for the watershed and sub-watershed, respectively. An extreme event in 2012 provided greater sediment yield for the sub-watershed (99.8 Mg km−2) than that for the watershed (51.7 Mg km−2). Rainfall events with a greater maximum intensity generated rapid discharge and suspended sediment concentration responses in the sub-watershed due to the smaller drainage area and steeper landscape. In the main watershed, the accumulation of flood waves occurred for most events, with less steep hydrographs, and a later occurrence of the discharge peak after that of the sub-watershed. The LISEM adequately reproduced the peak discharge and runoff for the calibrated events; however, the peak time and the shape of the hydrograph were not adequately represented. The hydrosedimentological patterns of the watershed and sub-watershed, both cultivated with eucalyptus, was characterized by sedimentographs preceding hydrographs during rainfall–runoff events where scale effects occur, with maximum discharge and specific sediment yield greater in the watershed than that in the sub-watershed. Empirical models based on hydrologic variables may be used for estimating the suspended sediment concentration and sediment yield. Therefore, LISEM may be used for the prediction of hydrological variables in these forested watersheds.

Agricultural adjustment, population dynamics and forests redistribution in a subtropical watershed of NW Argentina

Patterns of land-use and land-cover change are usually grouped into one of two categories defined by the dominant trend: (1) deforestation resulting from expanding agriculture and (2) forest expansion, usually related to the abandonment of marginal lands. At regional scale, however, both processes can occur simultaneously even in the absence of net change. Given the focus on net change, such redistribution of agricultural and natural and seminatural lands has been generally overlooked. The interaction between agriculture modernization, human demography and complex topographic gradients of northwestern Argentina has resulted in processes of both forest recovery and deforestation, thus providing the opportunity to analyze patterns and driving forces of land-cover redistribution. We analyzed 20 years (1986–2006) of land-cover change in a subtropical watershed in relation to topographic and demographic variables. Although net forest change represented <1 %, forests redistribution affected 7 % of forest lands. There was a consistent geographic segregation of deforestation and forest recovery, with forests expanding over steep highlands and agriculture expanding over lowland irrigated areas. Population trends were not associated to forest expansion in lowlands but they explained 32 % of forest recovery in highlands. Highland forest expansion and lowland deforestation, respectively, imply conservation opportunities for humid montane forests and the environmental services they provide (e.g., watershed conservation) and threats for the conservation of dry forests and its biodiversity. Our study exemplifies the importance of land-use redistribution (rather than net change) with relevant environmental consequences at regional scale.

ENSO impacts on Atlantic watersheds of South America

Interannual changes in precipitation cause significant effects on the Pacific watersheds of South America. Large rivers flowing to the Atlantic Ocean as the Paraná and Uruguay are also influenced by ENSO-triggered floods causing significant impacts on the economies of Argentina, Brazil and Uruguay. At the same time, the floodplains of small rivers are also sensitive to strong ENSOs. Floods occur approximately simultaneously in subtropical watersheds of Brazil and high-latitude watersheds of North Patagonia. ENSO-triggered floods were recorded during the years 1941-42, 1982-83 and 1997-98. Several floods impacted along the Itajaí River affecting the higher (Blumenau city) and lower estuary (Itajaí city). The Guaiba River, at the headlands of Lagoa dos Patos, has repeatedly flooded Porto Alegre. At the Río de la Plata basin, significant increases in the discharges were assigned to the Paraná and the Uruguay rivers. The small basin of the Quequén Grande River without extended floodplains is subject to periodic failure of its margins. The Colorado River, the northern limit of Patagonia, is allochthonous and its watershed was artificial and progressively modified; the ENSO event of 1983 caused an increase in its salinity due to the sudden reactivation of the Desaguadero-Curacó system.

Response of bacterial communities to environmental changes in a mesoscale subtropical watershed, Southeast China

This study used 16S rRNA gene-based pyrosequencing (16S-pyrotag) to investigate both planktonic and benthic bacterial communities in two main tributaries (North River and West River) of the Jiulong River Watershed (JRW), a mesoscale subtropical watershed that has experienced intensive human perturbation in recent decades. The results of 16S-pyrotag showed that benthic bacterial communities were clearly more diverse and uniform than surface bacterioplankton communities. The results of taxonomic assignments indicated that Betaproteobacteria, Actinobacteria and Firmicutes were significantly more abundant in planktonic than in benthic communities, whereas the relative abundances of Acidobacteria, Delta-, Gammaproteobacteria, Chloroflexi and Nitrospira were higher in sediment than in water samples. In particular, several sewer- and fecal-pollution bacterial indicators were observed in water samples, implying that the water bodies of the JRW were contaminated by fecal pollution. Using the typical freshwater bacteria (TFB) taxonomic framework, 57.6±10%, 27.6±10.9% and 10.4±6.9% of sequences recovered from planktonic communities could be assigned to lineages, clades and tribes of TFB, respectively. The relatively lower abundance of TFB implied that some unknown or unique autochthonous bacterioplankton populations occurred in the JRW. The principal coordinate analysis (PCoA) and one way analysis of similarity (ANOSIM) analysis demonstrated that planktonic bacterial community structures were significantly different between North River and West River, whereas benthic communities from these two tributaries were grouped together. Multivariate statistical analysis revealed that nutrient concentrations and stoichiometry were the key drivers of both α- and β-diversity patterns of bacterioplankton communities. Overall, our results indicate that the diversity, composition and structure of planktonic bacterial communities are sensitive to water chemistry (e.g., nutrient concentrations and stoichiometry) in the JRW, and therefore can serve as a good sentinel of environmental changes in this watershed.

Characterization of tetracycline-resistant bacteria in an urbanizing subtropical watershed

The objective of this study was to determine whether varying levels of urbanization influence the dominant bacterial species of mildly resistant (0·03 mmoll(-1) tetracycline) and highly resistant (0·06mmoll(-1) tetracycline) bacteria in sediment and water. Also, the level of urbanization was further evaluated to determine whether the diversity of tetracycline resistance genes present in the isolates and the capability of transferring their resistance were influenced. Sediment and water samples collected from five sampling sites were plated in triplicate on nutrient agar plates with a mild dose (0·03 mmoll(-1) ) and a high dose (0·06mmoll(-1) ) of tetracycline. Five colonies from each plate plus an additional five from each triplicate group were randomly selected and isolated on nutrient agar containing 0·03mmoll(-1) tetracycline (400 isolates). The isolates were identified by 16S rRNA gene sequencing and comparison to GenBank using blast. The isolates were also screened for 15 tetracycline resistance genes using a multiplex PCR assay and their ability to transfer resistance through conjugation experiments using a kanamycin-resistant Escherichia. coli K-12 strain labelled with a green fluorescent protein gene. Results from this study indicate that the dominant resistant organisms in this watershed are Acinetobacter spp., Chryseobacterium spp., Serratia spp., Pseudomonas spp., Aeromonas spp. and E.coli. All of these organisms are Gram negative and are closely related to pathogenic species. A majority of the isolates (66%) were capable of transferring their resistance, and there was a greater incidence of tet resistance transfer with increasing urbanization. Also, it was determined that the dominant resistance genes in the watershed are tet(W) and tet(A). Urbanization significantly affected dominant tetracycline-resistant bacteria species, but did not affect dominant resistance genes. There was correlation between increased urbanization with an increase in the ability to transfer tetracycline resistance. This indicates that urban areas may select for bacterial species that are capable of transferring resistance. These results indicate that urbanization influences the occurrence of tetracycline-resistant bacteria and the potential for transfer of resistance genes.

Distinct effects of temperature change on discharge and non-point pollution in subtropical southern China by SWAT simulation

Climate change impacts on the availability of water resources. Projection of hydrological response to temperature change is valuable for water management. Such response may be complex and uncertain at the watershed scale and differences may exist between low and high latitudes. A simulation experiment was achieved by using SWAT modelling in the upstream watershed of Dongjiang River, South China. After calibration, the model was found appropriate for hydrological simulation in the study area and was run from 1995 to 2004 under a series of temperature change scenarios to reveal the response of streamflow and loads of sediment and nutrients. For a temperature increase of 3°C, streamflow, sediment and total phosphorus decreased by 5.2, 7.7 and 2.2%, respectively. Linear temperature change seemed to have a linear hydrological response. Nutrient deficiency was still the primary vegetation stress compared with water availability and temperature stress under rising temperatures. Comparison with previous research showed that two southern subtropical watersheds (one upstream and one downstream) gave different hydrological responses. Sediment and inorganic nitrogen loads decreased in the upstream watershed, but increased in the downstream one, when temperature increased. Under the warming scenarios, streamflow and sediment loads decreased throughout the year, especially during the wet season, which is different from results at high latitudes. Nutrient export decreased in April–June, but increased in the remaining months. Simulation results should be applied with caution in water resources management, as simulated climate change had variable hydrological influence in different regions and seasons. Citation Xu, H. and Peng, S.L., 2013. Distinct effects of temperature change on discharge and non-point pollution in subtropical southern China by SWAT simulation. Hydrological Sciences Journal, 58 (5), 1032–1046. Editor Z.W. Kundzewicz; Associate editor C.-Y. Xu

Nitrate source apportionment in a subtropical watershed using Bayesian model

Nitrate (NO3−) pollution in aquatic system is a worldwide problem. The temporal distribution pattern and sources of nitrate are of great concern for water quality. The nitrogen (N) cycling processes in a subtropical watershed located in Changxing County, Zhejiang Province, China were greatly influenced by the temporal variations of precipitation and temperature during the study period (September 2011 to July 2012). The highest NO3− concentration in water was in May (wet season, mean±SD=17.45±9.50mgL−1) and the lowest concentration occurred in December (dry season, mean±SD=10.54±6.28mgL−1). Nevertheless, no water sample in the study area exceeds the WHO drinking water limit of 50mgL−1 NO3−. Four sources of NO3− (atmospheric deposition, AD; soil N, SN; synthetic fertilizer, SF; manure & sewage, M&S) were identified using both hydrochemical characteristics [Cl−, NO3−, HCO3−, SO42−, Ca2+, K+, Mg2+, Na+, dissolved oxygen (DO)] and dual isotope approach (δ15N–NO3− and δ18O–NO3−). Both chemical and isotopic characteristics indicated that denitrification was not the main N cycling process in the study area. Using a Bayesian model (stable isotope analysis in R, SIAR), the contribution of each source was apportioned. Source apportionment results showed that source contributions differed significantly between the dry and wet season, AD and M&S contributed more in December than in May. In contrast, SN and SF contributed more NO3− to water in May than that in December. M&S and SF were the major contributors in December and May, respectively. Moreover, the shortcomings and uncertainties of SIAR were discussed to provide implications for future works. With the assessment of temporal variation and sources of NO3−, better agricultural management practices and sewage disposal programs can be implemented to sustain water quality in subtropical watersheds.

Assessing ways to combat eutrophication in a Chinese drinking water reservoir using SWAT

Across China, nutrient losses associated with agricultural production and domestic sewage have triggered eutrophication, and local managers are challenged to comply with drinking water quality requirements. Evidently, the improvement of water quality should be targeted holistically and encompass both point sources and surface activities within the watershed of a reservoir. We expanded the ordinary Soil Water Assessment Tool – (SWAT) with a widely used empirical equation to estimate total phosphorus (TP) concentrations in lakes and reservoirs. Subsequently, we examined the effects of changes in land and livestock management and sewage treatment on nutrient export and derived consequences for water quality in the Chinese subtropical Kaiping (Dashahe) drinking water reservoir (supplying 0.4 million people). The critical load of TP was estimated to 13.5 tonnes yr–1 in order to comply with the minimum drinking water requirements, which corresponds to 87% of the simulated load to the reservoir at present. Both the implementation of buffer zones along rivers and removal of sewage discharges showed marked improvement in reservoir water quality. Future research should focus on both hydrological model performance and nutrient transport pathways, which are challenged by a complex artificially altered water infrastructure in the form of ditches, channels and ponds in monsoon-influenced subtropical watersheds.

Assessing the Influence of Land Use and Land Cover Datasets with Different Points in Time and Levels of Detail on Watershed Modeling in the North River Watershed, China

Land use and land cover (LULC) information is an important component influencing watershed modeling with regards to hydrology and water quality in the river basin. In this study, the sensitivity of the Soil and Water Assessment Tool (SWAT) model to LULC datasets with three points in time and three levels of detail was assessed in a coastal subtropical watershed located in Southeast China. The results showed good agreement between observed and simulated values for both monthly and daily streamflow and monthly NH4+-N and TP loads. Three LULC datasets in 2002, 2007 and 2010 had relatively little influence on simulated monthly and daily streamflow, whereas they exhibited greater effects on simulated monthly NH4+-N and TP loads. When using the two LULC datasets in 2007 and 2010 compared with that in 2002, the relative differences in predicted monthly NH4+-N and TP loads were −11.0 to −7.8% and −4.8 to −9.0%, respectively. There were no significant differences in simulated monthly and daily streamflow when using the three LULC datasets with ten, five and three categories. When using LULC datasets from ten categories compared to five and three categories, the relative differences in predicted monthly NH4+-N and TP loads were −6.6 to −6.5% and −13.3 to −7.3%, respectively. Overall, the sensitivity of the SWAT model to LULC datasets with different points in time and levels of detail was lower in monthly and daily streamflow simulation than in monthly NH4+-N and TP loads prediction. This research provided helpful insights into the influence of LULC datasets on watershed modeling.

Effect of storm events on riverine nitrogen dynamics in a subtropical watershed, southeastern China

Rain storms are predicted to increase in the subtropical region due to climate change. However, the effects of storm events on riverine nitrogen (N) dynamics are poorly understood. In this study, the riverine N dynamics and storm effects in a large subtropical river (North Jiulong River, southeastern China) were investigated through continuous sampling of two storm events which occurred in June 2010 and June 2011. The results disclosed a strong linkage between N dynamics and hydrological controls and watershed characteristics. The extreme storm in June 2010 resulted in more fluctuations in N concentrations, loads, and composition, compared with the moderate storm in June 2011. There were contrasting patterns (e.g., the hysteresis effect) between nitrate and ammonium behavior in storm runoff, reflecting their different supply source and transport mechanism. Overall, nitrate supply originated from subsurface runoff and was dominated by within-channel mobilization, while ammonium was mainly from over-land sources and flushed by surface runoff. Extreme storm runoff (2010) caused a four-fold increase in dissolved inorganic N fluxes (DIN), with a greater fraction of ammonium (up to 30% of DIN) compared with the moderate storm and background flow condition (less than 15%). Storm-driven sharp increases of N loads and changes in nutrient stoichiometry (more ammonium) might have been connected with algal blooms in the adjacent estuary and Xiamen Bay. Combined with the background flow measurement of N gradients along the main river and a stream together with anthropogenic N load information, the interactive effect of hydrological and biogeochemical process on riverine N was preliminarily revealed. Current results suggested that storm runoff N was controlled by rainfall, hydrological condition, antecedent soil moisture, spatial variability of land-based N source, and damming. These findings could be used as a reference for future water quality monitoring programs and the development of a pollution mitigation strategy.

Relationships between the Seasonal Variations of Macroinvertebrates, and Land Uses for Biomonitoring in the Xitiaoxi River Watershed, China

AbstractThe impacts of differences in watershed land uses, and differences in seasonality on benthic macroinvertebrate communities, were evaluated in 12 stream sites within the Xitiaoxi River watershed, China, from April 2009 to January 2010. The composition of macroinvertebrate community differed significantly among three land use types. Forested sites were characterized by high taxa richness, diversity and the benthic‐index of biotic integrity (B‐IBI), while farmland and urban disturbed stream sites presented contrary patterns. The percentage of urban land use, conductivity, dissolved oxygen, ammonia nitrogen and total phosphorus were the major drivers for the variations. The land use related water quality stress gradients of the four sampling seasons were determined by means of four independent Principal Component Analyses. The responses of macroinvertebrate community metrics, to anthropogenic stressors, were explored using Spearman Rank Correlation analyses. All the selected metrics, including total numbers of taxa, numbers of Ephemeroptera, Plecoptera and Trichoptera taxa, percentage of non‐insect abundance, percentage of scrapers abundance, Pielou’s evenness index, Simpson diversity index, and the Benthic Index of Biotic Integrity were correlated significantly with environmental gradients (PC1) in autumn. In other seasons such correlations were less pronounced. Our results imply that autumn is the optimal time to sample macroinvertebrate communities, and to conduct water quality biomonitoring in this subtropical watershed. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Land use effect and hydrological control on nitrate yield in subtropical mountainous watersheds

Abstract. Nitrate export in small subtropical watersheds is rarely observed and the estimation of individual land use nitrate yield from a mixed combination within catchments has scarcely been studied. In this study the nitrate concentrations at 16 nested catchments in the Chi-Chia-Wan watershed in Central Taiwan were measured during 2007–2008. A 3-layer TOPMODEL was applied to estimate daily discharge for ungauged sub-catchments. The observed nitrate concentrations and the simulated discharges were used for nitrate flux estimations through four flux methods. Meanwhile, a new deconvolution computation was developed to resolve the nitrate yield of each land use from within the mixed combinations. The results showed that the observed mean NO3-N concentration in relatively pristine catchments was approximately 0.145 ± 0.103 mg l−1, which is comparable with other forestry catchments around the world. However, the higher rainfall/runoff, substantial N deposition, and other nitrogen sources resulted in significantly higher annual export of approximately 238–1018 kg-N km−2 yr−1. Our deconvolution computation showed that the background yield of natural forestry was ~351 ±62 kg-N km−2 yr−1. On the other hand, the extremely high nitrate yield of active farmland was ~308, 170 ± 19 241 kg-N km−2 yr−1 due to over-fertilization. The deconvolution computation technique is capable of tracing the mixed signals at the outlet back to the nitrate productions from varied land use patterns. It advances the application of river monitoring network. The typical values of nitrate yields can serve as a guideline for land management. Comparing the nitrogen input and output, we found some nitrogen missing in the cycling which may indicate certain removal processes and we therefore suggest further study to be carried out to fully understand nitrogen cycling in subtropics.