Total mercury and methylmercury dynamics in upland–peatland watersheds during snowmelt

Wetlands, and peatlands in particular, are important sources of methylmercury (MeHg) to susceptible downstream ecosystems and organisms, but very little work has addressed MeHg production and export from peatland-dominated watersheds during the spring snowmelt. Through intensive sampling, hydrograph separation, and mass balance, this study investigated the total mercury (THg) and MeHg fluxes from two upland–peatland watersheds in Minnesota, USA during the 2005 spring snowmelt and proportionally attributed these fluxes to either peatland runoff or upland runoff. Between 26% and 39% of the annual THg flux and 22–23% of the annual MeHg flux occurred during the 12-days snowmelt study period, demonstrating the importance of large hydrological inputs to the annual mercury flux from these watersheds. Upland and peatland runoff were both important sources of THg in watershed export. In contrast to other research, our data show that peatland pore waters were the principal source of MeHg to watershed export during snowmelt. Thus, despite cold and mostly frozen surface conditions during the snowmelt period, peatland pore waters continued to be an important source of MeHg to downstream ecosystems.     

Forest harvest legacies control dissolved organic carbon export in small watersheds,western Oregon

This study examined physical and biological controls on dissolved organic carbon (DOC) fluxes from conifer-forest watersheds in the H.J. Andrews Experimental Forest of Oregon. We tested how DOC export was related to streamflow and legacies of wood on the forest floor three to five decades after harvest of old-growth forest in seven watersheds spanning the rain to snow elevation gradient. Three watersheds had old-growth forest and four had 30 to 50-year-old forest established after clearcutting of old-growth forest. Mean annual DOC flux in the watersheds was related to the biomass of forest floor wood, which was two or three times higher in watersheds with old-growth forest compared to young forest, and was inversely related to elevation, a measure of snowpack depth and duration. In contrast, fluxes of inorganic elements such as Si and Ca did not vary with harvest history or forest floor characteristics. Annual fluxes of DOC, Si, and Ca were linearly related to annual runoff, and annual volume-weighted concentrations of these ions declined by?<?0.6% with several-fold increases in annual runoff. Across all years, DOC concentrations peaked before the peak of the hydrograph in all watersheds, which we interpret as representing movement, likely via preferential and surficial flow, of organic materials mineralized and solubilized during the long dry summers in this ecosystem. DOC concentrations relative to stream flow exhibited clockwise hysteresis loops in each water year, also suggesting that soluble DOC produced in the dry summer is exported in the fall. DOC concentration differences between reference and harvested watersheds also peaked in late summer or early fall, suggesting that the source of the additional DOC from reference watersheds came from coarse woody debris that remains moist during the dry summers and that was significantly greater in watersheds with elevated DOC. Taken together, our results suggest that forest floor wood is a previously unappreciated control on the supply of DOC that can be exported, and runoff is a secondary control on total DOC flux to streams. The legacy of forest harvest on DOC flux can be observed for decades, as total ecosystem carbon stocks, especially coarse woody debris, may require centuries to develop after old-growth forest harvest.     

Climate variability masks the impacts of land use change on nutrient export in a suburbanizing watershed

Suburbanization negatively impacts aquatic systems by altering hydrology and nutrient loading. These changes interact with climate and aquatic ecosystem processes to alter nutrient flux dynamics. We used a long term data set (1993–2009) to investigate the influence of suburbanization, climate, and in-stream processes on nitrogen and phosphorus export in rivers draining the Ipswich and Parker River watersheds in northeastern MA, USA. During this timeframe population density increased in these watersheds by 14 % while precipitation varied by 46 %. We compared nutrient export patterns from the two larger watersheds with those from two nested headwater catchments collected over a nine year period (2001–2009). The headwater catchments were of contrasting, but stable, land uses that dominate the larger watersheds (suburban and forested). Despite ongoing land use change and an increase in population density in the mainstem watersheds, we did not detect an increase in dissolved inorganic nitrogen (DIN) or PO₄ concentration or export over the 16-year time period. Inter-annual climate and associated runoff variability was the major control. Annual DIN and PO₄ export increased with greater annual precipitation in the Ipswich and the Parker River watersheds, as well as the forested headwater catchment. In contrast, annual DIN export fluxes from the suburban headwater catchment were less affected by precipitation variability, with inter-annual export fluxes negatively correlated with mean annual temperature. The larger watershed exports diverged from headwater exports, particularly during summer, low-flow periods, suggesting retention of DIN and PO₄. Our study shows suburban headwater exports respond to inter-annual variation in runoff and climate differently than forested headwater exports, but the impacts from headwater streams could be buffered by the river network. The net effect is that inter-annual variation and network buffering can mitigate higher nutrient exports from larger suburbanizing watersheds over decadal time periods.     

The age of terrestrial carbon export and rainfall intensity in a temperate river headwater system

Riverine dissolved organic carbon (DOC) supports the production of estuaries and coastal ecosystems, constituting one of the most actively recycled pools of the global carbon cycle. A substantial proportion of DOC entering oceans is highly aged, but its origins remain unclear. Significant fluxes of old DOC have never been observed in temperate headwaters where terrestrial imports take place. Here, we studied the radiocarbon age of DOC in three streams draining forested headwater catchments of the river Mulde (Ore Mountains, Germany). In a 4 week summer precipitation event DOC aged at between 160 and 270 years was delivered into the watershed. In one stream, the DOC was modern but depleted in radiocarbon compared to other hydrological conditions. The yield was substantial and corresponded to 20–52 % of the annual DOC yields in wet and dry years, respectively. The analysis of long-term data suggested that the DOC export in extreme precipitation events added to the annual yield and was not compensated for by lower exports in remaining periods. We conclude that climate change, along with additional processes associated with human activities, channels old soil carbon into more rapidly cycled carbon pools of the hydrosphere.     

Release of aquatic carbon from two peatland catchments in E. Finland during the spring snowmelt period

Spring snowmelt in the arctic and boreal regions represents the most significant event in the hydrological year. We measured concentrations and fluxes of different carbon species in 2 small contrasting (control v drained) forested peatland catchments in E. Finland between April and June 2008 and compared these to long-term annual fluxes. Measurements were made using a combination of continuous sensors (CO₂, temperature, pH, discharge) and routine spot sampling (DOC, POC, DIC, CO₂, CH₄, N₂O). The highest concentrations of CO₂ and CH₄ in streamwater were observed under low flow conditions before the spring flood event, reflecting accumulation and downstream release of gaseous C at the end of the winter period. Over the length of the study mean CH₄ concentrations were 10× higher in the drained site. The snowmelt event was associated with a dilution of DOC and CO₂, with the drained catchment showing a much flashier hydrological response compared to the control site, and post-event, a slower recovery in DOC and CO₂ concentrations. Fluxes of all carbon species during the snowmelt event were significant and represented 37?C45% of the annual flux. This highlights the challenge of quantifying aquatic C fluxes in areas with large temporal variability and suggests that inability to ??capture?? the spring snowmelt event may lead to under-estimation of C fluxes in northern regions.     

Shining light on the storm: in-stream optics reveal hysteresis of dissolved organic matter character

The quantity and character of dissolved organic matter (DOM) can change rapidly during storm events, affecting key biogeochemical processes, carbon bioavailability, metal pollutant transport, and disinfection byproduct formation during drinking water treatment. We used in situ ultraviolet–visible spectrophotometers to concurrently measure dissolved organic carbon (DOC) concentration and spectral slope ratio, a proxy for DOM molecular weight. Measurements were made at 15-minute intervals over three years in three streams draining primarily agricultural, urban, and forested watersheds. We describe storm event dynamics by calculating hysteresis indices for DOC concentration and spectral slope ratio for 220 storms and present a novel analytical framework that can be used to interpret these metrics together. DOC concentration and spectral slope ratio differed significantly among sites, and individual storm DOM dynamics were remarkably variable at each site and among the three sites. Distinct patterns emerged for storm DOM dynamics depending on land use/land cover (LULC) of each watershed. In agricultural and forested streams, DOC concentration increased after the time of peak discharge, and spectral slope ratio dynamics indicate that this delayed flux was of relatively higher molecular weight material compared to the beginning of each storm. In contrast, DOM character during storms at the urban stream generally shifted to lower molecular weight while DOC concentration increased on the falling limb, indicating either the introduction of lower molecular weight DOM, the exhaustion of a higher molecular weight DOM sources, or a combination of these factors. We show that the combination of high-frequency DOM character and quantity metrics have the potential to provide new insight into short-timescale DOM dynamics and can reveal previously unknown effects of LULC on the chemical nature, source, and timing of DOM export during storms.

     

DOC and DIC in Flowpaths of Amazonian Headwater Catchments with Hydrologically Contrasting Soils

Organic and inorganic carbon (C) fluxes transported by water were evaluated for dominant hydrologic flowpaths on two adjacent headwater catchments in the Brazilian Amazon with distinct soils and hydrologic responses from September 2003 through April 2005. The Ultisol-dominated catchment produced 30% greater volume of storm-related quickflow (overland flow and shallow subsurface flow) compared to the Oxisol-dominated catchment. Quickflow fluxes were equivalent to 3.2 ± 0.2% of event precipitation for the Ultisol catchment, compared to 2.5 ± 0.3% for the Oxisol-dominated watershed (mean response ±1 SE, n = 27 storms for each watershed). Hydrologic responses were also faster on the Ultisol watershed, with time to peak flow occurring 10 min earlier on average as compared to the runoff response on the Oxisol watershed. These different hydrologic responses are attributed primarily to large differences in saturated hydraulic conductivity (K _s). Overland flow was found to be an important feature on both watersheds. This was evidenced by the response rates of overland flow detectors (OFDs) during the rainy season, with overland flow intercepted by 54 ± 0.5% and 65 ± 0.5% of OFDs for the Oxisol and Ultisol watersheds respectively during biweekly periods. Small volumes of quickflow correspond to large fluxes of dissolved organic C (DOC); DOC concentrations of the hydrologic flowpaths that comprise quickflow are an order of magnitude higher than groundwater flowpaths fueling base flow (19.6 ± 1.7 mg l⁻¹ DOC for overland flow and 8.8 ± 0.7 mg l⁻¹ DOC for shallow subsurface flow versus 0.50 ± 0.04,mg l⁻¹ DOC in emergent groundwater). Concentrations of dissolved inorganic C (DIC, as dissolved CO₂–C plus HCO₃⁻–C) in groundwater were found to be an order of magnitude greater than quickflow DIC concentrations (21.5 mg l⁻¹ DIC in emergent groundwater versus 1.1 mg l⁻¹ DIC in overland flow). The importance of deeper flowpaths in the transport of inorganic C to streams is indicated by the 40:1 ratio of DIC:DOC for emergent groundwater. Dissolved CO₂–C represented 92% of DIC in emergent groundwater. Results from this study illustrate a highly dynamic and tightly coupled linkage between the C cycle and the hydrologic cycle for both Ultisol and Oxisol landscapes: organic C fluxes strongly tied to flowpaths associated with quickflow, and inorganic C (particularly dissolved CO₂) transported via deeper flowpaths.     

Changes in the character of DOC in streams during storms in two Midwestern watersheds with contrasting land uses

Dissolved organic carbon (DOC) dynamics in streams is important, yet few studies focus on DOC dynamics in Midwestern streams during storms. In this study, stream DOC dynamics during storms in two Midwestern watersheds with contrasting land uses, the change in character of stream DOC during storms, and the usability of DOC as a hydrologic tracer in artificially drained landscapes of the Midwest are investigated. Major cation/DOC concentrations, and DOC specific UV absorbance (SUVA) and fluorescence index (FI) were monitored at 2–4 h intervals during three spring storms. Although DOC is less aromatic in the mixed land use watershed than in the agricultural watershed, land use has little impact on stream DOC concentration during storms. For both watersheds, DOC concentration follows discharge, and SUVA and FI values indicate an increase in stream DOC aromaticity and lignin content during storms. The comparison of DOC/major cation flushing dynamics indicates that DOC is mainly exported via overland flow/macropore flow. In both watersheds, the increase in DOC concentration in the streams during storms corresponds to a shift in the source of DOC from DOC originating from mineral soil layers of the soil profile at baseflow, to DOC originating from surficial soil layers richer in aromatic substances and lignin during storms. Results also suggest that DOC, SUVA and FI could be used as hydrologic tracers in artificially drained landscapes of the Midwest. These results underscore the importance of sampling streams for DOC during high flow periods in order to understand the fate of DOC in streams.     

Effects of native perennial vegetation buffer strips on dissolved organic carbon in surface runoff from an agricultural landscape

Dissolved organic carbon (DOC) constitutes a small yet important part of a watershed’s carbon budget because it is mobile and biologically active. Agricultural conservation practices such as native perennial vegetation (NPV) strips will influence carbon cycling of an upland agroecosystem, and could affect how much DOC enters streams in runoff, potentially affecting aquatic ecosystems. In a study conducted in Iowa (USA), four treatments with strips of NPV varying in slope position and proportion of area were randomly assigned among 12 small agricultural watersheds in a balanced incomplete block design. Runoff samples from 2008 to 2010 were analyzed for DOC and correlated with flow data to determine flow weighted DOC concentrations and loads. Data were analyzed for the entire 3 years, annually, seasonally, monthly, by flow event size and for one extreme storm event. Overall we found few differences in DOC concentration with the exception that concentrations were greater in the 10 % NPV at the footslope watersheds than the 20 % NPV in contours watersheds over the 3 years, and the 100 % agricultural treatment had higher DOC concentrations than all NPV treatments during the one extreme event. Because the NPV treatments reduced runoff, DOC export tended to be highest in the 100 % agricultural watersheds over the 3 years and during high flows. We also compared two watersheds that were restored to 100 % NPV and found decreases in DOC concentrations and loads indicating that complete conversion to prairie leads to less watershed DOC export. Regression results also support the contention that increases in the percentage of NPV in the watershed decreases watershed export of DOC. Further analysis indicated that DOC concentrations were diluted as flow event size increased, independent of any treatment effects. It appears groundwater sources become an important component to flow as flow event size increases in these watersheds.     

Stormflow Dynamics of Dissolved Organic Carbon and Total Dissolved Nitrogen in a Small Urban Watershed

We examined patterns of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) loading to a small urban stream during baseflow and stormflow. We hypothesized that lower DOC and TDN contributions from impervious surfaces would dilute natural hydrologic flowpath (i.e., riparian) contributions during storm events in an urban watershed, resulting in lower concentrations of DOC and TDN during storms. We tested these hypotheses in a small urban watershed in Portland, Oregon, over a 3-month period during the spring of 2003. We compared baseflow and stormflow chemistry using Mann–Whitney tests (significant at p<0.05). We also applied a mass balance to the stream to compare the relative significance of impervious surface contributions versus riparian contributions of DOC and TDN. Results showed a significant increase in stream DOC concentrations during stormflows (median baseflow DOC = 2.00 mg l⁻¹ vs. median stormflow DOC = 3.46 mg l⁻¹). TDN streamwater concentrations, however, significantly decreased with stormflow (median baseflow TDN = 0.75 mg l⁻¹ vs. median stormflow TDN = 0.56 mg l⁻¹). During storms, remnant riparian areas contributed 70–74% of DOC export and 38–35% of TDN export to the stream. The observed pattern of increased DOC concentrations during stormflows in this urban watershed was similar to patterns found in previous studies of forested watersheds. Results for TDN indicated that there were relatively high baseflow nitrogen concentrations in the lower watershed that may have partially masked the remnant riparian signal during stormflows. Remnant riparian areas were a major source of DOC and TDN to the stream during storms. These results suggest the importance of preserving near-stream riparian areas in cities to maintain ambient carbon and nitrogen source contributions to urban streams.     

土地利用变化对挠力河流域可溶性有机碳输出的影响

采用野外采样、室内分析、GIS及统计分析相结合的方法,研究了挠力河流域河水可溶性有机碳（DOC）浓度的季节性动态,以及年均尺度上全流域、100 m河岸带土地利用变化对河水DOC输出的影响.结果表明： 基流状态下,河水DOC浓度在春季、夏季显著高于秋季;有湿地存在的子流域DOC浓度的季节性动态与无湿地存在的子流域存在显著差异,且有湿地存在的子流域中DOC浓度的季节性变异与整个流域的趋势一致;年均尺度上,DOC浓度与全流域湿地以及100 m河岸带范围内的水田面积百分比呈显著正相关,而与全流域尺度的林地百分比呈显著负相关（P<0.05）.表明湿地的存在是影响挠力河流域河水DOC季节性变异的重要因素;全流域的湿地以及100 m河岸带范围内的水田对其具有显著的促进作用,而林地对其有显著的减缓效应,流域过去几十年的土地利用变化改变了河水DOC的平衡状况.     

Biogeochemistry of unpolluted forested watersheds in the Oregon Cascades: temporal patterns of precipitation and stream nitrogen fluxes

We analyzed long-term organic and inorganic nitrogen inputs and outputs in precipitation and streamwater in six watersheds at the H.J. Andrews Experimental Forest in the central Cascade Mountains of Oregon. Total bulk N deposition, averaging 1.6 to 2.0 kg N ha^–1 yr^–1, is low compared to other sites in the United States and little influenced by anthropogenic N sources. Streamwater N export is also low, averaging <1 kg ha^–1 yr^–1. DON is the predominant form of N exported from all watersheds, followed by PON, NH₄-N, and NO₃-N. Total annual stream discharge was a positive predictor of annual DON output in all six watersheds, suggesting that DON export is related to regional precipitation. In contrast, annual discharge was a positive predictor of annual NO₃-N output in one watershed, annual NH₄-N output in three watersheds, and annual PON output in three watersheds. Of the four forms of N, only DON had consistent seasonal concentration patterns in all watersheds. Peak streamwater DON concentrations occurred in November-December after the onset of fall rains but before the peak in the hydrograph, probably due to flushing of products of decomposition that had built up during the dry summer. Multiple biotic controls on the more labile nitrate and ammonium concentrations in streams may obscure temporal DIN flux patterns from the terrestrial environment. Results from this study underscore the value of using several watersheds from a single climatic zone to make inferences about controls on stream N chemistry; analysis of a single watershed may preclude identification of geographically extensive mechanisms controlling N dynamics.     

Seasonal variation of DOC concentration and annual loss of DOC from an Atlantic blanket bog in South Western Ireland

This paper represents the first continuous dissolved organic carbon (DOC) record, measured in a stream draining an Atlantic blanket bog in South West Ireland for the calendar year 2007. At 30-min intervals, the DOC concentration was automatically measured using an in-stream spectroanalyser whose variation compared well with laboratory analysed samples taken by a 24-bottle auto-sampler. The concentration of DOC ranged from 2.7 to 11.5 mg L^?1 with higher values during the summer and lower values during the winter. A simple linear regression model of DOC concentration versus air temperature of the previous day was found, suggesting that temperature more than discharge was controlling the DOC concentration in the stream. The change in DOC concentration with storm events showed two patterns: (1) in the colder period: the DOC concentration seemed to be independent of changes in stream flow; (2) in the warmer period: the DOC concentration was found to rise with increases in stream flow on some occasions and to decrease with increasing stream flow on other occasions. The annual export of DOC for 2007 was 14.1 (±1.5) g C m^?2. This value was calculated using stream discharge data that were determined by continuously recorded measurements of stream height. The flux of DOC calculated with the 30-min sampling was compared with that calculated based on lower sampling frequencies. We found that sampling frequency of weekly or monthly were adequate to calculate the annual flux of DOC in our study site in 2007.     

Dissolved organic carbon and nitrogen in urban and rural watersheds of south-central Texas: land use and land management influences

Dissolved organic carbon (DOC) and nitrogen (DON) concentrations were quantified in urban and rural watersheds located in central Texas, USA between 2007 and 2008. The proportion of urban land use ranged from 6 to 100% in our 12 study watersheds which included nine watersheds without waste water treatment plants (WWTP) and three watersheds sampled downstream of a WWTP. Annual mean DOC concentrations ranged 20.4–52.5 mg L^?1. Annual mean DON concentrations ranged 0.6–1.9 mg L^?1. Only the rural watersheds without a WWTP had significantly lower DOC concentrations compared to those watersheds with a WWTP but all the streams except two had significantly reduced DON compared to those with a WWTP. Analysis of the nine watersheds without a WWTP indicated that 68% of the variability in mean annual DOC concentration was explained by urban open areas such as golf courses, sports fields and neighborhood parks under turf grass. There was no relationship between annual mean DON concentration and any land use. Urban open area also explained a significant amount of the variance in stream sodium and stream sodium adsorption ratio (SAR). Ninety-four percent of the variance in annual mean DOC concentration was explained by SAR. Irrigation of urban turf grass with domestic tap water high in sodium (>181 mg Na⁺ L^?1) may be inducing sodic soil conditions in watershed soils in this region resulting in elevated mean annual DOC concentrations in our streams.     

高山峡谷地区森林流域分布式降雨-径流模型的构建与验证

根据岷江上游杂谷脑河流域典型的高山峡谷地区主要水文特点,选择通用性较强的水文过程模式,构建高山峡谷地区森林流域分布式降雨-径流过程模型,避免过多复杂的区域性模型参数率定,保证模型在相似地区的可移植性;并选择杂谷脑水文站上游地区进行降雨-径流过程模拟,得到1999年和2000年模拟时段长度为1000 h的两个径流过程,对模拟与实测的径流过程、累积径流量、洪峰流量与峰现时间等进行比较,其拟合效果较好.该模型结构简单,引入的经验参数较少,可推广应用到其它尺度流域.     

Nitrogen input–output budgets for lake-containing watersheds in the Adirondack region of New York

The Adirondack region of New York is characterized by soils and surface waters that are sensitive to inputs of strong acids, receiving among the highest rates of atmospheric nitrogen (N) deposition in the United States. Atmospheric N deposition to Adirondack ecosystems may contribute to the acidification of soils through losses of exchangeable basic cations and the acidification of surface waters in part due to increased mobility of nitrate (NO₃^–). This response is particularly evident in watersheds that exhibit nitrogen saturation. To evaluate the contribution of atmospheric N deposition to the N export and the capacity of lake-containing watersheds to remove, store, or release N, annual N input–output budgets were estimated for 52 lake-containing watersheds in the Adirondack region from 1998 to 2000. Wet N deposition was used as the N input and the lake N discharge loss was used as the N output based on modeled hydrology and measured monthly solute concentrations. Annual outputs were also estimated for dissolved organic carbon (DOC). Wet N deposition increased from the northeast to the southwest across the region. Lake N drainage losses, which exhibited a wider range of values than wet N deposition, did not show any distinctive spatial pattern, although there was some evidence of a relationship between wet N deposition and the lake N drainage loss. Wet N deposition was also related to the fraction of N removed or retained within the watersheds (i.e., the fraction of net N hydrologic flux relative to wet N deposition, calculated as [(wet N deposition minus lake N drainage loss)/wet N deposition]). In addition to wet N deposition, watershed attributes also had effects on the exports of NO₃^–, ammonium (NH₄⁺), dissolved organic nitrogen (DON), and DOC, the DOC/DON export ratio, and the N flux removed or retained within the watersheds (i.e., net N hydrologic flux, calculated as [wet N deposition less lake N drainage loss]). Elevation was strongly related with the lake drainage losses of NO₃^–, NH₄⁺, and DON, net NO₃^– hydrologic flux (i.e., NO₃^– deposition less NO₃^– drainage loss), and the fraction of net NO₃^– hydrologic flux, but not with the DOC drainage loss. Both DON and DOC drainage losses from the lakes increased with the proportion of watershed area occupied by wetlands, with a stronger relationship for DOC. The effects of wetlands and forest type on NO₃^– flux were evident for the estimated NO₃^– fluxes flowing from the watershed drainage area into the lakes, but were masked in the drainage losses flowing out of the lakes. The DOC/DON export ratios from the lake-containing watersheds were in general lower than those from forest floor leachates or streams in New England and were intermediate between the values of autochthonous and allochthonous dissolved organic matter (DOM) reported for various lakes. The DOC/DON ratios for seepage lakes were lower than those for drainage lakes. In-lake processes regulating N exports may include denitrification, planktonic depletion, degradation of DOM, and the contribution of autochthonous DOM and the influences of in-lake processes were also reflected in the relationships with hydraulic retention time. The N fluxes removed or stored within the lakes substantially varied among the lakes. Our analysis demonstrates that for these northern temperate lake-containing watershed ecosystems, many factors, including atmospheric N deposition, landscape features, hydrologic flowpaths, and retention in ponded waters, regulated the spatial patterns of net N hydrologic flux within the lake-containing watersheds and the loss of N solutes through drainage waters.     

黄土高原不同植被覆盖对流域水文的影响

以山西省吉县蔡家川流域为对象,研究了植被覆盖类型对流域水文的影响.结果表明:不同植被覆盖的流域年径流系数分别为:林地流域1.6%～2.3%,以农、牧为主的流域3.1%～3.9%;各流域基流系数差异显著,人工林流域为零,次生林为主的流域1.0%～1.5%,以农、牧为主的流域2.5%～2.8%;在雨季人工林流域的径流总量是次生林流域的3.37倍、农地流域的1.9倍,而农地流域的基流量是次生林流域的2.2倍;短历时高强度降雨条件下,人工林流域、次生林流域地表径流量分别是农地流域的10.8倍和2.2倍;在历时较长的暴雨条件下,人工林流域单位面积上的洪峰流量是农地流域的3.4倍,次生林流域的6.9倍;在长历时、大雨量条件下,农地流域的径流量是次生林流域的1.8倍.水平梯田的水源涵养功能与次生林植被相当,次生林植被的水源涵养功能远好于人工植被,在水资源短缺的黄土高原应提倡植被的自然恢复.     

老爷岭多年冻土小流域春季冻融期径流溶解性有机碳变化特征

溶解性有机碳（DOC）的输移过程是流域碳循环中重要的组成部分,对全球碳循环产生重要影响。以大兴安岭多年冻土区的典型森林小流域-老爷岭流域为研究对象,获得2021年4月9日到6月30日冻融期降雨量、气温、土温等气象数据及逐日径流量、径流DOC浓度,计算了冻融循环期（4月9日-28日）和融化期（4月29日-6月30日）流域径流DOC的输出通量,揭示了径流DOC浓度及输出通量的影响因素。结果表明：（1）研究时段内,老爷岭流域径流DOC浓度变化范围为3.88-33.75 mg/L,流域上游的径流DOC浓度变化趋势与下游基本一致,DOC浓度随着温度的升高呈现下降趋势,4月份平均径流DOC浓度明显高于5、6月份。（2）研究时段内流域径流DOC总输出通量为3215.48 kg/km²,其中5月径流DOC输出通量高于4、6月份。径流量与径流DOC输出通量存在显著正相关关系（P<0.05）,是流域DOC输出通量的主导因素。（3）研究时段内流域DOC浓度与平均气温呈极显著负相关（R²=0.5048,P<0.001）;降水样品中的DOC浓度变化范围为1.06-9.42 mg/L,显著低于径流DOC浓度;土壤中DOC含量变化趋势与径流DOC变化趋势一致,0-10 cm、10-20 cm土壤平均DOC浓度范围为77.57-133.99 mg/L。（4）冻融循环期平均日径流DOC浓度（24.02 mg/L）显著（P<0.05）高于融化期（14.64 mg/L）,而融化期平均日DOC输出通量（48.02 kg/km²）是冻融循环期（9.52 kg/km²）的5倍。研究结果揭示了大兴安岭多年冻土小流域春季冻融期径流DOC的输移特征及其影响因素,对理解多年冻土区碳循环有重要意义。     

Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia

Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast, stream water DOC concentrations did not exhibit seasonal variation. Higher DOC and NO₃ ⁻ concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there was an additional NO₃ ⁻ source or reduction of a NO₃ ⁻ sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated area.     

Dissolved and particulate nutrient flux from three adjacent agricultural watersheds: A five-year study

Fluxes of dissolved and particulate nitrogen (N) and phosphorus(P) from three adjacent watersheds were quantified with ahigh-resolution sampling program over a five-year period. The watershedsvary by an order of magnitude in area (12,875, 7968 and 1206 ha), and inall three watersheds intensive agriculture comprises > 90% ofland. Annual fluxes of dissolved N and P per unit watershed area (exportcoefficients) varied 2X among watersheds, and patterns were notdirectly related to watershed size. Over the five-year period, meanannual flux of soluble reactive P (SRP) was 0.583 kg P ·ha^–1 · yr^–1 from the smallestwatershed and 0.295 kg P · ha^–1 ·yr^–1 from the intermediate-sized watershed, which hadthe lowest SRP flux. Mean annual flux of nitrate was 20.53 kg N ·ha^–1 · yr^–1 in the smallestwatershed and 44.77 kg N · ha^–1 ·yr^–1 in the intermediate-sized watershed, which had thehighest nitrate flux. As a consequence, the export ratio of dissolvedinorganic N to SRP varied from 80 (molar) in the smallest watershed to335 in the intermediate-sized watershed. Because most N was exported asnitrate, differences among watersheds in total N flux were similar tothose for nitrate. Hence, the total N:P export ratio was 42(molar) for the smallest watershed and 109 for the intermediate-sizedwatershed. In contrast, there were no clear differences among watershedsin the export coefficients of particulate N, P, or carbon, even though> 50% of total P was exported as particulate P in allwatersheds. All nutrient fractions were exported at higher rates in wetyears than in dry years, but precipitation-driven variability in exportcoefficients was greater for particulate fractions than for dissolvedfractions.Examination of hydrological regimes showed that, for all nutrientfractions, most export occurred during stormflow. However, theproportion of nitrate flux exported as baseflow was much greater thanthe proportion of SRP flux exported as baseflow, for all threewatersheds (25–37% of nitrate exported as baseflow vs.3–13% of SRP exported as baseflow). In addition, baseflowcomprised a greater proportion of total discharge in theintermediate-sized watershed (43.7% of total discharge) than theother two watersheds (29.3 and 30.1%). Thus, higher nitrateexport coefficients in the intermediate-sized watershed may haveresulted from the greater contribution of baseflow in this watershed.Other factors potentially contributing to higher nitrate exportcoefficients in this watershed may be a thicker layer of loess soils anda lower proportion of riparian forest than the other watersheds. Theamong-watershed variability in SRP concentrations and exportcoefficients remains largely unexplained, and might represent theminimum expected variation among similar agriculturalwatersheds.