Spatial and temporal dynamics of base flow in semi-arid montane watersheds and the effects of landscape patterns and topography.

Base flow (BF) is harder to predict than other hydrological signatures. The lack of hydrologically relevant information or adequately broad spectrum of typically selected catchment attributes (particularly landscape and topography) hinders the explanatory power. Our goals were to identify the most influential controls on base flow spatially and temporally and to elucidate the response relationships. Base flow in 19 semi-arid sub-watersheds was separated by digital filtering. One hundred and fourteen sub-watershed attributes were related to base flow using random forest regression. The main results were as follows: (1) Annual BF significantly declined since 1999 due to decreased precipitation, increased air temperature, afforestation, urban expansion, and increasing water consumption. Annual base flow index (BFI), varying between 0.319 and 0.695, showed less noticeable temporal trends. (2) Precipitation (P) and underlying carbonate rocks primarily controlled the spatial variation of annual BF and total flow (TF), with the impacts being positive. Landscape was less influential. After the abrupt runoff decline, landscape composition rather than configuration exerted greater impacts on spatial BF and TF, and the importance of forest increased, whereas landscape configuration was decisive for BFI during the whole observation period. The absence of significant links between landscape configuration and water quantity may result from a scale issue. Concave profile curvatures were found to be topographic variables more important than slopes. The impact of soil was the least. This study would benefit the selection of catchment attributes and spatial extents to quantify these attributes in building BF predicting models in future studies.

A flexible framework for regionalization of base flow for river habit maintenance and its thresholds.

Information on base flow for river habit maintenance (BFRH) and its thresholds is necessary for water resource utilization and protection. BFRH and its thresholds have significant spatial differences; however, it is still unclear how to identify and assess these characteristics. In this study, a technical framework was proposed to clarify the specific procedures and methods for regionalization of BFRH and its thresholds in large-scale areas. The framework includes four parts: construction of controlling factor system, sub-region division, identification of dominant factors, and determination of the thresholds in sub-regions. The framework was then applied to China to analyzed the regionalized characteristics of BFRH and its thresholds from a national perspective. The results illustrate the following: (1) the country is divided into nine sub-regions, and the controlling factors and their action paths to BFRH vary greatly. The elements of climate, vegetation, soil, topography and morphology are satisfactory in explaining the variance of BFRH and its thresholds, as R2 of the partial least squares structural equation modeling is between 0.503 and 0.848. (2) The value of BFRH/MAF (i.e. the proportion of BFRH to mean annual natural flow) differs greatly among sub-regions. The mean value is the largest in the Northwest Region, reaching 20 %, while it is only 1.7 % in the Northeast Cold Region. (3) The dynamic and static thresholds are obtained by using the precipitation and other indices as the explanatory variables in the sub-regions. In general, the more abundant the water resources, the higher may be the threshold. Moreover, attention should be paid to the positive and negative effects of vegetation restoration on this threshold. The case study proves that the framework can guide the determination of BFRH, especially for ungagged rivers. Importantly, the framework is flexible and highly adaptable in different regions.

Attenuation of organic pollutants and the effects of salinity and seasonality in a Mediterranean micro-estuary.

Micro-estuaries are small ubiquitous transitional water bodies that are often located in semi-arid zones. Unlike the vastly studied large estuaries, micro-estuaries lack the ability to dilute and contain pollution from point and non-point sources due to low natural water discharges. Therefore, these diverse ecological systems are susceptible to pollutant loads due to prolonged water residence time and complex geochemical dynamics. Although this elevated anthropogenic stress limits their potential to provide ecological and recreational services, micro-estuaries have some traits similar to those found in wetlands, which provide a natural potential to retain and mitigate organic pollutants. A two consecutive years study conducted at the Alexander micro-estuary tracked the influx and outflux of a large organic pollutant mixture during base-flow and flood events. During the research period, 165 kg of active ingredients entered the micro-estuary and 160 kg flowed out to the Mediterranean Sea, suggesting negligible net attenuation. However, this broad picture conceals inner shifts in pollutant mixture loads, which contained 46 pesticides and 19 pharmaceuticals. Only a handful of pollutants were actually balanced, whereas most compounds were either removed or added to the flow, with no observed correlation to chemical properties. A prominent observation was the load increase along the flow for some pollutants during base-flow conditions. This trend, which was correlated with salinity elevation and was verified in lab experiments, suggests that seawater intrusion to the bottom of the estuary may increase desorption rates of pollutants from the estuary bed, creating an estuarine desorption magnification effect. The combination of strong anthropogenic stress with increased desorption rates severely limits the estuary's potential to mitigate pollutants, frequently transforming it into a pollution source rather than a sink.

Research on the exploitation and utilization degree of mineral water based on ecological base flow in the Changbai Mountain basalt area, northeast China.

Mineral water from the Changbai Mountain basalt area is China's most important source of drinking water. Mineral water with abundant output and enriched trace elements has driven the rapid development of the local economy. However, the extensive exploitation of mineral water and the neglect of ecological base flows threaten interdependent river ecosystems. In order to ensure the sustainable use of mineral water, it's necessary to calculate the exploitation and utilization of mineral water according to the ecological base flow. In this study, four hydrological methods were used to analyze the intra-annual and inter-annual ecological base flows, namely the Tennant method, the base flow ratio method, the driest monthly average flow method and the Texas method. The results show that the ecological base flow during the flood season is about 3-4 times that during the non-flood season. Affected by rainfall and runoff, the inter-annual ecological base flow also fluctuated within a small range. This study divides the mineral water exploitation coefficient into five categories and proposes an assessment of the mineral water exploitation potential based on the ecological base flow. It shows that the flow of Baijiang River spring group is the largest, but it's exploitation potential is normal. Huangni River spring group at weak level for exploitation because of its small flow rate. It's obvious that river runoff is not the only factor that determines the exploitation coefficient of the spring group. The proportion of ecological base flow and the proportion of river base flow are also important factors.

A Bayesian network approach for determining optimal ecological base flow of rivers in water shortage areas of Northwest China.

How to balance natural river base flow with water loss from agricultural activities such as irrigation has become an important global challenge in areas of water shortage. Using a Bayesian network model, we constructed a decision-making framework for river ecological base flow in the water shortage areas. This framework can be divided into three components: (1) calculation of economic losses caused when priority is given to protect river ecological base flow, (2) construction of a decision-making Bayesian network, and (3) determination of an optimum ecological base flow of rivers based on an acceptable probability of acceptable economic losses the decision-makers selected. We used the Baoji section of the Weihe River as a typical water shortage area case study. The results show that the optimum ecological base flows in the Baoji section of Weihe River in wet, normal, and dry years were 32.35%, 20.59%, and 14.71% of natural inflow in rivers, respectively. From an economic benefit perspective, the beneficiaries should provide more financial support to protect the ecological base flow of rivers. Our approach could be used in other water shortage areas where decisions need to be made between protection of river ecological base flows of rivers and agricultural use.

Base flow in the Yarlungzangbo River, Tibet, maintained by the isotopically-depleted precipitation and groundwater discharge.

The extension-induced rift systems on the Tibetan Plateau (TP) may convey large amount of groundwater to rivers, but sources and flow paths of such groundwater are unknown. The Yarlungzangbo River (YR) is the only large river that traverses the southern Tibetan plateau from west to east, following one major suture zone that is cut by extensional normal faults. The faults could influence the flow paths of groundwater discharging to the river. In this study, O and H isotopes, major ions and 222Rn concentrations are analyzed along the YR, and interpreted in relation to structural geology and tectonics. The YR exhibits an abrupt change of isotopic and chemical compositions along with a large increase in flow where the middle reach intersects NE-SW-trending rifts. Low values of δD and δ18O and high concentrations of major ions and 222Rn in the middle reach show that waters are modified isotopically and chemically by a variety of possible water origins, such as recharge of high-altitude glacier melt and discharge from groundwater. Groundwater contributes 27 to 40% of the river flow in the middle reach. Isotopically-light meltwater from high-altitude glacier melt cannot account for the isotope composition of the present outflow of groundwater. The O and H isotope data in the YR and discharging groundwater can be well explained by the groundwater originated as paleo-precipitation during a cooler time, such as the late Pleistocene to early Holocene. The paleo-groundwater discharge can account for about 36 × 108 m3 water budget unbalance in the middle reach. The study provides the first clear isotope evidence for the source of groundwater discharge into a large river through favorable conduits in large-scale active tensile fault zones and confirms the regional scale of groundwater flow on the Tibetan Plateau. Understanding the characteristics and changes of streamflow and surface-groundwater circulation on the Tibetan Plateau will help to manage water resources under a changing environment.

The impact of small-scale land cover and groundwater interactions on base flow solute and nutrient export in a small agricultural stream.

Central Kentucky could be considered a critical source area of nutrients in water ways because of low permeability soils, fast groundwater flow through bedrock fractures, and pervasive agriculture and development. Of particular concern is rising development in rural areas, which creates mixed land cover (MLC) watersheds, i.e., watersheds with development, agriculture, and other land cover types. MLC watersheds add complexity to spatial and temporal releases of dissolved constituents, leading to less predictable water quality patterns. The goal of this research was to examine the export of dissolved substances from a small, upland MLC catchment in central Kentucky with a focus on how the interaction between discharges from developed agricultural land cover and groundwater influence base flow water quality. Our approach was to spatially sample a representative catchment monthly over 1 year, characterize the major dissolved constituents, and evaluate catchment processes with statistical analyses and Piper diagrams. Principal component analysis, factor analysis, and Piper diagrams indicate base flow was composed of groundwater influenced by two different host rocks and an outfall draining a developed region. Base flow nutrient export was dominated by mixing nitrate-sulfate rich groundwater with ammonium-phosphate-chloride rich outfall drainage. High nitrate groundwater dominated nitrogen export in the winter, whereas high ammonium outfall drainage dominated summer export. Spatial analysis revealed that ~ 10% of the basin may have similar land cover and hydrologic processes, suggesting that MLC catchments are small but collectively significant nitrogen sources to river networks due to development and agriculturally impacted groundwater.

Precipitation recharges the shallow groundwater of check dams in the loessial hilly and gully region of China.

Check dams affect regional hydrological cycles and ecological environments. We conducted a field monitoring experiment in the Liudaogou Catchment on the Loess Plateau in China to determine the spatiotemporal response of shallow groundwater recharge and base flow by precipitation in check dams of this loessial hilly and gully region. The amount and seasonal distribution of precipitation directly affected the changes in shallow groundwater and base flow. The shallow groundwater was recharged by in situ vertical infiltration and lateral underground movement. Precipitation >30.0 mm d-1 recharged the shallow groundwater by piston flow when the water table in the check dam was <4.0 m. When the water table varied from the head (>4.0 m) to the middle and tail (<4.0 m) of the check dam, the influx of precipitation was by runoff in the catchment above the trench head, replenishing the groundwater vertically through the trench head and then moving laterally underground to the end of the dam. The response of the base flow and groundwater lagged the precipitation by a certain period. The lag time of the base flow was about 5-16 days, in which the response was more sensitive than for shallow groundwater. The lag time of the vertical supply for the response of the precipitation to the shallow groundwater was about 12-54 days, whereas the lag time for the lateral supply was about 72-93 days. The lag time may be associated with precipitation, temperature, solar radiation, vegetation water consumption and soil porosity. These findings will help elucidate the processes of groundwater recharge and provide new insight for managing the water balance in this loessial hilly and gully region.

Basic and target eco-environment water requirements of a dry inland river under typical flow frequencies in China.

Analysis of eco-environmental water requirements (EEWRs) and water resource allocation strategies for arid, inland river basins can provide the theoretical basis for sustainable water utilization and management. In this paper, an optimal water resource allocation strategy is proposed for Yarkand River Basin in Xinjiang, China, on the basis of a comprehensive analysis of runoff data collected between 1970 and 2016, three ecological environmental protection goals, basic eco-environmental water requirement (BEEWR) aimed at sustaining aquatic ecosystems within the river, and target eco-environmental water requirements (TEEWR) aimed at protecting various types of riparian vegetation along the river. The results showed that: (1) after the runoff in Kaqun reach subtracting the BEEWR, the annual average river loss (recharge), and the amount of water diversion for irrigation (51.43 × 108 m3) from flows along the Kaqun reach, the remaining water volume during wet years was able to meet all three TEEWRs; (2) during moderately wet years, the remaining water was capable of meeting the second and third TEEWRs; and (3) during dry and extremely dry years, there was little or no residual water available to meet TEEWRs. The proposed optimal water resource allocation strategy, based on the above findings, states that the water diversion requirement for irrigation and domestic use allocated from the total amount of runoff should not exceed the National Water Policy (Three Red Lines) standard first. Then, the BEEWR allocated from the runoff should be met second, and the annual average river loss, third. Depending on the amount of remaining water, the second and third TEEWRs can be fulfilled during wet years, but during moderately wet years, only the third TEEWR can be met. During dry and extremely dry years, only the BEEWR of the river can be met and only during the flood season.

Dinâmica da água em áreas de recarga de nascentes em dois ambientes na Região Alto Rio Grande, Minas Gerais / Water dynamics in springs recharge areas in two environments in the alto Rio Grande Region, Minas Gerais

RESUMO Apesar da importância da água na Terra, pouco se conhece sobre sua dinâmica no solo e, consequentemente, poucas são as informações sobre sua preservação em quantidade e qualidade. Diante disso, objetivou-se, neste trabalho, gerar informações que dessem subsídio ao entendimento da dinâmica da água no solo em áreas de recarga de nascentes. Para tanto, realizou-se o monitoramento da vazão e da precipitação nas áreas de recarga de duas pertencentes, verificando a relação existente entre os atributos físico-hídricos do solo intimamente relacionados à recarga subterrânea, além das características pedológicas, do uso do solo, da cobertura vegetal e da declividade. Os resultados mostraram que a sinalização de que os Cambissolos apresentam maior capacidade de infiltração e, consequentemente, recarga, em relação aos Latossolos, não se confirmou neste trabalho. Mesmo havendo uma grande discrepância nas classes de relevo entre as áreas de recarga das nascentes, esse efeito não foi suficiente para reduzir o processo de recarga. As condições de uso do solo não foram substancialmente diferentes a ponto de influenciar o processo de infiltração. O grande diferencial entre as áreas de recarga das nascentes em relação à capacidade de gerar deflúvio base se explica pelos atributos físico-hídricos e pelo índice pluviométrico, que, neste caso, resultaram em uma associação mais efetiva na área da nascente L1. A elevada capacidade de retenção de água no Latossolo, reflexo da maior microporosidade associada a sua maior profundidade, é uma característica desfavorável para o processo de recarga quando comparado ao Cambissolo, que tem horizonte B incipiente e menores profundidades.

ABSTRACT Despite the importance of water on earth, little is known about its dynamics in the soil, and consequently, there is little information about its preservation in quantity and quality. Therefore, the objective of this research was to generate information to support the understanding of water dynamics in soil springs in recharge areas. Thus, we carried out monitoring flow and precipitation in the areas of two belonging clearance, verifying the relationship between the physical and water attributes of the soil closely related to groundwater recharge, in addition to soil characteristics, land use, cover vegetation and slope. The results showed that the signaling that Cambisoils have higher infiltration capacity and consequently clearance in relation to Latosols was not confirmed in this study. Even though there is a large discrepancy in relief classes between the recharge areas of springs, this effect was not sufficient to reduce the recharge process. Land use conditions were not substantially different as to influencing the infiltration process. The big difference between the recharge areas of the springs on the ability to generate runoff base is explained by physical-water attributes and rainfall, which in this case resulted in a more effective association in the area of ​​spring L1. The higher water retention capacity of the Oxisol, reflecting the higher microporosity associated with its greater depth, is an unfavorable feature for the recharge process, when compared to Inceptisol, which has an incipient B horizon and lower depths.

Spring Water of an Alpine Karst Aquifer Is Dominated by a Taxonomically Stable but Discharge-Responsive Bacterial Community.

Alpine karst aquifers are important groundwater resources for the provision of drinking water all around the world. Yet, due to difficult accessibility and long-standing methodological limitations, the microbiology of these systems has long been understudied. The aim of the present study was to investigate the structure and dynamics of bacterial communities in spring water of an alpine limestone karst aquifer (LKAS2) under different hydrological conditions (base vs. event flow). The study was based on high-throughput 16S rRNA gene amplicon sequencing, study design and sample selection were guided by hydrology and pollution microbiology data. Spanning more than 27 months, our analyses revealed a taxonomically highly stable bacterial community, comprising high proportions of yet uncultivated bacteria in the suspended bacterial community fraction. Only the three candidate phyla Parcubacteria (OD1), Gracilibacteria (GN02), Doudnabacteria (SM2F11) together with Proteobacteria and Bacteroidetes contributed between 70.0 and 88.4% of total reads throughout the investigation period. A core-community of 300 OTUs consistently contributed between 37.6 and 56.3% of total reads, further supporting the hypothesis of a high temporal stability in the bacterial community in the spring water. Nonetheless, a detectable response in the bacterial community structure of the spring water was discernible during a high-discharge event. Sequence reads affiliated to the class Flavobacteriia clearly increased from a mean proportion of 2.3% during baseflow to a maximum of 12.7% during the early phase of the studied high-discharge event, suggesting direct impacts from changing hydrological conditions on the bacterial community structure in the spring water. This was further supported by an increase in species richness (Chao1) at higher discharge. The combination of these observations allowed the identification and characterization of three different discharge classes (Q1-Q3). In conclusion, we found a taxonomically stable bacterial community prevailing in spring waters from an alpine karst aquifer over the entire study period of more than 2 years. Clear response to changing discharge conditions could be detected for particular bacterial groups, whereas the most responsive group - bacteria affiliated to the class of Flavobacteriia - might harbor potential as a valuable natural indicator of "system disturbances" in karst aquifers.

Estimating groundwater recharge for Hanoi, Vietnam.

Determining groundwater recharge is in particular in areas with Monsoon rainfall a challenge. Several methods were used to estimate groundwater recharge for the first time ever for the urban area of Hanoi city, Vietnam. Water table fluctuation method (WTF), hydrograph analyses including recession curve displacement, graphical separation, the Automated Web GIS-Base Hydrology Analysis Tool (WHAT), and empirical formulas were utilized. The mean recharge is approximately 340 mm/year accounting for 20% of precipitation in the period 1996 to 2009 with a certain decrease during this period. Baseflow separation methods show the highest values, whereas the displacement recession curve obtains the lowest recharge. The WTF methods approximate values are close to the mean value of all other proxies. The differences between all proxies are around 10%, and Spearman correlation is statistically significant. This indicates that these methods can be used to estimate recharge for this area. With long-term data, however, the results of WTF appear more consistent and more reasonable than the other approaches.

Statistical Models to Predict and Assess Spatial and Temporal Low-Flow Variability in New England Rivers and Streams.

In the northern hemisphere, summer low flows are a key attribute defining both quantity and quality of aquatic habitat. I developed one set of models for New England streams/rivers predicting July/August median flows averaged across 1985 to 2015 as a function of weather, slope, % imperviousness, watershed storage, glacial geology and soils. These models performed better than most USGS models for summer flows developed at a statewide scale. I developed a second set of models predicting interannual differences in summer flows as a function of differences in air temperature, precipitation, the North Atlantic Oscillation Index (NAO), and lagged NAO. Use of difference equations eliminated the need for transformations and accounted for serial autocorrelations at lag 1. The models were used in sequence to estimate time series for monthly low flows and for two derived flow metrics (tenth percentile (Q10) and minimum 3-in-5 year average flows). The first metric is commonly used in assessing risk to low flow conditions over time while the second has been correlated with increased probability of localized extinctions for brook trout. The flow metrics showed increasing trends across most of New England for 1985-2015. However, application of summer flow models with average and extreme climate projections to the Taunton River, MA, a sensitive watershed undergoing rapid development, projected that low flow metrics will decrease over the next 50 years.

[Seasonal Changes of the Pathways of Nitrogen Export from an Agricultural Watershed in China].

Nonpoint source pollution has become a major factor influencing the water quality. Identifying the pathway of nitrogen (N) transport from the source to the watershed mouth is a critical step in taking measures to control this pollution. However, it is difficult to identify the pathway of N transport because the transport pathway varies among different watersheds depending on the difference in the terrain, hydrology, and land cover etc and changes over time. Additionally, there is little knowledge about the major pathway of N transport through agricultural watersheds in the Yunnan Plateau lake area. The pathways of N export and their temporal variations over time were investigated in this study based on a typical agriculture-dominated watershed in a plateau lake area, Yunnan Province, and two-year monitoring data (June 2011-May 2013) in combination with a base flow separation program. The results show that the base flow accounts for most of the streamflow discharge (80.0%) and N export (71.1%). The proportion of the stream flow discharge via storm flow increases significantly with increasing rainfall. Therefore, the percentage of total N (TN) export via storm flow increases with increasing storm flow, which is closely related to rainfall. The major pathway of N export shifts toward storm flow when the storm flow proportion of the stream flow discharge increases up to 40%. During the monitoring period, the proportion of the TN export via storm flow increases up to 65.6% in the rainy season. This study provides important information for the improvement of the management of nonpoint source pollution at the watershed scale.

The influence of lithology on surface water sources.

Understanding the temporal and spatial variability of water sources within a basin is vital to our ability to interpret hydrologic controls on biogeochemical processes and to manage water resources. Water stable isotopes can be used as a tool to determine geographic and seasonal sources of water at the basin scale. Previous studies in the Coastal Range of Oregon reported that the variation in the isotopic signatures of surface water did not conform to the commonly observed "elevation effect", which exhibits a trend of increasing isotopic depletion with rising elevation. The primary purpose of this research is to investigate the mechanisms governing seasonal and spatial variations in the isotopic signature of surface waters within the Marys River Basin, located in the leeward side of the Oregon Coastal Range. Surface water and precipitation samples were collected every 2-3 weeks for isotopic analysis for one year. Our results confirmed the lack of elevational variation of surface water isotopes within this leeward basin. While we find elevational variation in precipitation in the eastern portion of the watershed, this elevation effect is counteracted by rainout with distance from the Pacific coast. In addition we found significant variation in surface water isotope values between catchments underlain predominantly by basalt or sandstone. The degree of separation was strongest during the summer when low flows reflect deeper groundwater sources. This indicates that baseflow within streams drained by each lithology is being supplied from two distinctly separate water sources. In addition, the flow of the Marys River is dominated by water originating from the sandstone water source, particularly during the low flow summer months. We interpreted that the difference in water source results from sandstone catchments having highly fractured geology or locally tipping to the east facilitating cross-basin water exchange from the windward to the leeward side of the Coast Range. Our results challenge topographic derived watershed boundaries in permeable sedimentary rocks; highlighting the overwhelming importance of underlying geology.

Nuclear accident-derived (3)H in river water of Fukushima Prefecture during 2011-2014.

During 2011-2014, we measured (3)H concentrations in river water samples collected during base flow conditions and during several flood events from two small rivers in a mountainous area in Fukushima Prefecture, which received deposition of (137)Cs from the Fukushima Dai-ichi Nuclear Power Plant accident. (3)H concentrations above background levels were found in water samples collected during both base flow conditions and flood events in 2011. The (3)H concentrations during flood events were generally higher than those during base flow conditions. The (3)H concentrations in both rivers during base flow conditions and flood events decreased with time after the accident and reached almost background levels in 2013. We also measured (3)H concentrations in freshwater samples from 16 other rivers and one dam in eastern Fukushima Prefecture from 2012 to 2014 during base flow conditions. The measured (3)H concentrations were higher than the background level in 2012 and decreased with time. The (137)Cs inventory in the catchment area at each sampling point was estimated from air-borne monitoring results in the literature and compared with the (3)H concentrations. We found surprisingly good correlations between (137)Cs inventories in the catchment areas and (3)H concentrations in the water samples. Further studies will be necessary to clarify the reason for the good correlation.

Effects of radiocesium inventory on (137)Cs concentrations in river waters of Fukushima, Japan, under base-flow conditions.

To investigate the behavior of nuclear accident-derived (137)Cs in river water under base-flow conditions, concentrations of dissolved and particulate (137)Cs were measured at 16 sampling points in seven rivers of Fukushima Prefecture, Japan, in 2012 and 2013. The concentration of dissolved (137)Cs was significantly correlated with the mean (137)Cs inventory in the catchment area above each sampling point in both sampling years. These results suggest that the concentration of dissolved (137)Cs under base-flow conditions is primarily determined by the (137)Cs inventory of the catchment area above the sampling point. However, the concentration of particulate (137)Cs did not show a clear relationship with either the mean (137)Cs inventory or the dissolved (137)Cs concentration, thus indicating that particulate and dissolved forms do not effectively interact in rivers. To evaluate the contribution of the (137)Cs inventory within catchment areas, we analyzed relations between the (137)Cs concentration and the mean (137)Cs inventory over the area within certain flow path lengths that were traced along the river and slope above the sampling point. Coefficients of determination for dissolved (137)Cs concentrations were highest for the longest flow path, i.e., the whole catchment area, and lower for shorter flow paths. Coefficients of determination for particulate (137)Cs concentrations were only moderately high for the shortest flow path in 2012, whereas the values were quite low for all flow paths in 2013. These results suggest that dissolved (137)Cs can originate from a larger area of the catchment even under base-flow conditions; however, particulate (137)Cs did not show such behavior. The results also show that under base-flow conditions, dissolved and particulate (137)Cs behave independently during their transport from river catchments to the ocean.