Isotopic Hydrograph Separations Research Articles

Assessment of frequency and areal extent of overland flow generation in a forested mountain catchment

Short time rainfall and throughfall measurements in the period May–October 2009, and the calculated saturated hydraulic conductivities of soils and isotopic hydrograph separations in August 2009 were used to estimate the frequency of the infiltration excess overland flow generation and the extent of saturated areas producing the saturation excess overland flow in the forested mountain catchment of the Jalovecký Creek, Western Tatra Mountains, northern Slovakia. The rainfall intensities exceeding 0.2 mm per 10 min occurred only in 2–4% of all data measured. Saturated hydraulic conductivities (K<sub>S</sub>) of soils were calculated by means of four methods based on the relationship between K<sub>S</sub> and soil texture. The comparison of K<sub>S</sub> with the measured rainfall intensities indicated that the infiltration excess overland flow could have been generated for 0–10% of the rainfall intensities measured. Isotopic hydrograph separation by means of the deuterium isotope indicated that the areas, where the saturated excess overland flow could have occurred, represented about 2–13% of the catchment during the analysed events. Despite the uncertainties connected with the assessments, the results are consistent with empirical knowledge of the catchment and limited older data.  

Runoff generation in small catchments under a native rain forest and a rubber plantation in Xishuangbanna, southwestern China

AbstractFor the purpose of assessing the potential influence of tropical rain forest conversion to rubber plantation on catchment hydrology, a stable oxygen isotope was used as a tracer to identify the event water and prestorm water components of storm runoff during low‐ and high‐flow conditions in a native tropical rain forest catchment (TRFC) and an artificial rubber plantation catchment (ARPC) in SW China. Isotopic hydrograph separations for the storm of April and September 2004 reveal that the event water made up only a small proportion of discharge (29 and 31%, respectively) at peak flow in the TRFC, while it made up the dominant component at peak flow (62 and 69%, respectively) in the ARPC. The analyses suggest that in the ARPC, the universal presence of compacted terraced tracks associated with land management would encourage infiltration‐excess overland flow. This isotopic investigation demonstrates that tropical rain forest conversion to rubber plantation would markedly change the runoff generation processes in this area.

A field‐based study of soil water and groundwater nitrate release in an Adirondack forested watershed

Nitrate (NO3−) movement was studied using a combination of isotopic, chemical, and hydrometric data within the 135 ha Archer Creek watershed in the Adirondack Mountains of New York from January 1995 to December 1996. This research was conducted to identify sources of stream water NO3− and the mechanisms that deliver NO3− to the stream to test two hypotheses: (1) Soil water NO3− concentrations are highest after dry periods and subsequently lower with each storm. (2) Stream water NO3− concentrations are controlled by groundwater during growing season low flows and by soil water during the dormant season and during storms. Antecedent moisture conditions and season had little effect on mean soil water NO3− concentrations before storms (range of 1.1–5.1 μmol L−1 throughout the study). High soil water NO3− concentrations (up to 136 μmol L−1) were found only at the watershed ridge top during the 1996 snowmelt and early summer. Results from isotopic hydrograph separations and chemical end‐member mixing analysis showed that soil water and till groundwater dominated stream base flow and storm flow during six monitored storms. Near‐stream wetland groundwater and event water contributed little to streamflow during most conditions. Near‐stream groundwater contributions to streamflow were significant only during very low base flow (<0.05 mm h−1) during the summer and fall. Highest stream water NO3− concentrations coincided with peaks in the till groundwater contribution according to isotopic hydrograph separations using δ18O and chloride as conservative tracers. A conceptualization of streamflow generation and watershed NO3− release is described in which hillslope hollows are the principal zones of soil water and till groundwater mixing in the watershed and till groundwater is the main source of stream water NO3− during both base flow and storms.

Controls on old and new water contributions to stream flow at some nested catchments in Vermont, USA

AbstractFactors controlling the partitioning of old and new water contributions to stream flow were investigated for three events in four catchments (three of which were nested) at Sleepers River Research Watershed in Danville, Vermont. In the 1993 snowmelt period, two‐component isotopic hydrograph separations showed that new water (meltwater) inputs to the stream ranged widely from 41 to 74%, and increased with catchment size (41 to 11 125 ha) (with one exception) and with open land cover (0–73%). Peak dissolved organic carbon concentrations and relative alkalinity dilution in stream water ranked in the same order among catchments as the new water fractions, suggesting that new water followed shallow flow paths. During the 1994 snowmelt, despite similar timing and magnitude of melt inputs, the new‐water contribution to stream flow ranged only from 30 to 36% in the four catchments. We conclude that the uncommonly high and variable new water fractions in streamwater during the 1993 melt were caused by direct runoff of meltwater over frozen ground, which was prevalent in open land areas during the 1993 winter. In a high‐intensity summer rainstorm in 1993, new water fractions were smaller relative to the 1993 snowmelt, ranging from 28 to 46%, but they ranked in the identical catchment order. Reconciliation of the contrasting patterns of new–old water partitioning in the three events appears to require an explanation that invokes multiple processes and effects, including: topographically controlled increase in surface‐saturated area with increasing catchment size; direct runoff over frozen ground; low infiltration in agriculturally compacted soils; differences in soil transmissivity, which may be more relevant under dry antecedent conditions. These data highlight some of the difficulties faced by catchment hydrologists in formulating a theory of runoff generation at varying basin scales. Copyright © 2002 John Wiley & Sons, Ltd.

Runoff Production in a Forested, Shallow Soil, Canadian Shield Basin

Storm flow in forested basins on the Canadian Shield is largely supplied by subsurface water; however, mechanisms by which this water reaches the stream remain unclear. Side slope contributions to storm flow were studied using throughflow trenches on slopes in a headwater basin near Dorset, Ontario. Discharge, soil water content, and chemical and isotopic signatures of subsurface water were monitored at each site. Four hypotheses were tested: (1) most flow occurs at the soil‐bedrock interface on shield slopes with thin soil; (2) a significant fraction of event water moves vertically to bedrock via preferential flow pathways and laterally over the bedrock surface; (3) relative preevent water contribution to subsurface flow on shield slopes is a function of soil thickness; and (4) a significant portion of event water flux in storm flow from forested basins with shallow soil cover is supplied from side slopes via subsurface flow along the soil‐bedrock interface. Hypothesis 1 was confirmed from hydrometric observations during spring and fall rainstorms. Hypotheses 2 and 3 were supported by temporal trends in dissolved organic carbon and 18O in flow at the soil‐bedrock interface and by isotopic hydrograph separations (IHSs) of hillslope runoff. Comparison with the streamflow IHS indicated that event water flux from the basin in excess of that attributable to direct precipitation onto near‐channel saturated areas could be supplied by flow along the bedrock surface (hypothesis 4). Flow at the soil‐bedrock interface on side slopes also contributed ∼25% of preevent water flux from the basin. Much of the event water component of basin storm flow may travel considerable distances via subsurface routes and is not necessarily contributed by surface runoff processes (Horton flow or saturation overland flow). Therefore the assumption that event water undergoes little interaction with the soil during its passage downslope may be unwarranted here.

Applicability of isotopic hydrograph separation in a suburban basin during snowmelt

AbstractC. H. TAYLORMethodological issues associated with isotopic hydrograph separations (IHSs) in built‐up environments are explored using results from the 1990 spring melt in a suburban basin in Peterborough, Ontario, Canada. The hetrogeneous nature of suburban environments complicates the selection of appropriate isotopic signatures for event and pre‐event waters. Near‐stream groundwater δ18O sampled from wells was poorly mixed, such that the pre‐event water signature was best characterized by δ18O in pre‐melt baseflow or discharge from a headwater spring. The event water signature during snowmelt can be characterized using δ18O in the pre‐melt snowpack, surface runoff samples or meltwater from lysimeters. However, the use of snowpack δ18O may be inappropriate in suburban basins where meltwater from thin snowcover may exhibit pronounced responses to δ18O in rainfall contributions. Intensive sampling of the spatial variability of runoff or meltwater δ18O may be required to characterize the average event water signature adequately. Rainfall δ18O provided an appropriate event water signal during a large rain on snow event, and differences between this IHS and one generated using an event water signature that included meltwater contributions from snow‐covered surfaces were within the uncertainty attributable to the analytical error in δ18O values. Event water supplied 55‐63% of the peak discharge and 48‐58% of total runoff from the basin during the melt, which is consistent with the fraction of the basin that has been developed. These results contrast with IHSs conducted in forested basins that suggest that stormflow is dominated by pre‐event water contributions.

Examining the contributions of glacial till water to storm runoff using two‐ and three‐component hydrograph separations

Two‐ and three‐component hydrograph separations based on 18O and dissolved silica are used to investigate the contributions of glacial till water to the storm runoff of a headwater stream on the Canadian Shield. Two‐component isotopic hydrograph separations based on 18O indicate that the volume and flux of event water could be accounted for by direct precipitation onto saturated areas. Three‐component hydrograph separations distinguish between event water, preevent soil water, and preevent till water. These results show that groundwater flow through coarse‐textured glacial tills can make a significant contribution to stream discharge during runoff events (29 and 62% in this study) despite the lower hydraulic conductivities of the tills compared to the overlying soils. The three‐component hydrograph separations also demonstrate that the relative contributions of preevent soil water and preevent till water changed during one runoff event such that the average water chemistry of the preevent component varied during the event. Two‐component hydrograph separations using dissolved silica indicate that seasonal changes in the till water contributions also occur and are related to groundwater levels. Measurements of vertical hydraulic gradients during runoff events indicate that the increase in flow from the tills to the soils is minimal and cannot account for the large and rapid increase in till water flow into the stream. Till water that has discharged to the soils prior to the event is probably being flushed from the soils into the stream during events.

Isotope hydrograph separations and rapid delivery of pre-event water from drainage basins

Environmental isotopes, such as oxygen-18 and deuterium, have been used increasingly to separate stormflow into its event and pre-event components in order to elucidate the sources, pathways and residence times of water in drainage basins. The majority of isotopic hydrograph separations indicate that pre-event water supplies at least 50% of streamflow at peak discharge in small- and medium-sized basins; however, there is no consensus as to the means by which pre-event water is rapidly exported from drainage basins. The hydrological processes that have been invoked to explain the observed isotopic response of streamflow to rainfall and snowmelt inputs in various environments are reviewed. These processes include groundwater ridging, translatory flow, macropore flow, saturation overland flow, kinematic waves and release of water from surface storage. Tests of the ability of the hypothesized mechanisms to explain the isotopic signature of stormflow from drainage basins will require a more complete integration of hydrometric methods with the use of environmental isotopes than has been achieved previously. Along with various methodological issues associated with the isotopic hydrograph separation technique, the overall relevance of these hydrograph separations to the understanding and prediction of stream hydrochemistry must be evaluated critically.

Groundwater and Wetland Contributions to Stream Acidification: An Isotopic Analysis

Stream water pH may be influenced by (1) the flow paths and (2) the residence time of water that contributes to streamflow, when these hydrologic factors interact with the biogeochemical processes that neutralize H+ ions in the catchment. This paper presents measures of the volumes of groundwater contributing to streamflow, the groundwater residence times, and the sources of stream water acidity found during spring runoff in three basins on the Canadian Shield. Isotopic hydrograph separations were used to estimate the relative contributions of groundwater to spring runoff. The contributions of old (premelt) groundwater to spring runoff were greater (60%) in a well‐buffered, third‐order basin than in a more acidic first‐order basin (49%). Using a simple mixing model, a larger groundwater reservoir (420 mm unit depth) and longer residence time (162 days) were estimated in the third‐order basin. The lowest stream pH (4.8) was observed in a second‐order basin with a wetland that collects drainage from about 79% of the basin. In this basin the principal source of H+ ions was the conifer‐sphagnum wetland. We conclude that the hypotheses that the pH of these streams was proportional to (1) a fraction of streamflow contributed by groundwater or (2) the residence time of water in a basin are rejected. More attention must be focused upon the source of acidity generated in wetlands, since these are ubiquitous in small basins.

Groundwater-Runoff relationships

Summary Stable environmental isotope ( 2 HH, 8 O)O) techniques were applied to an Alpine watershed of 18.7 km 2 and 670–1801 m a.s.l. From this example some insights into the runoff generation in a natural hydrologic system are developed. Isotopic hydrograph separations into direct and indirect runoff fractions, based on these techniques, show that the indirect component contributes dominantly to the total runoff. Contributions of 70–80% are frequent even during peak discharge from rainstorms or snowmelt events. The subsurface reservoir spontaneously responds to an input impetus. This effect has been ascribed to rapid dislocations of the subsurface pressure head. The indirect component describes distinct hydrographs substantially similar to total discharge hydrographs. Further differentiations of the indirect component are restricted to special cases, and they involve increased experimental efforts. The isotope-discharge relationships occasionally give more detailed informations. The most reliable approximations to reality by two-component separations are limited to snowmelt periods with almost homogeneous isotope inputs from the melting snow covers. The long-term isotope studies, that yielded sinusoidal input and distinctly damped output functions, enable us to make in combination with hydrological basic data and specific modeling procedures assessments of the turnover dynamics in the subsurface system. The average contribution of the indirect component is 2/3 of the runoff during the observation period of 3 years. This then corresponds to an annual infiltration rate of 1000 mm. The traditional concept of runoff generation dominated by the surface respectively overland flow should in any case be fundamentally revised.