YanLong Kong , ZhongHe Pang

1. Key Laboratory of Engineering Geomechanics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing,100029, China

2. Graduate School of Chinese Academy of Sciences, Beijing 100029, China

*Correspondence to: YanLong Kong, Institute of Geology and Geophysics, Chinese Academy of Sciences. No. 19, Beituchengxilu, Chaoyang district, Beijing, 100029, China. Tel: 86-10-82998611; Email: kongyanlong917@163.com

Isotope hydrograph separation in alpine catchments:a review

YanLong Kong1,2*, ZhongHe Pang1

1. Key Laboratory of Engineering Geomechanics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing,100029, China

2. Graduate School of Chinese Academy of Sciences, Beijing 100029, China

*Correspondence to: YanLong Kong, Institute of Geology and Geophysics, Chinese Academy of Sciences. No. 19, Beituchengxilu, Chaoyang district, Beijing, 100029, China. Tel: 86-10-82998611; Email: kongyanlong917@163.com

Isotope hydrograph separation (IHS) is a basic tool in applied hydrology. Its application has expanded to surface water and groundwater interaction, and eco-hydrological processes from runoff generation processes. This paper reviews the progress made in IHS for alpine catchments, with emphasis on its significance in reflecting the impact of global change on water resources. Also,the principle of IHS and its uncertainties are explained in detail. The mechanism of variation of stable isotopes in snow-melt water is discussed, and then methods are presented to improve the separation during snow-melt such as volume weighted average method (VWA), current melt-water method (CMW) and runoff-corrected event water approach (RunCE), with their advantages and disadvantages explained. New approaches may extend the applications of IHS, for example, large basin studies combined with GIS, and develop new theories of runoff generation combined with other parameters such as deuterium excess and DOC.

isotope; hydrograph separation; alpine catchments; snow-melt water; uncertainty

1. Introduction

The earth’s climate system has been undergoing a significant change with warming as the main feature over the past century, particularly in the last three decades (Qin,2008). The fourth assessment of the Intergovernmental Panel on Climate Change (IPCC) points out that global surface temperature increased 0.74 °C in the past 100 years.Furthermore, the rate of global warming has accelerated significantly since 1990, and 11 years of the warmest 12 years since 1850 belong to the period from 1995 to 2006. In the next 100 years, global surface temperatures could rise by 1.1-6.4 °C (IPCC, 2007; Liet al., 2009). Glacier/snow is the most sensitive to global change (Watsonet al., 1995), and a temperature rise will inevitably lead to increased volume of glacier/snow-melt water, and further result in the increase of mountainous runoff. Hydrograph separation is used to quantitatively determine the ratio of compositions making up the stream runoff (Quet al., 2006). Therefore, using the method of hydrograph separation one can estimate the volume of snow/ice-melt water runoff in the total stream, and then study the process of runoff generation. The results can be used not only to analyze the precipitation-runoff relation,and identify the recharge source, but also to calculate the confluence runoff which could reflect global change.

Hydrograph separation methods include graphical,time-interval, and filtering methods, and hydrologic and water balance models, in which time-interval separation methods are compiled as programs named HYSEP (Yang,1985; Nathan and Mchamon, 1990; Ronald and Michele,1996; Amold and Allen, 1999; Yanget al., 2003). Thus,HYSEP can estimate base flow automatically, which was widely used. Although all the methods stated above have their advantages, graphical separation methods need complex calculations and treat the stream data too arbitrarily;HYSEP and filtering separation methods can not derive a hydrological basis even though they can avoid artificial arbitrariness; hydrological and water balance models can not guarantee the reliability and applicability when used in various regions for the model parameters are too difficult to satisfy the accuracy demand. At present, various separation methods are controversial, and the relative theories produced as a result of hydrograph separation can not be validated by experiments (Chenet al., 2006).

Isotopic techniques were first applied to the field of water cycle research in the 1950s, and subsequently have been proven to be effective and convenient methods, and play an important role in the qualitative and quantitative study of groundwater age, groundwater renewability, recharge elevation, and mixing ratio (Wang, 2006). Isotope hydrograph separation (IHS) was first applied in the 1970s to define the base flow component in the total runoff, and since then IHS has provided an irresistible temptation due to its hydrological basis. However, during the 1980s, this method was restricted because the variability of isotopes incorporated was involved in the whole process of the total runoff. From the 1990s to the present, discussions on the uncertainties of IHS flourished and great improvements were made in peer-reviewed papers. Burn (2002) reviewed the development of IHS and pointed out that IHS should be combined with new cutting-edge methods and improved database,which then can be used to study hydrological processes with different climatic and human disturbance regimes. Quet al.(2006) reviewed the study of IHS in China, pointing out the principle of IHS and the importance of sampling in IHS,especially with the current lack of researchers and funding for IHS in China. In recent years, increased attention has been paid to IHS research in alpine catchments. However,discussions on the mechanisms of isotope variability in glacier/snow-melt processes and its implications to IHS are still scarce, which limits the wider application of IHS in alpine catchments. This paper will (1) review the progress of IHS in alpine catchments; (2) discuss the uncertainties of IHS in alpine regions, and the mechanisms of isotope variability in the glacier/snow-melt processes; (3) propose a future direction of IHS.

2. Isotope hydrograph separation (IHS)

Isotope hydrograph separation is used to estimate the ratio of each component contributing to the total runoff based on the differences of isotopes incorporated in each component. The primary purpose of IHS is to determine the ratio of precipitation and base flow contribution quantitatively in a storm event, and further study the process of runoff generation (Fritzet al., 1976; Clark and Fritz, 1999; Gibsonet al.,2005). IHS methods can be applied to divide the total runoff into two-components, three-components or multiple components, and should satisfy the following assumptions(Sklashet al., 1976; Sklash and Farvolden, 1979, 1982): (1)isotopes of the components should be conservative; (2) isotopes incorporated in precipitation should be conservative or its variability can be expressed; (3) significant differences should exist in different components; (4) contributions from soil water can be negligible; (5) storage of the surface water to the total hydrograph can be negligible.

Based on the water balance and isotope equilibrium equations, we can get:

whereQis the runoff, andδis isotopic deviation (the difference between the measured ratios of the sample and reference over the measured ratio of the reference). The subscripts "t", "g" and "p" are total runoff, groundwater and precipitation, respectively. The two components (precipitation and groundwater) can also be expressed as event water(new water) and pre-event water (old water). On the basis of the described equations above, we can estimate the ratios of different sources recharging the runoff during a storm event.Fritzet al. (1976) defined the base flow using IHS and then various applications were conducted. Results show that groundwater contributes a greater amount to the total runoff during the storm event (Sklashet al., 1976; Rodhe, 1984;Turneret al., 1992). Dewalleet al. (1988) used a three-component hydrograph separation (direct precipitation,soil water, groundwater) to estimate the ratio of each component. Later, Hintonet al. (1994) summarized the mechanism of three-component hydrograph separation, and estimated the ratios based on the tracers ofδand SiO2. The equations are written as follows:

whereCis the concentration of SiO2, and the subscript 's'is soil water. The other symbols are the same as stated above. Subsequently, IHS using stable isotopes and conservative chemical parameters (such as Si, TDS, Cl-and EC) flourished. New results indicate that soil water should not be negligible and the choice of hydrochemical parameters should be based on the study of systematic hydrogeochemical analysis; different tracers can be applied in different catchments; and runoff generation mechanism was greatly improved (Turneret al., 1992; Laudon and Slaymaker, 1997; Trceket al., 2006; Blumeet al., 2008;Tammet al., 2008; Wagneret al., 2008). Except for the parameters stated above, DOC and deuterium excess can also be treated as tracers in some catchments (Gibsonet al.,2000, 2005).

In recent years, numerous studies on IHS have focused on parameter uncertainties and their errors (Gu and Xie,1997; Joerinet al., 2002; Tayloret al., 2002; Geyeret al.,2008). Gu and Xie (1997) used a two-component IHS in the Tengqiao catchments in Jiangxi Province, China, and pointed out that the assumption of conservativeδvalues in precipitation and groundwater is not appropriate. Uhlenbrook and Hoeg (2003) studied the variation of stable isotopes in the processes of precipitation and runoff, and concluded that the18O variability was caused by evaporation and adiabatic cooling during the process of precipitation.

Uhlenbrook and Hoeg (2003) pointed out that there are five errors causing the uncertainties of IHS: (1) tracer analysis and discharge measurement; (2) intra-storm variability of18O; (3) elevation effect of18O and silica; (4) solution of minerals during runoff formation; (5) general spatial heterogeneity of tracer concentrations. Gu and Xie (1997) also emphasized that the variability of tracers should be considered when using IHS. Golleret al. (2005) suggested a method to estimate the range of errors: (1) to estimate maximum deviation, the annual mean of theδ18O in rainfall of the entire monitored year could be used instead of theδ18O in the rainfall during the event and (2) to estimate minimum deviation, theδ18O in rainfall for the day after the event could be used.

3. Isotope hydrograph separation in alpine catchments

3.1. Principles

Alpine catchments are defined as the catchments where ice-melt water is one of the recharging sources. The runoff is mainly composed of land flow and groundwater derived from precipitation and ice-melt water, and direct precipitation and ice-melt water. The isotopes of land flow are close to precipitation and ice-melt water due to the recharging relation, while isotopes of groundwater have particular values because recharge in alpine catchments can be interfered by frozen soil.Therefore, runoff composed of precipitation, ice-melt water and groundwater should have its own special hydrograph: the total runoff often reaches a maximum in spring and/or summer. Due to the temperature effect, isotopes of ice-melt water are always depleted before that of precipitation and runoff,while isotopes of groundwater are always similar to the weighted average of the annual precipitation. Thus, there are significant isotopic differences between each component.Thus, the IHS method can be applied in alpine catchments with three components of precipitation, groundwater and ice-melt water. Using isotopes and chemical parameters of ice-melt water instead of those of soil water, the equations can be derived from the three-component model stated above(equations(3),(4)and(5)).

3.2. Applications of IHS in alpine catchments

In recent years, the application of IHS has been widely used in alpine catchments (Hoeget al., 2000; Blumeet al.,2008; Liuet al., 2008; Zhanget al., 2008). Cooperet al.(1991) used18O to identify water sources at Imnavait Creek,Alaska and concluded that the major recharging source of runoff is snow-melt water because18O in the snow-melt water is very close to that in the runoff. Huthet al. (2004)combined stable isotopes18O and2H, and hydrochemical parameters Si and Na to perform IHS for three nested snowmelt-dominated catchments (above 2,620 m.a.s.l.) in Sequoia National Park, USA. IHS was also conducted in a high-accumulation year (annual precipitation, 2.4 m) and an average year (annual precipitation, 1.3 m) using18O and2H to differentiate new (current snowmelt) and old water (water stored in the watershed during the previous year). Then Si and Na were used to estimate the ratio of reactive water,i.e.passing through soil and talus. Results show that old water accounts for 10%-20%, and in the snowmelt, reactive water accounts for 80%-100%.

Gu and Longinelli (1993) carried out IHS using18O as tracers and the water sources were divided as precipitation,snow-melt water, groundwater and ice-melt water at different locations in the Urumqi catchments, Eastern Tianshan,China. Results show that groundwater and snow-melt water are the major sources in Urumqi catchments. Liuet al. (2008)analyzed the convergence zone in Heishui catchment (over 122 km in length), and estimated the contributions of snow-melt water in the base flow. Zhanget al. (2008) calculated the average ratio of snow-melt water upriver of the Heihe catchments in China.

3.3. Progress in uncertainties study

As stated above, uncertainties should be considered when using IHS. In alpine catchments, these uncertainties are much more important due to the interference effect of altitude. Because of the complex topographic features, stable isotopes incorporated in the groundwater may not remain constant during a storm process. Therefore, two or three-component hydrograph separation model should be chosen with caution because of the differences of shallow and deep groundwater.

Another error is the variability of stable isotopes incorporated in the ice-melt water. Also, change of isotopes of snow-melt water along the flow path makes IHS much more complex (Rodhe, 1998). Unnikrishnaet al. (2002) pointed out that in winter when snow-melt water is minimal,18O of snow-melt water is enriched compared to the snow pack;while in summer when much of the snow has melted,18O of snow-melt water is depleted compared to the snow pack,and then is gradually enriched. Tayloret al. (2002) analyzed the variability of stable isotopes during the snow-melt process at four different locations: a warm, maritime snow pack in California; a temperate continental snow pack in Vermont;a cold continental snow pack in Colorado; and an Arctic snow pack in Alaska. Despite these very different climatic conditions,δ18O of melted-water from all four snowpacks increased as melting progressed which is consistent with the theory of isotope fraction (Clark and Fritz, 1999). Heet al.(2006) pointed out that although18O of new snow-melt water, residual snow, glacier and its melted water have few differences, the variability of18O is still complex in the snow/glacier-melted water, and the reasons can be attributed to: (1) the accumulated snow may be derive from different snowfall events; (2) the process of post-deposition including melting, evaporation and wind may change the distribution of18O in the residual snow pack.

Studies on uncertainties of IHS advanced the progress of sampling and error processing. In the majority of previous IHS studies, depth-integrated snow cores were used to define the event component (Rodhe, 1981; Bottomleyet al., 1986; Ingraham and Taylor, 1989), however, such a method did not account forδ18O enrichment that occurred during melting. Subsequent studies began to incorporateδ18O enrichment (Rodhe, 1987; Suekeret al., 2000), and now snow lysimeters are being used for sampling (Hooper and Shoemaker, 1986; Stichleret al., 1986; Shanleyet al.,2002). This not only accounts for the timing ofδ18O enriched water, but also includes rainfall contribution. Nevertheless, errors still exist because the value ofδ18O changes over time and this method can not include the variability (Mastet al., 1995; Laudonet al., 2002; Liuet al., 2008). In order to eliminate the uncertainties, two methods are always used: (1) use a volume of weighted average (VWA) value from the ice-melt water (Mastet al.,1995; Laudonet al., 2002; Liuet al., 2008); (2) use the current ice-melted waterδ18O (CMW) from the snow lysimeter at each time of sampling (Hooper and Shoemaker, 1986; Maule and Stein, 1990; Laudonet al., 2002;Shanleyet al., 2002). VWA can be written as:

whereδ18Oeandδ18Omare theδvalue of event and meltwater isotopic compositions;M(i) is the incrementally collected meltwater depth andtis each time step of hydrograph separation.

Both methods are very simple, but they can not totally eliminate the errors because VWA does not address the within storm/snowmelt variability of the input signal. Also,CMW has a limitation which assumes that a given meltwater increment is only resident in the soil until the next sampling occasion (Laudonet al., 2002). Laudonet al.(2002) presented a new method that accounts for the temporal change in the snowmelt isotopic signal. It can be written as:

whereM(i) is the incrementally collected meltwater depth,E(i) is the incrementally calculated event water discharged andδ18Om(i) andδ18Oe(i) are the isotopic composition of melted-water and event water, respectively. This method calculates theδ18O value of incremental melt water by dividing the calculated melt water at a precise time step from the total melt water. As the result at the next time step is dependent on the result at the last time step, iterative methods should be applied to estimate the ratios of each component. An application at spring time flood in the catchments of Northern Sweden shows that the above mentioned methods underestimated the ratio of pre-event (old) water.

RunCE can calibrate the uncertainties derived from the spatial variability if snow-melt water. In order to obtain a more accurate result, Tiper (1994) used the Gauss formula to study the rule of errors transfer and concluded all the variables should be independent of each other. Genereux (1998)proved that the assumption of independence between each variable was feasible. The complex isotope fraction processes in the snowfall and snow-melting make it very difficult to eliminate IHS errors, so the methods stated above need to be adopted when IHS is applied.

4. Conclusions

4.1. Elimination of uncertainties

There are two serious problems in studies of uncertainties which should be resolved: (1) first is to deal with the uncertainties of various parameters ofIHS as stated above,and then set a model excluding al l the uncertainties from gro undwater, precipitation and ice-melt water; (2) second,find a more significant tracer adjusting to specific regions to apply IHS, such as deuterium excess and DOC.

4.2. Extensive research on IHS

Over several decades, IHS has experienced two-component hydrograph separation (1970-1980s),three-component hydrograph separation (1980-1990s) and uncertainty analysis (1990s-present), and has been applied inmany catchments all over the world. However, IHS should not be limited to these applications. Burns (2002)pointed out that IHS can also be used in catchments with different climatic and human disturbance regimes. IHS can also be used to study the response of water resources to climate change. With the improvement of stable isotope analysis techniques, for example laser techniques, spatial scale can be increased by combining IHS with other new cutting-edge tools like GIS. Artificial tests of IHS can also be conducted to study the flow path through irrigation devices(Waddington and Devito, 2001).

In alpine catchments, the change of melted-water in the total runoff is a direct response of water resources to climate change. Therefore, calculating how much ice-melt water in the total runoff can evaluate the effect of climate change.IHS can also be combined with MRT models to increase the temporal scale, thus it can estimate the average recharging ratio of water sources in a hydrologic year (Harriset al.,1995).

4.3. Runoff generation mechanism

The features of runoff generation are dependent on the runoff generation types in the upland catchments. In order to identify the mechanism, we should analyze the climatic conditions, structures of unsaturated zone, the feature of outlet section, groundwater regime and precipitation-runoff relation (Rui, 2004). Kirchner (2003) pointed out that Horton overland flow, saturation overland flow and pipe flow are the usual assumptions, however, but how do these catchments store water for weeks or months, but then release it in minutes or hours in response to rainfall inputs? These questions are the challenge to the assumptions stated above.On the basis of special terrains, applying IHS in the alpine catchments can be useful in the research of runoff generation mechanism.

This study is supported by the National Natural Science Foundation of China (Grants 40672171), and the Innovation Program ofChinese Academy of Sciences (Grant kzcx2-yw-127). We thank the reviewers for their assistance in evaluating this paper.

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10.3724/SP.J.1226.2011.00086

20 June 2010 Accepted: 19 September 2010