Xiahong Feng

Fractal stream chemistry and its implications for contaminant transport in catchments

The time it takes for rainfall to travel through a catchment and reach the stream is a fundamental hydraulic parameter that controls the retention of soluble contaminants and thus the downstream consequences of pollution episodes. Catchments with short flushing times will deliver brief, intense contaminant pulses to downstream waters, whereas catchments with longer flushing times will deliver less intense but more sustained contaminant fluxes. Here we analyse detailed time series of chloride, a natural tracer, in both rainfall and runoff from headwater catchments at Plynlimon, Wales. We show that, although the chloride concentrations in rainfall have a white noise spectrum, the chloride concentrations in streamflow exhibit fractal 1/f scaling over three orders of magnitude. The fractal fluctuations in tracer concentrations indicate that these catchments do not have characteristic flushing times. Instead, their travel times follow an approximate power-law distribution implying that they will retain a long chemical memory of past inputs. Contaminants will initially be flushed rapidly, but then low-level contamination will be delivered to streams for a surprisingly long time.

Catchment-scale advection and dispersion as a mechanism for fractal scaling in stream tracer concentrations

Time series of chemical tracers in rainfall and streamflow can be used to probe the internal workings of catchments. We have recently proposed that catchments act as fractal filters for inert chemical tracers like chloride, converting ‘white noise’ rainfall chemistry inputs into fractal ‘’ chemical time series in runoff [Nature 403 (2000) 524]. This implies that catchments have long-tailed travel-time distributions, and thus retain soluble contaminants for unexpectedly long timespans. Here we show that these long-tailed travel-time distributions, and the fractal tracer time series that they imply, can be generated by advection and dispersion of spatially distributed rainfall inputs as they travel toward a channel. Tracer pulses that land close to the stream reach it promptly, with relatively little dispersion. Tracer pulses that land farther upslope must travel farther to reach the stream, and undergo more dispersion. The tracer signal in the stream will be the integral of the contributions from each point along the length of the hillslope, with a peak at short lag times (reflecting tracers landing near the stream) and a long tail (reflecting tracers landing farther from the stream). Here we integrate the advection–dispersion equation for rainfall tracers landing at all points on a simple model hillslope, and show that it yields fractal tracer behavior, as well as a travel-time distribution nearly equivalent to that found empirically [Nature 403 (2000) 524]. However, it does so only when the dispersion length scale approaches the length of the hillslope, implying that subsurface transport is dominated by large conductivity contrasts related to macropores, fracture networks, and similar large-scale heterogeneities in subsurface conductivity. Thus, the 1/f scaling observed at our study sites indicates that these catchments are dominated by flowpaths that exhibit macro-dispersion over the longest possible length scales.

Isotopic evolution of a seasonal snowpack and its melt

The study of isotopic variation in snowmelt from seasonal snowpacks is useful for understanding snowmelt processes and is important for accurate hydrograph separation of spring runoff. However, the complex and variable nature of processes within a snowpack has precluded a quantitative link between the isotopic composition of the original snow and its melt. This work studies the isotopic composition of new snow and its modification by snow metamorphism and melting. To distinguish individual snowstorms, we applied solutions of rare earth elements to the snow surface between storms. The snowmelt was isotopically less variable than the snowpack, which in turn was less variable than the new snow, reflecting isotopic redistribution during metamorphism and melting. The snowmelt had low d 18 O values early in the season and became progressively enriched in 18 O as the pack continued to melt. On a given day, meltwater d 18 O was systematically lower whenever melt rates were low than when melt rates were high. The progressive enrichment in d 18 O of snowmelt and the dependence of d 18 O on melt rates can be explained by isotopic exchange between liquid water and ice. A one-dimensional (1-D) model of the melting process, including advection and water-ice isotopic exchange kinetics, reproduces the observed progressive 18 O enrichment of snowmelt.

TRENDS IN INTRINSIC WATER-USE EFFICIENCY OF NATURAL TREES FOR THE PAST 100-200 YEARS : A RESPONSE TO ATMOSPHERIC CO2 CONCENTRATION

To evaluate how the land carbon reservoir has been acting as a sink to the anthropogenic CO{sub 2} input to the atmosphere, it is important to study how plants in natural forests physiologically adjust to the changing atmospheric conditions. This has been studied intensively using controlled experiments, but it has been difficult to scale short-term observations to long-term ecosystem-level response. This paper derives variations of plant intrinsic water-use efficiency from natural trees for the past 100--200 years using carbon isotope chronologies. This parameter may potentially cause an increase in plant growth rate by improving the efficiency of plant water use, especially in arid environments. Attempts were made to isolate the variations of intrinsic water-use efficiency as a function of only the CO{sub 2} concentration of the atmosphere. The intrinsic water-use efficiency of almost all trees increased with increasing atmospheric CO{sub 2} concentration. This is caused by an increase in the carbon assimilation rate (A) and/or a decrease in the stomatal conductance (g). The increase in plant intrinsic water-use efficiency may imply an increase in plant transpiration efficiency which may have a direct connection with changes in plant biomass.

The effect of soil hydrology on the oxygen and hydrogen isotopic compositions of plants’ source water

Abstract Many studies have demonstrated that the isotopic composition of plants’ source water is the main factor affecting the isotopic composition of tree rings. Because of soil hydrological processes, soil water as the source water for plants may isotopically differ from precipitation that contains climatic information (such as surface temperature). This study addresses the effects of soil hydrological processes on the isotopic compositions of soil water and discusses how these effects affect interpretations of tree ring data in isotopic dendroclimatology. We collected precipitation, soil gas at two depths (20 and 50 cm), and twigs from a maple tree ( Acer saccharum ) on a biweekly basis during the growing season in 1997–1999 at Hanover, NH, USA. Water was extracted from the twig samples by vacuum distillation. All water samples were analyzed for both δD and δ 18 O. Soil CO 2 was extracted from soil gas and measured for the δ 18 O values, and using the soil temperature and assuming isotopic equilibrium between CO 2 and H 2 O, we calculated the δ 18 O values of soil water. Comparisons among the isotopic time series of each type of sample indicate the following. (1) The isotopic composition of soil water is much less variable than that of precipitation, suggesting isotopic mixing between waters of different precipitation events. (2) In early spring, soil water at all depths is isotopically similar to winter precipitation, but with time the surface soil water becomes progressively enriched in deuterium and 18 O due to infiltration of summer rain and enrichment through soil water evaporation. (3) The influence of summer precipitation decreases with increasing depth, and soil at 50 cm can only receive water from large storms. (4) Replacement of old soil water with new infiltrating water is dependent upon frequency and intensity of growing season precipitation, and it is generally more efficient in a wet year than in a dry year. (5) The tree we studied uses water mainly from near-surface soil layers. (6) The δD–δ 18 O relationship in twig water indicates that soil water has experienced isotopic enrichment by evaporation. These results have important implications for selecting sites for paleoclimatic studies using isotopic data of tree rings.

A study of solute transport mechanisms using rare earth element tracers and artificial rainstorms on snow

Rare earth element (REE) tracers and three artificial rain-on-snow storms at the Central Sierra Snow Laboratory, California indicate that (1) tracers applied to the snow surface immediately prior to the storm quickly appear at the bottom of the pack, with the tracer traversing the pack faster when the snowpack is wetter; (2) unlike most previous studies in which low solute concentrations were observed at high flow in diurnal cycles, the concentrations of the REE tracers in the outflow are positively related with input water flux; and (3) at a constant input flux the concentrations of all the REE tracers decreased exponentially with time, and the rate of this decrease was greater at high flow than at low flow. These observations can be qualitatively simulated by partitioning liquid water in the snowpack into mobile and immobile phases. Transport of the mobile water phase is governed by the advection-dispersion equations, while the immobile water only moves by exchanging with the mobile water. The rate of exchange between mobile and immobile waters follows first-order kinetics. Unlike previous mobile-immobile models for snow, the exchange rate coefficient is assumed to increase exponentially with the effective water saturation. The model successfully simulates the positive concentration dependency on input water flux. However, it remains unclear how the exchange rate coefficient varies with the nature of the medium and with hydrological conditions. These observations suggest that tracer concentrations in the outflow are largely dominated by solute transport via fast flow channels. This surprising result implies that a spatially averaged flow rate may not be adequate for modeling solute transport properties in unsaturated media.

The variations in δD of tree rings and the implications for climatic reconstruction

Abstract The δD values of cellulose nitrate of tree rings (δDcn) have been used to reconstruct the climate records, because trees obtain hydrogen from meteorologically derived water that contains information on atmospheric conditions (e.g., temperature). However, many hydrological and biological factors may affect how hydrogen isotopic characteristics of meteoric water are recorded in tree rings as hydrogen from precipitation is transferred into cellulose. In addition, these factors may cause random isotopic variations within a single site or between different species. In this study, the δDcn was analyzed for Douglas-fir (Pseudotsuga menziesii) and subalpine fir (Abies lasiocarpa) trees from five sites within the Olympic Mountains located in the northwest of Washington State, USA. These sites are distributed on both west and east sides of the mountains, which differ in climatic regimes (e.g., dry and wet) and non-climatic conditions (e.g., hydrology, topography and soil permeability). We found that within-site variations of δDcn differ from one site to another, depending upon the degree of hydrological homogeneity. A relatively flat and wet site has smaller isotopic variability than a dry, rocky, and hilly site. For limited observations, the variation between Douglas-fir and subalpine fir is no greater than the within-species variation of Douglas-fir. A one-to-one relationship between the δDcn and the δD of source water (δDsw) exists among sites. Most of our δDcn time series significantly correlate with annual mean temperature, but temperature only explains up to 26% of the total variance in δDcn. The sensitivity of the δDcn response to temperature ranges from 4.7 to 13.4‰/°C, which is typically higher and more variable compared to the isotopic response of precipitation to temperature (4.0–5.2‰/°C). We argue that the more sensitive response of δDcn to temperature is caused by inter-annual covariation of humidity with temperature.

Influence of sea ice on Arctic precipitation

Significance There has been a growing consensus that a decrease in sea ice would cause an increase in Arctic precipitation because of the potential for increased local evaporation. We quantify the effect of sea ice on the percentage of moisture sourced from the Arctic, using measurements of the isotopic composition of precipitation at six sites across the Arctic. These moisture proportion changes are important in that they indicate systematic adjustment and/or reorganization of the global hydrological cycle with climate change and provide validation for climate models. We explore how much these changes may increase Arctic precipitation and its impact on the energy balance. Global climate is influenced by the Arctic hydrologic cycle, which is, in part, regulated by sea ice through its control on evaporation and precipitation. However, the quantitative link between precipitation and sea ice extent is poorly constrained. Here we present observational evidence for the response of precipitation to sea ice reduction and assess the sensitivity of the response. Changes in the proportion of moisture sourced from the Arctic with sea ice change in the Canadian Arctic and Greenland Sea regions over the past two decades are inferred from annually averaged deuterium excess (d-excess) measurements from six sites. Other influences on the Arctic hydrologic cycle, such as the strength of meridional transport, are assessed using the North Atlantic Oscillation index. We find that the independent, direct effect of sea ice on the increase of the percentage of Arctic sourced moisture (or Arctic moisture proportion, AMP) is 18.2 ± 4.6% and 10.8 ± 3.6%/100,000 km2 sea ice lost for each region, respectively, corresponding to increases of 10.9 ± 2.8% and 2.7 ± 1.1%/1 °C of warming in the vapor source regions. The moisture source changes likely result in increases of precipitation and changes in energy balance, creating significant uncertainty for climate predictions.

The Changes in North American atmospheric circulation patterns indicated by wood cellulose

General circulation model simulations suggest that during the Last Glacial Maximum, the northern circumpolar vortex intensifi ed and enlarged, a glacial anticyclone developed over the Laurentide Ice Sheet, and the position of the jet stream was shifted southward. However, observations directly related to shifts in wind patterns across the North American continent have not yet been reported. We examined tree-ring cellulose from the Holocene and the last glacial period for: (1) covariation between precipitation δ 18 O (and δD) and relative humidity, and (2) variation of cellulose δ 18 O and δD with longitude. Holocene isotopic features are consistent with modern moisture trajectories. The isotopic features during the last glaciation are dissimilar to those in the Holocene, and constitute direct evidence for an expansion of the polar easterlies to latitudes as low as 40°N. This is the fi rst time that moisture transport patterns have been inferred from covariation between isotopic composition in precipitation and relative humidity, a technique that holds much promise for future studies of atmospheric circulation.

The diel cycle of water vapor in west Greenland

We present a study of the dynamics of small-scale (~100 km) atmospheric circulation in west Greenland which is dominated by interactions of marine and continental air masses. Water vapor concentration and isotopic ratios measured continuously over a 25 day period in Kangerlussuaq, Greenland were used to monitor the convergence of easterly katabatic winds and westerly sea breezes that form a front between the dry, isotopically depleted, glacial air mass and the moist, isotopically enriched, marine air mass. During the latter 16 days of the measurement period, an interval with no large-scale synoptic interference, the inland penetration of the sea breeze controlled the largest day-to-day humidity and vapor isotopic variations. Kangerlussuaq experienced sea breezes in the afternoon on 9 days, consistent with the long-term average of such occurrences on 56% of days in July and August. The inland position of the sea breeze front is controlled by the katabatic wind strength, which is stronger during times of reduced cloud coverage and/or higher-pressure gradient between the coast and the Greenland ice sheet. The position and movement of the front will likely respond to changes in the general atmospheric circulation and regional radiation balance resulting from global warming, which will, in turn, impact the local hydrological cycle and ecosystem processes.