John J. Gibson

Terrestrial water fluxes dominated by transpiration

Renewable fresh water over continents has input from precipitation and losses to the atmosphere through evaporation and transpiration. Global-scale estimates of transpiration from climate models are poorly constrained owing to large uncertainties in stomatal conductance and the lack of catchment-scale measurements required for model calibration, resulting in a range of predictions spanning 20 to 65 per cent of total terrestrial evapotranspiration (14,000 to 41,000 km3 per year) (refs 1, 2, 3, 4, 5). Here we use the distinct isotope effects of transpiration and evaporation to show that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration. On the basis of our analysis of a global data set of large lakes and rivers, we conclude that transpiration recycles 62,000 ± 8,000 km3 of water per year to the atmosphere, using half of all solar energy absorbed by land surfaces in the process. We also calculate CO2 uptake by terrestrial vegetation by connecting transpiration losses to carbon assimilation using water-use efficiency ratios of plants, and show the global gross primary productivity to be 129 ± 32 gigatonnes of carbon per year, which agrees, within the uncertainty, with previous estimates. The dominance of transpiration water fluxes in continental evapotranspiration suggests that, from the point of view of water resource forecasting, climate model development should prioritize improvements in simulations of biological fluxes rather than physical (evaporation) fluxes.

Climate change effects on hydroecology of arctic freshwater ecosystems.

Abstract In general, the arctic freshwater-terrestrial system will warm more rapidly than the global average, particularly during the autumn and winter season. The decline or loss of many cryospheric components and a shift from a nival to an increasingly pluvial system will produce numerous physical effects on freshwater ecosystems. Of particular note will be reductions in the dominance of the spring freshet and changes in the intensity of river-ice breakup. Increased evaporation/evapotranspiration due to longer ice-free seasons, higher air/water temperatures and greater transpiring vegetation along with increase infiltration because of permafrost thaw will decrease surface water levels and coverage. Loss of ice and permafrost, increased water temperatures and vegetation shifts will alter water chemistry, the general result being an increase in lotic and lentic productivity. Changes in ice and water flow/levels will lead to regime-specific increases and decreases in habitat availability/quality across the circumpolar Arctic.

Flow‐weighted values of runoff tracers (δ18O, DOC, Ba, alkalinity) from the six largest Arctic rivers

dissolved organic carbon (DOC), dissolved barium and total alkalinity from the six largest Arctic rivers: the Ob’, Yenisey, Lena, Kolyma, Yukon and Mackenzie. These data, which can be used to trace runoff, are based upon coordinated collections between 2003 and 2006 that were temporally distributed to capture linked seasonal dynamics of river flow and tracer values. Individual samples indicate significant variation in the contributions each river makes to the Arctic Ocean. Use of these new flow-weighted estimates should reduce uncertainties in the analysis of freshwater transport and fate in the upper Arctic Ocean, including the links to North Atlantic thermohaline circulation, as well as regional water mass analysis. Additional improvements should also be possible for assessing the mineralization rate of the globally significant flux of terrigenous DOC contributed to the Arctic Ocean by these major rivers. Citation: Cooper, L. W., J. W. McClelland, R. M. Holmes, P. A. Raymond, J. J. Gibson, C. K. Guay, and B. J. Peterson (2008), Flow-weighted values of runoff tracers (d 18 O, DOC, Ba, alkalinity) from the six largest Arctic rivers, Geophys. Res. Lett., 35, L18606, doi:10.1029/2008GL035007.

Quantitative comparison of lake throughflow, residency, and catchment runoff using stable isotopes: modelling and results from a regional survey of Boreal lakes

An isotope-based approach for water balance assessment is presented and applied to estimate throughflow, residence time and catchment runoff to 70 headwater lakes on the Boreal plain and uplands of northern and north-central Alberta, Canada. The survey reveals a complex hydrologic regime with systematic variability in water balance due to local site characteristics. On average, runoff to lakes in wetland-dominated catchments is found to be significantly higher than runoff to upland-dominated lakes, with generally higher contributions from catchments with low bog/fen ratios. The isotope method, which relies primarily on water sampling and isotopic analysis, can be easily integrated in routine water quality surveys and is shown to be a practical alternative to conventional hydrological modelling for comparative analysis of water balance controls on hydrochemistry and aquatic ecology of lakes, particularly in low-relief wetland-rich terrain.

Short-term evaporation and water budget comparisons in shallow Arctic lakes using non-steady isotope mass balance

Shallow lakes in a continental, low Arctic setting are found to undergo substantial fluctuations in heavy-isotope content during the annual cycle due to extreme seasonality in water balance processes. Progressive isotope enrichment during the icefree period occurs as a consequence of isotope exchange during evaporation under seasonally arid conditions, while enhanced input and flushing by heavy-isotope depleted precipitation and snowmelt during late fall and spring tend to deplete the lakes in heavy isotopes once again. Time-series sampling surveys in a group of nearby lakes, ranging in volume from 41,000 to 2,250,000 m 3 , was carried out for two consecutive ice-free periods, in conjunction with a comprehensive hydrological measurement program in a detailed study lake, to assess models that describe isotopic enrichment in lakes and to test their suitability for comparing evaporation rates and water balance. From a non-steady isotope balance analysis, it is found that isotope enrichment rates in lakes during the ice-free period are determined primarily by evaporation rates and volume of the lakes, and are less sensitive to water balance variations for short time intervals when evaporation is less than 50 mm or so. A basic assessment of best-fit and step-wise models is presented which suggests that the former are useful only for predicting evaporation and water balance during month-long periods with relatively stable atmospheric and hydrologic conditions. A stepwise isotope balance approach is presented which demonstrates how isotope-based estimates of evaporation rates can be applied to effectively compare short-term (weekly) water balance in nearby lakes. Practical applications in the region include water balance assessments to assist in design and maintenance of tailings ponds for gold and diamond mining operations. q 2002 Elsevier Science B.V. All rights reserved.

DEVELOPMENT AND VALIDATION OF AN ISOTOPIC METHOD FOR ESTIMATING LAKE EVAPORATION

A study designed to test the validity of an isotopic method for estimating evaporation was conducted within a small, tundra lake situated in the continental Arctic of Canada. Evaporation was determined using an isotope mass balance approach which accounted quantitatively for isotopic fractionation, progressive evaporative enrichment of δ 18 O and δ 2 H in lake water, and attenuation of enrichment by liquid inputs and atmospheric moisture. Concurrent determinations made using standard mass balance, energy balance, aerodynamic profile and class A pans permitted rigorous comparisons between methods. Results are presented for two summers which had distinct weather conditions and hydrological balances. Overall, the δ 18 O balance was found to be in good agreement with standard methods during both years over time intervals greater than about one week. Owing to a less systematic response of δ 2 H over short time periods, it use is not recommended for quantitative mass balance determinations over time intervals of less than about 50 days in this setting.

Development of a Pan‐Arctic Database for River Chemistry

More than 10% of all continental runoff flows into the Arctic Ocean. This runoff is a dominant feature of the Arctic Ocean with respect to water column structure and circulation. Yet understanding of the chemical characteristics of runoff from the pan-Arctic watershed is surprisingly limited. The Pan- Arctic River Transport of Nutrients, Organic Matter, and Suspended Sediments ( PARTNERS) project was initiated in 2002 to help remedy this deficit, and an extraordinary data set has emerged over the past few years as a result of the effort. This data set is publicly available through the Cooperative Arctic Data and Information Service (CADIS) of the Arctic Observing Network (AON). Details about data access are provided below.

Isotope studies in large river basins: A new global research focus

Rivers are an important linkage in the global hydrological cycle, returning about 35%of continental precipitation to the oceans. Rivers are also the most important source of water for human use. Much of the worlds population lives along large rivers, relying on them for trade, transportation, industry, agriculture, and domestic water supplies. The resulting pressure has led to the extreme regulation of some river systems, and often a degradation of water quantity and quality For sustainable management of water supply agriculture, flood-drought cycles, and ecosystem and human health, there is a basic need for improving the scientific understanding of water cycling processes in river basins, and the ability to detect and predict impacts of climate change and water resources development.

Reconstruction of paleohydrology and paleohumidity from oxygen isotope records in the Bolivian Andes

Cellulose-inferred lake water N 18 O( N 18 Olw) records from Lago Potosi (LP), a seasonally closed lake in a watershed that is not currently glaciated, and Lago Taypi Chaka Kkota (LTCK) [previously reported in Abbott et al., 2000. Quat. Sci. Rev. 19, 1801^1820], an overflowing lake in a glaciated watershed, provide the basis for late Pleistocene and Holocene paleoclimatic reconstruction in the Bolivian Andes. Deconvolution of the histories of changing evaporative isotopic enrichment from source water N 18 O in the lake sediment records is constrained by comparison to the Sajama ice core oxygen isotope profile, whereas local hydrological influence is distinguished from the regional moisture balance history by the response of the different catchments to climate change. Overall, variations in the LP N 18 Olw record appear to be dominantly controlled by evaporative 18 O-enrichment, reflecting shifts in local effective moisture. This record is used to generate a preliminary quantitative reconstruction of summer relative humidity spanning the past 11 500 cal yr on the basis of an isotope-mass balance model. Results indicate that the late Pleistocene was moist with summer relative humidity values estimated at 10^20% greater than present. Increasing aridity developed in the early Holocene with maximum prolonged dryness spanning 7500^6000 cal yr BP at LP, an interval characterized by summer relative humidity values that may have been 20% lower than present. Highly variable but dominantly arid conditions persist in the mid- to late Holocene, with average summer relative humidity values estimated at 15% below present, which then increase to about 10^20% greater than present by 2000 cal yr BP. Slightly more arid conditions characterize the last millennium with summer relative humidity values ranging from 5^10% lower than present. Similar long-term variations are evident in the LTCK N 18 Olw profile, except during the early Holocene when lake water evaporative 18 O-enrichment in response to low relative humidity appears to have been offset by enhanced inflow from 18 O-depleted snowmelt or groundwater from the large catchment. Although some temporal offset is evident, significant correspondence occurs between the isotope-inferred paleohumidity reconstruction and other paleohydrological proxies from the region. These

Forest-tundra water balance signals traced by isotopic enrichment in lakes

Abstract Enrichment of oxygen-18 and deuterium in surface waters is shown to be a useful indicator of water balance variations in remote, permafrost regions of northern Canada where hydroclimate monitoring networks are limited. Necessarily, such indicators must be applied with care as isotopic signals in each lake trace water balance and atmospheric conditions integrated over different time-periods and spatial areas, as determined by lake water-residence time and catchment drainage area. Isotopic enrichment in large lakes is found to be relatively stable in time, and, as interpreted in the context of steady-state models, will yield information more representative of regional climatological-scale processes. In contrast, seasonal isotopic enrichment in shallow lakes, which occurs due to extreme seasonality of the northern climate and short water residence times, may be useful for estimating short-term ‘point’ evaporation rates.