Energy balance and evaporation loss of an irrigation reservoir equipped with a suspended cover in a semiarid climate (south‐eastern Spain)

The main objective of this study was to assess the impact of a suspended cover on the evaporation loss of an agricultural water reservoir (AWR). To this aim, a detailed data collection was carried out in a typical AWR located in south‐eastern Spain during 2 consecutive years. During the first year, the reservoir remained uncovered, while during the second year it was covered with a double black polyethylene (PE) shade cloth. On an annual scale, it was observed that the cover can provide a reduction of evaporation loss of 85%. Two approaches, energy balance and mass transfer, were used to analyse the effect of the cover on the evaporation process. Important modifications were observed on the magnitude, sign, annual trend and relative weight of the components of the energy balance. The changes were ascribed to the strong reduction of net radiation and to the substantial weight of the heat storage and sensible heat flux in the energy balance. A relevant finding was the contrast between the patterns of the annual evaporation curve for open‐water and covered conditions. The mass transfer approach allowed discriminating between the wind‐ and radiation‐shelter effects on the evaporation term. The reduction in water‐to‐air vapour deficit was the main factor explaining the high efficiency of the cover, whereas the reduction of the mass transfer coefficient was a modulating factor that accounted for the wind‐shelter effect. Overall, both approaches provided a sound basis to describe and explain the physical mechanisms underlying the high performance of the tested cover. Copyright © 2010 John Wiley & Sons, Ltd.     

Spatial interpolation of snow water equivalency using surface observations and remotely sensed images of snow‐covered area

As demand for water continues to escalate in the western Unites States, so does the need for accurate monitoring of the snowpack in mountainous areas. In this study, we describe a simple methodology for generating gridded‐estimates of snow water equivalency (SWE) using both surface observations of SWE and remotely sensed estimates of snow‐covered area (SCA). Multiple regression was used to quantify the relationship between physiographic variables (elevation, slope, aspect, clear‐sky solar radiation, etc.) and SWE as measured at a number of sites in a mountainous basin in south‐central Idaho (Big Wood River Basin). The elevation of the snowline, obtained from the SCA estimates, was used to constrain the predicted SWE values. The results from the analysis are encouraging and compare well to those found in previous studies, which often utilized more sophisticated spatial interpolation techniques. Cross‐validation results indicate that the spatial interpolation method produces accurate SWE estimates [mean R² = 0·82, mean mean absolute error (MAE) = 4·34 cm, mean root mean squared error (RMSE) = 5·29 cm]. The basin examined in this study is typical of many mid‐elevation mountainous basins throughout the western United States, in terms of the distribution of topographic variables, as well as the number and characteristics of sites at which the necessary ground data are available. Thus, there is high potential for this methodology to be successfully applied to other mountainous basins. Copyright © 2010 John Wiley & Sons, Ltd.     

Controls on surface water chemistry in two lake‐watersheds in the Adirondack region of New York: differences in nitrogen solute sources and sinks

The southwestern Adirondack region of New York receives among the highest rates of atmospheric nitrogen (N) deposition in the USA. Atmospheric N deposition to sensitive ecosystems, like the Adirondacks, may increase the acidification of soils through losses of exchangeable nutrient cations, and the acidification of surface waters associated with enhanced mobility of nitrate (NO₃^?). However, watershed attributes, including surficial terrestrial characteristics, in‐lake processing, and geological settings, have been found to complicate the relationships between atmospheric N deposition and N drainage losses. We studied two lake‐watersheds in the southwestern Adirondacks, Grass Pond and Constable Pond, which are located in close proximity (～26 km) and receive similarly high N deposition, but have contrasting watershed attributes (e.g. wetland area, geological settings). Since the difference in the influence of N deposition was minimal, we were able to examine both within‐ and between‐watershed influences of land cover, the contribution of glacial till groundwater inputs, and in‐lake processes on surface water chemistry with particular emphasis on N solutes and dissolved organic carbon (DOC). Monthly samples at seven inlets and one outlet of each lake were collected from May to October in 1999 and 2000. The concentrations of NO₃^? were high at the Grass Pond inlets, especially at two inlets, and NO₃^? was the major N solute at the Grass Pond inlets. The concentrations of likely weathering products (i.e. dissolved Si, Ca²⁺, Mg²⁺, Na⁺) as well as acid neutralizing capacity and pH values, were also particularly high at those two Grass Pond inlets, suggesting a large contribution of groundwater inputs. Dissolved organic N (DON) was the major N solute at the Constable Pond inlets. The higher concentrations of DON and DOC at the Constable Pond inlets were attributed to a large wetland area in the watershed. The DOC/DON ratios were also higher at the Constable Pond inlets, possibly due to a larger proportion of coniferous forest area. Although DON and DOC were strongly related, the stronger relationship of the proportion of wetland area with DOC suggests that additional factors regulate DON. The aggregated representation of watershed physical features (i.e. elevation, watershed area, mean topographic index, hypsometric‐analysis index) was not clearly related to the lake N and DOC chemistry. Despite distinctive differences in inlet N chemistry, NO₃^? and DON concentrations at the outlets of the two lakes were similar. The lower DOC/DON ratios at the lake outlets and at the inlets having upstream ponds suggest the importance of N processing and organic N sources within the lakes. Although an inverse relationship between NO₃^? and DOC/DON has been suggested to be indicative of a N deposition gradient, the existence of this relationship for sites that receive similar atmospheric N deposition suggest that the relationship between NO₃^? and the DOC/DON ratio is derived from environmental and physical factors. Our results suggest that, despite similar wet N deposition at the two watershed sites, N solutes entering lakes were strongly affected by hydrology associated with groundwater contribution and the presence of wetlands, whereas N solutes leaving lakes were strongly influenced by in‐lake processing. Copyright © 2006 John Wiley & Sons, Ltd.     

Preliminary spectral characterization of snow in a high altitude tropical glacier and potential effects of impurities in snow on albedo of tropical glaciers

This paper presents some preliminary measurements of snow spectral reflectance on the tropical Bolivian Zongo glacier. Measurements show a correct agreement with theoretical spectral albedo of pure snow in the near infrared region, but lower values in the visible region (by 10–20%) probably due to aerosols contained in snow. Impurity contents ranged from 10 to 100 ppmw in one‐week‐old snow collected from the Zongo glacier, but measurements are scarce. Large amounts of snowfall partly compensate the proximity of dust sources in mid‐latitude glaciers, whereas on outer‐tropical glaciers precipitations are not abundant and are very seasonal, and sources of aerosols are proximate. Copyright © 2007 John Wiley & Sons, Ltd.     

Permafrost‐thaw‐induced land‐cover change in the Canadian subarctic: implications for water resources

Climate warming and human disturbance in north‐western Canada have been accompanied by degradation of permafrost, which introduces considerable uncertainty to the future availability of northern freshwater resources. This study demonstrates the rate and spatial pattern of permafrost loss in a region that typifies the southern boundary of permafrost. Remote‐sensing analysis of a 1·0 km² area indicates that permafrost occupied 0·70 km² in 1947 and decreased with time to 0·43 km² by 2008. Ground‐based measurements demonstrate the importance of horizontal heat flows in thawing discontinuous permafrost, and show that such thaw produces dramatic land‐cover changes that can alter basin runoff production in this region. A major challenge to northern water resources management in the twenty‐first century therefore lies in predicting stream flows dynamically in the context of widely occurring permafrost thaw. The need for appropriate water resource planning, mitigation, and adaptation strategies is explained. Copyright © 2010 John Wiley & Sons, Ltd.     

Impact of the Kuroshio Current on the South China Sea based on a 115 000 year diatom record

We used the tropical oceanic planktonic diatom species Nitzschia marina, Rhizosolenia bergonii and Azpeitia africana/Azpeitia neocrenulata, most commonly found in the surface sediments of the northeasternmost South China Sea (SCS) where the Kuroshio Current enters the SCS through the Bashi Strait north of Luzon, to analyse the influence of the the Kuroshio Current on the SCS. The impact of the Kuroshio Current seems to be relatively strong during major warm periods and, to a lesser degree, in minor warm periods during the last 115 000 years. The strongest influence took place during the main part of the Holocene and during the very late phase of Marine Isotope Stage (MIS) 5e. It is possible to distinguish two magnitudes of change in the impact of the Kuroshio Current on the SCS: large changes occurred at shifts between glacial and interglacial conditions, while smaller changes seem to have recurred in both glacial and interglacial episodes as well as during the Holocene. Climatic/oceanographic changes and sea‐level variations were possibly the two most important mechanisms for the varying influences of the Kuroshio Current on the SCS. The interaction between the Kuroshio Current and monsoon‐related processes may also have played a role. However, because of restricted knowledge of the present‐day Kuroshio Current and the absence of a modern analogue to the ancient SCS due to the marked changes in palaeogeography, this relationship is difficult to establish. Copyright © 2006 John Wiley & Sons, Ltd.     

A theory of pressure sensor performance in snow

A theory of pressure sensor response in snow is derived and used to examine the sources of measurement errors in snow water equivalent (SWE) pressure sensors. Measurement errors in SWE are caused by differences in the compressibility of the pressure sensor and the adjacent snow layer, which produces a shear stress along the perimeter of the sensor. When the temperature at the base of the snow cover equals 0 °C, differences in the snowmelt rate between the snow–SWE sensor interface and the adjacent snow–soil interface may also produce a shear stress along the sensor's perimeter. This shear stress perturbs the pressure field over the sensor, producing SWE measurement errors. Snow creep acts to reduce shear stresses along the SWE sensor's perimeter at a rate that is inversely proportional to the snow viscosity. For sustained periods of differential snowmelt, a difference in the mass of snow over the sensor compared with the surrounding soil will develop, producing additional permanent errors in SWE measurements. The theory indicates that SWE pressure sensor performance can be improved by designing a sensor with a high Young's modulus (low compressibility), low aspect ratio, large diameter and thermal properties that match those of the surrounding soil. Simulations of SWE pressure sensor errors using the theory are in close agreement with observed errors and may provide a means to correct historical SWE measurements for use in hydrological hindcast or climate studies. Published in 2003 by John Wiley & Sons, Ltd.     

Estimation of snow ablation under a dust layer covering a wide range of albedo

Ablation processes of snow under a thin dust cover are complicated compared with those under a thick cover, mainly owing to the effects of aggregation (redistribution) of dust particles on the conditions of surface melting. Aggregation of dust particles causes the snow surface to brighten after the initial dust configuration, thus affecting the relationship between initial dust concentration and surface albedo. In order to estimate snow ablation rate under a thin dust cover, we used a composite energy balance model in which the surface albedo is taken as a measured input variable. The estimated results of snow ablation agreed reasonably well with the observation, considering the measurement errors inherited in the snow depressions. Comparison of the two cases, that is, one considering the aggregation of dust particles (observation: albedo variable) and the other without aggregation (assumption: albedo constant), showed that the ablation rates were noticeably lower on the former case. This suggests that the aggregation of dust particles induces a reduction of snow ablation. Copyright © 2002 John Wiley & Sons, Ltd.     

Norwegian sea‐surface palaeoenvironments of marine oxygen‐isotope stage 3: the paradoxical response of dinoflagellate cysts

High‐resolution marine palynological data have been obtained from two very long sediment cores (MD952009 and MD952010) retrieved from the southern Norwegian Sea. The dinoflagellate cyst assemblages show pronounced fluctuations in composition, which correlate strongly with magnetic susceptibility records and also mimic the δ¹⁸O signal of the GISP2 Greenland ice‐core. If focusing on the period from 48 to 30 cal. kyr BP, this correlation suggests a paradoxical response of the sea‐surface environments to the atmospheric conditions over Greenland: when the Greenland δ¹⁸O signal reflects warm interstadial conditions, the Norwegian Sea depicts cold sea‐surface temperatures with quasi‐perennial sea‐ice cover (based on dinoflagellate cysts). In contrast, when the Greenland δ¹⁸O records cold stadial periods, the Norwegian Sea‐surface temperatures are warm (based on dinoflagellate cysts), probably linked to inflow of the North Atlantic Drift. These results, similar in both cores, are contrary to those of previous studies and shed light on a possible decoupling of Norwegian sea surface‐water conditions and atmospheric conditions over Greenland. This decoupling could be linked to an atmosphere–ocean system behaving similar to that which the Northern Hemisphere is experiencing at present, i.e. strongly variable owing to the North Atlantic Oscillation. Copyright © 2002 John Wiley & Sons, Ltd.