Distribution by Scientific Domains

Kinds of Evapotranspiration

  • actual evapotranspiration
  • potential evapotranspiration
  • total evapotranspiration

  • Selected Abstracts

    Spatiotemporal dynamics of evapotranspiration at the Glacial Ridge prairie restoration in northwestern Minnesota

    Assefa M. Melesse
    Abstract Among the various indicators of success in wetland restoration, hydrology is the most important and relatively easy to monitor. Evapotranspiration (ET) was used to assess the ecohydrologic changes at the Glacial Ridge prairie restoration site in northwestern Minnesota. Twelve Landsat images from 2000,03 for the months of June, July and August were used to study the spatial ET changes. Spatial monthly and seasonal ET were estimated using a surface energy budget technique from Landsat images. Five sub-basins within the study area were delineated to represent the different conditions of the wetlands. Their spatial and temporal ET responses to the restoration activities (native species planting, burning and ditch closures) were studied. Spatial statistics showing mean and standard deviation of monthly ET were computed. Comparisons were made between these watersheds and the preserved sub-watershed of the study area. The average annual ET increases for the five sub-basins were in the range of 9% (2002,03) to 25% (2001,02). Over the study period, ET increased by nearly 50%. After considering the effect of variations in precipitation, wind speed and solar radiation on the resulting ET, the results show that ET increased in recent years as result of the restoration activities. Copyright © 2005 John Wiley & Sons, Ltd. [source]

    Daily streamflow modelling and assessment based on the curve-number technique

    Jin-Yong Choi
    Abstract A cell-based long-term hydrological model (CELTHYM) that can be integrated with a geographical information system (GIS) was developed to predict continuous stream flow from small agricultural watersheds. The CELTHYM uses a cell-by-cell soil moisture balance approach. For surface runoff estimation, the curve number technique considering soil moisture on a daily basis was used, and release rate was used to estimate baseflow. Evapotranspiration was computed using the FAO modified Penman equation that considered land-use-based crop coefficients, soil moisture and the influence of topography on radiation. A rice paddy field water budget model was also adapted for the specific application of the model to East Asia. Model sensitivity analysis was conducted to obtain operational information about the model calibration parameters. The CELTHYM was calibrated and verified with measured runoff data from the WS#1 and WS#3 watersheds of the Seoul National University, Department of Agricultural Engineering, in Hwaseong County, Kyounggi Province, South Korea. The WS#1 watershed is comprised of about 35·4% rice paddy fields and 42·3% forest, whereas the WS#3 watershed is about 85·0% forest and 11·5% rice paddy fields. The CELTHYM was calibrated for the parameter release rate, K, and soil moisture storage coefficient, STC, and results were compared with the measured runoff data for 1986. The validation results for WS#1 considering all daily stream flow were poor with R2, E2 and RMSE having values of 0·40, ,6·63 and 9·69 (mm), respectively, but validation results for days without rainfall were statistically significant (R2 = 0·66). Results for WS#3 showed good agreement with observed data for all days, and R2, E2 and RMSE were 0·92, 0·91 and 2·23 (mm), respectively, suggesting potential for CELTHYM application to other watersheds. The direct runoff and water balance components for watershed WS#1 with significant areas of paddy fields did not perform well, suggesting that additional study of these components is needed. Copyright © 2002 John Wiley & Sons, Ltd. [source]

    A Calibrated, High-Resolution GOES Satellite Solar Insolation Product for a Climatology of Florida Evapotranspiration,

    Simon J. Paech
    Paech, Simon J., John R. Mecikalski, David M. Sumner, Chandra S. Pathak, Quinlong Wu, Shafiqul Islam, and Taiye Sangoyomi, 2009. A Calibrated, High-Resolution GOES Satellite Solar Insolation Product for a Climatology of Florida Evapotranspiration. Journal of the American Water Resources Association (JAWRA) 45(6):1328-1342. Abstract:, Estimates of incoming solar radiation (insolation) from Geostationary Operational Environmental Satellite observations have been produced for the state of Florida over a 10-year period (1995-2004). These insolation estimates were developed into well-calibrated half-hourly and daily integrated solar insolation fields over the state at 2 km resolution, in addition to a 2-week running minimum surface albedo product. Model results of the daily integrated insolation were compared with ground-based pyranometers, and as a result, the entire dataset was calibrated. This calibration was accomplished through a three-step process: (1) comparison with ground-based pyranometer measurements on clear (noncloudy) reference days, (2) correcting for a bias related to cloudiness, and (3) deriving a monthly bias correction factor. Precalibration results indicated good model performance, with a station-averaged model error of 2.2 MJ m,2/day (13%). Calibration reduced errors to 1.7 MJ m,2/day (10%), and also removed temporal-related, seasonal-related, and satellite sensor-related biases. The calibrated insolation dataset will subsequently be used by state of Florida Water Management Districts to produce statewide, 2-km resolution maps of estimated daily reference and potential evapotranspiration for water management-related activities. [source]

    Effects of Land-Use and Land-Cover Change on Evapotranspiration and Water Yield in China During 1900-2000,

    Mingliang Liu
    Abstract:, China has experienced a rapid land-use/cover change (LUCC) during the 20th Century, and this process is expected to continue in the future. How LUCC has affected water resources across China, however, remains uncertain due to the complexity of LUCC-water interactions. In this study, we used an integrated Dynamic Land Ecosystem Model (DLEM) in conjunction with spatial data of LUCC to estimate the LUCC effects on the magnitude, spatial and temporal variations of evapotranspiration (ET), runoff, and water yield across China. Through comparisons of DLEM results with other model simulations, field observations, and river discharge data, we found that DLEM model can adequately catch the spatial and seasonal patterns of hydrological processes. Our simulation results demonstrate that LUCC led to substantial changes in ET, runoff, and water yield in most of the China's river basins during the 20th Century. The temporal and spatial patterns varied significantly across China. The largest change occurred during the second half century when almost all of the river basins had a decreasing trend in ET and an increasing trend in water yield and runoff, in contrast to the inclinations of ET and declinations of water yield in major river basins, such as Pearl river basin, Yangtze river basin, and Yellow river basin during the first half century. The increased water yield and runoff indicated alleviated water deficiency in China in the late 20th Century, but the increased peak flow might make the runoff difficult to be held by reservoirs. The continuously increasing ET and decreasing water yield in Continental river basin, Southwest river basin, and Songhua and Liaohe river basin implied regional water deficiency. Our study in China indicates that deforestation averagely increased ET by 138 mm/year but decreased water yield by the same amount and that reforestation averagely decreased ET by 422 mm/year since most of deforested land was converted to paddy land or irrigated cropland. In China, cropland-related land transformation is the dominant anthropogenic force affecting water resources during the 20th Century. On national average, cropland expansion was estimated to increase ET by 182 mm/year while cropland abandonment decreased ET by 379 mm/year. Our simulation results indicate that urban sprawl generally decreased ET and increased water yield. Cropland managements (fertilization and irrigation) significantly increased ET by 98 mm/year. To better understand LUCC effects on China's water resources, it is needed to take into account the interactions of LUCC with other environmental changes such as climate and atmospheric composition. [source]

    Estimating Evapotranspiration and Seepage for a Sinkhole Wetland From Diurnal Surface-Water Cycles,

    A. Jason Hill
    Abstract:, This study used measured diurnal surface-water cycles to estimate daily evapotranspiration (ET) and seepage for a seasonally flooded sinkhole wetland. Diurnal surface-water cycles were classified into five categories based on the relationship between the surface-water body and the surrounding ground-water system (i.e., recharge/discharge). Only one class of diurnal cycles was found to be suitable for application of this method. This subset of diurnal cycles was used to estimate ET and seepage and the relative importance of each transfer process to the overall water budget. The method has limited utility for wetlands with erratic hydrologic regimes (e.g., wetlands in urban environments). This is due to violation of the critical assumption that the inflow/outflow rate remains constant throughout the day. For application to surface-water systems, the method is typically applied with an assumed specific yield of 1.0. This assumption was found to be invalid for application to surface-water systems with a noncylindrical pond geometry. An overestimation of ET by as much as 60% was found to occur under conditions of low pond stage and high water loss. The results demonstrate the high ET rates that can occur in isolated wetlands due to contrasting roughness and moisture conditions (oasis and clothesline effects). Estimated ET rates ranged from 4.1 to 18.7 mm/day during the growing season. Despite these large ET rates, seepage (recharge) was found to be the dominant water loss mechanism for the wetland. [source]


    Philip J. Gerla
    ABSTRACT: In areas of the Red River Valley that overlie permeable Paleozoic sediments, wetlands and salinization have developed where unregulated flowing wells discharge brackish water. Field data were collected to assess the fate of water and salt from a well 25 km northwest of Grand Forks. Drilled during the drought of the 1930s, discharge was used to replenish water in a small oxbow pond used by livestock. The unregulated well discharges about 56 m3/day, measured since 1993. This discharge exceeds ground water flow from the site, thereby forming a ground water mound with a maximum height of 1 m and a diameter of about 300 m. Most soil and underlying sediments near the well have a hydraulic conductivity of 0.3 m3/day. Flow net analysis suggests that less than 25 percent infiltrates, with the remaining water lost to surface flow and evapotranspiration (ET). Evapotranspiration and slow infiltration has led to increased salinization, with shallow soils exhibiting EC to 500 milliSiemens/m. The most pronounced soil salinization occurs along the margins of the oxbow pond and meander scars. Wetland vegetation with low diversity comprises three zones, with species associations similar to those of closed basin prairie potholes to the west. [source]

    Remote sensing of protected areas to derive baseline vegetation functioning characteristics

    Martín F. Garbulsky
    Abstract: Question: How can we derive baseline/reference situations to evaluate the impact of global change on terrestrial ecosystem functioning? Location: Main biomes (steppes to rain forests) of Argentina. Methods: We used AVHRR/NOAA satellite data to characterize vegetation functioning. We used the seasonal dynamics of the Normalized Difference Vegetation Index (NDVI), a linear estimator of the fraction of the photosynthetic active radiation intercepted by vegetation (fPAR), and the surface temperature (Ts), for the period 1981,1993. We extracted the following indices: NDVI integral (NDVI -I), NDVI relative range (Rrel), NDVI maximum value (Vmax), date of maximum NDVI (Dmax) and actual evapotranspiration. Results: fPAR varied from 2 to 80%, in relation to changes in net primary production (NPP) from 83 to 1700 g.m- 2.yr -1. NDVI -I, Vmax and fPAR had positive, curvilinear relationships to mean annual precipitation (MAP), NPP was linearly related to MAP. Tropical and subtropical biomes had a significantly lower seasonality (Rrel) than temperate ones. Dmax was not correlated with the defined environmental gradients. Evapotranspiration ranged from 100 to 1100 mm.yr -1. Interannual variability of NDVI attributes varied across the temperature and precipitation gradients. Conclusions: Our results may be used to represent baseline conditions in evaluating the impact of land use changes across environmental gradients. The relationships between functional attributes and environmental variables provide a way to extrapolate ecological patterns from protected areas across modified habitats and to generate maps of ecosystem functioning. [source]

    Evapotranspiration covers: An innovative approach to remediate and close contaminated sites

    REMEDIATION, Issue 1 2003
    Kelly L. Madalinski
    A new, cost-effective option for the remediation and final closure of contaminated landfill sites gaining interest among environmental professionals is the evapotranspiration (ET) cover. Unlike current covers that use hydraulic barriers to prevent water from reaching the waste, ET covers rely on the simple concept of using water balance components to meet this goal. Depending on site conditions and other factors, ET covers may be less costly to construct, while still offering performance equivalent to conventional covers. While ET covers are being proposed, tested, or installed at a number of contaminated sites, field performance data remain limited. To address the need for more information, the U.S. Environmental Protection Agency (EPA) has been conducting several efforts to make available more information about this promising alternative solution. This article summarizes these efforts and provides information on the current status of using ET covers at contaminated sites. © 2003 Wiley Periodicals, Inc. [source]

    GIS-based niche models identify environmental correlates sustaining a contact zone between three species of European vipers

    F. Martínez-Freiría
    ABSTRACT The current range of European vipers is mostly parapatric but local-scale allopatric distribution is common and few cases of sympatry are known. In the High Course of Ebro River, northern Spain, there is a contact zone between Vipera aspis, V. latastei, and V. seoanei. Sympatry was detected between aspis and latastei and also specimens with intermediate morphological traits. Presence-data at a local scale (1 × 1 km) and ecological niche-based models manipulated in a GIS were used to (1) identify how environmental factors correlate with the distribution of the three vipers and with the location of the sympatry area, and (2) identify potential areas for viper occurrence and sympatry. Ensemble for casting with 10 Maximum Entropy models identified a mixture of topographical (altitude, slope), climatic (precipitation, evapotranspiration, and minimum and maximum temperature), and habitat factors (land cover) as predictors for viper occurrence. Similar predicted probabilities according to the variation of some environmental factors (indicating probable sympatry) were observed only for aspis-latastei and aspis-seoanei. In fact, areas of probable occurrence of vipers were generally allopatric but probable sympatry between vipers was identified for aspis-latastei in 76 UTM 1 × 1 km squares, for aspis-seoanei in 23 squares, and latastei-seoanei in two squares. Environmental factors correlate with the location of this contact zone by shaping the species range: some enhance spatial exclusion and constrain distribution to spatially non-overlapping ranges, while others allow contact between species. The distribution in the contact zone apparently results from the balance between the pressures exerted by the different environmental factors and in the sympatry area probably by interspecific competition. Further ecological and genetical data are needed to evaluate the dynamics of the probable hybrid zone. GIS and niche-modelling tools proved to be powerful tools to identify environmental factors sustaining the location of contact zones. [source]

    Long-term InHM simulations of hydrologic response and sediment transport for the R-5 catchment

    Christopher S. Heppner
    Abstract The physics-based model known as the Integrated Hydrology Model (InHM) is used to simulate continuous hydrologic response and event-based sediment transport for the R-5 catchment (Oklahoma, USA). For the simulations reported herein the R-5 boundary-value problem was refined, from that reported by Loague et al. (2005), to include (i) an improved conceptualization of the local hydrogeologic setting, (ii) a more accurate topographical representation of the catchment, (iii) improved boundary conditions for surface-water outflow, subsurface-water outflow and evapotranspiration, (iv) improved characterization of surface and subsurface hydraulic parameters and (v) improved initial conditions. The hydrologic-response simulations were conducted in one-year periods, for a total of six years. The sediment-transport simulations were conducted for six selected events. The multi-year water-balance results from the hydrologic-response simulations match the observed aggregate behavior of the catchment. Event hydrographs were generally simulated best for the larger events. Soil-water content was over-estimated during dry periods compared with the observed data. The sediment-transport simulations were more successful in reproducing the total sediment mass than the peak sediment discharge rate. The results from the effort reported here reinforce the contention that comprehensive and detailed datasets are crucial for testing physics-based hydrologic-response models. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    Why do mountains support so many species of birds?

    ECOGRAPHY, Issue 3 2008
    Adriana Ruggiero
    Although topographic complexity is often associated with high bird diversity at broad geographic scales, little is known about the relative contributions of geomorphologic heterogeneity and altitudinal climatic gradients found in mountains. We analysed the birds in the western mountains of the New World to examine the two-fold effect of topography on species richness patterns, using two grains at the intercontinental extent and within temperate and tropical latitudes. Birds were also classified as montane or lowland, based on their overall distributions in the hemisphere. We estimated range in temperature within each cell and the standard deviation in elevation (topographic roughness) based on all pixels within each cell. We used path analysis to test for the independent effects of topographic roughness and temperature range on species richness while controlling for the collinearity between topographic variables. At the intercontinental extent, actual evapotranspiration (AET) was the primary driver of species richness patterns of all species taken together and of lowland species considered separately. In contrast, within-cell temperature gradients strongly influenced the richness of montane species. Regional partitioning of the data also suggested that range in temperature either by itself or acting in combination with AET had the strongest "effect" on montane bird species richness everywhere. Topographic roughness had weaker "effects" on richness variation throughout, although its positive relationship with richness increased slightly in the tropics. We conclude that bird diversity gradients in mountains primarily reflect local climatic gradients. Widespread (lowland) species and narrow-ranged (montane) species respond similarly to changes in the environment, differing only in that the richness of lowland species correlates better with broad-scale climatic effects (AET), whereas mesoscale climatic variation accounts for richness patterns of montane species. Thus, latitudinal and altitudinal gradients in species richness can be explained through similar climatic-based processes, as has long been argued. [source]

    The mid-latitude biodiversity ridge in terrestrial cave fauna

    ECOGRAPHY, Issue 1 2006
    David C. Culver
    The world's obligate cave-dwelling fauna holds considerable promise for biogeographic analysis because it represents a large number of independent evolutionary experiments in isolation in caves and adaptation to subterranean life. We focus on seven north temperate regions of at least 2000 km2, utilizing more than 4300 records of obligate cave-dwelling terrestrial invertebrates. In North America, highest diversity was found in northeast Alabama while in Europe highest diversity was found in Aričge, France, and in southeast Slovenia. Based on these regions as well as more qualitative data from 16 other regions, we hypothesize that a ridge (ca 42°,46° in Europe and 34° in North America) of high biodiversity occurs in temperate areas of high productivity and cave density. This may reflect a strong dependence of cave communities on long term surface productivity (as reflected in actual evapotranspiration), because the subterranean fauna relies almost entirely on resources produced outside caves. This dependence may explain the unique biodiversity pattern of terrestrial cave invertebrates. [source]

    Ecohydrology of a semi-arid forest: partitioning among water balance components and its implications for predicted precipitation changes

    ECOHYDROLOGY, Issue 2 2010
    Naama Raz Yaseef
    Abstract The distribution of precipitation inputs into different hydrological components of water-limited forest ecosystems determines water availability to trees and consequently forest productivity. We constructed a complete hydrological budget of a semi-arid pine forest (285 mm annual precipitation) by directly measuring its main components: precipitation (P), soil water content, evapotranspiration (ET, eddy covariance), tree transpiration (sap flux), soil evaporation (soil chambers), and intercepted precipitation (calculated). Our results indicated that on average for the 4-year study period, ET accounted for 94% of P, varying between 100% when P < 250 mm and 85% when P > 300 mm (with indications for losses to subsurface flow and soil moisture storage in wetter years). Direct measurements of the components of the ET flux demonstrated that both transpiration and soil evaporation were significant in this dry forest (45% and 36% of ET, respectively). Comparison between ecosystem ET (eddy covariance measurements) and the sum of its measured components showed good agreement on annual scales, but up to 30% discrepancies (in both directions) on shorter timescales. The pulsed storm pattern, characteristics of semi-arid climates, was sufficient to maintain the topsoil layer wet during the whole wet season. Only less often and intensive storms resulted in infiltration to the root zone, increasing water availability for uptake by deeper roots. Our results indicate that climate change predictions that link reduced precipitation with increased storm intensity may have a smaller effect on water availability to forest ecosystems than reduced precipitation alone, which could help forests' survival and maintain productivity even under drier conditions. Copyright © 2009 John Wiley & Sons, Ltd. [source]

    Topographic controls on spatial patterns of conifer transpiration and net primary productivity under climate warming in mountain ecosystems

    ECOHYDROLOGY, Issue 4 2009
    C. Tague
    Abstract The response of forests to a warmer climate depends upon the direct impacts of temperature on forest ecophysiology and indirect effects related to a range of biogeophysical processes. In alpine regions, reduced snow accumulation and earlier melt of seasonal snowpacks are expected hydrologic consequences of warming. For forests, this leads to earlier soil moisture recharge, and may increase summer drought stress. At the same time, increased air temperature alters plant net primary productivity. Most models of climate change impacts focus either on hydrologic behaviour or ecosystem structure or function. In this study we address the interactions between them. We use a coupled model of eco-hydrologic processes to estimate changes in evapotranspiration and vegetation productivity under temperature warming scenarios. Results from Yosemite National Park, in the California Sierra Nevada, suggest that for most snow-dominated elevations, the shift in the timing of recharge is likely to lead to declines in productivity and vegetation water use, even with increased water-use efficiency associated with elevated atmospheric CO2 concentrations. The strength of this effect, however, depends upon interactions between several factors that vary substantially across elevation gradients, including the initial timing of melt relative to the summer growing season, vegetation growth, and the extent to which initial vegetation is water-limited or temperature-limited. These climate-driven changes in vegetation water use also have important implications for summer streamflow. Results from this analysis provide a framework that can be used to develop strategic measurement campaigns and to extrapolate from local measurements of vegetation responses to watershed scale patterns. Copyright © 2009 John Wiley & Sons, Ltd. [source]

    Assessing the impact of riparian processes on streambank stability,

    ECOHYDROLOGY, Issue 3 2009
    Eddy J. Langendoen
    Abstract The series of biennial United States (US) National Water Quality Inventory surveys shows no reduction in the percentage of degraded miles of streams since the early 1990s despite an exponential increase in river restoration projects to improve water quality, enhance in-stream habitat and manage the riparian zone. This may suggest that many river restoration projects fail to achieve their objectives. This is partly due to a lack of understanding of the dynamics of the degraded riverine system and its interaction with the riparian zone. These projects could, therefore, benefit from using proven models of stream and riparian processes to guide restoration design and to evaluate indicators of ecological integrity. The US Department of Agriculture has developed two such models: the channel evolution computer model CONCEPTS and the riparian ecosystem model REMM. These models have been integrated to evaluate the impact of edge-of-field and riparian conservation measures on stream morphology and water quality. Vegetative riparian conservation measures are commonly used to stabilize failing streambanks. The shear strength of bank soils is greatly affected by the degree of saturation of the soils and root reinforcement provided by riparian vegetation. The integrated model was used to study the effectiveness of woody and herbaceous riparian buffers in controlling streambank erosion of an incised stream in northern Mississippi. Comparison of model results with observations showed that pore-water pressures are accurately predicted in the upper part of the streambank, away from the groundwater table. Simulated pore-water pressures deviate from those observed lower in the streambank near the phreatic surface. These discrepancies are mainly caused by differences in the simulated location of the phreatic surface and simulated evapotranspiration in case of the woody buffer. The modelling exercise further showed that a coarse rooting system, e.g. as provided by trees, significantly reduced bank erosion rates for this deeply incised stream. Published in 2009 by John Wiley & Sons, Ltd. [source]

    Ecohydrological controls on snowmelt partitioning in mixed-conifer sub-alpine forests

    ECOHYDROLOGY, Issue 2 2009
    Noah P. Molotch
    Abstract We used co-located observations of snow depth, soil temperature, and moisture and energy fluxes to monitor variability in snowmelt infiltration and vegetation water use at mixed-conifer sub-alpine forest sites in the Valles Caldera, New Mexico (3020 m) and on Niwot Ridge, Colorado (3050 m). At both sites, vegetation structure largely controlled the distribution of snow accumulation with 29% greater accumulation in open versus under-canopy locations. Snow ablation rates were diminished by 39% in under-canopy locations, indicating increases in vegetation density act to extend the duration of the snowmelt season. Similarly, differences in climate altered snow-season duration, snowmelt infiltration and evapotranspiration. Commencement of the growing season was coincident with melt-water input to the soil and lagged behind springtime increases in air temperature by 12 days on average, ranging from 2 to 33 days under warmer and colder conditions, respectively. Similarly, the timing of peak soil moisture was highly variable, lagging behind springtime increases in air temperature by 42 and 31 days on average at the Colorado and New Mexico sites, respectively. Latent heat flux and associated evaporative loss to the atmosphere was 28% greater for the year with earlier onset of snowmelt infiltration. Given the large and variable fraction of precipitation that was partitioned into water vapour loss, the combined effects of changes in vegetation structure, climate and associated changes to the timing and magnitude of snowmelt may have large effects on the partitioning of snowmelt into evapotranspiration, surface runoff and ground water recharge. Copyright © 2009 John Wiley & Sons, Ltd. [source]

    Wide-area estimates of saltcedar (Tamarix spp.) evapotranspiration on the lower Colorado River measured by heat balance and remote sensing methods,,

    ECOHYDROLOGY, Issue 1 2009
    Pamela L. Nagler
    Abstract In many places along the lower Colorado River, saltcedar (Tamarix spp) has replaced the native shrubs and trees, including arrowweed, mesquite, cottonwood and willows. Some have advocated that by removing saltcedar, we could save water and create environments more favourable to these native species. To test these assumptions we compared sap flux measurements of water used by native species in contrast to saltcedar, and compared soil salinity, ground water depth and soil moisture across a gradient of 200,1500 m from the river's edge on a floodplain terrace at Cibola National Wildlife Refuge (CNWR). We found that the fraction of land covered (fc) with vegetation in 2005,2007 was similar to that occupied by native vegetation in 1938 using satellite-derived estimates and reprocessed aerial photographs scaled to comparable spatial resolutions (3,4 m). We converted fc to estimates of leaf area index (LAI) through point sampling and destructive analyses (r2 = 0·82). Saltcedar LAI averaged 2·54 with an fc of 0·80, and reached a maximum of 3·7 with an fc of 0·95. The ranges in fc and LAI are similar to those reported for native vegetation elsewhere and from the 1938 photographs over the study site. On-site measurements of water use and soil and aquifer properties confirmed that although saltcedar grows in areas where salinity has increased much better than native shrubs and trees, rates of transpiration are similar. Annual water use over CNWR was about 1·15 m year,1. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    A dam problem: simulated upstream impacts for a Searsville-like watershed

    ECOHYDROLOGY, Issue 4 2008
    Christopher S. Heppner
    Abstract The integrated hydrology model (InHM), a physics-based hydrologic-response model with sediment-transport capabilities, was used to simulate upstream impacts from dam construction/removal for a generalized approximation of the Searsville watershed in Portola Valley, California. Four 10-year simulation scenarios (pre-dam, early dam, current and post-dam) were considered. Each scenario was simulated using the same sequence of synthetically generated rainfall and evapotranspiration. For each scenario the boundary-value problem was constructed based on the available watershed information (e.g. topography, soils, geology, reservoir bathymetry and land use). The results from the simulations are presented in terms of the temporal and spatial characteristics of hydrologic response and sediment transport. The commonalities/differences between the four Searsville-like watershed scenarios are discussed. The effort demonstrates that heuristic physics-based simulation can be a useful tool for the characterization of dam-related impacts at the watershed scale. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Comparison of soil moisture and meteorological controls on pine and spruce transpiration

    ECOHYDROLOGY, Issue 3 2008
    Eric E. Small
    Abstract Transpiration is an important component of the water balance in the high elevation headwaters of semi-arid drainage basins. We compare the importance of soil moisture and meteorological controls on transpiration and quantify how these controls are different at a ponderosa pine site and a spruce site in the Jemez river drainage basin of northern New Mexico, a sub-basin of the Rio Grande. If only soil moisture controls fluctuations in transpiration, then simple hydrologic models focussed only on soil moisture limitations are reasonable for water balance studies. If meteorological controls are also critical, then more complex models are required. We measured volumetric water content in the soil and sap velocity, and assumed that transpiration is proportional to sap velocity. Ponderosa sap velocity varies with root zone soil moisture. Nearly all of the scatter in the ponderosa sap velocity,soil moisture relationship can be predicted using a simple model of potential evapotranspiration (ET), which depends only on measured incident radiation and air temperature. Therefore, simple hydrologic models of ponderosa pine transpiration are warranted. In contrast, spruce sap velocity does not clearly covary with soil moisture. Including variations in potential evapotranspiration does not clarify the relationship between sap velocity and soil moisture. Likewise, variations in radiation, air temperature, and vapour pressure do not explain the observed fluctuations in sap velocity, at least according to the standard models and parameters for meteorological restrictions on transpiration. Both the simple and more complex models commonly used to predict transpiration are not adequate to model the water balance in the spruce forest studied here. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Local hydrologic effects of introducing non-native vegetation in a tropical catchment

    ECOHYDROLOGY, Issue 1 2008
    Maite Guardiola-Claramonte
    Abstract This study investigates the hydrologic implications of land use conversion from native vegetation to rubber (Hevea brasiliensis) in Southeast Asia. The experimental catchment, Nam Ken (69 km2), is located in Xishuangbanna Prefecture (22°N, 101°E), in the south of Yunnan province, in southwestern China. During 2005 and 2006, we collected hourly records of 2 m deep soil moisture profiles in rubber and three native land-covers (tea, secondary forest and grassland), and measured surface radiation above the tea and rubber canopies. Observations show that root water uptake of rubber during the dry season is controlled by day-length, whereas water demand of the native vegetation starts with the arrival of the first monsoon rainfall. The different dynamics of root water uptake in rubber result in distinct depletion of soil moisture in deeper layers. Traditional evapotranspiration and soil moisture models are unable to simulate this specific behaviour. Therefore, a different conceptual model, taking in account vegetation dynamics, is needed to predict hydrologic changes due to land use conversion in the area. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Diel variation in surface and subsurface microbial activity along a gradient of drying in an Australian sand-bed stream

    FRESHWATER BIOLOGY, Issue 10 2003
    Cecile Claret
    Summary 1. Microbes play key roles in nutrient transformation and organic matter mineralisation in the hyporheic zone but their short-term responses to diel variations in discharge and temperature are unknown. Rates of microbial esterase activity were hypothesised to vary vertically and along a gradient of moisture in a drying sand-bed stream where discharge fluctuated daily in response to evapotranspiration. 2. At ,fully saturated', ,moist' and ,dry' locations in three sites along a drying Australian sand-bed stream, microbial activity at three depths (surface, 10 and 30 cm) was assessed using fluorescein diacetate hydrolysis. Samples were collected in mid-summer in the late afternoon and again at dawn to assess diel variation in hydrolytic activity at each site and depth. Data loggers tracked diel variations in temperature at each depth. 3. Hydrolytic activity was up to 10-fold greater in the surface sediments in late afternoon than at dawn in all habitats, and was correlated with surface sediment temperature. Diel differences in activity were not detected at 10 cm, although daily thermal cycles were evident at this depth. Unexpectedly, activity was marginally higher at dawn at 30 cm in all habitats, perhaps reflecting lags in temperature at that depth. 4. Overall, microbial activity declined with depth, strongly correlated with vertical trends in total organic matter and concentrations of dissolved phosphorus. Particulate organic matter, probably buried during a flood 35 days earlier, appeared largely responsible for these vertical trends. On the other hand, there was little evidence for hydrological exchange between much of the hyporheic zone and the surface stream, implying that processes in the subsurface zone of this stream are effectively isolated during baseflow in mid-summer. 5. Diel cycles of wetting and drying in the moist habitats did not enhance esterase activity relative to the dry or fully saturated habitats. Sediment moisture was not correlated with microbial activity, and mats of senescent algae appeared to inhibit water loss from surface sediments in the moist habitat. In this sand-bed stream, local diel fluctuations in water level appear to have less influence on microbial activity and mineralisation of organic matter in the sediments than occasional floods that bury leaf litter and renew many hyporheic zone functions. Subreach-scale processes seem to be the major driving force of microbial processes and nutrient cycling in this sand-bed river. [source]

    C3,C4 composition and prior carbon dioxide treatment regulate the response of grassland carbon and water fluxes to carbon dioxide

    FUNCTIONAL ECOLOGY, Issue 1 2007
    H. W. POLLEY
    Summary 1Plants usually respond to carbon dioxide (CO2) enrichment by increasing photosynthesis and reducing transpiration, but these initial responses to CO2 may not be sustained. 2During May, July and October 2000, we measured the effects of temporarily increasing or decreasing CO2 concentration by 150,200 µmol mol,1 on daytime net ecosystem CO2 exchange (NEE) and water flux (evapotranspiration, ET) of C3,C4 grassland in central Texas, USA that had been exposed for three growing seasons to a CO2 gradient from 200 to 560 µmol mol,1. Grassland grown at subambient CO2 (< 365 µmol mol,1) was exposed for 2 days to an elevated CO2 gradient (> 365 µmol mol,1). Grassland grown at elevated CO2 was exposed for 2 days to a subambient gradient. Our objective was to determine whether growth CO2 affected the amount by which grassland NEE and ET responded to CO2 switching (sensitivity to CO2). 3The NEE per unit of leaf area was greater (16,20%) and ET was smaller (9,20%), on average, at the higher CO2 concentration during CO2 switching in May and July. The amount by which NEE increased at the higher CO2 level was smaller at elevated than subambient growth concentrations on both dates, but relationships between NEE response and growth CO2 were weak. Conversely, the effect of temporary CO2 change on ET did not depend on growth CO2. 4The ratio of NEE at high CO2 to NEE at low CO2 during CO2 change in July increased from 1·0 to 1·26 as the contribution of C3 cover to total cover increased from 26% to 96%. Conversely, in May, temporary CO2 enrichment reduced ET more in C4 - than C3 -dominated grassland. 5For this mesic grassland, sensitivity of NEE and ET to brief change in CO2 depended as much on the C3,C4 composition of vegetation as on physiological adjustments related to prior CO2 exposure. [source]

    The microstratigraphic record of abrupt climate changes in cave sediments of the Western Mediterranean

    Marie-Agnčs Courty
    The purpose of this paper is to illustrate how calcareous sediments from Pleistocene and Holocene rockshelters and open caves of the Western Mediterranean can provide a stratigraphic record of abrupt climate change. The method proposed here is based on microstratigraphic examination of sedimentary sequences using microscopic techniques. The most important processes for characterizing the sensitivity of each cave to climate variables are: (1) the modes and rate of carbonate sediment production, (2) the nature and intensity of the pedogenic processes responsible for the synchronous alteration of carbonate materials (either those derived from the cave walls or those deposited on the ground surface), and (3) the supply of allogenic sediments, particularly by eolian activity. The cave sediment sequences presented record the marked coolings known as Dansgaard-Oeschger stadials and Heinrich events that occurred during the Pleistocene and the Holocene, as demonstrated by the high resolution records from ice and deep sea cores. At Abric Romaně in northeastern Spain, a series of sharp climatic deteriorations of increasing severity is shown to have occurred synchronously with the transition from the Middle to the Upper Paleolithic, with a period of seasonal frost and strong winds at ca. 37,000 yr B.P., tentatively correlated with Heinrich event 4. At Pigeon Cave, Taforalt (northern Morocco), the transition from the Aterian to Ibero-Maurusian/Epipalaeolithic cultures is dated to around 24,000,20,000 yr B.P. and is punctuated by a series of short cold pulses with evidence for seasonal freezing, soil erosion, and minimal evapotranspiration. In El Miron cave in north-central Spain, the exceptional nature of the Younger Dryas cooling produced a marked destabilization of the cave walls and roof. At El Miron, the stratigraphic evidence for sediment removal due to the rapid percolation of snow melt under a degraded soil cover allows us to reconstruct the nature of the negative excursion at ca. 8200 yr B.P. This example also illustrates how climate-controlled pedogenic processes can create a stratigraphic signature which has often been confused with a sedimentary hiatus. We conclude that cave sediments provide a valuable record of Pleistocene and Holocene climate changes. In appropriate contexts, these sequences allow us to examine the ecological stress generated by these unique global events at a local and regional level and improve our understanding of the complex anthropological processes that occurred at the same time. © 2001 John Wiley & Sons, Inc. [source]

    Connecting Atmosphere and Wetland: Energy and Water Vapour Exchange

    Peter M. Lafleur
    Wetlands are ubiquitous over the globe, comprise a vast array of ecosystem types and are of great ecological and social importance. Their functioning is intimately tied to the atmosphere by the energy and mass exchanges that take place across the wetland,atmosphere boundary. This article examines recent research into these exchanges, with an emphasis on the water vapour exchange. Although broad classes of wetland type, such as fen, bog and marsh, can be defined using ecological or hydrologic metrics, distinct difference in energy exchanges between the classes cannot be found. This arises because there are many factors that control the energy exchanges and interplay of these factors is unique to every wetland ecosystem. Wetlands are more similar in their radiation balances than in the partitioning of this energy into conductive and turbulent heat fluxes. This is especially true of evapotranspiration (ET) rates, which vary considerably among and within wetland classes. A global survey of wetland ET studies shows that location has little to do with ET rates and that variation in rates is largely determined by local climate and wetland characteristics. Recent modelling studies suggest that although wetlands occupy a small portion of the global land surface, their water and energy exchanges may be important in regional and global climates. Although the number of studies of wetland,atmosphere interactions has increased in recent years more research is needed. Five key areas of study are identified: (i) the importance of moss covers, (ii) lack of study in tropical systems, (iii) inclusion of wetlands in global climate models, (iv) importance of microforms in wetlands and their scaling to the whole ecosystem, and (v) the paucity of annual ET measurements. [source]

    Greater seed production in elevated CO2 is not accompanied by reduced seed quality in Pinus taeda L.

    Abstract For herbaceous species, elevated CO2 often increases seed production but usually leads to decreased seed quality. However, the effects of increased atmospheric CO2 on tree fecundity remain uncertain, despite the importance of reproduction to the composition of future forests. We determined how seed quantity and quality differed for pine trees grown for 12 years in ambient and elevated (ambient+200 ,L L,1) CO2, at the Duke Forest free-air CO2 enrichment (FACE) site. We also compared annual reproductive effort with yearly measurements of aboveground net primary productivity (ANPP), precipitation (P), potential evapotranspiration (PET) and water availability [precipitation minus potential evapotranspiration (P,PET)] to investigate factors that may drive interannual variation in seed production. The number of mature, viable seeds doubled per unit basal area in high-CO2 plots from 1997 to 2008 (P<0.001), but there was no CO2 effect on mean seed mass, viability, or nutrient content. Interannual variation in seed production was positively related to ANPP, with a similar percentage of ANPP diverted to reproduction across years. Seed production was negatively related to PET (P<0.005) and positively correlated with water availability (P<0.05), but showed no relationship with precipitation (P=0.88). This study adds to the few findings that, unlike herbaceous crops, woody plants may benefit from future atmospheric CO2 by producing larger numbers of seeds without suffering degraded seed quality. Differential reproductive responses between functional groups and species could facilitate woody invasions or lead to changes in forest community composition as CO2 rises. [source]

    Litter decomposition in grasslands of Central North America (US Great Plains)

    Abstract One of the major concerns about global warming is the potential for an increase in decomposition and soil respiration rates, increasing CO2 emissions and creating a positive feedback between global warming and soil respiration. This is particularly important in ecosystems with large belowground biomass, such as grasslands where over 90% of the carbon is allocated belowground. A better understanding of the relative influence of climate and litter quality on litter decomposition is needed to predict these changes accurately in grasslands. The Long-Term Intersite Decomposition Experiment Team (LIDET) dataset was used to evaluate the influence of climatic variables (temperature, precipitation, actual evapotranspiration, and climate decomposition index), and litter quality (lignin content, carbon : nitrogen, and lignin : nitrogen ratios) on leaf and root decomposition in the US Great Plains. Wooden dowels were used to provide a homogeneous litter quality to evaluate the relative importance of above and belowground environments on decomposition. Contrary to expectations, temperature did not explain variation in root and leaf decomposition, whereas precipitation partially explained variation in root decomposition. Percent lignin was the best predictor of leaf and root decomposition. It also explained most variation in root decomposition in models which combined litter quality and climatic variables. Despite the lack of relationship between temperature and root decomposition, temperature could indirectly affect root decomposition through decreased litter quality and increased water deficits. These results suggest that carbon flux from root decomposition in grasslands would increase, as result of increasing temperature, only if precipitation is not limiting. However, where precipitation is limiting, increased temperature would decrease root decomposition, thus likely increasing carbon storage in grasslands. Under homogeneous litter quality, belowground decomposition was faster than aboveground and was best predicted by mean annual precipitation, which also suggests that the high moisture in soil accelerates decomposition belowground. [source]

    Water savings in mature deciduous forest trees under elevated CO2

    GLOBAL CHANGE BIOLOGY, Issue 12 2007
    Abstract Stomatal conductance of plants exposed to elevated CO2 is often reduced. Whether this leads to water savings in tall forest-trees under future CO2 concentrations is largely unknown but could have significant implications for climate and hydrology. We used three different sets of measurements (sap flow, soil moisture and canopy temperature) to quantify potential water savings under elevated CO2 in a ca. 35 m tall, ca. 100 years old mixed deciduous forest. Part of the forest canopy was exposed to 540 ppm CO2 during daylight hours using free air CO2 enrichment (FACE) and the Swiss Canopy Crane (SCC). Across species and a wide range of weather conditions, sap flow was reduced by 14% in trees subjected to elevated CO2, yielding ca. 10% reduction in evapotranspiration. This signal is likely to diminish as atmospheric feedback through reduced moistening of the air comes into play at landscape scale. Vapour pressure deficit (VPD)-sap flow response curves show that the CO2 effect is greatest at low VPD, and that sap flow saturation tends to occur at lower VPD in CO2 -treated trees. Matching stomatal response data, the CO2 effect was largely produced by Carpinus and Fagus, with Quercus contributing little. In line with these findings, soil moisture at 10 cm depth decreased at a slower rate under high-CO2 trees than under control trees during rainless periods, with a reversal of this trend during prolonged drought when CO2 -treated trees take advantage from initial water savings. High-resolution thermal images taken at different heights above the forest canopy did detect reduced water loss through altered energy balance only at <5 m distance (0.44 K leaf warming of CO2 -treated Fagus trees). Short discontinuations of CO2 supply during morning hours had no measurable canopy temperature effects, most likely because the stomatal effects were small compared with the aerodynamic constraints in these dense, broad-leaved canopies. Hence, on a seasonal basis, these data suggest a <10% reduction in water consumption in this type of forest when the atmosphere reaches 540% ppm CO2. [source]

    Climatic controls on the carbon and water balances of a boreal aspen forest, 1994,2003

    Abstract The carbon and water budgets of boreal and temperate broadleaf forests are sensitive to interannual climatic variability and are likely to respond to climate change. This study analyses 9 years of eddy-covariance data from the Boreal Ecosystem Research and Monitoring Sites (BERMS) Southern Old Aspen site in central Saskatchewan, Canada and characterizes the primary climatic controls on evapotranspiration, net ecosystem production (FNEP), gross ecosystem photosynthesis (P) and ecosystem respiration (R). The study period was dominated by two climatic extremes: extreme warm and cool springs, which produced marked contrasts in the canopy duration, and a severe, 3-year drought. Annual FNEP varied among years from 55 to 367 g C m,2 (mean 172, SD 94). Interannual variability in FNEP was controlled primarily by factors that affected the R/P ratio, which varied between 0.74 and 0.96 (mean 0.87, SD 0.06). Canopy duration enhanced P and FNEP with no apparent effect on R. The fraction of annual photosynthetically active radiation (PAR) that was absorbed by the canopy foliage varied from 38% in late leaf-emergence years to 51% in early leaf-emergence years. Photosynthetic light-use efficiency (mean 0.0275, SD 0.026 mol C mol,1 photons) was relatively constant during nondrought years but declined with drought intensity to a minimum of 0.0228 mol C mol,1 photons during the most severe drought year. The impact of drought on FNEP varied with drought intensity. Years of mild-to-moderate drought suppressed R while having little effect on P, so that FNEP was enhanced. Years of severe drought suppressed both R and P, causing either little change or a subtle reduction in FNEP. The analysis produced new insights into the dominance of canopy duration as the most important biophysical control on FNEP. The results suggested a simple conceptual model for annual FNEP in boreal deciduous forests. When water is not limiting, annual P is controlled by canopy duration via its influence on absorbed PAR at constant light-use efficiency. Water stress suppresses P, by reducing light-use efficiency, and R, by limiting growth and/or suppressing microbial respiration. The high photosynthetic light-use efficiency showed this site to be a highly productive boreal deciduous forest, with properties similar to many temperate deciduous forests. [source]

    Ecohydrological impacts of woody-plant encroachment: seasonal patterns of water and carbon dioxide exchange within a semiarid riparian environment

    Abstract Across many dryland regions, historically grass-dominated ecosystems have been encroached upon by woody-plant species. In this paper, we compare ecosystem water and carbon dioxide (CO2) fluxes over a grassland, a grassland,shrubland mosaic, and a fully developed woodland to evaluate potential consequences of woody-plant encroachment on important ecosystem processes. All three sites were located in the riparian corridor of a river in the southwest US. As such, plants in these ecosystems may have access to moisture at the capillary fringe of the near-surface water table. Using fluxes measured by eddy covariance in 2003 we found that ecosystem evapotranspiration (ET) and net ecosystem exchange of carbon dioxide (NEE) increased with increasing woody-plant dominance. Growing season ET totals were 407, 450, and 639 mm in the grassland, shrubland, and woodland, respectively, and in excess of precipitation by 227, 265, and 473 mm. This excess was derived from groundwater, especially during the extremely dry premonsoon period when this was the only source of moisture available to plants. Access to groundwater by the deep-rooted woody plants apparently decouples ecosystem ET from gross ecosystem production (GEP) with respect to precipitation. Compared with grasses, the woody plants were better able to use the stable groundwater source and had an increased net CO2 gain during the dry periods. This enhanced plant activity resulted in substantial accumulation of leaf litter on the soil surface that, during rainy periods, may lead to high microbial respiration rates that offset these photosynthetic fluxes. March,December (primary growing season) totals of NEE were ,63, ,212, and ,233 g C m,2 in the grassland, shrubland, and woodland, respectively. Thus, there was a greater disparity between ecosystem water use and the strength of the CO2 sink as woody plants increased across the encroachment gradient. Despite a higher density of woody plants and a greater plant productivity in the woodland than in the shrubland, the woodland produced a larger respiration response to rainfall that largely offset its higher photosynthetic potential. These data suggest that the capacity for woody plants to exploit water resources in riparian areas results in enhanced carbon sequestration at the expense of increased groundwater use under current climate conditions, but the potential does not scale specifically as a function of woody-plant abundance. These results highlight the important roles of water sources and ecosystem structure on the control of water and carbon balances in dryland areas. [source]

    Impact of land use and land cover change on groundwater recharge and quality in the southwestern US

    GLOBAL CHANGE BIOLOGY, Issue 10 2005
    Bridget R. Scanlon
    Abstract Humans have exerted large-scale changes on the terrestrial biosphere, primarily through agriculture; however, the impacts of such changes on the hydrologic cycle are poorly understood. The purpose of this study was to test the hypothesis that the conversion of natural rangeland ecosystems to agricultural ecosystems impacts the subsurface portion of the hydrologic cycle by changing groundwater recharge and flushing salts to underlying aquifers. The hypothesis was examined through point and areal studies investigating the effects of land use/land cover (LU/LC) changes on groundwater recharge and solute transport in the Amargosa Desert (AD) in Nevada and in the High Plains (HP) in Texas, US. Studies use the fact that matric (pore-water-pressure) potential and environmental-tracer profiles in thick unsaturated zones archive past changes in recharging fluxes. Results show that recharge is related to LU/LC as follows: discharge through evapotranspiration (i.e., no recharge; upward fluxes <0.1 mm yr,1) in natural rangeland ecosystems (low matric potentials; high chloride and nitrate concentrations); moderate-to-high recharge in irrigated agricultural ecosystems (high matric potentials; low-to-moderate chloride and nitrate concentrations) (AD recharge: ,130,640 mm yr,1); and moderate recharge in nonirrigated (dryland) agricultural ecosystems (high matric potentials; low chloride and nitrate concentrations, and increasing groundwater levels) (HP recharge: ,9,32 mm yr,1). Replacement of rangeland with agriculture changed flow directions from upward (discharge) to downward (recharge). Recent replacement of rangeland with irrigated ecosystems was documented through downward displacement of chloride and nitrate fronts. Thick unsaturated zones contain a reservoir of salts that are readily mobilized under increased recharge related to LU/LC changes, potentially degrading groundwater quality. Sustainable land use requires quantitative knowledge of the linkages between ecosystem change, recharge, and groundwater quality. [source]