Model-based estimates of water loss from “patches” of the understory mosaic of the Hartheim Scots pine plantation

Summary During the Hartheim Experiment (HartX) 1992 conducted in the Upper Rhine Valley, Germany, we estimated water vapor flux from the understory and the forest floor by several methods. At the vegetation patch level, direct estimates were made with small weighing lysimeters, and water loss was scaled-up to the stand level based on vegetation patchtype distribution. At the leaf level, transpiration flux was determined with a CO₂/H₂O porometer for the dominant understory plant species,Brachypodium pinnatum, Carex alba, andCarex flacca. Measured leaf transpiration was scaled-up to patch level with a canopy light interception and leaf gas exchange model, and then to stand level as in the case of lysimeter data, but with further consideration of patchtype leaf area index (LAI). On two days, total understory latent heat flux was estimated by eddy correlation methods below the tree canopy.The understory vegetation was subdivided into five major patch-types which covered 62% of the ground area and resulted in a cumulative LAI of approx. 1.54 when averaged over total stand ground area and compared to the average tree canopy LAI of 2.8. The remaining 38% of ground area was unvegetated bare soil and/or covered by moss (mainly byScleropodium purum) or litter. The evapotranspiration from the understory and unvegetated areas equaled approx. 20% of total forest stand transpiration during the HartX period. The understory vegetation transpired about 0.4 mm d^–1 (13%) estimated over the period of May 13 to 21, whereas evaporation from moss and soil patches amounted 0.23 mm d^–1 (7.0%). On dry, sunny days, total water vapor flux below the tree canopy exceeded 0.66 mm d^–1. Using the transpiration rates derived from the GAS-FLUX model together with estimates of evaporation from moss and soil areas and a modified application of the Penman-Monteith equation, the average daily maximum conductance of the understory and the forest floor was 1.7 mm s^–1 as compared to 5.5 mm s^–1 for the tree canopy.With 6 Figures     

A Water Balance Model for a Subarctic Sedge Fen and its Application to Climatic Change

A model to calculate the water balance of a hummocky sedge fen in the northern Hudson Bay Lowland is presented. The model develops the potential latent heat flux (evaporation) as a function of net radiation and atmospheric temperature. It is about equally sensitive to a 2% change in net radiation and a 1°C change in temperature. The modelled potential evaporation agrees well with the Priestley–Taylor formulation of evaporation under conditions of a non-limiting water supply. The actual evaporative heat flux is modelled by expressing actual/potential evaporation as a function of potential accumulated water deficit. Model evaporation agrees well with energy balance calculations using 7 years of measured data including wet and dry extremes. Water deficit is defined as the depth of water below reservoir capacity. Modelled water table changes concur with measurements taken over a 4 year period. When net radiation, temperature and precipitation measurements are available the water balance can be projected to longer time periods. Over a 30 year interval (1965–1994) the water balance of the sedge fen showed the following. During the growing season, there was an increase in precipitation, no change in temperature and a decrease in net radiation, evapotranspiration and water deficit. There was also a decrease in winter snow depths. The fen was brought back to reservoir capacity during final snowmelt every year but one. Summer rainfall was the most important single factor affecting the water balance and the ratio actual/potential evaporation emerged as a linear function of rainfall amount. A 2 × CO₂ climate warming scenario with an annual temperature increase of 4°C and no precipitation change indicates lesser snow amounts and a shorter snow cover period. A greater summer water deficit, triggered mainly by greater evaporation during the month of May, is partially alleviated by lesser evaporation magnitudes in July. The greater water deficit would be counterbalanced by a 23% increase in summer rainfall. On average, the fen's water reservoir would still be recharged after winter snowmelt but the ground would remain at reservoir capacity for a shorter time. The warming scenario with a 10% decline in summer rainfall would create a large increase in the longevity and severity of the water deficit and this would be particularly evident during drier years. The carbon budget and peat accumulation and breakdown rates are strongly affected by changes in the water balance. Some evidence implies that greater water deficits lead to an increase in net carbon emissions. This implies that the sedge peatland could lose biomass under such conditions. An example is given where increased water deficit results in large decreases in local wetland streamflow.     

Energy balance and transpiration from tussock grassland in New Zealand

The energy balance was measured for the dry canopy of narrow-leaved snow tussock (Chionochloa rigida), and measurements of transpiration were obtained from a large weighing lysimeter.Typical maximum summer transpiration rates of 0.21–0.43 mmhr^-1 (140–290 W m^-2) were recorded. The latent heat flux accounted for less than 40% of net radiation. The estimated value of the bulk stomatal resistance (r _ST) for 29 days was 158 s m^-1, and the decoupling parameter () was 0.17. Transpiration rates were not driven directly by net radiation, but were closely linked to the size of the regional saturation deficit imposed at the level of the canopy by efficient overhead mixing, and were constrained by a large bulk stomatal resistance. A linear relationship between r _ST and the saturation deficit is proposed as a realistic method for estimating transpiration for water yield studies of tussock catchments.     

Comparisons of xylem sap flow and water vapour flux at the stand level and derivation of canopy conductance for Scots pine

Summary Simultaneous measurements of xylem sap flow and water vapour flux over a Scots pine (Pinus sylvestris) forest (Hartheim, Germany), were carried out during the Hartheim Experiment (HartX), an intensive observation campaign of the international programme REKLIP. Sap flow was measured every 30 min using both radial constant heating (Granier, 1985) and two types of Cermak sap flowmeters installed on 24 trees selected to cover a wide range of the diameter classes of the stand (min 8 cm; max 17.5 cm). Available energy was high during the observation period (5.5 to 6.9 mm.day^–1), and daily cumulated sap flow on a ground area basis varied between 2.0 and 2.7 mm day^–1 depending on climate conditions. Maximum hourly values of sap flow reached 0.33 mm h^–1, i.e., 230 W m^–2.Comparisons of sap flow with water vapour flux as measured with two OPEC (One Propeller Eddy Correlation, University of Arizona) systems showed a time lag between the two methods, sap flow lagging about 90 min behind vapour flux. After taking into account this time lag in the sap flow data set, a good agreement was found between both methods: sap flow = 0.745^* vapour flux,r ² = 0.86. The difference between the two estimates was due to understory transpiration.Canopy conductance (g _c ) was calculated from sap flow measurements using the reverse form of Penman-Monteith equation and climatic data measured 4 m above the canopy. Variations ofg _c were well correlated (r ² = 0.85) with global radiation (R) and vapour pressure deficit (vpd). The quantitative expression forg _c =f (R, vpd) was very similar to that previously found with maritime pine (Pinus pinaster) in the forest of Les Landes, South Western France.With 6 Figures     

基于碳水通量耦合原理改进Penman-Monteith蒸散发模型

陆地蒸散(ET)涵括地表和潮湿叶片的蒸发和植物的蒸散发,是陆地水循环的重要组成部分。Penman-Monteith方程是估算陆地蒸散的重要方法,方程中的叶片或冠层气孔导度是提高估算精度的关键因子。根据碳水循环的耦合原理,植物光合作用模型可用于估算叶片或冠层气孔导度。植物光合作用模型可分为三类:1)使用总冠层导度的大叶模型(BL),2)区别阴、阳叶冠层导度的双大叶模型(TBL),3)区别阴、阳叶叶片导度的双叶模型(TL)。与这三类光合作用模型相对应,衍生出基于不同导度计算方法的三种蒸散估算模型。三种蒸散模型之间的主要区别在于是否进行从叶片尺度到冠层尺度的气孔导度集成。这三种模型中,双叶模型使用叶片尺度的气孔导度,集成度最低。反之,大叶模型使用冠层尺度的气孔导度,集成度最高。由于在Penman-Monteith中,蒸腾和气孔导度之间的关系是非线性的,气孔导度的集合会导致负偏差。因此,与通量测量相比,大叶蒸散模型的估算偏差最大,而双叶蒸散模型的估算偏差最小。     

Energy Fluxes of an Open Water Area in a mid-Latitude Prairie Wetland

Concurrent measurements of the surface energy balance components (net radiation, heat storage, and sensible and latent heat fluxes) were made in three communities (open water, Phragmites australis, Scirpus acutus) in a wetland in north-central Nebraska, U.S.A., during May-October, 1994. The Bowen ratio – energy balance method was used to calculate latent and sensible heat fluxes. This paper presents results from the open water area. The heat stored in water (G) was found to play a major role in the energy exchange over the water surface. During daytime, G consumed 45–60% of R _n , the net radiation (seasonally averaged daytime G was about 127 W m^–2). At night, G was a significant source of energy (seasonally averaged nighttime G was about -135 Wm^–). The diurnal pattern of latent heat flux ( E) did not follow that of R _n . On some days, E was near zero during midday periods with large R _n . The diurnal variability in E seemed to be significantly affected by temperature inversions formed over the cool water surface. The daily evaporation rate (E) ranged from 2 to 8 mm during the measurement period, and was generally between 70 and 135% of the equilibrium rate.     

Water vapor flux at the sea surface

Methods and instrumentation for determining the rate of evaporation at the sea surface are reviewed. At experimental sites free of local influences, there is a consensus that the evaporation coefficient in neutral conditions C _EN = 1.2 × 10^–3 at low and moderate wind speeds. Combining this with Businger-Dyer flux-gradient formulas, a parameterization scheme is proposed. Evaporation of spray droplets from breaking waves is expected to cause C _EN to increase at high wind speeds, but no direct observations of this are found. Recently it has become possible to estimate water vapor flux in tropical regions from satellite data, opening the possiblity of studying large-scale evaporative events as a function of both time and space.Visiting scholar, Department of Atmospheric Sciences AK-40, University of Washington, Seattle, Washington, 98195, U.S.A. (until June, 1989).     

Methane emissions from a landfill measured by eddy correlation using a fast response diode laser sensor

We describe a fast response methane sensor based on the absorption of radiation generated with a near-infrared InGaAsP diode laser. The sensor uses an open path absorption region 0.5 m long; multiple pass optics provide an optical path of 50 m. High frequency wavelength modulation methods give stable signals with detection sensitivity (S/N=1, 1 Hz bandwidth) for methane of 65 ppb at atmospheric pressure and room temperature. Improvements in the optical stability are expected to lower the current detection limit. We used the new sensor to measure, by eddy correlation, the CH₄ flux from a clay-capped sanitary landfill. Simultaneously we measured the flux of CO₂ and H₂O. From seven half-hourly periods of data collected after a rainstorm on November 23, 1991, the average flux of CH₄ was 17 mmol m^–2 hr^–1 (6400 mg CH₄ m^–2 d^–1) with a coefficient of variation of 25%. This measurement may underrepresent the flux by 15% due to roll-off of the sensor response at high frequency. The landfill was also a source of CO₂ with an average flux of 8.1 mmol m^–2 hr^–1 (8550 mg CO₂ m^–2 d^–1) and a coefficient of variation of 26%. A spectral analysis of the data collected from the CH₄, CO₂, and H₂O sensors showed a strong similarity in the turbulent transfer mechanisms.     

On Transpiration and Soil Moisture Content Sensitivity to Soil Hydrophysical Data

Sensitivity of evapotranspiration E and root zone soil moisture content θ to the parameterization of soil water retention Ψ(θ) and soil water conductivity K(Ψ), as well as to the definition of field capacity soil moisture content, is investigated by comparing Psi1-PMSURF and Theta-PMSURF models. The core of PMSURF (Penman–Monteith Surface Fluxes) consists of a 3-layer soil moisture prediction module based on Richard’s equation in combination with the Penman–Monteith concept for estimating turbulent heat fluxes. Psi1- PMSURF and Theta-PMSURF differ only in the parameterization of the moisture availability function F_ma. In Psi1,F_ma is parameterized by using Ψ(θ) and K(Ψ) hydrophysical functions; in Theta, F_ma is parameterized by using hydrophysical parameters: the field capacity θ_f and wilting point θ_w soil moisture contents. Both Psi1 and Theta are based on using soil hydrophysical data, that is, there is no conceptual difference between them in the parameterization of E even if in Psi1F_ma depends on 12 parameters, while in Theta only on two soil/vegetation parameters. Sensitivity tests are performed using the Cabauw dataset. Three soil datasets are used: the vG (van Genuchten), CH/vG (Clapp and Hornberger/van Genuchten) and CH/PILPS (Clapp and Hornberger/Project for Intercomparison of Land-surface Parameterization Schemes) datasets. The vG dataset is used in van Genuchten’s parameterization, while in Clapp and Hornberger’s the CH/vG and CH/PILPS datasets are used. It is found that the consistency of soil hydrophysical data in the simulation of transpiration is quite important. The annual sum of E obtained by Psi1E^Psi1, differs from the annual sum of E obtained by Theta, E^Theta, because of the inconsistency between the fitting parameters of Ψ(θ) and K(Ψ) and the θ_f, and not because of the differencies in the parameterization of F_ma. Further, θ_f can be estimated not only on the basis of using soil hydrophysical functions (the θ_f so obtained is θ^Soil_f) but also on the basis of analysing the transpiration process (the θ_fso obtained is θ^tr_f). θ^tr_f values estimated from the condition E^Theta ≈ E^Psi1 are in acceptable accordance with the θ^Soil_f values proposed by Wösten and co-workers. The results are useful in optimizing the parameterization of transpiration in land-surface schemes.     

Further studies of the temperature dependence of the rate coefficients for the reactions of OH with a series of ethers under simulated atmospheric conditions

The following temperature-dependent rate coefficients (k/cm³ molecule^–1 s^–1) of the reactions of hydroxyl radicals with aliphatic ethers have been determined over the temperature range 247–373 K by a competitive flow technique: diethyl ether,k _OH=5.2×10^–12 exp[(262±150)/T]; methyln-butyl ether,k _OH=5.4×10^–12 exp[(309±150)/T]; ethyln-butyl ether,k _OH=7.3×10^–12 exp[(335±150)/T]; di-n-butyl ether,k _OH=5.5×10^–12 exp[(502±150)/T] and di-n-pentyl ether,k _OH=8.5×10^–12 exp[(417±150)/T]. The data have been measured relative to the rate coefficientk(OH + 2,3-dimethylbutane)=6.2×10^–12 cm³ molecule^–1 s^–1 independent of temperature.Previous discrepancies in the room-temperature rate coefficients for the OH reactions with ethyln-butyl ether and di-n-butyl ether, obtained in the flow and static experiments of Bennett and Kerr (J. Atmos. Chem. 8, 87–94, 1989;10, 29–38, 1990) compared with those of Wallingtonet al. (Int. J. Chem. Kinet. 20, 541–547, 1988;21, 993–1001, 1989) and of Nelsonet al. (Int. J. Chem. Kinet. 22, 1111–1126, 1990) have been resolved. The results are considered in relation to the available literature data and evaluated rate expressions are deduced where possible. The data are also discussed in terms of structure-activity relationships.     

Net carbon dioxide exchange in canopies of burned and unburned tallgrass prairie

Summary Net carbon dioxide exchange (NCE) rates were measured in a tallgrass prairie, a grassland with high productivity, to determine photosynthetic rates of the canopy. Canopy measurements were made in large, plexiglass chambers (1.21 m long; 0.91 m wide; 1.40 m tall) placed on burned and unburned areas of the prairie. The NCE rates of the canopy were compared with those of individual leaves ofAndropogon gerardii Vitman (big bluestem). In addition, CO₂ flux from the soil was quantified and compared with net photosynthetic flux. The canopy NCE rates were generally lower than those made on individual leaves. In mid-summer (11 July 1987), the maximum canopy NCE rates were 55% and 64% of those measured on individual leaves in burned and unburned treatments, respectively. Canopy NCE rates were lower than individual-leaf NCE rates for two reasons. First, the individualleaf measurements were made on young, unshaded, healthy leaves, while the canopy measurements were made on all types of leaves including senescing, shaded, and damaged leaves. Second, soil CO₂ flux into the chambers lowered NCE values. The CO₂ flux from the soil ranged from 7.2% to 28.4% of the total NCE. One needs to add soil CO₂ flux rates to the measured canopy NCE rates to obtain canopy NCE rates closer to individual-leaf NCE rates. Soil CO₂ flux decreased when conditions became dry, reaching a low of 0.06 mg CO₂m^–2s^–1, but increased after rain to 0.16 mg CO₂m^–2s^–1. Also, after rain, when plants were well watered, they were not light saturated at 1 900 µEm^–2s^–1. The NCE rates on the burned treatment were either higher or similar to those on the unburned treatment. For example, on 11 July 1987, NCE rates were higher on the burned treatment (0.66 mg CO₂m^–2s^–1) compared to the unburned treatment (0.47 mg CO₂m^–2s^–1). During the rest of July and August, the rates of the two treatments were not significantly different. But in September and October, the NCE rates were again higher on the burned treatment compared to the unburned treatment. The results indicated that canopy NCE rates may be more indicative of the productivity of the prairie than individual-leaf measurements made only on young, highly productive leaves.Contribution No. 89-82-J from the Kansas Agricultural Experiment Station. This research was supported, in part, by Grant No. DE-FG02-84ER60253.A000.With 4 Figures     

The HartX-synthesis: An experimental approach to water and carbon exchange of a Scots pine plantation

Summary In May 1992 during the interdisciplinary measurement campaign HartX (Hartheim eXperiment), several independent estimates of stand water vapor flux were compared at a 12-m high Scots pine (Pinus silvestris) plantation on a flat fluvial terrace of the Rhine close to Freiburg, Germany. Weather during the HartX period was characterized by ten consecutive clear days with exceptionally high input of available energy for this time of year and with a slowly shifting diurnal pattern in atmospheric variables like vapor pressure deficit. Methods utilized to quantify components of stand water flux included porometry measurements on understory graminoid leaves and on pine needles and three different techniques for determining individual tree xylem sap flow. Micrometeorological methods included eddy covariance and eddy covariance energy balance techniques with six independent systems on two towers separated by 40 m. Additionally, Bowen ratio energy balance estimates of water flux were conducted and measurements of the gradients in water vapor, CO₂, and trace gases within and above the stand were carried out with an additional, portable 30 m high telescoping mast.Biologically-based estimates of overstory transpiration were obtained by up-scaling tree sap flow rates to stand level via cumulative sapwood area. Tree transpiration contributed between 2.2 and 2.6 mm/day to ET for a tree leaf area index (LAI) of 2.8. The pine stand had an understory dominated by sedge and grass species with overall average LAI of 1.5. Mechanistic canopy gas exchange models that quantify both water vapor and CO₂ exchange were applied to both understory and tree needle ecosystem compartments. Thus, the transpiration by graminoid species was estimated at approximately 20% of total stand ET. The modelled estimates for understory contribution to stand water flux compared well with micrometeorologically-based determinations. Maximum carbon gain was estimated from the canopy models at approximately 425 mmol/(m²day) for the tree needles and at 100 mmol/(m²day) for the understory. Carbon gain was suggested by the modelling analysis to remain relatively constant during the HartX period, while water use efficiency in carbon fixation increased with decreasing vapor pressure deficit. Biologically- and micrometeorologically-based estimates of stand water flux showed good general agreement with variation of up to 20% that reflects both errors due to the inherent assumptions associated with different methods as well as natural spatial variability in fluxes. The various methods support a reliable estimate of average ET from this homogeneous canopy during HartX of about 2.6 mm/day (a maximum of about 3.1 mm/day) with an insignificant decreasing trend in correlation with decreasing vapor pressure deficit and possibly soil moisture.Findings during HartX were embedded in local scale heterogeneity with greater roughness over the forest and much higher ET over the surrounding agricultural fields which results in weak but clearly existant circulation patterns. A variety of measurements were continued after the HartX campaign. They allow us to extend our findings for six months with changing environmental conditions, including shortage of soil moisture. Hydrological estimates of soil water extractions and micrometeorological estimates of ET by the one-propeller eddy covariance (OPEC) system were in very good agreement, supporting the use of this robust eddy covariance energy balance technique for long-term monitoring.With 5 Figures     

Estimates of water vapor flux and canopy conductance of Scots pine at the tree level utilizing different xylem sap flow methods

Summary During the Hartheim Experiment (HartX) 1992 conducted in the upper Rhine Valley, Germany, three different methods were used to measure sap flow in Scots pine trees via heating of water transported in the xylem: (1) constant heating applied radially in the sapwood (Granier-system-G), (2) constant heating of a stem segment (ermák-system-C), and (3) regulated variable heating of a stem segment that locally maintains a constant temperature gradient in the trunk (ermák/Schulze-system-CS). While the constant heating methods utilize changes in the induced temperature gradient to quantify sap flux, the CS-system estimates water flow from the variable power requirement to maintain a 2 or 3 degree Kelvin temperature gradient over a short distance between inserted electrodes and reference point. The C- and CS-systems assume that all transported water is encompassed and equally heated by the electrodes. In this case, flux rate is determined from temperature difference or energy input and the heat capacity of water. Active sapwood area need not be determined exactly. In contrast, the G-system requires an empirical calibration of the sensors that allows conversion of temperature difference into sap flow density. Estimates of sapwood area are used to calculate the total flux. All three methods assume that the natural fluctuation in temperature of the trunk near the point of insertion of heating and sensing elements is the same as that where reference thermocouples are inserted.Using all three systems, 24 trees were simultaneously monitored during the HartX campaign. Tree size within the stand ranged between 18 and 61 cm circumference at breast height, while sample trees ranged between 24 and 55 cm circumference. The smallest trees could only be measured by utilizing the G-system. Sap flow rates of individual trees measured at breast height increased rapidly in the morning along with increases in irradiance and vapor pressure deficit (D), decreased slowly during the course of the afternoon with continued increase inD, and decreased more slowly during the night.Ignoring potential effects introduced by the different methods, maximum flow rates of individual trees ranged between 0.5 and 2.5 kg H₂O h^–1 tree^–1 or 0.3 and 0.6 mm h^–1 related to projected crown area of trees and daily sums of sap flow for individual trees varied between 4.4 and 24 kg H₂O tree^–1 d^–1 or 1.1 and 6.0 mm d^–1. Maximum sap flow rates per sapwood area of trees varied least for the G-system (11–17 g cm^–2 h^–1) and was of similar magnitude as the C- (8–21 g cm^–2 h^–1) and CS-system (4–14 g cm^–2 h^–1).Regressions of total tree conductance (g _t ) derived from sap flow estimates demonstrated the same linear increase of conductance with increasing irradiance, however decrease of conductance with increasingD under non-limiting light conditions was different for the three systems with strongest reduction ofg _t measured with the CS-system followed by the C- and G-system. This led to different estimates of daily sap flow rates especially during the second part of the measurement period.Variation in sap flow rates is explained on the basis of variation in leaf area index of individual trees, heterogeneity in soil conditions, and methodological differences in sap flow measurements. Despite the highly uniform plantation forest at the scale of hectares, the heterogeneity in tree size and soil depth at the scale of square meters still make it difficult to appropriately and efficiently select sample trees and to scale-up water flux from individual trees to the stand level.With 5 Figures     

Regional hydrologic and carbon balance responses of forests resulting from potential climate change

The projected response of coniferous forests to a climatic change scenario of doubled atmospheric CO₂, air temperature of +4 °C, and +10% precipitation was studied using a computer simulation model of forest ecosystem processes. A topographically complex forested region of Montana was simulated to study regional climate change induced forest responses. In general, increases of 10–20% in LAI, and 20–30% in evapotranspiration (ET) and photosynthesis (PSN) were projected. Snowpack duration decreased by 19–69 days depending on location, and growing season length increased proportionally. However, hydrologic outflow, primarily fed by snowmelt in this region, was projected to decrease by as much as 30%, which could virtually dry up rivers and irrigation water in the future.To understand the simulated forest responses, and explore the extent to which these results might apply continentally, seasonal hydrologic partitioning between outflow and ET, PSN, respiration, and net primary production (NPP) were simulated for two contrasting climates of Jacksonville, Florida (hot, wet) and Missoula, Montana (cold, dry). Three forest responses were studied sequentially from; climate change alone, addition of CO₂ induced tree physiological responses of-30% stomatal conductance and +30% photosynthetic rates, and finally with a reequilibration of forest leaf area index (LAI), derived by a hydrologic equilibrium theory. NPP was projected to increase 88%, and ET 10%, in Missoula, MT, yet dcrease 5% and 16% respectively for Jacksonville, FL, emphasizing the contrasting forest responses possible with future climatic change.     

Eddy correlation measurements of CO2, latent heat,and sensible heat fluxes over a crop surface

Eddy correlation equipment was used to measure mass and energy fluxes over a soybean crop. A rapid response CO₂ sensor, a drag anemometer, a Lyman-alpha hygrometer and a fine wire thermocouple were used to sense the fluctuating quantities.Diurnal fluxes of sensible heat, latent heat and CO₂ were calculated from these data. Energy budget closure was obtained by summing the sensible and latent heat fluxes determined by eddy correlation which balanced the sum of net radiation and soil heat flux. Peak daytime CO₂ fluxes were near 1.0 mg m^–2 (ground area) s^–1.The eddy correlation technique was also employed in this study to measure nocturnal CO₂ fluxes caused by respiration from plants, soil, and roots. These CO₂ fluxes ranged from - 0.1 to - 0.25 mg m^–2s^–1.From the data collected over mature soybeans, a relationship between CO₂ flux and photosynthetically active radiation (PAR) was developed. The crop did not appear to be light-saturated at PAR flux densities < 1800 Ei m^–2 s^–1. The light compensation point was found to be about 160 Ei m^–2 s^–1.Published as Paper No. 7402, Journal Series, Nebraska Agricultural Experiment Station. The work reported here was conducted under Nebraska Agricultural Experiment Station Project 27-003 and Regional Research Project 11–33.Post-doctoral Research Associate, Professor and Professor, respectively. Center for Agricultural Meteorology and Climatology, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68583-0728.     

Evaporation from the reservoir of the High Aswan Dam, Egypt: A new comparison of relevant methods with limited data

Summary Previous estimates of average annual evaporation from the lake formed by the High Dam at Aswan, Egypt, fall in the range from 4.65 mm d^–1 to 7.95 mm d^–1. The difference between these limits, more than 7 billion m³ yr^–1 at the highest storage level, is nearly one-eighth the share by treaty of Egypt, and more than one-third of the share of the Sudan. It is also more than the estimated increase of the annual water need for Egypt between 1990 and 2000. This state of affairs renders proper management of the river flow for the sake of Egypt and the Sudan quite difficult. This paper compares the relevant methods of estimating evaporation from the limited data available.These methods are:water-balance, energy budget, bulk aerodynamic (Dalton),combination (Penman) andComplementary Relations Lake Evaporation (CRLE) model (Morton). The new estimates have a much narrower range, from 5.70 mm d^–1 to 7.05 mm d^–1, or only a bit more than 4% of the annual Nile flow below the High Aswan Dam. The average of these annual estimates of evaporation, after excluding the bulk aerodynamic method because of its severe limitations, is 6.0 ± 0.3 mm d^–1 or 20% less than the 7.5 mm d^–1 adopted by the irrigation authorities in Egypt and the Sudan. This difference corresponds to 3 billion m³ yr^–1 at the highest storage level or more than 5% of the annual outflow from the reservoir. Even when the higher estimates from the bulk aerodynamic method and from the Penman method with its usual wind function are included, the new average is still 15% less than the figure of 7.5 mm d^–1. The monthly distribution of the annual evaporation varies more widely with the method applied. Similar comparative studies in future, aiming at obtaining improved estimates of evaporation, require all the data relevant to all the methods to be collected properly for a common period of several years at relatively stable lake level.With 1 Figure     

Global distribution of natural freshwater wetlands and rice paddies,their net primary productivity,seasonality and possible methane emissions

A global data set on the geographic distribution and seasonality of freshwater wetlands and rice paddies has been compiled, comprising information at a spatial resolution of 2.5° by latitude and 5° by longitude. Global coverage of these wetlands total 5.7×10⁶ km² and 1.3×10⁶ km², respectively. Natural wetlands have been grouped into six categories following common terminology, i.e. bog, fen, swamp, marsh, floodplain, and shallow lake. Net primary productivity (NPP) of natural wetlands is estimated to be in the range of 4–9×10¹⁵ g dry matter per year. Rice paddies have an NPP of about 1.4×10¹⁵ g y^–1. Extrapolation of measured CH₄ emissions in individual ecosystems lead to global methane emission estimates of 40–160 Teragram (1 Tg=10¹² g) from natural wetlands and 60–140 Tg from rice paddies per year. The mean emission of 170–200 Tg may come in about equal proportions from natural wetlands and paddies. Major source regions are located in the subtropics between 20 and 30° N, the tropics between 0 and 10° S, and the temperate-boreal region between 50 and 70° N. Emissions are highly seasonal, maximizing during summer in both hemispheres. The wide range of possible CH₄ emissions shows the large uncertainties associated with the extrapolation of measured flux rates to global scale. More investigations into ecophysiological principals of methane emissions is warranted to arrive at better source estimates.     

Emission and flux of DMS from the Australian Antarctic and Subantarctic Oceans during the 1988/89 summer

DMS emissions and fluxes from the Australasian sector of the Antarctic and Subantarctic Oceans, bound by 46–68° S and 65.5–142.6° E, were determined from a limited number of samples (n=32) collected during three summer resupply voyages to Australian Antarctic continental research bases between November 1988 and January 1989 (a 92 day period). The maximum DMS emission from this sector of the Antarctic Ocean was in an area near the Antarctic Divergence (60–63° S) and the minimum DMS emission was from the Antarctic coastal and offshelf waters. The greatest emission of DMS from this sector of the Southern Ocean was from the Subantarctic waters. DMS flux from the Australasian Antarctic Ocean was 64.3×10⁶ (±115) mol d^–1 or 5.9 (±10.6)×10⁹ mol based on an emission of 10.9 (±19.5) µmol m^–2 d^–1 (n=26). The flux of DMS from the Australasian sector of the Subantarctic Ocean was probably twice the flux of DMS from the adjacent Antarctic Ocean.     

Parameter sensitivity study of the saline circulation

Saline circulation forced by fresh water alone is studied for a broad region of parameter space by varying the amplitude and profile of evaporation minus precipitation, the vertical and horizontal mixing of salt, vertical and horizontal dissipation of momentum, and the horizontal resolution. The model is a modified Bryan-Cox model with a freshwater flux as the natural boundary condition for the salinity balance. For a model forced by a linear freshwater flux profile, as the amplitude of freshwater flux is increased from 0.01 m year ^–1 to 1 m year^–1 with other parameters fixed, the system evolves from a steady state of no oscillation to a state of periodic oscillation whose frequency increases almost linearly with the amplitude of freshwater flux. When the freshwater flux is fixed and the vertical mixing coefficient is increased from 0.5 to 2.5 cm²s^–1, the system evolves from a steady state to a state of single-period oscillation, chaotic, a single period, and finally to a chaotic state when the vertical mixing coefficient is larger than 2 cm² s^–1. One set of numerical experiments forced by a cosine shape of freshwater flux clearly reveals the transition from a state of single period oscillation to period doubling, period quadrupling, and a state of chaotic oscillation. Simple scaling analysis and numerical experiments indicate that the strength of the meridional overturning increases with the square-root of the vertical mixing and the 1/4 power of the freshwater flux. The mean sea surface salinity (deviation from 35 psu) increases with the 3/4 power of the freshwater flux and decreases with the 1/2 power of the vertical salt mixing.Contribution No. 8191 from the Woods Hole Oceanographic Institution     

A model for predicting actual evapotranspiration under soil water stress in a Mediterranean region

Summary In this paper a model for estimating actual evapotranspiration is developed and tested for field crops (grain sorghum and sunflower) maintained under water stress conditions. The model is based on the Penman-Monteith formulation of ET in which canopy resistance (r _c) is modeled with respect to the crop water status and local climatological conditions. The model was previously tested on reference grass; in this last case no reference was made to soil water conditions andr _c was modeled only as a function of climatological parameters. Herer _c is expressed as a function of available energy, vapour pressure deficit, aerodynamic resistance and crop water status by means of predawn leaf water potential. Results, obtained with various crop water stress intensities, show that, on a daily scale, calculated ET is 98% and 95% of the measured ET for sorghum and sunflower respectively. The correlation between daily calculated and measured ET is very high (r ² = 0.95 for sorghum andr ² = 0.98 for sunflower). On an hourly scale, the model works very well when the crops were not stressed and during the senescence stage. In case of weak and strong stress the model has to be used with some precautions.With 9 Figures