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The impact of site factors and climate variability on the calculation of potential evaporation at Moel Cynnedd, Plynlimon
S. B. Crane,J. A. Hudson
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 1997,
Abstract: The meteorological record from the manual Moel Cynnedd climate station at Plynlimon in the Welsh Uplands has been supplemented with solar radiation data, initially from the Institute of Hydrology's Dolydd Office, and later from an adjacent automatic weather station, in order to calculate Penman potential evapotranspiration for the entire 27 year data set, 1969-1995. The methods of data capture are consistent with Meteorological Office criteria throughout the entire record, establishing an unbiased and probably unique indicator of climatic variability and change for this type of environment. Values of Penman Et calculated from these data provides an independent index of atmospheric demand for moisture as an adjunct to the hydrological studies being carried out in the Plynlimon and neighbouring catchments. Analysis of the long term data indicates considerable year-to year variability in the component variables, including some cyclical changes and possible long term trends in measured temperature. Annual variability in Et is less than in the component variables, and there is an indication of a possible long term cycle, but no evidence of an overall trend in Et during this particular study period. The results indicate that some of the observed variability can be explained by inevitable changes in exposure within this forest clearing site rather than changes in regional or global climatic patterns. A single meteorological station sited in a forest clearing at a relatively low altitude may underestimate potential evaporation across the catchment, as this will also include areas of exposed hillside and forest canopy.
Controls on open water evaporation
R. J. Granger,N. Hedstrom
Hydrology and Earth System Sciences Discussions , 2010, DOI: 10.5194/hessd-7-2709-2010
Abstract: The paper presents the initial results of a field study of boundary layer behaviour and open water evaporation carried out on two small- to medium-sized lakes in Western and Northern Canada. Meteorological and boundary layer measurements were made over the water surfaces and over the upwind land surface, allowing for an examination of the effect of lake-land contrasts of temperature on the wind speed over the open water and on the evaporation rates. Lake evaporation was measured directly using eddy covariance equipment. The study showed that, for time periods shorter than daily, the open water evaporation bears no relationship to the net radiation. The wind speed is the most significant factor governing the evaporation rates, followed by the land-water temperature contrast and the land-water vapour pressure contrast. The effect of the stability on the wind field is demonstrated; stability over the water and adjacent land surfaces are, for the most part, out of phase. The derived relationships will be used to develop a model for estimating the hourly evaporation rates from open water. Examination of the seasonal trends shows that the open water period can be separated into two distinct evaporative regimes: the warming period in the Spring, when the land temperature is greater than the water temperature, the turbulent fluxes over water are suppressed; and the cooling period, when the water temperature is greater than the air temperature, and the turbulent fluxes over water are enhanced.
Estimating actual, potential, reference crop and pan evaporation using standard meteorological data: a pragmatic synthesis
T. A. McMahon, M. C. Peel, L. Lowe, R. Srikanthan,T. R. McVicar
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2013,
Abstract: This guide to estimating daily and monthly actual, potential, reference crop and pan evaporation covers topics that are of interest to researchers, consulting hydrologists and practicing engineers. Topics include estimating actual evaporation from deep lakes and from farm dams and for catchment water balance studies, estimating potential evaporation as input to rainfall-runoff models, and reference crop evapotranspiration for small irrigation areas, and for irrigation within large irrigation districts. Inspiration for this guide arose in response to the authors' experiences in reviewing research papers and consulting reports where estimation of the actual evaporation component in catchment and water balance studies was often inadequately handled. Practical guides using consistent terminology that cover both theory and practice are not readily available. Here we provide such a guide, which is divided into three parts. The first part provides background theory and an outline of the conceptual models of potential evaporation of Penman, Penman–Monteith and Priestley–Taylor, as well as discussions of reference crop evapotranspiration and Class-A pan evaporation. The last two sub-sections in this first part include techniques to estimate actual evaporation from (i) open-surface water and (ii) landscapes and catchments (Morton and the advection-aridity models). The second part addresses topics confronting a practicing hydrologist, e.g. estimating actual evaporation for deep lakes, shallow lakes and farm dams, lakes covered with vegetation, catchments, irrigation areas and bare soil. The third part addresses six related issues: (i) automatic (hard wired) calculation of evaporation estimates in commercial weather stations, (ii) evaporation estimates without wind data, (iii) at-site meteorological data, (iv) dealing with evaporation in a climate change environment, (v) 24 h versus day-light hour estimation of meteorological variables, and (vi) uncertainty in evaporation estimates. This paper is supported by a Supplement that includes 21 sections enhancing the material in the text, worked examples of many procedures discussed in the paper, a program listing (Fortran 90) of Morton's WREVAP evaporation models along with tables of monthly Class-A pan coefficients for 68 locations across Australia and other information.
Climate Change and Open Science  [PDF]
Ian Percival
Physics , 2013,
Abstract: Obtaining reliable answers to the major scientific questions raised by climate change in time to take appropriate action gives added urgency to the open access program.
Numerical study of the evaporation process and parameter estimation analysis of an evaporation experiment
K. Schneider-Zapp, O. Ippisch,K. Roth
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2010,
Abstract: Evaporation is an important process in soil-atmosphere interaction. The determination of hydraulic properties is one of the crucial parts in the simulation of water transport in porous media. Schneider et al. (2006) developed a new evaporation method to improve the estimation of hydraulic properties in the dry range. In this study we used numerical simulations of the experiment to study the physical dynamics in more detail, to optimise the boundary conditions and to choose the optimal combination of measurements. The physical analysis exposed, in accordance to experimental findings in the literature, two different evaporation regimes: (i) a soil-atmosphere boundary layer dominated regime (regime I) close to saturation and (ii) a hydraulically dominated regime (regime II). During this second regime a drying front (interface between unsaturated and dry zone with very steep gradients) forms which penetrates deeper into the soil as time passes. The sensitivity analysis showed that the result is especially sensitive at the transition between the two regimes. By changing the boundary conditions it is possible to force the system to switch between the two regimes, e.g. from II back to I. Based on this findings a multistep experiment was developed. The response surfaces for all parameter combinations are flat and have a unique, localised minimum. Best parameter estimates are obtained if the evaporation flux and a potential measurement in 2 cm depth are used as target variables. Parameter estimation from simulated experiments with realistic measurement errors with a two-stage Monte-Carlo Levenberg-Marquardt procedure and manual rejection of obvious misfits lead to acceptable results for three different soil textures.
Numerical study of the evaporation process and parameter estimation analysis of an evaporation experiment
K. Schneider-Zapp,O. Ippisch,K. Roth
Hydrology and Earth System Sciences Discussions , 2009,
Abstract: Evaporation is an important process in soil-atmosphere interaction. The determination of hydraulic properties is one of the crucial parts in the simulation of water transport in porous media. Schneider et al. (2006) developed a new evaporation method to improve the estimation of hydraulic properties in the dry range. In this study we used numerical simulations of the experiment to study the physical dynamics in more detail, to optimise the boundary conditions and to choose the optimal combination of measurements. The physical analysis exposed, in accordance to experimental findings in the literature, two different evaporation regimes, a soil-atmosphere boundary layer dominated regime (regime I) in the saturated region and a hydraulically dominated regime (regime II). During this second regime a drying front forms which penetrates deeper into the soil as time passes. The sensitivity analysis showed that the result is especially sensitive at the transition between the two regimes. By using boundary condition changes it is possible to force the system to switch between the two regimes, e.g. from II back to I. Based on this findings a multistep experiment was developed. The response surfaces for all parameter combinations are flat and have a unique, localised minimum. Best parameter estimates are obtained if the evaporation flux and a potential measurement in 2 cm depth are used as target variables. Parameter estimation from simulated experiments with realistic measurement errors with a two-stage Monte-Carlo Levenberg-Marquardt procedure and manual rejection of obvious misfits lead to acceptable results for three different soil textures.
Effects of Climate Variability on Evaporation in Dongping Lake, China, during 2003–2010  [PDF]
Yuan Rong,Hongbo Su,Renhua Zhang,Zheng Duan
Advances in Meteorology , 2013, DOI: 10.1155/2013/789290
Abstract: Based on two long-term, hourly (10:30–11:30 and 13:10–14:10) meteorological over-lake observations and data from Shenxian meteorological station, nearby Dongping Lake, the Penman-Monteith equation and reference evaporation ratio algorithm were used to calculate lake evaporation in Dongping Lake, China, from 2003 to 2010. The variation trend of evaporation of Dongping Lake was analyzed, and the influences that caused changes in lake evaporation were also discussed. The results show that (1) the total annual evaporation in Dongping Lake increased at 18.24?mm/a during 2003–2010. The major climatic factors accounting for this increase are the rising net radiation and the rising air temperature; (2) the total annual evaporation in a particular hour (13:10–14:10) in Dongping Lake increased at 4.55?mm/a during 2003–2010—the major climate factors that accounted for this increase are rising net radiation, followed by air temperature, wind velocity, and air humidity; (3) against the background of global warming, the climate of Dongping Lake tended to be dry during 2003–2010; the largest contribution to this comes from air temperature, followed by wind velocity and relative humidity; and (4) the monthly evaporation in Dongping Lake has seasonal variability. 1. Introduction Lakes and reservoirs store valuable fresh water and make them available for use in domestic, industrial, irrigation, hydropower, wetlands, and environmental applications [1]. The availability of fresh water stored in lakes and reservoirs is closely tied to variations in climate and human activities [2]. Evaporation is a key component in the water and energy cycles of lakes and reservoirs [2, 3]. In most situations, lake evaporation represents major water loss. Therefore, information on lake evaporation is essential for the water management of lakes and reservoirs. Climate variability affects lake evaporation, and in turn, evaporation also has an effect on the local climate [2, 4]. Some studies stated that lake evaporation can be considered to be a basic type of reference data for studies on land evaporation, climate changes, and water cycles [4]. The causes of lake evaporation changes have been extensively studied. For instance, hydrological models were used to calculate the water surface evaporations of many endorheic lakes and closed lakes, and how the regional climate changes affect the lake evaporation was analyzed [5, 6]. A Penman equation and CRLE (complementary relationship lake evaporation) model were used to calculate the evaporation of Ziway Lake in Africa, and how the air temperature
Climate indicators for Italy: calculation and dissemination
F. Desiato, G. Fioravanti, P. Fraschetti, W. Perconti,A. Toreti
Advances in Science and Research (ASR) , 2011, DOI: 10.5194/asr-6-147-2011
Abstract: In Italy, meteorological data necessary and useful for climate studies are collected, processed and archived by a wide range of national and regional institutions. As a result, the density of the stations, the length and frequency of the observations, the quality control procedures and the database structure vary from one dataset to another. In order to maximize the use of those data for climate knowledge and climate change assessments, a computerized system for the collection, quality control, calculation, regular update and rapid dissemination of climate indicators was developed. The products publicly available through a dedicated web site are described, as well as an example of climate trends estimates over Italy, based on the application of statistical models on climate indicators from quality-checked and homogenised time series.
Nonlinearly combined impacts of initial perturbation from human activities and parameter perturbation from climate change on the grassland ecosystem
G. Sun,M. Mu
Nonlinear Processes in Geophysics (NPG) , 2011,
Abstract: Human activities and climate change are important factors that affect grassland ecosystems. A new optimization approach, the approach of conditional nonlinear optimal perturbation (CNOP) related to initial and parameter perturbations, is employed to explore the nonlinearly combined impacts of human activities and climate change on a grassland ecosystem using a theoretical grassland model. In our study, it is assumed that the initial perturbations and parameter perturbations are regarded as human activities and climate change, respectively. Numerical results indicate that the climate changes causing the maximum effect in the grassland ecosystem are different under disparate intensities of human activities. This implies the pattern of climate change is very critical to the maintenance or degradation of grassland ecosystem in light of high intensity of human activities and that the grassland ecosystem should be rationally managed when the moisture index decreases. The grassland ecosystem influenced by the nonlinear combination of human activities and climate change undergoes abrupt change, while the grassland ecosystem affected by other types of human activities and climate change fails to show the abrupt change under a certain range of perturbations with the theoretical model. The further numerical analyses also indicate that the growth of living biomass and the evaporation from soil surface shaded by the wilted biomass may be crucial factors contributing to the abrupt change of the grassland equilibrium state within the theoretical model.
Does evaporation paradox exist in China?
Z. T. Cong, D. W. Yang,G. H. Ni
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2009,
Abstract: One expected consequence of global warming is the increase in evaporation. However, lots of observations show that the rate of evaporation from open pans of water has been steadily decreasing all over the world in the past 50 years. The contrast between expectation and observation is called "evaporation paradox". Based on data from 317 weather stations in China from 1956 to 2005, the trends of pan evaporation and air temperature were obtained and evaporation paradox was analyzed. The conclusions include: (1) From 1956 to 2005, pan evaporation paradox existed in China as a whole while pan evaporation kept decreasing and air temperature became warmer and warmer, but it does not apply to Northeast and Southeast China; (2) From 1956 to 1985, pan evaporation paradox existed narrowly as a whole with unobvious climate warming trend, but it does not apply to Northeast China; (3) From 1986 to 2005, in the past 20 years, pan evaporation paradox did not exist for the whole period while pan evaporation kept increasing, although it existed in South China. Furthermore, the trend of other weather factors including sunshine duration, windspeed, humidity and vapor pressure deficit, and their relations with pan evaporation are discussed. As a result, it can be concluded that pan evaporation decreasing is caused by the decreasing in radiation and wind speed before 1985 and pan evaporation increasing is caused by the decreasing in vapor pressure deficit due to strong warming after 1986. With the Budyko curve, it can be concluded that the actual evaporation decreased in the former 30 years and increased in the latter 20 year for the whole China.
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