Direct Measurements Of Evaporation Research Articles

Lake Superior evaporation: A long-term eddy covariance dataset at Stannard Rock Lighthouse (2008–2022)

Robust, accurate, and direct measurements of evaporation and related energy fluxes on the Laurentian Great Lakes are necessary to understand the large historical range in water levels, regional climatology, lake hydrodynamics, and lake-effect snowfall, all of which inform water management. Despite the societal and scientific importance of this information, few long-term, full-year, in situ measurements exist due to logistical, financial, and safety-related challenges. We present 15 years (2008–2022) of eddy covariance data from Stannard Rock, a historic lighthouse on Lake Superior located 38 km southeast of Manitou Island and 72 km north of Marquette, Michigan. We provide information about the site and instrumentation, as well as data availability and processing. Analysis of this unique long-term dataset, available through the AmeriFlux network (US-GL1), will improve our ability to understand the drivers and patterns of large-lake surface energy fluxes and will advance predictions of evaporative regimes over Lake Superior.

Controls and rates of evaporation from a water supply reservoir in the Colorado Front Range

Evaporation can be a major component of a reservoir’s water balance in arid, populated regions such as the western United States, yet accurate measurements are scarce. In an effort to fill this observation gap, direct measurements of evaporation from Standley Lake, a water supply reservoir in the Colorado Front Range, were collected via the eddy covariance method from March 1, 2020 through February 28, 2021. Evaporation followed a seasonal pattern, with the spring, summer, and early fall experiencing daily cycles that peaked just after midday, with measurable evaporation occurring each day for approximately 4–6 h. In the late fall and winter, the daily cycles decayed and episodic enhancements in evaporation were common, most often lasting between 18 and 36 h, some longer. Evaporation peaked in June, aligning with peak net radiation, and summertime evaporation was predominantly driven by diurnal variations in wind speed and the vapor pressure gradient. Interestingly, fall and winter saw multiple periods of negative (downward) sensible heat transfer which helped drive wintertime evaporation. The study period evaporation totaled 1377 mm, with 35 % of that occurring in summer, 27 % in fall, 21 % in spring, and 17 % in winter. This study shows that evaporation is a significant part of the energy balance of Standley Lake, and with continued observations, enables a better understanding of the impacts a changing climate and increased water demands have on water resources.

Transpiration of a Tropical Dry Deciduous Forest in Yucatan, Mexico

The study of forest hydrology and its relationships with climate requires accurate estimates of water inputs, outputs, and changes in reservoirs. Evapotranspiration is frequently the least studied component when addressing the water cycle; thus, it is important to obtain direct measurements of evaporation and transpiration. This study measured transpiration in a tropical dry deciduous forest in Yucatán (Mexico) using the thermal dissipation method (Granier-type sensors) in representative species of this vegetation type. We estimated stand transpiration and its relationship with allometry, diameter-at-breast-height categories, and previously published equations. We found that transpiration changes over time, being higher in the rainy season. Estimated daily transpiration ranged from 0.562 to 0.690 kg m–2 d–1 in the late dry season (April–May) and from 0.686 to 1.29 kg m–2 d–1 in the late rainy season (September–October), accounting for up to 51% of total evapotranspiration in the rainy season. These daily estimates are consistent with previous reports for tropical dry forests and other vegetation types. We found that transpiration was not species-specific; diameter at breast height (DBH) was a reliable way of estimating transpiration because water use was directly related to allometry. Direct measurement of transpiration would increase our ability to accurately estimate water availability and assess the responses of vegetation to climate change.

TEMPERATURE-DRIVEN EVAPORATION ANALYSIS OVER BALLIKAYA CATCHMENT

Hydrometeorological analysis such as terrain characteristics, flood forecasts and water potentials have become possible through computer models today in parallel with rapidly ever-evolving technology. While the surface-runoff during a storm has linearly and highly correlation with effective precipitation, it is inversely proportional to hydrological losses, including interception, infiltration, transpiration and evaporation. However, flood estimation analysis, in especially rural areas in Turkey, is still not widely and accurately usable, due to difficulties in accessing the necessary ground-based data records such as evaporation, which affects much of the rainfall. Although direct measurement of evaporation is not available now, indirect methods such as evaporation pans have been developed to give acceptable results. On the one hand, observation networks are built by Turkish State Meteorological Service for the estimation of evaporation from open water bodies, these measurements, on the other hand, are only obtained at large climatological stations such as city centers and generally for only six months of the year. In this sense, evaporation values, in order to apply rainfall-runoff analysis, were estimated by Kharrufa, Blaney-Criddle, and modified Blaney-Criddle methods to be used in Monthly Average models. While the highest underestimations were obtained in modified Blaney-Criddle method, Kharrufa method outperformed the other Blaney-Criddle products. The results, confirmed by analysis of the temperature-based Kharrufa approach in two urban areas close to the study basin, show to be appliable for estimation of long-term hydrographs. The analyzed method can be applied over rural watersheds lacking in-situ evaporation measurements as well as enables more accurate rainfall-runoff simulations processes with calibrated and verified hydrological models.

Výpar z výparoměrných stanic VÚV TGM

Evaporation from water surface significantly affects the overall hydrological balance of a river basin. Direct measurement of evaporation is complicated and not as common as the measurement of other quantities (e.g. precipitation or air temperature). Therefore, evaporation is often determined using formulas requiring available meteorological quantities as input data.

Water temperature control on CO2 flux and evaporation over a subtropical seagrass meadow revealed by atmospheric eddy covariance

AbstractSubtropical seagrass meadows play a major role in the coastal carbon cycle, but the nature of air–water CO2 exchanges over these ecosystems is still poorly understood. The complex physical forcing of air–water exchange in coastal waters challenges our ability to quantify bulk exchanges of CO2 and water (evaporation), emphasizing the need for direct measurements. We describe the first direct measurements of evaporation and CO2 flux over a calcifying seagrass meadow near Bob Allen Keys, Florida. Over the 78‐d study, CO2 emissions were 36% greater during the day than at night, and the site was a net CO2 source to the atmosphere of 0.27 ± 0.17 μmol m−2 s−1 (x̅ ± standard deviation). A quarter (23%) of the diurnal variability in CO2 flux was caused by the effect of changing water temperature on gas solubility. Furthermore, evaporation rates were ~ 10 times greater than precipitation, causing a 14% increase in salinity, a potential precursor of seagrass die‐offs. Evaporation rates were not correlated with solar radiation, but instead with air–water temperature gradient and wind shear. We also confirm the role of convective forcing on night‐time enhancement and day‐time suppression of gas transfer. At this site, temperature trends are regulated by solar heating, combined with shallow water depth and relatively consistent air temperature. Our findings indicate that evaporation and air–water CO2 exchange over shallow, tropical, and subtropical seagrass ecosystems may be fundamentally different than in submerged vegetated environments elsewhere, in part due to the complex physical forcing of coastal air–sea gas transfer.

Dynamics of crop coefficients for citrus orchards of central India using water balance and eddy covariance flux partition techniques

Accurate quantification of crop water demand and characterizing its temporal variability is essential in evaluating the role of water saving strategies for sustainable production. Representing evapotranspiration (ET) as a proxy to crop water needs is often misleading, particularly during the periods of high non-stomatal exchanges. This study is aimed at modeling crop water requirements for citrus orchards of central India using single and dual crop coefficient approaches. ET fluxes derived from eddy-covariance (EC) technique were used to develop single crop coefficient (Kc) curves at daily, weekly, and seasonal scales. Site-specific Kc values for initial, mid, and late season were found to be 0.43, 0.78, and 0.80 respectively. ET partitioning was done by estimating soil evaporation coefficient (Ke), and basal crop coefficient in the presence of water stress (Ks×Kcb) using soil-water balance (SIMDualKc) and EC flux partitioning (EC FP) methods. SIMDualKc model was calibrated against electrical resistivity tomography (ERT) derived soil water contents (R2 = 0.81, RMSE = 0.021 cm3 cm−3). Energy based flux partitioning was done by considering the correlation between high-frequency water vapour and carbon fluxes and applying flux variance similarity principles. Direct measurement of evaporation (E) and transpiration (T) at four citrus trees (using micro-lysimeters and sap flow meters) was used to assess the performance of two models. Three-stage basal crop coefficients from SIMDualKc and EC FP methods were found to be 0.18, 0.57, 0.63 and 0.26, 0.51, 0.59 respectively. Both methods were effectively partitioning the ET fluxes at daily scale (R2 > 0.5, RMSE < 0.6 mm, NSE > 0.3), however, variability in estimated fluxes between the two methods is high during the initial stage and gradually diminished with time.

Seasonal evaporation cycle in oxbow lakes formed along the Tisza River in Hungary for flood control

AbstractAs a result of water regulation, dykes (i.e., embankments against floods) were constructed along the Tisza River and meanders were cut to control the floods in the region. These cut‐off meanders resulted in oxbow lakes that are important locations for nature conservation. We collected water samples over 5 years in 7 campaigns to measure the δ18O (‰) and δ2H (‰) ratios in 45 oxbow lakes from the Upper Tisza Region (NE‐Hungary). We applied Random Forest Regression involving climatic data to reveal the connection with the stable isotopes. We determined that isotope ratios changed as a function of time, due to evaporation and a varying water supply (precipitation and groundwater). The average difference in the isotopic ratios for the river and oxbows increased from spring to winter, but decreased between the oxbows on both sides of the dyke. We found that isotope ratios were determined by the maximum monthly temperature in the case of oxbows in the active floodplain, whereas in case of oxbows on the reclaimed side, this was also influenced by the maximum monthly temperature, and the cumulative evaporation. As direct measurement of evaporation is difficult to evaluate, stable isotope measurements provided an effective quantitative alternative to estimate evaporation. Measuring the seasonality of the δ18O and δ2H is important to interpret the results and these data are useful to water management experts to identify the lakes at risk of running dry.

An estimation of evapotranspiration by simple models in the semi arid Mediterranean zone of the North-West of Algeria

Four temperature-based evapotranspiration models were applied and compared to daily measurements obtained by a Piche evaporimeter in a Mediterranean climate zone of the semi arid type in the North-West of Algeria. Direct measurements of evaporation obtained by Piche on five stations were corrected by the Bouchet model. Evapotranspiration was calculated according to the following models: Blaney-Criddle, Blaney-Criddle (SCS), Linacre and Romanenko. The results showed that the best model is the Blaney-Criddle (SCS) one which gave excellent results for almost all of the stations studied, presenting the lowest statistical parameters [root mean squared error (RMSE), mean bias error (MBE) and t-statistic and a higher index of agreement (d). The Blaney-Criddle (SCS) also showed the lowest overestimation of evapotranspiration compared with other models. Therefore, the Blaney-Criddle (SCS) model is the most suitable model for estimating the daily evapotranspiration when direct evaporation measurements are not available.

An estimation of evapotranspiration by MHBJHHHsimple models in the semi arid Mediterranean zone of the North-West of Algeria

Four temperature-based evapotranspiration models were applied and compared to daily measurements obtained by a Piche evaporimeter in a Mediterranean climate zone of the semi arid type in the North-West of Algeria. Direct measurements of evaporation obtained by Piche on five stations were corrected by the Bouchet model. Evapotranspiration was calculated according to the following models: Blaney-Criddle, Blaney-Criddle (SCS), Linacre and Romanenko. The results showed that the best model is the Blaney-Criddle (SCS) one which gave excellent results for almost all of the stations studied, presenting the lowest statistical parameters [root mean squared error (RMSE), mean bias error (MBE) and t-statistic and a higher index of agreement (d). The Blaney-Criddle (SCS) also showed the lowest overestimation of evapotranspiration compared with other models. Therefore, the Blaney-Criddle (SCS) model is the most suitable model for estimating the daily evapotranspiration when direct evaporation measurements are not available.

Estimation of evaporation over the upper Blue Nile basin by combining observations from satellites and river flow gauges

AbstractReliable estimates of regional evapotranspiration are necessary to improve water resources management and planning. However, direct measurements of evaporation are expensive and difficult to obtain. Some of the difficulties are illustrated in a comparison of several satellite‐based estimates of evapotranspiration for the Upper Blue Nile (UBN) basin in Ethiopia. These estimates disagree both temporally and spatially. All the available data products underestimate evapotranspiration leading to basin‐scale mass balance errors on the order of 35 percent of the mean annual rainfall. This paper presents a methodology that combines satellite observations of rainfall, terrestrial water storage as well as river‐flow gauge measurements to estimate actual evapotranspiration over the UBN basin. The estimates derived from these inputs are constrained using a one‐layer soil water balance and routing model. Our results describe physically consistent long‐term spatial and temporal distributions of key hydrologic variables, including rainfall, evapotranspiration, and river‐flow. We estimate an annual evapotranspiration over the UBN basin of about 2.55 mm per day. Spatial and temporal evapotranspiration trends are revealed by dividing the basin into smaller subbasins. The methodology described here is applicable to other basins with limited observational coverage that are facing similar future challenges of water scarcity and climate change.

Comparison of diurnal dynamics in evaporation rate between bare soil and moss-crusted soil within a revegetated desert ecosystem of northwestern China

Effects of biological soil crusts (BSCs) on soil evaporation is quite controversial in literature, being either facilitative or inhibitive, and therein few studies have actually conducted direct evaporation measurements. Continuous field measurements of soil water evaporation were conducted on two microlysimeters, i.e., one with sand soil collected from bare sand dune area and the other with moss-crusted soil collected from an area that was revegetated in 1956, from field capacity to dry, at the southeastern edge of the Tengger Desert. We mainly aimed to quantify the diurnal variations of evaporation rate from two soils, and further comparatively discuss the effects of BSCs on soil evaporation after revegetation. Results showed that in clear days with high soil water content (Day 1 and 2), the diurnal variation of soil evaporation rate followed the typical convex upward parabolic curve, reaching its peak around mid-day. Diurnal evaporation rate and the accumulated evaporation amount of moss-crusted soil were lower (an average of 0.90 times) than that of sand soil in this stage. However, as soil water content decreased to a moderately low level (Day 3 and 4), the diurnal evaporation rate from moss-crusted soil was pronouncedly higher (an average of 3.91 times) than that of sand soil, prolonging the duration of this higher evaporation rate stage; it was slightly higher in the final stage (Day 5 and 6) when soil moisture was very low. We conclude that the effects of moss crusts on soil evaporation vary with different evaporation stages, which is closely related to soil water content, and the variation and transition of evaporation rate between bare soil and moss-crusted soil are expected to be predicted by soil water content.

Inundation and groundwater dynamics for quantification of evaporative water loss in tropical wetlands

Abstract. Characterizing hydrological processes within tropical wetlands is challenging due to their remoteness, complexity and heterogeneity. In particular, estimates of evaporative water loss are inherently uncertain. In view of the large influence on the local and regional climate, the quantification of evaporation is essential for the determination of the water balance of permanent and intermittent water bodies. Data for tropical wetlands are scarce where their remoteness impedes direct evaporation measurements. Seasonal inundation dynamics affect evaporation processes in tropical wetlands, which can be analysed in two stages: the first stage during the wet season and the second stage during the dry season. As yet no adequate method exists for determining second-stage evaporation in a data-scarce environment that additionally allows for a transfer of simulated actual evaporation (AET) to other locations. Our study aimed at developing a process-based model to simulate first- and second-stage evaporation in tropical wetlands. We selected a set of empirical potential evaporation (PET) models of varying complexity, each based on different assumptions and available data sets, and evaluated the models with pan evaporation observations in the Pantanal of South America, one of the largest tropical wetlands in the world. We used high-resolution measurements of surface and groundwater levels at different locations to determine the water available for evaporation. AET was derived by constraining simulated PET based on available water. The model of best fit was applied to different types of water bodies with varying hydroperiods to capture first- and second-stage evaporation across a range of wetland types. With our new model we could quantify evaporative water loss in the dry and the wet season for different locations in the Pantanal. This new spatially explicit approach represents an improvement in our understanding of the role of evaporation in the water balance of the Pantanal. We recommend the application of this model in other remote tropical wetlands, since only a minimum of input data is necessary.

In situ measurement shows ocean boundary layer physical processes control catastrophic global warming.

The infrared greenhouse gas heat trap at the top of the atmosphere controls anthropogenic global warming (AGW) heat balance. Processes at the top of the ocean similarly control the 93% of AGW in the oceans. The tropics are a global year-round ocean heat source. Heat is transported in the ocean by sinking brine from tropical evaporation and polar freezing. Buoyant freshwater and ice barriers limit heat loss from the surface layer. The almost completely unstudied ocean surface skin is critically important to understanding global warming and climate change processes. Studies to date have concentrated on atmospheric warming mainly from land-air data. In this paper we present the first hourly meridional 3m and surface observations in the equatorial Pacific from Tahiti to Hawaii for direct measurement of evaporation and ocean boundary layer heat trapping. We relate this to poleward heat and freshwater transport and ocean warming moderation by basal icemelt of floating ice explored in a second paper [1].We show heat sequestration below 3m in the hypersaline (&gt;35.5‰) southern hemisphere (SH) is limited to ~6MJm-2day-1 but evaporation is 7.3mmm-2day--1, at salinity ~36.4‰ and temperature &gt;28ºC. In the northern hemisphere (NH) tropics the corresponding figures are ~12 MJm-2day-1 and ~4.5mmm-2day--1. Equatorial upwelling and the 50m deep Bering Strait limit buoyant surface outflow from the North Pacific.We found pairs of counter-rotating vertical meridional tropical cells (MTCs), ~300-1200km wide, ~100m deep form separate SH and NH systems with little cross-equatorial flux. Counter-rotating Lagrangian wind-driven gyres transport heat and freshwater polewards in seasonally and tidally moderated stratified surface waters. The zonal geostrophic balance is maintained by the Equatorial Undercurrent (EUC) with an eastbound core ~140cms-1 and density ~25.0 at 50-150m.Global warming and polar icemelt has been underestimated from wrong assumptions of the processes in the top 3m of oceans. These are the unverified beliefs that ocean evaporation depends on windspeed and relative humidity that the ocean is well mixed to 10m depths, and by neglect of water density determined by both salinity and temperature. Temperature measurement to ±0.01ºC is required to account for the 3000x greater volumetric heat capacity of seawater to air (3.9x106: 1.3x103Jm-3°C-1). Most SST data are to atmospheric standards (&gt;±0.5°C). Evaporation depends only on temperature (Clausius-Clapeyron). Heat sequestration depends on the buoyant surface layer processes and underlying density gradient. Eleven interconnected counter-rotating Lagrangian wind-driven surface gyres form a global circulation system that carries buoyant surface water masses at speeds much higher than Eulerian geostrophic currents. Polar ice may erode year-round from basal melting from warm subsurface water.This explains contrasting Arctic/Antarctic warming impacts. We suggest many more in situ 3m timeseries especially meridional ones are needed to confirm our findings. In a second paper on centennial daily surface timeseries we show ocean surface warming trend rate post about 1976-1986 is ~0.037ºCyr-1, i.e. &gt;1ºC in 20 years [1]. We suggest global warming research be concentrated on the top of the ocean through multidisciplinary timeseries fieldwork verification, monitoring and modeling. This would best be conducted through a cost-efficient dynamic adaptive scientific management for rapid determination of mitigation and adaptation strategies. Reducing troposphere greenhouse gases can only reduce warming. Mitigation maybe possible through heat energy extraction from geothermal, ocean, tidal and solar sources.

Quantification of surface energy fluxes from a small water body using scintillometry and eddy covariance

[1] Accurate quantification of evaporation from small water storages is essential for water management and planning, particularly in water-scarce regions. In order to ascertain suitable methods for direct measurement of evaporation from small water bodies, this study presents a comparison of eddy covariance and scintillometry measurements from a reservoir in southeast Queensland, Australia. The work presented expands on a short study presented by McJannet et al. (2011) to include comparisons of eddy covariance measurements and scintillometer-derived predictions of surface energy fluxes under a wide range of seasonal weather conditions. In this study, analysis was undertaken to ascertain whether important theoretical assumptions required for both techniques are valid in the complex environment of a small reservoir. Statistical comparison, energy balance closure, and the relationship between evaporation measurements and key environmental controls were used to compare the results of the two techniques. Reasonable agreement was shown between the sensible heat flux measurements from eddy covariance and scintillometry, while scintillometer-derived estimates of latent heat flux were approximately 21% greater than eddy covariance measurements. We suggest possible reasons for this difference and provide recommendations for further research for improving measurements of surface energy fluxes over small water bodies using eddy covariance and scintillometry.

River evaporation and corresponding heat fluxes in forested catchments

AbstractRiver water temperature is a very important variable in ecological studies, especially for the management of fisheries and aquatic resources. Temperature can impact on fish distribution, growth, mortality and community dynamics. River evaporation has been identified as an important heat loss and a key process in the thermal regime of rivers. However, its quantification remains a challenge, mainly because of the difficulty of making direct measurements. The objectives of this study were to characterize the evaporative heat flux at different scales (brook vs river) and to improve the estimation of the evaporative heat flux in a stream temperature model at the hourly timescale. Using a mass balance approach with floating minipans, we measured river evaporation at an hourly timescale in a medium‐sized river (Little Southwest Miramichi) and a small brook (Catamaran Brook) in New Brunswick, Canada. With these direct measurements of evaporation, we developed mass transfer equations to estimate hourly evaporation rates from microclimate conditions measured 2 m above the stream. During the summer 2012, river evaporation was more important for the medium‐sized river with a mean daily evaporation rate of 3.0 mm day−1 in the Little Southwest Miramichi River compared with that of 1.0 mm day−1 in Catamaran Brook. Evaporation was the main heat loss mechanism in the two studied streams and was responsible for 42% of heat losses in the Little Southwest Miramichi River and 34% of heat losses in Catamaran Brook during the summer. Copyright © 2013 John Wiley &amp; Sons, Ltd.

The Importance of Spring and Autumn Atmospheric Conditions for the Evaporation Regime of Lake Superior

AbstractFeedbacks between ice extent and evaporation have long been suspected to be important for Lake Superior evaporation because it is during autumn and winter when latent heat fluxes are highest. Recent direct measurements of evaporation made at the Stannard Rock Lighthouse have provided new information on the physical controls on Lake Superior evaporation, in particular that evaporation can react within hours to days to a change in synoptic conditions. However, the large heat capacity of the lake creates a strong seasonal cycle of energy storage and release. There is a complex interaction among heat storage, evaporation, and ice cover that is highly dependent on atmospheric conditions in the spring and autumn “shoulder seasons.” Small changes in conditions in November and March caused by synoptic-scale events can have profound impacts on annual evaporation, the extent of ice cover, and the length of the ice-covered period. Early winter air temperatures in November and December dictate the nature of ice formation and much of the winter evaporative flux. Decreased ice cover, by itself, does not necessarily lead to enhanced annual evaporation losses. Rather, a combination of low ice cover and warm spring air temperatures, leading to an early breakup, can significantly lengthen the next evaporation season and cause greater cumulative water loss the subsequent year. The influence of individual synoptic events on annual evaporation is notable enough that the research community should ensure that their role is properly captured in numerical models to provide sound predictions of future Laurentian Great Lakes evaporation regimes.

Evaporation from Lake Superior: 2: Spatial distribution and variability

Evaporation is a critical component of the water balance of each of the Laurentian Great Lakes, and it is expected that because of their shear size, evaporation cannot be spatially or temporally uniform. Despite this, examples of spatially distributed estimates of evaporation in the scientific literature are rare for most of the lakes and non-existent for Lake Superior. Direct measurements of evaporation taken at an offshore site on Lake Superior from June 2008 to October 2010 were used with concurrent satellite and climate model data to extrapolate evaporation measurements across the entire lake. Evaporation rates, large scale forcing mechanisms, and spatial patterns and variability are presented. Spatial patterns of evaporation tend to follow synoptic-scale air masses traveling over the lake. While most evaporation occurs during relatively short term events, these episodes tend not to occur in isolated locations on the lake, but are spatially widespread. The exceptions to this rule are during periods of a stable atmosphere and low evaporation, and when a transient ice cover can limit evaporation from some areas.

Estimating evaporation from a wet grassland

Abstract. Wet grasslands are being restored across the UK and Europe to reinstate their high biodiversity following over 50 years of drainage and conversion to arable agriculture. The water balance of many wet grasslands is dominated by precipitation and evaporation and it is essential to quantify evaporation rates to understand the hydrological functioning of wetlands and the implications for water resources in catchments where wetlands are being restored. This paper considers data from direct measurements of evaporation from the Pevensey Levels wet grassland using the eddy correlation method. Equations are derived to predict actual evaporation using meteorological data on the site or from standard meteorological station observations. It was found that evaporation could be estimated reliably from meteorological variables, such as wind speed, temperature and humidity and by water availability. It was also found that when water availability is high, evaporation is high and may exceed reference evaporation values, raising questions over the deployment of the two-step Penman-Monteith model unless reliable crop coefficients and relative evaporation figures can be determined.

Measured and estimated evaporation and soil moisture deficit for growers and the water industry

AbstractMeasurements and estimates of evaporation have been routinely carried out for over a century. In the United Kingdom the most well‐known method of estimating evaporation and its derivatives such as soil moisture deficit (SMD) is the Meteorological Office Rainfall and Evaporation Calculation System (MORECS) which is based on the Penman‐Monteith equation. Direct measurements of evaporation have become less common in recent years. A comparison of measurements and estimates of evaporation and SMD at a single site and over two catchment areas is presented in this paper. Measurements of actual evaporation from a grassed lysimeter have been achieved for the period 1995–98 and they agree well with catchment losses over the same time period in two natural catchments, the Upper Brue and Upper Cary. In contrast to this, MORECS estimates of actual evaporation are different by an average of +20% for the Upper Brue catchment and +27% for the Upper Cary catchment. The largest difference is nearly +40% for the Upper Cary catchment. Similarly, there are substantial differences in the estimates of SMD, often up to 50 mm. When local rainfall is used together with MORECS estimates of potential evaporation the discrepancy is reduced but significant differences remain during much of the growing season, although improvements do occur. The results from this paper will be of great importance to growers, water resource planners, engineers who give flood warnings, and to river flow modellers. Copyright © 2002 Royal Meteorological Society.