Calculation Of Evapotranspiration Research Articles

Quantifying mesoscale soil moisture with the cosmic-ray rover

Abstract. Soil moisture governs the surface fluxes of mass and energy and is a major influence on floods and drought. Existing techniques measure soil moisture either at a point or over a large area many kilometers across. To bridge these two scales we used the cosmic-ray rover, an instrument similar to the recently developed COSMOS probe, but bigger and mobile. This paper explores the challenges and opportunities for mapping soil moisture over large areas using the cosmic-ray rover. In 2012, soil moisture was mapped 22 times in a 25 km × 40 km survey area of the Tucson Basin at an average of 1.7 km2 resolution, i.e., a survey area extent comparable to that of a pixel for the Soil Moisture and Ocean Salinity (SMOS) satellite mission. The soil moisture distribution is dominated by climatic variations, notably by the North American monsoon, that results in a systematic increase in the standard deviation, observed up to 0.022 m3 m−3, as a function of the mean, between 0.06 m3 m−3 and 0.14 m3 m−3. Two techniques are explored to use the cosmic-ray rover data for hydrologic applications: (1) interpolation of the 22 surveys into a daily soil moisture product by defining an approach to utilize and quantify the observed temporal stability producing an average correlation coefficient of 0.82 for the soil moisture distributions that were surveyed, and (2) estimation of soil moisture profiles by combining surface moisture from satellite microwave sensors (SMOS) with deeper measurements from the cosmic-ray rover. The interpolated soil moisture and soil moisture profiles allow for basin-wide mass balance calculation of evapotranspiration, which amounted to 241 mm in 2012. Generating soil moisture maps with a cosmic-ray rover at this intermediate scale may help in the calibration and validation of satellite soil moisture data products and may also aid in various large-scale hydrologic studies.

Monitoring daily evapotranspiration in Northeast Asia using MODIS and a regional Land Data Assimilation System

AbstractWe applied an approach for daily estimation and monitoring of evapotranspiration (ET) over the Northeast Asia monsoon region using satellite remote sensing observations from the Moderate Resolution Imaging Spectroradiometer (MODIS). Frequent cloud cover results in a substantial loss of remote sensing information, limiting the capability of continuous ET monitoring for the monsoon region. Accordingly, we applied and evaluated a stand‐alone MODIS ET algorithm for representative regional ecosystem types and an alternative algorithm to facilitate continuous regional ET estimates using surface meteorological inputs from the Korea Land Data Assimilation System (KLDAS) in addition to MODIS land products. The resulting ET calculations showed generally favorable agreement (root‐mean‐square error < 1.3 mm d−1) with respect to in situ measurements from eight regional flux tower sites. The estimated mean annual ET for 3 years (2006 to 2008) was approximately 362.0 ± 161.5 mm yr−1 over the Northeast Asia domain. In general, the MODIS and KLDAS‐based ET (MODIS‐KLDAS ET) results showed favorable performance when compared to tower observations, though the results were overestimated for a forest site by approximately 39.5% and underestimated for a cropland site in South Korea by 0.8%. The MODIS‐KLDAS ET data were generally underestimated relative to the MODIS (MOD16) operational global terrestrial ET product for various biome types, excluding cropland; however, MODIS‐KLDAS ET showed better agreement than MOD16 ET for forest and cropland sites in South Korea. Our results indicate that MODIS ET estimates are feasible but are limited by satellite optical‐infrared remote sensing constraints over cloudy regions, whereas alternative ET estimates using continuous meteorological inputs from operational regional climate systems (e.g., KLDAS) provide accurate ET results and continuous monitoring capability under all‐sky conditions.

Comparison of Several Methods for Calculation of Reference Evapotranspiration

Abstract The knowledge of the evapotranspiration of natural ecosystems and plant populations is of fundamental importance in several branches of science, research and practical uses. Nevertheless, the harmonisation of the large number of methods and user needs often causes problems. The objective of the analyses was to explore the output range and sensitivity of models of different physical approaches under local conditions. We performed descriptive statistical and sensitivity analysis of 10 commonly used estimation models - one of them with two variants. Correlation between modelled and measured evapotranspiration data series was assessed. The magnitude of the model outputs, their variability and responses to the changes of selected atmospheric parameters were evaluated. Priestley-Taylor, Penman-Monteith-FAO-56, Shuttleworth-Wallace (parameterized with alternative radiation balance), Szász and Makkink proved to be the most sensitive methods. As regards the systematic error, Makkink and Shuttleworth-Wallace showed the best agreement with pan evaporation, while Shuttleworth-Wallace, Blaney-Criddle and Makkink models were found to be the closest to the Penman-Monteith-FAO-56 method as a reference value.

Optimizing Irrigation of Fresh Market Tomato Grown in the Mid-Atlantic United States

Determining irrigation requirements for fresh market tomato (Solanum lycopersicum) production is essential to obtain optimum yields, cost-effective water use, and minimize nitrate leaching. The objective of this study was to determine the appropriate irrigation rate for polyethylene-mulched fresh market tomato grown in Virginia. This study investigated irrigation regimes by applying water based on evapotranspiration (ET) calculations in three spring and three fall seasons. Plants were grown using 0.0 × ET, 0.5 × ET, 1.0 × ET, 1.5 × ET, and 2.0 × ET. Additional irrigation treatments involved tensiometers installed at 12-inch depth in the bed, programmed to irrigate at soil moisture set points of −20, −40, and −60 kPa. Tensiometer treatments were able to irrigate up to nine times per day if soil moisture fell below the designated moisture set point. Measurements included fruit yield, plant and fruit nitrogen (N) uptake, and inorganic soil nitrate-N (NO3-N) at 0 to 10-, 10 to 20-, and 20 to 30-inch depths. Overall, the 0.5 × ET treatment provided optimum yields in all growing seasons except Spring 2010, which was unseasonably hot and dry. A tensiometer treatment (−40 kPa) provided optimum yields in all growing seasons, and was able to adjust irrigation in a hot and dry season. Residual soil NO3-N at 0 to 10 inches generally exhibited an inverse relationship with yield; greater yields resulted in less residual soil NO3-N. In most treatments throughout the duration of this study, plant N uptake + fruit N uptake accounted for most of the N fertilizer applied (68% to 151%). In conclusion, an irrigation rate of 0.5 × ET and a tensiometer treatment (−40 kPa) provided minimal irrigation inputs to obtain optimum marketable yields while also minimizing residual soil nitrate that may be prone to leaching after the season.

Evamapper: A Novel Matlab Toolbox For Evapotranspiration Mapping

Abstract. Water consumption has been exceeding as the world population increases. Therefore, it is very important to manage water resources with care as it is not an endless resource. The Water loss in regional scale is the key phenomena to accomplish this goal. One of the main components of this phenomenon is evapotraspiration (ET) due to being one of the most important parameter for the management of water resources. Until recent years, evapotranspiration calculations were performed locally, using data obtained from weather stations. But for a successful water management, regional evapotranspiration maps are required. Different approaches are used to compute regional ETs. Among them, the direct measurement methods are not cost-effective and regionalized. For costeffective and regional ET mapping, Surface Energy Balance Algorithm (SEBAL) is the most known and effective technique. In this study, EvaMapper Toolbox which is based on SEBAL approach are developed for regional evapotranspiration mapping in MATLAB. By this toolbox, researchers can apply SEBAL technique which has a very complex structure to their study area easily through entering regional parameter values.

Comparison of six evapotranspiration models for a surface irrigated maize agro-ecosystem in Northern Italy

The approaches for the estimation of evapotranspiration (ET) can be classified in “direct” methods, based on the original Penman–Monteith (PM) equation, in which the canopy resistance rc is modelled, and “indirect” methods, based on the preliminary calculation of ET for a well-watered reference grass (ETo) with a constant rc, which is then multiplied by a crop coefficient Kc to obtain ET. Even if the latter approaches are more widely adopted for their practical simplicity, many authors show that the former often provide better ET estimates in absence of calibration of crop parameters. In this study the performances of different direct and indirect methods were evaluated in the case of a surface irrigated maize grown in the Padana Plain (Northern Italy). The “one-layer” original PM equation with three different models for rc (Monteith, Jarvis, Katerji–Perrier), the “two-layers” PM model proposed by Shuttleworth and Wallace, the “single” and “double crop coefficient” models illustrated in the Paper FAO-56 were compared to latent heat fluxes measured in 2006 by eddy-covariance techniques. Results confirm that direct methods are more performing. The FAO-56 models with generalized crop coefficients overestimate ET, especially during the middle growth stage.

Multiscale spectral analysis of temporal variability in evapotranspiration over irrigated cropland in an arid region

The temporal patterns of evapotranspiration (ET) and its biophysical and physiological controls (e.g., soil water content, solar radiation, and canopy conductance) occur over a wide range of time-scales ranging from seconds to decades. Thus, there is increasing interest in understanding at which scales the main temporal correlations between ET and its controlling factors occur across different ecosystems. For this study, we used eddy covariance measurements of ET over 2 years at an irrigated maize field in an arid inland region of northwest China. We applied the wavelet transform as a novel technique to examine spectral characteristics of ET and its controlling factors. The ET power spectra displayed a −1 power law in turbulent inertial subrange at <1-h time-scale, and showed substantial power at daily, seasonal and annual time-scales. The cospectra of ET and soil water content (SWC) showed significant temporal correlation at 5-days, which has implications for calculation of ET using the soil water balance method in this region. We found that ET synchronized with the change of net radiation, and led vapor pressure deficit and air temperature for ∼2h at the 1-day time-scale (i.e., positive lags), but the phase relationship between ET and SWC was influenced by irrigation patterns. Canopy conductance influenced ET variability at the 1-day time-scale, but the effect was not consistent across the growing seasons. Our results show the importance of irrigation practices and its influence on the multi-temporal correlations of ET and its controlling factors. Irrigation can sharply change the phase angle relationship of ET and SWC from −10h to 10h. These results are important for understanding water cycle processes, improving water management, and address food security issues across irrigated croplands in arid regions.

Evapotranspiration models for a maize agro-ecosystem in irrigated and rainfed conditions

A high level of accuracy in the estimation of crop evapotranspiration (ET) may lead to significant savings of economic and water resources in irrigated agriculture. Although ET is a fundamental process in many applications, it cannot be directly measured but it has to be estimated by monitoring the exchange of energy/water above the vegetated surface (micrometeorological methods), or as a residual term of the hydrological balance (lysimeters, soil water budget). The techniques to be adopted are often complex, costly and require specific equipment. Thus, since the ‘50s, many researchers have devoted their activity to the development of models for its estimation. The available approaches can be classified in “direct” methods, based on the original Penman-Monteith (PM) equation, in which the canopy resistance rc is modelled, and “indirect” methods, based on the preliminary calculation of ET for a well-watered reference grass (ET0) with a constant rc, which is then multiplied by a crop coefficient Kc and, in case, by a stress coefficient Ks to obtain ET. Even if the latter approaches are more widely adopted for their practical simplicity, many authors show that the former often provide better ET estimates in absence of calibration of crop parameters. In this study the performances of different direct and indirect methods were evaluated in the case of a surface irrigated and of a rainfed maize grown in the Padana Plain (Northern Italy). The following models were considered: the “one-layer” original PM equation with three different models for rc (Monteith, Jarvis, Katerji-Perrier), the “two-layers” PM model proposed by Shuttleworth and Wallace, the “single” and “double” crop coefficient models illustrated in the Paper FAO-56. Latent heat fluxes measured in 2006 and 2011 in an experimental maize field by eddy-covariance were used to evaluate the models accuracy. Crop, soil and meteo data monitored contextually were used for the implementation of the different models. Results confirm that direct methods are more performing for both irrigated (2006) and rainfed (2011) conditions, with the SW model providing the best results and the FAO-56 models with generalized crop coefficients overestimating ET, especially during the middle growth stage.

A multi-disciplinary assessment of the southeastern United States climate

The southeastern United States (SUS) is a region that exhibits considerable climate variability throughout the year along with high-impact weather extremes of severe weather and land-falling Atlantic tropical cyclones in the summer and fall, freeze events in the winter season, and tornadoes in the spring season. The SUS region has rapidly grown in terms of human population for last several decades that further exposes vulnerabilities to the frailties of nature. Furthermore, in the recent times, the agricultural production of the region has grown significantly with the rise in consumption and demand for bio fuels, increase in price and demand for commodity and food crops and increasing irrigation infrastructure that is subsidized by the state governments in the SUS. Similarly, the coastal development and capitalization of natural marine resource environment have also risen with the growing wealth of the population. All of these developments have led to an increasing interest in the understanding of the climate variations and change on the built and the natural environments of the SUS. This special issue is a manifestation of such an interest in the SUS with contributions from several climate-related inter-disciplinary groups including the Southeast Climate Consortium (SECC; http://seclimate. org/), the Florida Climate Institute (FCI; http://floridacli mateinstitute.org/), the University of Florida Water Institute (http://waterinstitute.ufl.edu/), and the Florida Water & Climate Alliance (FloridaWCA; http://www.floridawca.org). There are 15 research papers in this special issue covering climate science and its applications in hydrology, ecology, economy, crop science, and social science. It is quite apparent from the diverse set of papers that the demand for ‘‘reliable’’ regional-scale climate data over the SUS is of considerable interest for climate impact assessment. As a result, there are several papers in this collection which dwell on developing the regional-scale climate information from dynamic downscaling (e.g., Misra et al. 2012), or from statistical downscaling (e.g., Asefa and Adams 2013) or from a combination of statistical and dynamical downscaling (e.g., Hwang et al. 2013). Asefa and Adams (2013) introduced in their paper a new statistical bias correction technique for regional climate projections over central Florida based on a Bayesian approach that weights on the reliability of the global climate model in reproducing the observed climate. Hwang et al. (2013) highlight the merit of using dynamically downscaled and statistically bias-corrected climate data for hydrological applications over the Tampa Bay watershed. Misra et al. (2012) show the advantage and fidelity of dynamically downscaling the twentieth-century global atmospheric reanalysis (20CR) to 10 km grid resolution. For example, they demonstrate that data inhomogeneity issues over SUS in the 20CR are greatly ameliorated by the internal variations resolved by the dynamic downscaling to 10-km grid resolution. The interest in regional-scale climate information over SUS is also evidenced in a number of papers in this special issue investigating the reliability of some of the existing high-resolution regional climate datasets. LaRow (2012) analyzes the surface wind and precipitation in land-falling Atlantic tropical cyclones over the SUS from one of the existing global reanalyses downscaled to 10 km grid resolution. Similarly, Obeysekera (2013) interrogates the reliability of the meteorological variables used in the calculation of the surface evapotranspiration from the V. Misra (&) Department of Earth, Ocean and Atmospheric Science & Florida Climate Institute & Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, FL, USA e-mail: vmisra@fsu.edu

FAO-56 methodology for determining water requirement of irrigated crops: critical examination of the concepts, alternative proposals and validation in Mediterranean region

The present study evaluates firstly the ability of the FAO-56 methodology, based on the two-step approach “reference evapotranspiration (ET0)—crop coefficient (K c),” to accurately determine the actual evapotranspiration (ET) of irrigated crops and proposes, secondly, the alternative approaches for improving this determination. The FAO-56 methodology is supported by two hypotheses: (1) ET0 represents all effects of weather and (2) K c varies predominately with specific crop characteristics and only marginally with climate, which enables the transfer of K c standard values among locations and climates. On the base of the theoretical analysis and experimental observations, a critical examination of the previous hypotheses demonstrates that they are not verified by reality. The first hypothesis is not verified for two reasons: (a) The formulation adapted by the Penman–Monteith equation and proposed in FAO-56 methodology for calculating ET0 uses climatic variables determined at a 24-h average scale. However, in principle it is only valid in permanent regime, in other words at least on an hourly scale. (b) The FAO-56-proposed formulation attributes a constant value to the canopy resistance of the reference surface; but in reality, this resistance is variable in relation to the climatic variables. The second hypothesis, concerning the two-step approach, is also not verified because the values of K c largely vary in relation to climatic variables (radiation, air vapour pressure deficit and wind speed). This fact does not support the possibility of the transferability of K c values into locations where the local conditions deviate from the conditions where the adjusted values of K c were determined. The weakness of the ET estimation, observed on several crops cultivated in the Mediterranean region, through the application of the FAO-56 methodology, is the result of errors accumulation, associated with that affects the determination of either ET0 or K c. The present study underlines the advantage of using a one-step approach in the calculation of ET, since it is based on fewer computation steps and, consequently, on fewer error sources than the two-step model. Two models adopting this approach are proposed and validated, one of which can be considered as operational, i.e. it only needs standard meteorological data as input. The use of these models enables an improvement of the ET estimation. This objective is a key component of any strategy to improve agricultural water management in Mediterranean region.

Coupling simulation of water-carbon processes for catchment―calibration and validation of the WaSSI-C model

Aims The water-carbon coupling model has become a major approach of eco-hydrology study under the complicated situation of climate changes. Our objective was to explore the applicability of the WaSSI-C model for the humid region in southwestern China. Methods We focused on a large watershed, the upper Zagunao River watershed, situated in the upper reach of the Minjiang River, Yangtze River Basin. Applicability of the WaSSI-C model was assessed by evaluating the modeling results with two indexes: determination coefficient (R2) and Nash-Sutcliffe efficiency coefficient (NS). In order to make it better suited for the study area, two main improvements were introduced regarding to the snowmelt and evapotranspiration (ET) calculation. The calibration period was set as 1988–1996, and the validation period was set as 1997–2006. We validated the model using observed runoff and gross ecosystem productivity (GEP) and ET production of moderate resolution imaging spectroradiometer (MODIS) in the calibration and validation periods, respectively. Important findings There was a good performance of our application of the WaSSI-C model in the study area. The R2 of calibration period and NS for the total runoff of the study watershed were 0.86 and 0.82, respectively, and 0.78 and 0.67, respectively, for the validation period. Moreover, the R2 of the GEP and ET were 0.89 and 0.78, respectively. We discussed the modeling process and results, and the future direction of improvements of the ET calculation in the end.

Sensitivity analysis of different evapotranspiration methods using a new sensitivity coefficient

The estimation of evapotranspiration is essential in water resources management. Among a group of methods, the Penman–Monteith has been commonly applied to calculate reference evapotranspiration as this method has been also recommended by the Food and Agriculture Organization of the U.N. (FAO). Other methods widely used are: the FAO 24 Penman, the modified Blaney and Criddle, the FAO 24 Makkink, and the Hargreaves. Sensitivity analysis is required to gain a better understanding of the meteorological systems; particularly to indicate the physical meaning of each meteorological parameter used in the estimation of the reference evapotranspiration. Several dimensionless sensitivity coefficients have been proposed, based on the partial derivative of the dependent variable (reference evapotranspiration) to the independent variables (meteorological variables). In this paper, a new sensitivity coefficient is proposed to drive sensitivity analysis of the evapotranspiration methods. The new sensitivity coefficient uses the partial derivative and the standard deviation of each independent variable. The meteorological variables, whose influence has been examined, are all the necessary meteorological parameters for the calculation of reference evapotranspiration, such as temperature, solar radiation, wind speed and relative humidity for each method. Data from the automatic meteorological station of Aminteo in the Prefecture of Florina, Western Macedonia, were used. The sensitivity coefficients were calculated for each month, year and irrigation period. The comparison of the sensitivity coefficients is performed for the month of water peak demand (July), the irrigation period and the year for each evapotranspiration method. Results show that the influence of the variables to evapotranspiration is not the same for each period, and also the order that the variables influence evapotranspiration is changing. A comparison between the five evapotranspiration methods shows that solar radiation and temperature are the main parameters that affect evapotranspiration, while relative humidity and wind speed are not so important for the calculation of evapotranspiration.

Water Requirement of Grape (Vitis vinifera) in the Northern Highlands of Yemen

Grape is a major fruit crop which occupies 33% of the total area of fruit cultivation in Yemen. Grape vines are cultivated under both irrigated and rainfed production systems. The irrigation practices in grape orchards are traditional with low efficiency due to high losses of water. In order to obtain rapid and reliable results, the comparison of five equations for calculation of evapotranspiration and obtaining the Kc values by utilizing actual evapotranspiration of grape became necessary. Crop water requirement of grape trees in Sawan, Bani Hushaish District in Sana'a Governorate was studied and two methods of irrigation were investigated for two years (2005-2006). The investigated irrigation methods were: bubbler (localized) irrigation and basin irrigation. Results indicated the significant superiority of bubbler irrigation over the basin irrigation. The actual water requirements reached 601 and 736 mm water depth respectively with application efficiency reaching 82.6% and 69.8% respectively. The irrigation water productivity of the bubbler irrigation was significant (3.8 kg/m3) while it was less under basin irrigation (1.8 kg/m3). Results indicated that the average crop coefficient throughout the growing season ranged from 0.42 in the case of using Ivanov equation and 0.75 in the case of Hargreaves. In addition to standard FAO Penman-Monteith equation, the Hargreaves and Blaney-Criddle are the best equations that can be used in determination of crop water requirements and irrigation scheduling of grapes. It was also observed that the highest crop coefficient was recorded in the months of May and June in all treatments.

Hyperspectral remote sensing for growth-stage-specific water use in wheat

Precise application of irrigation water to crops requires an accurate calculation of daily crop evapotranspiration (ET), which has always remained a challenge to the scientific community. Reflectance-based crop coefficients approach has a strong theoretical base, as both the crop coefficient [ratio of actual crop (ETc) and reference ET (ET0)] and remote sensing of crop follow a similar response curve mediated by crop growth stages and crop health conditions. This paper investigates the feasibility of linking the evolution of basal crop coefficient (Kcb) of wheat to the hyperspectral remote sensing derived vegetation indices, through leaf area index (LAI), the principal plant growth parameter. Two years field experiments were conducted with three cultivars of wheat (Triticum aestivum L.) under adequate (6-cm each irrigation) and limited (4-cm each irrigation) water supply. Ground based observations on profile water balance components, hyperspectral remote sensing, fractional ground coverage, LAI, water potential and relative water content in leaves were monitored periodically. Biomass and yields were recorded at harvest. Limited water application forced the crop to attain its peak crop coefficients and LAI values early (at flowering, 80–95DAS), compared to at milking stage (90–105DAS) under adequate water supply. Basal crop coefficients (Kcb), indicative of transpiration in plants were able to generate a better estimate of the stage-specific crop water use. The prospect of its retrieval through hyperspectral remote sensing was demonstrated. A reduction in Kcb could be primarily due to reduction in LAI in wheat, especially when soil moisture was not a limiting factor. Exclusion of residual evaporation and minimizing background effect of soil made the evolution of Kcb similar to LAI and LAI similar to Soil Adjusted Vegetation Index (SAVI). These imply that the transpiration and light absorption profile of the crop increase or decrease with nearly the same rate throughout its growth period. The LAI saturated at a value of 3 in limited and 4 in adequate irrigation treatments suggesting that once the canopy coverage is complete, further increase in LAI might not lead to an increase in single crop coefficient values. Interestingly, SAVI showed a linear response to Kcb, and also did not saturate before the LAI reached to 4.5 (LAI>4.0 is reported in full developed canopies of wheat). This makes SAVI superior than NDVI (Normalized Difference Vegetation Index), which saturates at LAI>3.5, for retrieving crop coefficient; and improving the accuracy in predicting crop water use at specific stages. These relations have high potential at an operational scale for irrigation scheduling over extended wheat growing areas like Indo-Gangetic Plains, through use of high resolution earth observation satellite data.

Evapotranspiration estimation methods in hydrological models

Actual evapotranspiration is a key process of hydrological cycle and a sole term that links land surface water balance and land surface energy balance. Evapotranspiration plays a key role in simulating hydrological effect of climate change, and a review of evapotranspira- tion estimation methods in hydrological models is of vital importance. This paper firstly summarizes the evapotranspiration estimation methods applied in hydrological models and then classifies them into the integrated converting methods and the classification gathering methods by their mechanism. Integrated converting methods are usually used in hydrological models and two differences exist among them: one is in the potential evaporation estimation methods, while the other in the function for defining relationship between potential evapora- tion and actual evapotranspiration. Due to the higher information requirements of the Pen- man-Monteith method and the existing data uncertainty, simplified empirical methods for calculating potential and actual evapotranspiration are widely used in hydrological models. Different evapotranspiration calculation methods are used depending on the complexity of the hydrological model, and importance and difficulty in the selection of the most suitable evapotranspiration methods is discussed. Finally, this paper points out the prospective de- velopment trends of the evapotranspiration estimating methods in hydrological modeling.

Evapotranspiration models for a maize agro-ecosystem in irrigated and rainfed conditions

A high level of accuracy in the estimation of crop evapotranspiration (ET) may lead to significant savings of economic and water resources in irrigated agriculture. Although ET is a fundamental process in many applications, it cannot be directly measured but it has to be estimated by monitoring the exchange of energy/water above the vegetated surface (micrometeorological methods), or as a residual term of the hydrological balance (lysimeters, soil water budget). The techniques to be adopted are often complex, costly and require specific equipment. Thus, since the ‘50s, many researchers have devoted their activity to the development of models for its estimation. The available approaches can be classified in “direct” methods, based on the original Penman-Monteith (PM) equation, in which the canopy resistance rc is modelled, and “indirect” methods, based on the preliminary calculation of ET for a well-watered reference grass (ET0) with a constant rc, which is then multiplied by a crop coefficient Kc and, in case, by a stress coefficient Ks to obtain ET. Even if the latter approaches are more widely adopted for their practical simplicity, many authors show that the former often provide better ET estimates in absence of calibration of crop parameters. In this study the performances of different direct and indirect methods were evaluated in the case of a surface irrigated and of a rainfed maize grown in the Padana Plain (Northern Italy). The following models were considered: the “one-layer” original PM equation with three different models for rc (Monteith, Jarvis, Katerji-Perrier), the “two-layers” PM model proposed by Shuttleworth and Wallace, the “single” and “double” crop coefficient models illustrated in the Paper FAO-56. Latent heat fluxes measured in 2006 and 2011 in an experimental maize field by eddy-covariance were used to evaluate the models accuracy. Crop, soil and meteo data monitored contextually were used for the implementation of the different models. Results confirm that direct methods are more performing for both irrigated (2006) and rainfed (2011) conditions, with the SW model providing the best results and the FAO-56 models with generalized crop coefficients overestimating ET, especially during the middle growth stage.

Comparison of Reference Evapotranspiration Calculations for Southeastern North Dakota

Potential water consumption for irrigation scheduling in North Dakota was typically calculated from a reference Evapotranspiration (ETref) using the Jensen-Haise method and its associated crop coefficient (Kc) curves developed in the 1970’s and 1980’s. The ETref method proposed by the American Society of Civil Engineers, Environmental and Water Research Institute (ASCE-EWRI) reference evapotranspiration task force has shown to be more accurate and therefore more widely used than any other methods. However, to apply the ASCE-EWRI method for irrigation scheduling requires a corresponding change of the Kc curves associated with the Jensen-Haise method. In this paper, a comparison of ETref estimates for 11 methods, including the ASCE-EWRI and the Jensen-Haise methods, was conducted using 18 years of data collected in southeastern North Dakota. The results show that the annual ETref by the Jensen-Haise method was nearly the same as the ASCE-EWRI grass ETref, but with a higher Root Mean Square Deviation (RMSD), 0.903 mm d-1, and a lower coefficient of determination (R2) 0.8659. The ETref comparison for the growing season only shows an RMSD of 1.007 mm d-1, R2 of 0.7996 and 8.13% overestimation. The ETref by the Jensen-Haisemethod has a higher monthly ETref than the ASCE-EWRI in June, July, and August, and a lower monthly ETref for all other months in an 18 year period. The ETref comparisons also show that the modified Penman method used by the High Plains Regional Climate Center (HPRCC Penman) has the best accuracy and correlation with the ASCE-EWRI ETref method. Indeed, all alfalfa based ETref methods, including Kimberly Penman and HPRCC Penman, show better performance than the grass based ETref methods, including FAO24 Penman, FAO24 Radiation, FAO24 Blaney-Criddle, Priestley-Taylor, Hargreaves, and the Jensen-Haise methods.

Determination of water balance components with high precision weighing lysimeter in Kleče

Components of the basic water balance equation and water balance calculation for July 2011 for weighing lysimeter on water supply pumping station in Kleče, Ljubljana the presented. Lysimeter and outflow mass measured with high precision weighing cells and precipitation as determined from changes in the mass of the lysimeter were used in calculation. Precipitation measurements in the same time resolution as the lysimeter mass measurements would be needed for correct calculation of actual evapotranspiration. In time of high plant water requirements only substantial precipitation events directly contribute to immediate groundwater recharge. The low water retention of the aquifer sediments shows susceptibility of the aquifer to ground water pollution.

Estimation method of grass net radiation on the determination of potential evapotranspiration

Evapotranspiration depends primarily on the local energy balance, which is the most important variable of the physical environment when it comes to irrigation scheduling, especially throughout the production formation stages that are limited by water factors. The current manuscript will verify the applicability of estimates of grass net radiation to the performance of estimation methods of potential evapotranspiration under the climatic conditions in Piracicaba, Sao Paulo, Brazil. In order to proceed to simple linear regression studies, meteorological data were monitored by an automatic weather station throughout a period of 87 days for the calculation of potential evapotranspiration, taking into account both the estimated and calculated grass net radiation values. The use of grass net radiation estimated by a model based on the local energy balance showed an excellent performance for estimating potential evapotranspiration by means of the Penman–Monteith, adjusted Priestley–Taylor, and simplified Penman approaches, along with a good agreement (R2 > 0.91) related to the potential demand values obtained with grass net radiation measured by net radiometers. Copyright © 2012 Royal Meteorological Society

Multilayer perceptron for reference evapotranspiration estimation in a semiarid region

Calculation of reference evapotranspiration (ETo) is essential in hydrology and agriculture. ETo plays an important role in planning and management of water resources and irrigation scheduling. The results of many studies strongly support the use of the Penman–Monteith FAO 56 (PMF-56) method as the standard method of estimating ETo. The basic obstacle to using this method widely is the numerous meteorological variables required. Multilayer perceptron (MLP) networks optimized with different learning algorithms and activation functions were applied for estimating ETo in a semiarid region in Iran. Four MLP models comprising various combinations of meteorological variables are developed. The MLP model which needs all of the meteorological parameters performed best for ETo estimation amongst the other MLP models. It was also found that the ConjugateGradient, DeltaBarDelta, DeltaBarDelta and Levenberg–Marquardt were the best algorithms for training the MLP1, MLP2, MLP3 and MLP4 models, respectively.