Calculation Of Evapotranspiration Research Articles

Estimation and comparison of reference evapotranspiration using different methods to determine olive trees irrigation schedule in different bioclimatic stages of Tunisia

The study of olive trees water requirements allows a better water management by using more accurate methods including maximum parameters of the continuum soil-plant- atmosphere. The Penman-Monteith equations is consideredas the most rational approach and the most reliable for calculating evapotranspiration. Only this approach necessarily requires an important number of climate parameters. The use of other equations, less complicated and using less climate parameters may be a reliable and efficient alternative. This experimental study was carried out on two cultivars cv. "Meski" and cv. "Chemlali" conducted in the intensive system in different bioclimatic stages (Subhumid, Semi-Arid and Arid) in Tunisia. This work aims to estimate olive trees water needs using evapotranspiration calculation in three different bioclimatic stages. For that, we compared the Penman-Monteith formula with Blaney-Criddel, Hargreaves-Temperature, Hargreaves- Radiation and Priestley-Taylor formulas to estimate reference evapotranspiration (ET0). Results show that ET0 values calculated by Priestley-Taylor and Blaney-Criddel formulas were more or less similar to Penman-Monteith. The ET0 values found by Hargreaves-Temperature and Hargreaves-Radiation were twice the values calculated by Penman-Monteith formula. We also found good correlations between the reference evapotranspiration calculated by the Penman-Monteith equation and that calculated by Priestley-Taylor and Blaney-Criddel equations in all bioclimatic stages (R2 more than 0.85, p < 1%). The ET0 sensitivity analysis has shown that solar radiation and air temperature (energetic climatic parameters) have the dominant effect on the ET0 at the level of the different climatic regions. Accordingly, in the case of lack of some climatic parameters and in sub-humid, semi-arid and arid conditions and for the different phenological stages of the olive tree, we can use Priestley-Taylor and/or Blaney-Criddle formulas to estimate water needs.

Water footprint of regional products: the case of white cachama (Piaractus brachypomus)

In response to the demand for water resources, the ecological indicator “Water Footprint” appears, which is measured in terms of the water consumed and / or contaminated per liter of water / kg / harvest. The Water Footprint helps to optimize the use of water resources by reducing water changes, increasing planting densities, employing closed systems with recirculation and / or green water technologies. For the calculation of the Water Footprint in fish ponds in the Piedmont Llanero, the climatology, meteorology and hydrological regime in the municipality of Restrepo-Meta-Colombia were characterized. Calculations of potential evapotranspiration were carried out, as well as effective precipitation. The volume necessary for the assimilation of the contaminants by the water bodies and the virtual water for the elaboration of the concentrate was also considered. For the cultivation of white cachama (Piaractus brachypomus), a Water Footprint of 3848.5 L of water / kg / harvest was calculated, a value lower than that reported for other fish cultures and, even, for other agricultural activities. It was also found that the Indirect Water Footprint (2913.3 L / kg) is the one that contributes the most to the total value of Water Footprint. This demonstrates the difference in water consumption in cachama production, bringing it closer to a sustainable production system.

A reference evapotranspiration map for Bosnia and Herzegovina

There are three major challenges for climatic data availability for Reference Evapotranspiration (ET0) calculations in Bosnia and Herzegovina: limited data availability, discontinuity of data records, and low station density. The main objective of this study was to apply reliable methods to calculate and spatially distribute ET0, while considering the impact of the elevation. A 20 m spatial resolution map was created for mean ET0 values at monthly, vegetation period (April–September) and annual temporal resolution at municipality (143 municipalities), regional (4 regions) and national spatial resolutions. 108 weather stations for the period 1961–2016 (56 years) were utilized for spatial interpolation of ET0 using kriging with external drift method. The required elevation for ET0 estimation at each grid node was extracted from the digital elevation model of Bosnia and Herzegovina. ET0 was calculated using FAO-56 Penman-Monteith, and in cases where only minimum and maximum temperature data were available the Hargreaves-Samani equation adjusted with locally appropriate empirical radiation coefficient was used. A gradual decrease of mean ET0 values from the southern to central and northern to central part of the country is notable. For the all seasons (monthly, vegetation and annually), the southern region has greater ET0 than the other three regions (north, west and central-east), which are similar. The long-term mean annual ET0 for Bosnia and Herzegovina is 716 mm, approximately 78% (559 mm) of which occurs during in the vegetation period.

Evaluation of Temperature-Based Methods for the Estimation of Reference Evapotranspiration in the Yucatán Peninsula, Mexico

Estimation of reference evapotranspiration (ETo) using air temperature is particularly attractive for places where solar radiation, wind speed, and air humidity data are not readily available. In this study, seven temperature-based (TET) models and the standardized reference evapotranspiration equation for short canopies method were compared. Using only temperature data from the Yucatán Peninsula, México, the Food and Agriculture Organization of the United Nations (FAO)-Penman-Monteith (PMT) model was used to estimate ETo. Results from the temperature-based models are compared with FAO-56 daily ETo calculations using the Nash-Sutcliffe model efficiency coefficient (NSE), the coefficient of determination (R2), mean absolute bias error (MAE), mean absolute percentage error (MAPE) and root mean square (RMSE). The results show that the noncalibrated PMT expression using temperatures alone produced the best results, with RMSE=0.7 mm d−1. The Hargreaves-Samani calibrated (RMSE=0.74 mm·d−1) and Camargo calibrated (RMSE=0.78) models exhibited the next best performance. RMSE values were as high as 1.56 mm·d−1 for the other models.

Assessment of different reference evapotranspiration models to estimate the actual evapotranspiration of corn (Zea mays L.) in a semiarid region (case study, Karaj, Iran)

In this study, an experiment was performed to assess and rank different evapotranspiration models. This was done to estimate the daily actual evapotranspiration of corn using a single (Kc-single) and dual (Kc-dual) crop coefficients in the semiarid climate of Karaj, Iran, in 2014. Daily evapotranspiration calculations using one combination-based model, one pan evaporation-based model, nine temperature-based models, ten radiation-based models, and seven mass transfer-based models were compared to the lysimeter measurements. Considering the single-crop coefficient, the Hargreaves-M3 model (RMSE = 1.89 mm/day) in the temperature-based models, the Caprio (1974) model (RMSE = 1.99 mm/day) in the radiation-based models, and the Albrecht (1950) model (RMSE = 4.33 mm/day) in the mass transfer-based models were ranked first place. Moreover, the Hargreaves-M2 model (RMSE = 0.88 mm/day) in the temperature-based models, the Caprio (1974) model (RMSE = 1.17 mm/day) in the radiation-based models, as well as the Albrecht (1950) model (RMSE = 3.76 mm/day) in the mass transfer-based models using the dual-crop coefficient, provided the most accurate estimation of daily corn evapotranspiration as compared to the lysimeter measurements.

Computationally Efficient Multivariate Calibration and Validation of a Grid-Based Hydrologic Model in Sparsely Gauged West African River Basins

The prediction of freshwater resources remains a challenging task in West Africa, where the decline of in situ measurements has a detrimental effect on the quality of estimates. In this study, we establish a series of modeling routines for the grid-based mesoscale Hydrologic Model (mHM) using Multiscale Parameter Regionalization (MPR). We provide a computationally efficient application of mHM-MPR across a diverse range of data-scarce basins using in situ observations, remote sensing, and reanalysis inputs. Model performance was first screened for four precipitation datasets and three evapotranspiration calculation methods. Subsequently, we developed a modeling framework in which the pre-screened model is first calibrated using discharge as the observed variable (mHM Q), and next calibrated using a combination of discharge and actual evapotranspiration data (mHM Q/ET). Both model setups were validated in a multi-variable evaluation framework using discharge, actual evapotranspiration, soil moisture and total water storage data. The model performed reasonably well, with mean discharge KGE values of 0.53 (mHM Q) and 0.49 (mHM Q/ET) for the calibration; and 0.23 (mHM Q) and 0.13 (mHM Q/ET) for the validation. Other tested variables were also within a good predictive range. This further confirmed the robustness and well-represented spatial distribution of the hydrologic predictions. Using MPR, the calibrated model can then be scaled to produce outputs at much smaller resolutions. Overall, our analysis highlights the worth of utilizing additional hydrologic variables (together with discharge) for the reliable application of a distributed hydrologic model in sparsely gauged West African river basins.

Estimation of Daily Evapotranspiration Using Instantaneous Decoupling Coefficient From the MODIS and Field Data

Daily evapotranspiration (ET) is of great significance among various practical applications including water management, drought monitoring, and climate change study. The ET estimations from remotely sensed models are usually instantaneous values. Various upscaling methods have been developed to extrapolate the instantaneous ET to daily scale. In the applications of these methods, the accuracy of daily ET estimation relies on both the accuracy of instantaneous ET calculation and the upscaling methods. This paper used the decoupling model to estimate daily ET directly, according to the constancy of the decoupling coefficient (Ω) in the model in a diurnal cycle, without the calculation of instantaneous ET. The estimated daily ET was compared with the Eddy covariance measurements which were corrected by the Bowen ratio method to close the energy imbalances. The result from field data alone showed that the coefficient of determination (R2) of daily ET estimation was 0.860, with a root-mean-square error (RMSE) of 18.2 W/m2, and a bias of −4.7 W/m2. Combining MODIS data and field data, the estimated daily ET had a R2 of 0.860, a RMSE of 21.6 W/m2, and a bias of −5.0 W/m2 . Therefore, it is feasible and effective to obtain daily ET using remote-sensing based instantaneous Ω to replace daily value in the decoupling model.

Calibration and evaluation of the FAO56-Penman-Monteith, FAO24-radiation, and Priestly-Taylor reference evapotranspiration models using the spatially measured solar radiation across a large arid and semi-arid area in southern Iran

Crop evapotranspiration (ET) is one of the main components in calculating the water balance in agricultural, hydrological, environmental, and climatological studies. Solar radiation (Rs) supplies the available energy for ET, and therefore, precise measurement of Rs is required for accurate ET estimation. However, measured Rs and ET and are not available in many areas and they should be estimated indirectly by the empirical methods. The Angstrom-Prescott (AP) is the most popular method for estimating Rs in areas where there are no measured data. In addition, the locally calibrated coefficients of AP are not yet available in many locations, and instead, the default coefficients are used. In this study, we investigated different approaches for Rs and ET calculations. The daily measured Rs values in 14 stations across arid and semi-arid areas of Fars province in south of Iran were used for calibrating the coefficients of the AP model. Results revealed that the calibrated AP coefficients were very different and higher than the default values. In addition, the reference ET (ETo) was estimated by the FAO56 Penman–Monteith (FAO56 PM) and FAO24-radiation methods by using the measured Rs and were then compared with the measured pan evaporation as an indication of the potential atmospheric demand. Interestingly and unlike many previous studies, which have suggested the FAO56 PM as the standard method in calculation of ETo, the FAO24-radiation with the measured Rs showed better agreement with the mean pan evaporation. Therefore, the FAO24-radiation with the measured Rs was used as the reference method for the study area, which was also confirmed by the previous studies based on the lysimeter data. Moreover, the accuracy of calibrated Rs in the estimation of ETo by the FAO56 PM and FAO24-radiation was investigated. Results showed that the calibrated Rs improved the accuracy of the estimated ETo by the FAO24-radiation compared with the FAO24-radiation using the measured Rs as the reference method, whereas there was no improvement in the estimation of ETo by the FAO56 PM method compared with the FAO24-radiation using the measured Rs. Moreover, the empirical coefficient (α) of the Priestley and Taylor (PT) ETo estimation method was calibrated against the reference method and results indicated ca. 2 or higher α values than the recommended α = 1.26 in all stations. An empirical equation was suggested based on yearly mean relative humidity for estimation of α in the study area. Overall, this study showed that (1) the FAO24-radiation method with the either measured or calibrated Rs is more accurate than the FAO56 PM, (2) the spatially calibrated AP coefficients are very different from each other over an arid and semi-arid area and are higher than those proposed by the FAO56, (3) the original PT model is not applicable in arid and semi-arid area and substantially underestimates the ETo, and (4) the coefficient of the PT should be locally calibrated for each station over an arid and semi-arid area.

Trading Natural Riparian Forests for Urban Shelterbelt Plantations—A Sustainability Assessment of the Kökyar Protection Forest in NW China

Cities at the fringe of the Taklimakan desert in NW China are prone to dust and sand storms with serious consequences for human well-being. The Kökyar Protection Forest was established in the 1980s as an ecological engineering project with the intent of protecting the city of Aksu, NW China, from these impacts. It is designed as a combination of poplar shelterbelts and orchards, irrigated by river water from the Aksu River, the main tributary of the Tarim River. Prevalent literature describes it as an afforestation project for combatting desertification with manifold positive effects for the economic, social, and environmental dimension of sustainable development. This paper sets out to challenge these claims by a sustainability assessment in which the plantation is examined from a broader perspective, embedding it to the wider context of social and environmental problems in South Xinjiang. Methods comprise evapotranspiration calculations, interviews, a socioeconomic household survey, stakeholder dialogues, and literature research. Results affirm its economic sustainability, but see a mixed record for the social sphere. From the nature conservation point of view, it has to be classified as unsustainable because its high irrigation water consumption results in the downstream desiccation and desertification of natural riparian forests along the Tarim River, causing a forest loss in the downstream area twice the size of the forest gain in the upstream area. There is a trade-off between artificial shelterbelt plantations for urban ecosystem services on the one hand side, and natural riparian forests and their biodiversity on the other hand side. The paper recommends restricting agricultural extension, and using locally adapted less water consuming agroforestry schemes to protect urban dwellers from dust stress.

Evaporation and abstraction determined from stable isotopes during normal flow on the Gariep River, South Africa

Changes in the stable isotope composition of water can, with the aid of climatic parameters, be used to calculate the quantity of evaporation from a water body. Previous workers have mostly focused on small, research catchments, with abundant data, but of limited scope. This study aimed to expand such work to a regional or sub-continental scale. The first full length isotope survey of the Gariep River quantifies evaporation on the river and the man-made reservoirs for the first time, and proposes a technique to calculate abstraction from the river. The theoretically determined final isotope composition for an evaporating water body in the given climate lies on the empirically determined local evaporation line, validating the assumptions and inputs to the Craig-Gordon evaporation model that was used. Evaporation from the Gariep River amounts to around 20% of flow, or 40 m3/s, of which about half is due to evaporation from the surface of the Gariep and Vanderkloof Reservoirs, showing the wastefulness of large surface water impoundments. This compares well with previous estimates based on evapotranspiration calculations, and equates to around 1300 GL/a of water, or about the annual water consumption of Johannesburg and Pretoria, where over 10 million people reside. Using similar evaporation calculations and applying existing transpiration estimates to a gauged length of river, the remaining quantity can be attributed to abstraction, amounting to 175 L/s/km in the lower middle reaches of the river. Given that high water demand and climate change are global problems, and with the challenges of maintaining water monitoring networks, stable isotopes are shown to be applicable over regional to national scales for modelling hydrological flows. Stable isotopes provide a complementary method to conventional flow gauging for understanding hydrology and management of large water resources, particularly in arid areas subject to significant evaporation.

Satellite Psychrometric Formulation of the Operational Simplified Surface Energy Balance (SSEBop) Model for Quantifying and Mapping Evapotranspiration

Abstract.Remote sensing-based evapotranspiration (ET) can be derived using various methods, from soil moisture accounting to vegetation-index based approaches to simple and complex surface energy balance techniques. Due to the complexity of fully representing and parameterizing ET sub-processes, different models tend to diverge in their estimations. However, most models appear to provide reasonable estimations that can meet user requirements for seasonal water use estimation and drought monitoring. One such model is the Operational Simplified Surface Energy Balance (SSEBop). This study presents a formulation of the SSEBop model using the psychrometric principle for vapor pressure/relative humidity measurements where the “dry-bulb” and “wet-bulb” equivalent readings can be obtained from satellite-based land surface temperature estimates. The difference in temperature between the dry (desired location) and wet limit (reference value) is directly correlated to the soil-vegetation composite moisture status (surface humidity) and thus producing a fractional value (0-1) to scale the reference ET. The reference ET is independently calculated using available weather data through the standardized Penman-Monteith equation. Satellite Psychrometric Approach (SPA) explains the SSEBop model more effectively than the energy balance principle because SSEBop does not solve all terms of the surface energy balance such as sensible and ground-heat fluxes. The SPA explanation demonstrates the psychrometric constant for the air can be readily adapted to a comparable constant for the surface, thus allowing the creation of a “surface” psychrometric constant that is unique to a location and day-of-year. This new surface psychrometric constant simplifies the calculation and explanation of satellite-based ET for several applications in agriculture and hydrology. The SPA formulation of SSEBop was found to be an enhancement of the ET equation formulated in 1977 by pioneering researchers. With only two key parameters, improved model results can be obtained using a one-time calibration for any bias correction. The model can be set up quickly for routine monitoring and assessment of ET at landscape scales and beyond. Keywords: Dry-bulb, ET fraction, ET modeling, Remote sensing, Satellite psychrometry, Wet-bulb.

Modifying Hargreaves-Samani Equation for Estimating Reference Evapotranspiration in Dryland Regions of Amudarya River Basin

Reference evapotranspiration (ETo) is a key factor in determining the amount of water needed for crops, which is crucial to correct irrigation planning. FAO Penman-Monteith (EToPM) is among the most popular method to estimate ETo. Apparently sometimes it is difficult to compute ETo using Penman-Monteith due to challenges on data availability. FAO Penman-Monteith method requires many parameters (solar radiation, air temperature, wind speed and humidity), while Hargreaves-Samani method calculates ETo based on air temperature. Because Central Asia is a data limited region with weather stations unable to provide all required parameters for the PM method, this study aimed to estimate ETo using the Hargreaves and Samani (HS) method in Karshi Steppe, in Kashkadarya province, in southern Uzbekistan, based on data from 2011 to 2017. Reference evapotranspiration calculated by non-modified HS method is underestimated during the summer months. The reason for this underestimation might be higher air temperature and wind speed during these months. Therefore, the HS method in its original form cannot be used in our study area to estimate ETo. Modification of the EToHS, through application of a bias correction factor, had better performance and allowed improving the accuracy of the ETo calculation for this region. The calculated ETo values can inform decision making and management practices regarding water allocation, irrigation scheduling and crop selection in dry land regions of Amudarya river basin and the greater Central Asia area.

RAPESEED (BRASSICA NAPUS, L.) – BIOLOGICAL REQUIREMENTS, GROWING CONDITIONS AND NEED FOR IRRIGATION

Rapeseed is one of the most important energy and food crops. The European agricultural producers are highly interested in it due to the obligations under the EU Directive for replacing the fossil fuels with biofuels as much as 20% by 2020, which has drawn the purchase prices considerably up. Rapeseed is widely used for production of cooking oil and rich in protein feed too. As regards to the environment, it contributes for restoration of degraded and contaminated lands owing to its capability for improving soil structure and leaving the area free of weeds. Rapeseed is one of the best pre-crops of winter wheat and contributes for some 20-30% increase of its yield. The paper contains an overview of different aspects of rapeseed growing: its biological requirements and the abiotic stressing factors in the Balkan geographic region; its sensibility to water and the impact of the water deficit on the yield and yield structural components; its yearly and monthly evapotranspiration and crop coefficients at different empirical evapotranspiration calculation methods; world data on its yields and the agricultural practices such as proper irrigation scheduling for its yield increase. The conclusions show that the soil and the climatic conditions on the Balkans are suitable for rapeseed growing and irrigation can contribute for obtaining sustainable yields from this crop.

Modeling the impact of climate change on water resources and soil erosion in a tropical catchment in Burkina Faso, West Africa

Soil erosion is recognized as one main reason for soil degradation in West Africa. However, predictions on the impact of climate change on soil erosion are rare for most West African countries including Burkina Faso.This study assesses the impact of climate change on water resources and soil erosion in a small catchment (126km2) in southwestern Burkina Faso. Climate data from an ensemble of six regional (RCM) and global (GCM) climate models were used to run the physically based spatially distributed hydrological and soil erosion model SHETRAN. The Representative Concentration Pathways (RCPs) 4.5 and 8.5 were selected as future climate scenarios.Bias corrected precipitation and temperature required for the calculation of potential evapotranspiration were used as input for the SHETRAN model to simulate total discharge and specific suspended sediment yield (SSY). Discharge and SSY from simulations run with climate data were able to reproduce discharge and SSY from a simulation that used observed precipitation and temperature from the historical period (1971–2000).The impact of climate change on hydrology and soil erosion was assessed by comparing the historical period with the future climate scenarios (2021–2050). Most of the used climate models predict an increase of temperature between 0.9°C and 2.0°C. The bias correction did not alter the climate change signal of temperature. Large uncertainties among the RCMs-GCMs exist regarding the climate change signal of future precipitation. Some climate models predict an increased (5.9%–36.5%) others a decreased (6.4%–10.9%) or mixed signal. The applied bias correction did not reverse the climate change signal in most cases but it influenced magnitude and timing of precipitation. The ensemble mean suggests an increased discharge between 19.5% (RCP 8.5) and 36.5% (RCP4.5) and an increased SSY of the same order. In general, the climate change signal and the corresponding discharge and SSY predictions are afflicted with large uncertainties. These uncertainties impede direct conclusions regarding future development of discharge and erosion. As a consequence of the mixed signals, potential increase and decrease of future discharge and soil erosion have to be incorporated in climate change adaption strategies.

Correction to: Evaluation of Changes in RDIst Index Effected by Different Potential Evapotranspiration Calculation Methods

Due to an oversight, 10 (ten) references were cited incorrectly in the reference list of the original publication. The first names were used instead of the last names of the authors. The correct presentation of these references is shown below.

Development and Analysis of a New Solar Radiation Atlas for Argentina from Ground-Based Measurements and CERES_SYN1deg data

Currently, quantifying global solar radiation at surface in Argentina is crucial for the development of projects related to solar energy, calculation of evapotranspiration and eco-sustainability architecture, among other environmental issues. In recent years, several models have been developed to estimate the solar energy resources by means of various techniques, e.g. satellite imaging, kriging, or Artificial Neural Networks. The use of satellite data allows for a better spatial representation, being of great relevance in areas with lack of terrain measurements. In this paper, we use the CERES_SYN1deg to develop a new Global Solar Radiation Atlas for Argentina. In this study, we developed maps of annual and monthly mean daily global solar radiation using CERES_SYN1deg data between 2000 and 2016. In order to validate the global solar radiation data provided by CERES_SYN1deg, they were compared with ground-based measurements in the time overlap of both instruments, in four monitoring sites of the SAVER-Net project and an additional site in Tandil, which belongs to the Remote Sensing Group of IHLLA. The maps show the spatial and temporal variation of global solar radiation in Argentina. Comparisons with ground-based pyranometers reveal relative differences of around 3% at a monthly scale for all sites, while the biases can be neglected. Therefore, it is possible to conclude that the maps could be very useful for different technical and scientific purposes, and the comparison with ground-based data demonstrates CERES_SYN1deg’s reliability.

Life Cycle Assessment of Two Vineyards after the Application of Precision Viticulture Techniques: A Case Study

Precision viticulture is the application of site-specific techniques to vineyard production to improve grape quality and yield and minimize the negative effects on the environment. While there are various studies on the inherent spatial and temporal variability of vineyards, the assessment of the environmental impact of variable rate applications has attracted limited attention. In this study, two vineyards planted with different grapevine cultivars (Sauvignon Blanc and Syrah) were examined for four consecutive growing seasons (2013–2016). The first year, the two vineyards were only studied in terms of soil properties and crop characteristics, which resulted in the delineation of two distinct management zones for each field. For the following three years, variable rate nutrient application was applied to each management zone based on leaf canopy reflectance, where variable rate irrigation was based on soil moisture sensors, meteorological data, evapotranspiration calculation, and leaf canopy reflectance. Life cycle assessment was carried out to identify the effect of variable rate applications on vineyard agro-ecosystems. The results of variable rate nutrients and water application in the selected management zones as an average value of three growing seasons were compared to the conventional practice. It was found that the reduction of product carbon footprint (PCF) of grapes in Sauvignon Blanc between the two periods was 25% in total. Fertilizer production and distribution (direct) and application (indirect) was the most important sector of greenhouse gas (GHG) emissions reduction, accounting for 17.2%, and the within-farm energy use was the second ranked sector with 8.8% (crop residue management increase GHG emissions by 1.1%, while 0.1% GHG reduction is obtained by pesticide use). For the Syrah vineyard, where the production was less intensive, precision viticulture led to a PCF reduction of 28.3% compared to conventional production. Fertilizers contributed to this decrease by 27.6%, while within-farm energy use had an impact of 2.2% that was positive even though irrigation was increased, due to yield rise. Our results suggest that nutrient status management offers the greatest potential for reducing GHG emissions in both vineyard types. Variable rate irrigation also showed differences in comparison to conventional treatment, but to a lesser degree than variable rate fertilization. This difference between conventional practices and precision viticulture is noteworthy, and shows the potential of precision techniques to reduce the effect of viticulture on GHG emissions.

Comparison of sum-of-hourly and daily time step standardized ASCE Penman-Monteith reference evapotranspiration

The objective of this study was to quantify differences associated with using 24-h time step reference evapotranspiration (ETo), as compared with the sum of hourly ETo computations with the standardized ASCE Penman-Monteith (ASCE-PM) model for semi-arid dry conditions at Fanaye and Ndiaye (Senegal) and semiarid humid conditions at Sapu (The Gambia) and Kankan (Guinea). The results showed that there was good agreement between the sum of hourly ETo and daily time step ETo at all four locations. The daily time step overestimated the daily ETo relative to the sum of hourly ETo by 1.3 to 8% for the whole study periods. However, there is location and monthly dependence of the magnitude of ETo values and the ratio of the ETo values estimated by both methods. Sum of hourly ETo tends to give higher ETo during winter time at Fanaye and Sapu, while the daily ETo was higher from March to November at the same weather stations. At Ndiaye and Kankan, daily time step estimates of ETo were high during the year. The simple linear regression slopes between the sum of 24-h ETo and the daily time step ETo at all weather stations varied from 1.02 to 1.08 with high coefficient of determination (R 2 ≥ 0.87). Application of the hourly ETo estimation method might help on accurate ETo estimation to meet irrigation requirement under precision agriculture.

SENSITIVITY ANALYSIS AND REGIONALIZATION OF REFERENCE EVAPOTRANSPIRATION FOR THE AMAZON REGION.

Sensitivity Analysis (SA) is important to understand the relative importance of climate variables in the reference evapotranspiration (ETo) computation. In this study, a sensitivity coefficient was used to predict ETo responses to disturbances of five climatic variables in the Amazonian Hydrographic Region - AHR (Brazilian Amazon). The ETo was estimated using the standardized equation of Penman-Monteith-FAO (PM-FAO). A 15-year meteorological data set of 38 surface meteorological stations were used in the study. An additional analysis was also presented to determine homogeneous regions of ETo by means of Cluster Analysis. The results showed that seven homogeneous sub-regions are sufficient to divide the AHR into different ETo patterns which were separated considering the intensity and the seasonal pattern of ETo. By the SA, the variables that contribute most to the computation of ETo using the PM-FAO method were the balance of radiation (Rn) and wind speed (u2). These results demonstrate that, in general, it should be emphasized to precise measures of insolation, since the precise estimation of Rn is directly associated with the measurement of this variable as well as of u2, which proved to be the second most influential variable in the ETo computation.

Evaluation of changes in RDIst index effected by different Potential Evapotranspiration calculation methods

Investigation of drought event has a great importance in the natural erctt management and water resources management planning. One of the important indexes of drought severity assessment is RDIst index, this index is B35based on ratio of precipitation (P) and potential evapotranspiration (PET) factors. Many different methods have been introduced for PET calculation. In this study results of monthly RDIst index (1 month) based on some popular PET calculation methods include: Jensen-Haise, Modified Jensen-Haise, Thornthwaite, Hargreaves-Samani, Blaney-Criddle and FAO Penman-Monteith (this method was used as reference method) with minimum data requirements were compared. In this research climatic data of 17 synoptic stations (with different climate conditions) in Iran during 1967–2014 were used to estimate PET and monthly RDIst index. Results of this paper showed that different methods of PET calculation had significant impact on RDIst index. According to results based on RMSE index Hargreaves- Samani had the most similarities with FAO Penman-Monteith method. Based on means comparison test, Blaney-Criddle and Modified Jensen-Haise methods had the most similarities with FAO Penman-Monteith (mean of RDIst index in 58.8% of stations in Thornthwaite method, 52.9% of stations in Jensen-Haise and Hargreaves-Samani methods and 71% of stations in Blaney-Criddle and Modified Jensen- Haise methods had not different by FAO Penman-Monteith).