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Non-Lambertian Corrected Albedo and Vegetation Index for Estimating Land Evapotranspiration in a Heterogeneous Semi-Arid Landscape  [PDF]
Isabella Mariotto,Vincent P. Gutschick
Remote Sensing , 2010, DOI: 10.3390/rs2040926
Abstract: The application of energy balance algorithms to remotely sensed imagery often fails to account for surface roughness variation with diverse land cover, resulting in poor resolution of evapotranspiration (ET) variations. Furthermore, the assumption of a horizontally homogeneous Lambertian surface reflecting energy equally in all directions affects the calculations of albedo and vegetation index. The primary objective of this study is to improve the accuracy of the estimation and discrimination of ET among different land cover types in Southern New Mexico from ASTER datasets, by formulating the spatial variation of non-Lambertian reflectance using a wavelength-dependent Minnaert function.
Species factor and evapotranspiration for an Ash (Fraxinus rotundifolia) and Cypress (Cupressus arizonica) in an arid region  [PDF]
B.Mostafazadeh-Fard,M. Heidarpour,S. E. Hashemi
Australian Journal of Crop Science , 2009,
Abstract: The determination of crop coefficients (species factor) and evapotranspiration are important for estimating irrigation water requirements in order to have better irrigation scheduling and water management. The aim of this study was to determine the species factor and evapotranspiration for a reference crop of grass (Lolium perenne) and for two typical landscape crops of Ash (Fraxinus rotundifolia) and Cypress (Cupressus arizonica) using field drainage lysimeters in an arid region of Isfahan in central part of Iran. The potential evapotranspiration was estimated using nine different common methods. Among these methods, the FAO-Radiation, Turc-Radiation-Grass and FAO-Blaney-Criddle methods showed very close agreement with the lysimeter data. The Penman-Monteith 56 and FAO-Corrected-Penman methods showed moderate agreement with the lysimeter data. The Hargreaves, Priestley-Taylor, Makkink 1957 and Penman-Kimberley did not show close agreement with the lysimeter data. The adjustment factors were suggested to overlap the estimated values to the lysimetric values. The values of the species factor for Ash for four different growth stages (first-stage, crop-development, reproductive stage and late-season) were 0.24, 0.56, 0.73 and 0.37, respectively. The values of the species factor for Cypress for the above four different growth stages were 0.32, 0.44, 0.58 and 0.34.
Distribution of Actual Evapotranspiration over Qaidam Basin, an Arid Area in China  [PDF]
Xiaomei Jin,Renhong Guo,Wei Xia
Remote Sensing , 2013, DOI: 10.3390/rs5126976
Abstract: Evapotranspiration is a major loss flux of the water balance in arid and semi-arid areas. The estimation of actual evapotranspiration has significance for hydrological and environmental purposes. The Surface Energy Balance System (SEBS) algorithm was applied to estimate actual evapotranspiration in the Qaidam Basin and its eight hydrological sub-regions, Northwest China. There were 3,036 cloud-free and atmospherically corrected MODIS satellite images from 2001 to 2011 used in the SEBS algorithm to determine the?actual evapotranspiration. The result indicated that the estimated annual actual evapotranspiration of the basin increased with time and the value varied from 72.7 to 182.3?mm. SEBS estimates were 7.5% and 14.1% of observed pan evaporation over the western and eastern areas, respectively. The variation of SEBS actual evapotranspiration is influenced by climate factors, vegetation, net radiation, land cover type and water table depth. The analysis of the evaporative behavior of different land cover types in the basin presented that water bodies, marsh, and farmland had relatively higher mean actual evapotranspiration though these land cover types make up less than 3.5% of the total basin. Bare soil has very low evapotranspiration and covered almost 60% of the study area. The actual evapotranspiration was observed to be decreased with an increase of water table depth. Overall, the SEBS algorithm proved to be useful and has potential for estimating spatial actual evapotranspiration on a regional scale.
The integration of FAO-CropWat Model and GIS Techniques for Estimating Irrigation Water Requirement and Its Application in the Gaza Strip  [PDF]
Husam Al-Najar
Natural Resources (NR) , 2011, DOI: 10.4236/nr.2011.23020
Abstract: In the Gaza Strip irrigation practices are only based on the farmer’s own experience, they determine when and how to irrigate crops based on the appearance of the soil and the climatic conditions. Even though FAO-CropWat model is used for many countries to estimate irrigation water requirements, it is rarely used for Gaza Strip. In the current re-search, it is the first attempt to model the historical available meteorological data to estimate the irrigation water requirements for the most common cultivated crops (citrus, almonds, date palms, grapes) and to compare the results with the farmer irrigation practices. The model results show that, the reference evapotranspiration accounts for 1451 ± 5 mm/year. Therefore the irrigation water requirements estimated to be 763, 722, 1083, 591 mm/year in average for Citrus, Almonds, Date palm, Grapes, respectively. The farmer irrigation practice exceeding the irrigation water requirement by 30%. The spatial distribution of irrigation water requirements in the entire area of Gaza Strip is shown on maps derived by GIS technique based on data from eight meteorological stations. Irrigation water quality is not optimal in the Gaza Strip, chemical analysis of irrigation wells indicate high salinity and SAR ratio. The obtained results from the model could be a good management tool for the planners and decision makers to minimize the overexploitation of the groundwater and to build fair and strict regulations to optimize the water use in agricultural sector in the Gaza Strip which characterized by semi-arid region.
Evaluation of Sensitivity of Some Existing Evapotranspiration Models to Climate Change Signals in Cold Semi-arid Climate of Hamedan  [cached]
A Sabziparvar,F Tafazoli,H Zare Abianeh,H Banzhad
Journal of Science and Technology of Agriculture and Natural Resources , 2009,
Abstract: Under different climatic conditions of Iran, the evaluation of evapotranspiration (ETo) models sensitivity to meteorological parameters, prior to introducing the superior performance model, seems quite necessary. Using a 35-year (1971-2005) climatological observations in Hamedan, this study compares the sensitivity of different commonly used evapotranspiration models to different meteorological parameters within the IPCC recommended variability range of 10 to 20% during the growing season (April-October). The radiation and temperature-based ETo models include: Penman-Monteith -FAO56 [PMF56], Jensen-Haise [JH1,2], Humid Turc [TH], Arid (semi) arid Turc [TA], Makkink [MK], Hansen [HN], and Hargreaves-Samani [HS]. Results indicate that all the above-mentioned ETo models show the highest sensitivity to radiation and temperature parameters. This implies that special care is required when we apply model-generated radiation and albedo parameters in such ETo models. It is predicted that by 2050, as a result of global warming, the cold semi-arid climates of Iran will cause an average evapotranspiration rise of about 8.5% in crop reference during the growing season.
Evaluation of Clear-Sky Incoming Radiation Estimating Equations Typically Used in Remote Sensing Evapotranspiration Algorithms  [PDF]
Zhigang Sun,Mekonnen Gebremichael,Qinxue Wang,Junming Wang,Ted W. Sammis,Alecia Nickless
Remote Sensing , 2013, DOI: 10.3390/rs5104735
Abstract: Net radiation is a key component of the energy balance, whose estimation accuracy has an impact on energy flux estimates from satellite data. In typical remote sensing evapotranspiration (ET) algorithms, the outgoing shortwave and longwave components of net radiation are obtained from remote sensing data, while the incoming shortwave ( R S) and longwave ( R L) components are typically estimated from weather data using empirical equations. This study evaluates the accuracy of empirical equations commonly used in remote sensing ET algorithms for estimating R S and R L radiation. Evaluation is carried out through comparison of estimates and observations at five sites that represent different climatic regions from humid to arid. Results reveal (1) both R S and R L estimates from all evaluated equations well correlate with observations ( R 2 ≥ 0.92), (2) R S estimating equations tend to overestimate, especially at higher values, (3) R L estimating equations tend to give more biased values in arid and semi-arid regions, (4) a model that parameterizes the diffuse component of radiation using two clearness indices and a simple model that assumes a linear increase of atmospheric transmissivity with elevation give better R S estimates, and (5) mean relative absolute errors in the net radiation ( R n) estimates caused by the use of R S and R L estimating equations varies from 10% to 22%. This study suggests that R n estimates using recommended incoming radiation estimating equations could improve ET estimates.
Effect of actual vapor pressure on estimating evapotranspiration at Serbia  [PDF]
Trajkovi? Slavi?a,?ivkovi? Svetlana
Facta Universitatis Series : Architecture and Civil Engineering , 2009, DOI: 10.2298/fuace0902171t
Abstract: Actual vapor pressure (VP) is an important parameter that is used in many evapotranspiration equations. However, vapor pressure is difficult to measure accurately. In the humid climate, the actual vapor pressure can be derived from minimum air temperature. The objectives of this study were: first, to estimate errors that can arise if VP data are not available and have to be estimated; second, to compare the Priestley-Taylor ET0 values computed under various levels of VP data availability; and third, to evaluate the reliability of Priestley-Taylor equation as compared to the FAO-56 Penman-Monteith method. The following main conclusions can be drawn: Estimated VP values generally were in closest agreement with measured VP values. The measurements of air humidity at humid locations are not indispensable for estimating reference evapotranspiration. The Priestley-Taylor method (with measured or estimated VP) provides the very good agreement with the evapotranspiration obtained by the FAO-56 Penman-Monteith method except windless locations.
Minimum weather data requirements for estimating reference evapotranspiration  [PDF]
Trajkovi? Slavi?a,Stojni? Vladimir,Goci? Milan
Facta Universitatis Series : Architecture and Civil Engineering , 2011, DOI: 10.2298/fuace1102335t
Abstract: The International Commission for Irrigation and Drainage (ICID) and Food and Agriculture Organization of the United Nations (FAO) have proposed using the Penman-Monteith method as the standard method for estimating reference evapotranspiration (ET0), and for evaluating other methods. The FAO-56 Penman-Monteith (FAO-56 PM) method requires the numerous weather data that are not available in the most of the stations. The objectives of this study were: first, to estimate errors that can arise if some weather data are not available and have to be estimated; second, to compare the FAO-56 PM ET0 values computed under various levels of data availability; and third, to determine minimum weather data requirements for estimating ET0 without decreasing the acceptable accuracy. For this study, full weather data sets were collected from six humid weather stations from Serbia (Southeast Europe).The main conclusion is that the minimum and maximum air temperature and "local default" value of wind speed are the minimum data requirements necessary to apply the FAO-56 PM method in humid climate.
Evaluation of evaporation-measuring equipments for estimating evapotranspiration within a greenhouse
Blanco, Flávio F.;Folegatti, Marcos V.;
Revista Brasileira de Engenharia Agrícola e Ambiental , 2004, DOI: 10.1590/S1415-43662004000200004
Abstract: with the objective of evaluating the performance of simple evaporation measuring equipments in estimating the evapotranspiration in greenhouse, an experiment was conducted in piracicaba, sp, during a tomato-growing season. daily water evaporation rate from piche atmometer, modified atmometer and a reduced evaporation pan installed inside the greenhouse and a class a pan installed outside were compared to the evapotranspiration rates calculated with penman-monteith equation. results showed that atmometers had the best performance in estimating the crop evapotranspiration in greenhouse and could be used advantageously in relation to the evaporation pans.
Developing an Integrated Complementary Relationship for Estimating Evapotranspiration  [PDF]
Homin Kim, Jagath J. Kaluarachchi
Natural Resources (NR) , 2018, DOI: 10.4236/nr.2018.94007
The complementary relationship for estimating evapotranspiration (ET) is a simple approach requiring only commonly available meteorological data; however, most complementary relationship models decrease in predictive power with increasing aridity. In this study, a previously developed Granger and Gray (GG) model by using Budyko framework is further improved to estimate ET under a variety of climatic conditions. This updated GG model, GG-NDVI, includes Normalized Difference Vegetation Index (NDVI), precipitation, and potential evapotranspiration based on the Budyko framework. The Budyko framework is consistent with the complementary relationship and performs well under dry conditions. We validated the GG-NDVI model under operational conditions with the commonly used remote sensing-based Operational Simplified Surface Energy Balance (SSEBop) model at 60 Eddy Covariance AmeriFlux sites located in the USA. Results showed that the Root Mean Square Error (RMSE) for GG-NDVI ranged between 15 and 20 mm/month, which is lower than for SSEBop every year. Although the magnitude of agreement seems to vary from site to site and from season to season, the occurrences of RMSE less than 20 mm/month with the proposed model are more frequent than with SSEBop in both dry and wet sites. Another finding is that the assumption of symmetric complementary relationship is a deficiency in GG-NDVI that may introduce an inherent limitation under certain conditions. We proposed a nonlinear correction function that was incorporated into GG-NDVI to overcome this limitation. As a result, the proposed model produced much lower RMSE values, along with lower RMSE across more sites, as compared to SSEBop.
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