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Bernard Itier - One of the best experts on this subject based on the ideXlab platform.
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Operational limits to the Priestley-Taylor Formula
Irrigation Science, 1996Co-Authors: K.j. Mcaneney, Bernard ItierAbstract:The Penman-Monteith equation (PM) provides a direct and logical route to explaining rates of crop water consumption without the need of resorting to the artificiality of a reference crop and ill-defined crop factors. However as an operational irrigation tool, the PM is often impractical because of uncertainties about stomatal behaviour and turbulent transport. While recent progress is providing better descriptions of these processes, it is questionable whether such detail is always warrented for practical irrigation. Another option for evaporation prediction is the Priestley and Taylor equation (PT). This approach enjoys reasonable empirical support in humid regions and our efforts here are devoted to better defining its limits of applicability. We begin by presenting previously published and new evidence in support of Priestley and Taylors' value of 1.26 for the coefficient α indicating proportionality between evaporation rate and available energy. While α may depart from this value depending on windspeed and saturation deficit, it is shown to be fairly insensitive to both over a reasonable range of conditions. A daytime mean saturation deficit of 10 g m–3 is suggested as a likely upper limit for the range of applicability of the PT; below this limit, the extra data inputs required for the PM will not be rewarded with a better estimate of evaporation. In arid regions and under stable conditions, fluxes well beyond the PT estimate have been measured and no simple prediction alternative to the PM exists.
K.j. Mcaneney - One of the best experts on this subject based on the ideXlab platform.
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Operational limits to the Priestley-Taylor Formula
Irrigation Science, 1996Co-Authors: K.j. Mcaneney, Bernard ItierAbstract:The Penman-Monteith equation (PM) provides a direct and logical route to explaining rates of crop water consumption without the need of resorting to the artificiality of a reference crop and ill-defined crop factors. However as an operational irrigation tool, the PM is often impractical because of uncertainties about stomatal behaviour and turbulent transport. While recent progress is providing better descriptions of these processes, it is questionable whether such detail is always warrented for practical irrigation. Another option for evaporation prediction is the Priestley and Taylor equation (PT). This approach enjoys reasonable empirical support in humid regions and our efforts here are devoted to better defining its limits of applicability. We begin by presenting previously published and new evidence in support of Priestley and Taylors' value of 1.26 for the coefficient α indicating proportionality between evaporation rate and available energy. While α may depart from this value depending on windspeed and saturation deficit, it is shown to be fairly insensitive to both over a reasonable range of conditions. A daytime mean saturation deficit of 10 g m–3 is suggested as a likely upper limit for the range of applicability of the PT; below this limit, the extra data inputs required for the PM will not be rewarded with a better estimate of evaporation. In arid regions and under stable conditions, fluxes well beyond the PT estimate have been measured and no simple prediction alternative to the PM exists.
Nilson Augusto Villa Nova - One of the best experts on this subject based on the ideXlab platform.
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sap flow leaf area net radiation and the priestley Taylor Formula for irrigated orchards and isolated trees
Agricultural Water Management, 2007Co-Authors: Antonio Roberto Pereira, Steve Green, Nilson Augusto Villa NovaAbstract:This paper describes the goodness of fit for two simple methods to estimate the daily sap flow of irrigated, non-stressed apple and olive trees in an orchard, and a walnut tree in isolation. The required inputs for the calculation are the tree leaf area (LA, in m2 tree-1), the net (all-wave) radiation over grass (RN, in MJ m-2 day-1) and the average air temperature. Data are presented for mid-summer when daily RN ranged between 2 and 20 MJ m-2 day-1. Tree leaf area ranged between 8.65 m2 for a dwarf apple and 35.5 m2 for a large apple. With the first method, daily sap flow (S, MJ tree-1 day-1) was empirically found to equal approximately 1/4 of RN times LA (R2 = 0.92, n = 72 days). The second method used the Priestley�Taylor equation with tree canopy net radiation term (A, in MJ tree-2 day-1) empirically computed as A = 0.32RNLA. Estimates of S based on the original a value of 1.26 did not differ significantly from a linear relationship (R2 = 0.91; n = 72; p < 0.05), for sap flows up to 56 L tree-1 day-1. However, there was a small leaf-area dependence for the �best-fit� a value i.e., a = 1.41 - 0.0064LA (R2 = 0.94; n = 4 trees). On average, the daily sap flow equated to about 2/3 of A. Both relationships appear robust and capable of providing a simple working alternative to the traditional crop-coefficient approach that relates crop water use to the potential evapotranspiration rate. The problem then shifts to that of obtaining a reliable estimate of tree leaf area either by destructive sampling or using a remote sensing method such as light transmission
Atul Sharma - One of the best experts on this subject based on the ideXlab platform.
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maximal helicity violating scattering of gluons and gravitons in chiral strong fields
Physical Review Letters, 2020Co-Authors: Tim Adamo, Lionel Mason, Atul SharmaAbstract:We present all-multiplicity Formulas for the tree-level scattering of gluons and gravitons in the maximal helicity violating (MHV) helicity configuration, calculated in certain chiral strong fields. The strong backgrounds we consider are self-dual plane waves in gauge theory and general relativity, which are treated exactly and admit a well-defined S matrix. The gauge theory background-coupled MHV amplitude is simply a dressed analog of the familiar Parke-Taylor Formula, but the gravitational version has nontrivial new structures due to graviton tails. Both Formulas have just one residual integral rather than the n-2 expected at n points from space-time perturbation theory; this simplification arises from the integrability of self-dual backgrounds and their corresponding twistor description. The resulting Formulas pass several consistency checks and limit to the well-known expressions for MHV scattering of gluons and gravitons when the background becomes trivial.
Wolf Martin - One of the best experts on this subject based on the ideXlab platform.
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Scattering Amplitude Recursion Relations in BV Quantisable Theories
'American Physical Society (APS)', 2019Co-Authors: Macrelli Tommaso, Saemann Christian, Wolf MartinAbstract:Tree-level scattering amplitudes in Yang-Mills theory satisfy a recursion relation due to Berends and Giele which yields e.g. the famous Parke-Taylor Formula for MHV amplitudes. We show that the origin of this recursion relation becomes clear in the BV formalism, which encodes a field theory in an $L_\infty$-algebra. The recursion relation is obtained in the transition to a smallest representative in the quasi-isomorphism class of that $L_\infty$-algebra, known as a minimal model. In fact, the quasi-isomorphism contains all the information about the scattering theory. As we explain, the computation of such a minimal model is readily performed in any BV quantisable theory, which, in turn, produces recursion relations for its tree-level scattering amplitudes.Comment: 33 pages, minor improvements, typos corrected, references added, published versio
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Scattering amplitude recursion relations in Batalin-Vilkovisky–quantizable theories
'American Physical Society (APS)', 2019Co-Authors: Macrelli Tommaso, Sämann Christian, Wolf MartinAbstract:Tree-level scattering amplitudes in Yang-Mills theory satisfy a recursion relation due to Berends and Giele which yields e.g., the famous Parke-Taylor Formula for maximally helicity violating amplitudes. We show that the origin of this recursion relation becomes clear in the Batalin-Vilkovisky (BV) formalism, which encodes a field theory in an L∞-algebra. The recursion relation is obtained in the transition to a smallest representative in the quasi-isomorphism class of that L∞-algebra, known as a minimal model. In fact, the quasi-isomorphism contains all the information about the scattering theory. As we explain, the computation of such a minimal model is readily performed in any BV quantizable theory, which, in turn, produces recursion relations for its tree-level scattering amplitudes
