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Monthly estimated potential evapotranspiration and measured pan evaporation for two locations in Hawaii, Hilo and Pahala. Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such as free water (for lakes and oceans), the soil type for bare soil, and also the density and diversity of vegetation.
Potential evapotranspiration is higher in the summer, on clearer and less cloudy days, and closer to the equator, because of the higher levels of solar radiation that provides the energy (heat) for evaporation.
s2 (Large summer surplus) : Ih ≥ 33.3 and the surplus in the summer is larger than in the winter; w2 (Large winter surplus) : Ih ≥ 33.3 and the surplus in the winter is larger than in the summer; The deficiency of water in the soil is calculated as the difference between the potential evapotranspiration and the actual evapotranspiration. [2]
In 1948, C. W. Thornthwaite proposed an AI defined as: = where the water deficiency is calculated as the sum of the monthly differences between precipitation and potential evapotranspiration for those months when the normal precipitation is less than the normal evapotranspiration; and where stands for the sum of monthly values of potential evapotranspiration for the deficient months (after ...
Specifically the Penman–Monteith equation refines weather based potential evapotranspiration (PET) estimates of vegetated land areas. [1] It is widely regarded as one of the most accurate models, in terms of estimates. [citation needed] The original equation was developed by Howard Penman at the Rothamsted Experimental Station, Harpenden, UK.
A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate.
The Standardized Precipitation Evapotranspiration Index (SPEI) is a multiscalar drought index based on climatic data. It was developed by Vicente-Serrano et al. (2010) at the Institute Pirenaico de Ecologia in Zaragoza, Spain . [ 1 ]
Given the limited data input to the equation, the calculated evapotranspiration should be regarded as only broadly accurate. Rather than a precise measure of evapotranspiration, the output of the equation is better thought of as providing an order of magnitude. [2] The inaccuracy of the equation is exacerbated by extreme variants of weather.