petdocumentation
Thepetgives the potential reference evapotranspiration following Hargreaves-Samani.
See also:spei.
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Contents
Syntax
pevap = pet(Ra,tmax,tmin,tmean)
Description
pevap = pet(Ra,tmax,tmin,tmean)computes the Hargreaves equation for the potential evapotranspiration (PET) for extraterrestrial radiationRa(比如从solar_radiation函数), and temperature maxima, minima, and means. Temperaturestmax,tmin, andtmeancan be vectors, or can be cubes in which the first two dimensions are spatial and the third dimension corresponds to time. The inputstmax,tmin, andtmeanrepresent maximinum, minimum, and mean temperatures over a given time period.
Example
This example is an abbreviated form of the example given in the documentation for thespei函数. You may wish to check out that page for a little more context and in-depth explanation.
Begin by loading data and converting units. Precipitation comes in units kg^2/m^2/s and we change these units to mm/day. Temperatures are in K and we change them to degrees C.
loadncep-ncarP = P*3600*24; T = T-273.15; TMAX = TMAX-273.15; TMIN = TMIN-273.15;
Calculate extraterrestrial solar radiation
哈格里夫斯和萨马尼的PET equation requires extraterrestrial radiation (Ra) as input. Ra is the solar radiation on a horizontal surface at the top of the Earth's atmosphere and is computed based on the orbital parameters of the Earth which are dependent on latitude. We wish to have the temporal variations of solar radiation integrated over daily time intervals which can be calculated using the function
[Lon,Lat] = meshgrid(lon,lat); Ra = solar_radiation(t,Lat);% Plot grid cell 5,5:plot(t,squeeze(Ra(5,5,:))) ylabel('Ra [MJ m^2 day^{-1}]') title(['Solar radiation at 'num2str(lat(5))'\circN'])
Calculate potential evapotranspiration
SPEI requires precipitation and potential evapotranspiration (PET). PET is calculated based on the formula of Hargreaves and Samani (1985) which estimates reference crop evapotranspiration based on temperature and extraterrestrial radiation.
There are numerous approaches to calculating potential evaporation. A compilation is foundhere(see supplements). Here we use the formula of Hargreaves-Samani (1985) which is a temperature based method to calculate the potential evapotranspiration (PET) and is implemented in the function pet. The main advantage of using the Hargreaves-Samani equation lies in its simplicity and low requirements regarding input parameters.
pevap = pet(Ra,TMAX,TMIN,T); plot(t,squeeze(pevap(5,5,:))) ylabel('PET [mm day^{-1}]') title(['Potential evaporation at 'num2str(lat(5))'\circN, 'num2str(lon(5))'\circE'])%
References
- Hargreaves, George H., and Zohrab A. Samani. "Reference crop evapotranspiration from temperature." Applied Engineering in Agriculture 1.2 (1985): 96-99.doi:10.13031/2013.26773.
- Samani, Zohrab. "Estimating solar radiation and evapotranspiration using minimum climatological data." Journal of Irrigation and Drainage Engineering 126.4 (2000): 265-267.10.1061/(ASCE)0733-9437(2000)126:4(265).
Author Info
This function and supporting documentation were written by José Delgado and Wolfgang Schwanghart (University of Potsdam), February 2019, for the Climate Data Toolbox for Matlab.