SEBS Estimation of Evapotranspiration Fraction

sebs computes Su (2002) Surface Energy Balance System (SEBS) for estimation of turbulent heat fluxes

Usage

sebs(
  albedo,
  Ts,
  Tmax,
  Tmin,
  RHmax = NULL,
  RHmin = NULL,
  n = NULL,
  NDVI,
  SAVI,
  iter.max = 7,
  xyhot,
  xycold,
  DOY,
  sunelev,
  welev,
  zx,
  u,
  zomw,
  zom = NULL,
  LAI = NULL,
  DEM = NULL,
  lapse = 0.0065,
  Rn24 = NULL,
  wmo = NULL,
  airport = NULL,
  Krs = 0.16,
  surface = "grass",
  latitude = NULL,
  model = "SEBS",
  clip = NULL,
  folder = NULL
)

# Default S3 method
sebs(
  albedo = NULL,
  Ts,
  Tmax,
  Tmin,
  RHmax = NULL,
  RHmin = NULL,
  n = NULL,
  NDVI,
  SAVI,
  iter.max = 7,
  xyhot = "auto",
  xycold = "auto",
  DOY = NULL,
  sunelev,
  welev,
  zx = 10,
  u = 2,
  zomw = 2,
  zom = NULL,
  LAI = NULL,
  DEM = NULL,
  lapse = 0.0065,
  Rn24 = NULL,
  wmo = NULL,
  airport = NULL,
  Krs = 0.16,
  surface = "grass",
  latitude = NULL,
  model = "SEBS",
  clip = NULL,
  folder = NULL
)

Arguments

albedo: A RasterLayer data that has albedo values. You can also provide file path or location on your computer
Ts: A RasterLayer data that indicates radiometric surface temperature values from remote sensing image, preferably in Kelvin (K). You can also point to raster file on your computer.
Tmax: Numeric. Maximum air Temperature in degree Celsius
Tmin: Numeric. Minimum air Temperature in degree Celsius
RHmax: Maximum Relative Humidity in percent
RHmin: Minimum Relative Humidity in percent
n: Sunshine hours
NDVI: A RasterLayer data that indicates NDVI (Normalized Difference Vegetation Index) values. You can also point to a file on your computer.
SAVI: A RasterLayer data that indicates SAVI(Soil-adjusted Vegetation Index) values. You can also point to file on your computer.
iter.max: maximum iterations of sensible heat calculation.
xyhot: numeric or "auto". A list of x and y coordinates of a hot pixel in the form c(x,y). If it is set to "auto", hotTs will be used to compute it.
xycold: numeric or "auto". A list of x and y coordinates of a cold pixel in the form of c(x,y). If it set to "auto", coldTs will be used to compute it.
DOY: Numeric or Date [YYYY-mm-dd]. Day of the Year. If you give data in the form of date [YYYY-mm-dd], it will be converted to DOY
sunelev: Angle of Sun elevation in degrees, as found in the meta data of the satellite image
welev: Weather station elevation in meters
zx: The height above the weather station where the wind speed is measured [m]
u: The satellites overpass time wind speed at the weather station [m/s]
zomw: the roughness length for the weather station surface [m]
zom: the momentum roughness length for each pixel [m]. You can also point to a raster file on your computer
LAI: Leaf Area Index, dimensionless
DEM: A digital elevation model[m]
lapse: A local lapse rate that is applied to correct DEM [K/m]. Default value is 0.0065
Rn24: A 24 hour net radiation [W/m2]. It is needed to estimate daily ET. It can be computed with ETo with even a minimal data of Tmax and Tmin. You can also point to raster file on your computer
wmo: numeric. World Meteorological Organization weather station code. You can use the Worldwide Station List at https://www.wunderground.com/ [Accessed on 2016-5-6] or https://www.wetterzentrale.de/ [Accessed on 2016-5-6]
airport: numeric. IATA or ICAO code. They are alphabetically listed at https://en.wikipedia.org/wiki/List_of_airports_by_IATA_code:_A [Accessed on 2016-5-6]
Krs: Relationship between the fraction of extraterrestrial radiation that reaches the earth's surface, Rs/Ra, and the air temperature difference Tmax - Tmin for interior (Krs = 0.16) and coastal (Krs = 0.19) regions
surface: character. It is either "grass" or "alfalfa"
latitude: geographical coordinates in decimal degrees. It should be negative for southern hemisphere
model: character. The type of model. It takes either "SEBAL" or "METRIC"
clip: extent object or raster object or polygon from which an Extent object can be extracted. A polygon or raster will be reprojected to conform the data to be cropped. for example clip can take the form of c(xmin, xmax, ymin, ymax)
folder: An original directory of the images that contains all landsat 5,7 or 8 bands and metadata. At the moment only Landsat folder is supported

Value

EF Evapotranspiration Fraction

Details

Function ETo will be automatically used to estimate parameters y (psychrometric constant), slope (slope of saturation vapour pressure curve) and vpd (Vapour pressure deficit). For information on data input procedure see 'details' at sebal

References

Su, Z. 2002. The Surface Energy Balance System (SEBS) for estimation of turbulent heat fluxes. Hydrol. Earth Syst. Sci., 6(1): 85-100.

Author

George Owusu

Examples

if (FALSE) { # \dontrun{
#define Input data
albedo=raster(system.file("extdata","albedo.grd",package="sebkc"))
Ts=raster(system.file("extdata","Ts.grd",package="sebkc"))
NDVI=raster(system.file("extdata","NDVI.grd",package="sebkc"))
LAI=raster(system.file("extdata","LAI.grd",package="sebkc"))
Tmax=31
Tmin=28
RHmax=84
RHmin=63
#perform simulation
modsebs=sebs(albedo=albedo,Ts=Ts,Tmax=Tmax,Tmin=Tmin,RHmax=RHmax,
RHmin=RHmin,NDVI=NDVI,SAVI=NULL,iter.max=7,xyhot="full",
xycold="full",DOY=37,sunelev=50.71154048,welev=317.1,zx=10,
u=2,zomw=2,zom=NULL,LAI=LAI,Rn24=NULL,model="SEBS")


#use original landsat 7 data by specifying folder path
folder=system.file("extdata","stack",package="sebkc")
modauto=sebs(folder = folder,welev = 380,Tmax=Tmax,
Tmin=Tmin,RHmax=RHmax,RHmin=RHmin)
#plot ET fraction
plot(modauto$EF)

#get Ts, Albedo, NDVI, SAVI, sunelev, DOY from landsat data 
welev=278
data=landsat578(rawdata,welev=welev)
 #perform semi-auto simulation 
#Determine xyhot. Digitize polygon on the Ts map
modhot=hotTs(data,welev=300,extent="auto",cluster=2)
#determine the cold. Digitize polygon on the Ts map
modcold=coldTs(data,welev=275,extent="auto",cluster=2)
xyhot=modhot$xyhot
xycold=modcold$xycold
#use object of \code{\link{coldTs}} or \code{\link{hotTs}} in different ways
modsebs=sebs(data,xyhot=modhot,xycold=xycold,welev=welev,
Tmax=Tmax,Tmin=Tmin,RHmax=RHmax,RHmin=RHmin)
 } # }