Iterative function to estimate H and R.ah

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Description

generates an iterative solution to estimate r.ah and H because both are unknown at each pixel.

Usage

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calcH(anchors, method = "mean", Ts, Z.om, WeatherStation, ETp.coef = 1.05,
  Z.om.ws = 0.03, mountainous = FALSE, DEM, Rn, G, verbose = FALSE,
  maxit = 20)

Arguments

anchors

anchors points. Can be the result from calcAnchors() or a spatialPointDataframe o Dataframe with X, Y, and type. type should be "cold" or "hot"

method

Method when using more than 1 pair of anchors pixels. method = "mean" will use the mean value for the cold pixels vs the mean value for the hot pixels.

Ts

Land surface temperature in K. See surfaceTemperature()

Z.om

momentum roughness lenght. See momentumRoughnessLength()

WeatherStation

WeatherStation data at the satellite overpass. Can be a waterWeatherStation object calculated using read.WSdata and MTL file

ETp.coef

ETp coefficient usually 1.05 or 1.2 for alfalfa

Z.om.ws

momentum roughness lenght for WeatherStation. Usually a value of 0.03 might be reasonable for a typical agricultural weather station sited over vegetation that is about 0.3 m tall. For clipped grass, use 0.015 m

mountainous

Logical. If TRUE heat transfer equation will be adjusted for mountainous terrain

DEM

Digital Elevation Model in meters.

Rn

Net radiation. See netRadiation()

G

Soil Heat Flux. See soilHeatFlux()

verbose

Logical. If TRUE will print information about every iteration to console

maxit

Maximun number of iteration. Default 20.

Details

Sensible heat flux is the rate of heat loss to the air by convection and conduction, due to a temperature difference.This parameter is computed using the following one-dimensional, aerodynamic,temperature gradient based equation for heat transport, this method is difficult to solve because there are two unknowns, rah and dT. To facilitate this computation, METRIC utilize the two "anchor" pixels and solve for dT that satisfies eq. given the aerodynamic roughness and wind speed at a given height. Aerodynamic resistance, and heat transfer is impacted by buoyancy of heated, light air at the surface, especially when H is large. Therefore, correction to rah is needed to account for buoyancy effects. However, H is needed to make this correction. An iterative solution for both H and rah is used.

Author(s)

Guillermo Federico Olmedo

de la Fuente-Saiz, Daniel

Fernando Fuentes PeƱailillo

References

R. G. Allen, M. Tasumi, and R. Trezza, "Satellite-based energy balance for mapping evapotranspiration with internalized calibration (METRIC) - Model" Journal of Irrigation and Drainage Engineering, vol. 133, p. 380, 2007

Allen, R., Irmak, A., Trezza, R., Hendrickx, J.M.H., Bastiaanssen, W., Kjaersgaard, J., 2011. Satellite-based ET estimation in agriculture using SEBAL and METRIC. Hydrol. Process. 25, 4011-4027. doi:10.1002/hyp.8408

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