Implementation of "dual crop coefficient" FAO method

This method allows to take into account the area covered by the crop vs bare ground.

This method is described in: https://www.fao.org/4/X0490E/x0490e0c.htm#chapter%207%20%20%20etc%20%20%20dual%20crop%20coefficient%20(kc%20=%20kcb%20+%20ke)

And is used in: https://theses.hal.science/tel-01804084v2/file/2017TOU30037B.pdf

This method allows to adapt ETc in function of soil cover opening possibilities to simulate more accurately:

• ETc associated with bare ground before sowing and after harvesting
• inter-row cropping effect
• differentiate local drip irrigation from other techniques