WaterBalance model - calculate change in soil water for one day
watbal.update(WAT0, RAIN, ETr, param, WP, FC): Water at the beginning of the day (mm).
: Rainfall of day (mm)
: Evapotranspiration of day (mm)
: a vector of parameters
: Water content at wilting Point (cm^3.cm^-3)
: Water content at field capacity (cm^3.cm^-3)
WAT1 : Water at the beginning of the day+1 (mm).