All California irrigation districts that receive either federal or state water are now required to prepare Water Conservation Plans. For the first time in the history of most districts, they are developing an elementary water balance. The term "elementary" should be emphasized, because there are significant weaknesses in our knowledge of subsurface flows and some components of Evapotranspiration (ET). Irrigation districts generally use published "typical" values of ET for their water balance computations.
Available at: http://works.bepress.com/djhowes/7/