Estimation methods for crop water requirements in the Gediz Basin of western Turkey

As part of an intercomparison study on evapotranspiration (ET), six methods for estimating the reference ET have been applied on a cotton field in the Gediz Basin, Turkey. Using a simple water balance model, the values for actual ET were obtained in two different ways. In the first case capillary rise was ignored, while in the second case capillary rise was included indirectly by updating the simulation model with measure soil moisture data. In terms of the reference ET, the FAO-24 methods gave the highest values and Hagreaves and Penman–Monteith gave the lowest values, ranging from 885 mm for Blaney–Criddle down to 697 for Penman–Monteith. Differences between the actual ET, including or excluding capillary rise, were between 121 and 267 mm. The methodology applied here, including capillary rise by using measured soil moisture contents to update the simulation model, is promising for areas with shallow groundwater tables.