The ET area represents the area of phreatophytes estimated to exist towards the end of the simulation period. This area is adjusted within each sub-basin according to a set of sub-basin factors to account for basin to basin differences in phreatophytes as well as changes in phreatophyte areas over time. In particular, the basin factors represents the re-growth of phreatophytes after the 1935 flood and changes in flood behavior following the construction of reservoirs.
The maximum evapotranspiration rate is calculated as the product of the estimated area of phreatophytes appropriately adjusted for that stress period and the estimated evapotranspiration rate. This evapotranspiration rate is calculated using an equivalent crop at three stations across the basin and then linearly interpolated in an east-west direction to each cell.