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The cooling tower objects have inputs for water evaporation, drift and blow down. See example file CoolingTower:SingleSpeed:MultiCell. See the Engineering Reference for equations used to calculate water usage.

Near the end of the cooling tower inputs are these fields:

CoolingTower:SingleSpeed,
  SaturatedExit,           !- Evaporation Loss Mode
  ,                        !- Evaporation Loss Factor {percent/K}
  0.0080,                  !- Drift Loss Percent {percent}
  ConcentrationRatio,      !- Blowdown Calculation Mode
  3.0000,                  !- Blowdown Concentration Ratio
  ,                        !- Blowdown Makeup Water Usage Schedule Name

When you run the simulation you should see these report variables in the rdd file:

Output:Variable,*,Cooling Tower Make Up Water Volume Flow Rate,hourly; !- HVAC Average [m3/s]
Output:Variable,*,Cooling Tower Make Up Water Volume,hourly; !- HVAC Sum [m3]
Output:Variable,*,Cooling Tower Make Up Mains Water Volume,hourly; !- HVAC Sum [m3]
Output:Variable,*,Cooling Tower Water Evaporation Volume Flow Rate,hourly; !- HVAC Average [m3/s]
Output:Variable,*,Cooling Tower Water Evaporation Volume,hourly; !- HVAC Sum [m3]
Output:Variable,*,Cooling Tower Water Drift Volume Flow Rate,hourly; !- HVAC Average [m3/s]
Output:Variable,*,Cooling Tower Water Drift Volume,hourly; !- HVAC Sum [m3]
Output:Variable,*,Cooling Tower Water Blowdown Volume Flow Rate,hourly; !- HVAC Average [m3/s]
Output:Variable,*,Cooling Tower Water Blowdown Volume,hourly; !- HVAC Sum [m3]