Schematic drawing of the water vapor emitting regions for Class 0, Class I, and Class II protostellar systems. A significant fraction of the water vapor resides in the warm inner envelope of Class 0 objects. Meanwhile, various physical and chemical models of Class II disks indicate three major water reservoirs from hot (T_dust > 160 K, region 1) to warm (T_gas > T_dust, region 3) to cold (T_dust < 20 K, region 2). The most abundant water vapor is located in Region 1. From this work, the water vapor reservoir in Class I objects that can be probed with the HO mm data is mostlikely similar to that of Class II disks that resides in the inner 10 au.