When landing your wheels aren’t spun up, this is akin to water-skiing, hence you get a lower initial value for commencement of hydroplaning. During take-off, you are initially at zero speed, and the wheel is firmly planted on the ground with a fairly decent coefficient of friction. As you increase speed the wheel spins up slowly so the tire has minimal slip, and it has rotation when at the first onset of hydroplaning which ameliorates its effects, hence the higher speed for take-off. I would not view these equations as absolutes, they’re more heuristic. To wit, let’s say you have main wheels at 100 psi, that would lead you to believe that 77kts is your hydroplaning speed, well you might be at 75kts and experience hydroplaning, or you may never experience it. I noticed in a previous post that you like mathematical models, the data from hydroplaning was derived empirically by NASA, I think way back in ’66 (maybe earlier) see NASA TN-D 3515.
This is basically what FL said, again I prove my ability to interpret English into English