Dehydration headaches?

I've gone through much of my life being told that where I'm not otherwise sick or have some significant external stimuli, such a bright light or disruptive noise, then the cause of a headache is often dehydration.

Our body's have a natural way of telling us to drink more water - the feeling of thirst.

Where drinking a lot of water can alleviate the headache ~1 hour later, why do I have a 'dehydration headache' but not feel thirsty at that time?