If the top is open, heat and humidity will just rise up and out. You can add more water to the substrate, run humidifiers, but if the top is open, physics dictates the the heat and humidity in the enclosure will try to equalize with that of the room.
In that case, you should just need to pour some water in to your substrate and add some heat.
It is possible that your gauge is off? What kind are you using. The little stick on dial type are worthless. You need $10-20 dollar digital one with a remote probe from the hardware store. Those are usually pretty close.
Also, that water bowl is a death trap. Terra cotta plant saucers are much safer and more inviting.
I have made modifications to the above design by adding a false bottom. This keeps the standing water separate from the substrate to avoid plastron rot - it lets the surface dry out.
Like a humidifier. Some use them to maintain the humidity of their enclosure. Or you can use the pump sprayer and just spray the enclosure a couple times a day.
As Tom mentioned (I've never tried anything else) pour water over your substrate and mix it up. I just looked quickly, but what kind if substrate do you have? Is that some sort of bark? I was using a bark substrate, which is fine by the way, but changed to coco coir. Where I live in central California, humidity gets around 20-30 %. Since I have been using coco coir, I have no problems with humidity. It holds water very well, I only have to pour water on it maybe once a day, and it's easy to keep clean. Good luck!
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For substrate, I'm using a 50/50 mix of organic top soil and peat moss. When creating a humid environment for a tortoise, it is very important to maintain proper temps. Too cool and humid equals a sick tortoise. I would go with a combination digital temperature/humidity gauge. Very common item.
Usually planting soil has those little white things in it to help keep it fluffy. Tortoises, for whatever reasons, find these to be irresistible and want to eat them. Bad things happen when tortoises eat non food items. Even perilit will negatively effect a tortoises gut track. Can you get to a Walmart or like a Kmart type store? You could get what I believe is a better priced gauge system. I like the ones that have a wireless remote unit to them. This way, while watching tv, I can see what is going on it my habitats. For the most part all digital gauges are created equally.