The trays are designed with the perfect volume of water in mind so you should only have to add water once (at the very start). The roots will grow faster than the water depletes. However, if you'd like grow the greens longer (past the microgreens stage), topping up the water may be required. Water should not cover more than half the roots. The roots require access to oxygen to be healthy.  In hotter weather, you may need to top up the water so the mats and roots don't dry out.