Ginkgo leaves turning brown from middles out. What do I do?

Many examples of Dwarf Ginkgo biloba cultivars are actually grafted clones, which can affect optimal care. Watering twice per day is in general too frequent: although only ten minutes, soil retains moisture, which results in more time of wetness. Watering thoroughly, as necessary, yet less often, and then allowing the soil to to dry somewhat between waterings, allows aereation of the roots. Watering once per day or every other day, or once every several days, may be sufficient. If the tree is in a container as a Bonsai, additional attention regarding watering frequency can be even more important.

Since Ginkgo biloba benefit from good drainage, maybe check to be sure that the soil is of a type that drains well, and if in a container, check that there is a sufficient hole or holes in the bottom of the container to allow adequate drainage, and some coarse gravel in the bottom of the container to help drainage & aereation. The sunny environment is good, though if in constant very strong sun that could be a bit difficult for it, especially if very different from the environment the tree was in previously. And, it might be too hot: If it's very hot, the leaves could be drying out with the soil remaining moist/ wet; if that is occurring, gently misting in the evening could be beneficial.

Also, avoid adding nutrient until the tree is thriving, and then sparingly yet adequately; there might be too much nutrient present already. If the tree is known to have been given a lot of nutrient recently, that could be causing the difficulty, or at least contributing to it.

Perhaps replanting the Ginkgo biloba in fresh good draining soil that doesn't have nutrient added, and then watering adequately yet less frequently, could be helpful; if in a container/ bonsai, when replanting, some coarse gravel in the bottom of the container, and a hole or holes in the bottom of the container for adequate drainage & aereation. Also, careful attention to placement of roots, and maintaining the same soil depth about the trunk when replanting, is good.