Unless you start drilling oil, nothing in your little fort is worth hauling several tons of water every year. This idea rests solely on exploiting the broken economy and actively ignoring realistic water needs of irrigation, drinking and washing
Deserts forts must be founded on either an
oasis or an
underground river. Either that or they need a nearby supply depot on these features to haul them water. Anything else is a laughably unrealistic of a long term solution
Settling on a desert should provide it's unique challenges, not just be like any other fort except you have to buy 10 barrels of water every year. Caravans probably wouldn't even show up every year unless you have world class regional specialties like high quality glass. Enemies would be scarce. Relying on a supply depot for your water would probably leave you vulnerable to being cut off by enemies using said water to supply their own
siegeCan you even use wagons in a sand desert? Good luck hauling meaningful amounts of water with camels long distance. It'll take time to dig out an underground tunnel, until then moisture would be in short supply
Goblins need a new mount if they intend to cross a desert. Giant lizards maybe? I can see them setting up settlements at the desert for the sole purpose of raiding yours