South Texas has high evaporation rates. If you lose 1/2" per day, for your 30' diameter pool, that's 220 gallons per day if you don't use a pool cover. You can find your area's evaporation rate in this link (pan evaporation rates are higher than the lake one's shown and pool evaporation rates are generally higher as well due to higher water temps).
Though a pool cover will virtually eliminate evaporation, you'll want it to be a white or silver reflective cover if you don't want to heat the pool.
Though a pool cover will virtually eliminate evaporation, you'll want it to be a white or silver reflective cover if you don't want to heat the pool.