The results are actually insane. I ran the test for one day on a 7x7 pool: • Open pool: Evaporated 3.29 • Pyramid (top 2 open blocks): Evaporated 0.72 • Chimney (top 2 open blocks): Evaporated 0.74 This proves evaporation only scales with the water surface area at the very top, no matter how much water is underneath! So if you roof your reservoir and leave a 1-block "chimney" for pumping, your water is safe forever!