Question

The water in an open pond remains cool even in hot summer because

Answer 1

by the process of evaporation

Answer 2

In an earthen pot, water gets evaporated quickly through the pores. Cooling is caused by evaporation. Some heat energy is utilised during the process of evaporation. 

Since this energy is taken from the water itself, it leads to a lowering of temperature in the remaining amount of water.