When you have water features in your garden its very important to keep them topped up during dry weather. But its also good to not waste water. With this in mind I suggest that providing shade is the best protective measure that will minimise evaporation. The best sort of shade is natural plant life and water lilies are excellent on ponds - but if you have no organic shade then try a shadecloth cover. You can stretch it over the water and it will definitely help keep the surface temperature down and therefore minimse evaporation.