The purpose of this letter is to describe a possible way to reduce drought and wildfires in targeted locations. This should not be interpreted as an excuse to relax efforts to reduce greenhouse gasses, pollution, or resource waste. Solar energy is used in communities along the Rio Grande to distill drinking water from polluted water available to the people. Solar stills are suitable for families and small communities. Their performance is limited by the necessity to evaporate supplied water, condense the water vapor, and recover the condensate in the same apparatus. What I propose is a large scale apparatus to evaporate water from oceans, bays, lakes, and rivers. Evaporator performance is not compromised by attempting to recover the water vapor. Instead it is released into the atmosphere to produce rain or humidity, depending on the location. High water surface temperature is required to make water molecules sufficiently energetic to enter the atmosphere as water vapor. Sunlight is absorbed by unmodified water bodies, but it usually penetrates to great depth. The solar energy is absorbed by a large volume of water, resulting in a negligible temperature rise and little water vapor released into the atmosphere.
My remedy for this problem is a raft composed of many black-painted wooden panels, each about 18 inches by 8 inches, with their top surfaces floating just below the water surface. Solar energy is absorbed by the black surface and heats the thin layer of water above the absorber, resulting in a large temperature rise and a useful rate of evaporation. A wooden cross piece fastened across the absorber plate at each end protrudes above the water surface. These cross pieces control the floating depth. If some force pushes downward on the raft, the cross pieces displace more water and resist the tendency to sink. The raft can be oriented so that the cross pieces block cold water from flowing over the solar absorber. This is important when rafts are placed in rapidly flowing rivers and streams.
River locations such as the Twisp and Methow in Washington State would probably not produce rainfall from the water vapor. If the water vapor is carried into the surrounding vegetation by the prevailing winds, it would supply humidity that would decrease the incidence and severity of wildfires. Firefighters were killed in this area. I am able to calculate the rate of evaporation for rafts in these river locations, but I need to study the wind regime to know where the water vapor will go.
For evaporator rafts located in San Francisco Bay, it is certain that the water vapor will follow the prevailing winds in summer, going east and then turning southeast to follow the San Joaquin Valley down to the Mojave and Sonoran deserts to replace air that is heated and rising from the desert. The temperature inversion layer over the bay area in summer and the high wind speed make rainfall unlikely. The benefit will be from the water vapor reducing the desiccation of the soil and vegetation. Much study and calculation lies ahead to justify evaporator rafts in San Francisco Bay.
Evaporator raft location in the Arabian Sea off the Indian coast near Bombay could enhance rainfall in the summer monsoon by supplying water vapor to the wind coming from the southwest. This air rises up the western Ghatt mountains, expands, cools, and moisture condenses to make rain. The plains inland fom the coastal Ghatts are suffering severe drought. The aquifers have been almost pumped out and arsenic contaminates the water. Evaporator raft location in the open sea requires protection from wave action. I have been looking at floating breakwater design, perhaps suitably spaced rings of logs surrounding the rafts.
The reader will note that every aspect of my work is incomplete. Help is needed to advance these ideas.