 Why does Evans give up his drinking water?
The light from the star that died when the meteor was created fell on Earth before the first lungfish ventured from the sea. It was two days to sunrise, where Evans stood. The shadow line between day and night could be seen from the Moon to be drifting across Australia. Evans, who had no watch, thought of the time as a quarter after Australia. Evans was a prospector, and like all prospectors, a sort of jackknife geologist, selenologist, rather. Evans was about three hundred miles east of Williamson Town, the site of the first landing on the Moon. Evans was due back at Williamson Town at about sunset, that is, in about sixteen days. His drinking water—kept separate from the water in the reactor—might conceivably last just as long. But his oxygen was too carefully measured; there was a four-day reserve. Four days reserve—plus one is five—plus sixteen days normal supply equals twenty-one days to live. By the light of this, he reinspected the steam system, and found about three gallons of water frozen in the condenser. This boiling lowered the temperature, and the condenser demonstrated its efficiency by quickly freezing the water in the tank. Evans sealed the turbine from the rest of the steam system by closing the shut-off valves. Carefully he poured a cup of his drinking water into a pipe that led to the boiler, and resealed the pipe. The water that he had poured into the boiler quickly turned into steam, and the steam turned the generator briefly. For example, because of an order of his stating that there would be no alcoholic beverages within the survey building, the entire survey was assured of a constant supply of home-made, but passably good liquor. Morning to him had no meaning at all, but he thought in the strictest sense that it would be morning on the Moon for another week. It was used mostly, from sun-up to sun-down, to supplement the nuclear power plant. "That could hurt," Cowalczk admitted, "but there was only one of them." After the sun rose, Evans returned to the lava cave that he had been exploring when the meteor hit. All of these needed water to form, and their existence on the Moon puzzled him for a while. He made trips from and to the Moon about once a month, carrying supplies in and metal and ores out. That's from here to Earth, of course.
 Using the water is the only way for his transportation to work
 Using the water is the only way for his transportation to work  He knows he will be able to find more soon in one of the caves  He knows his rescuers will come find him and bring water  He knows he does not have enough to survive so he uses it to save his equipment