I have done several tours with the temperature in the high 30C and low 40C. All off road and only rarely passing by towns. Water was something we thought about and carried plenty. 10 litres each and we also took the oppurtunity if we passed somewhere we could fill up not far from where we intended to camp to get another 4 or 5 litres, as you can use lots in camp. Even with so much and filling up every time we passed a supply point somewhere several times we got down to our last water bottle and were worried about running out. We now always fill up everything as we go and only empty excess out when we know we are going to reach a refill point quickly.