(Al Qamar) 54 : 28
وَنَبِّئْهُمْ أَنَّ الْمَاءَ قِسْمَةٌ بَيْنَهُمْ ۖ كُلُّ شِرْبٍ مُحْتَضَرٌ
And you inform them that indeed the water is divided between them, each should take water according to their turn fixed.
See explanation 26: 155-158.