As far as use of water for Wuzu and performance of Tayammum is concerned. These matters have been discussed explicitly in the Quran itself, therefore, as far as their status as a pre-requisite for Salah and conditions under which each is to be adopted are concerned, there’s no other way around it:
يَا أَيُّهَا الَّذِينَ آمَنُواْ إِذَا قُمْتُمْ إِلَى الصَّلاةِ فاغْسِلُواْ وُجُوهَكُمْ وَأَيْدِيَكُمْ إِلَى الْمَرَافِقِ وَامْسَحُواْ بِرُؤُوسِكُمْ وَأَرْجُلَكُمْ إِلَى الْكَعْبَينِ وَإِن كُنتُمْ جُنُبًا فَاطَّهَّرُواْ وَإِن كُنتُم مَّرْضَى أَوْ عَلَى سَفَرٍ أَوْ جَاء أَحَدٌ مِّنكُم مِّنَ الْغَائِطِ أَوْ لاَمَسْتُمُ النِّسَاء فَلَمْ تَجِدُواْ مَاء فَتَيَمَّمُواْ صَعِيدًا طَيِّبًا فَامْسَحُواْ بِوُجُوهِكُمْ وَأَيْدِيكُم مِّنْهُ مَا يُرِيدُ اللّهُ لِيَجْعَلَ عَلَيْكُم مِّنْ حَرَجٍ وَلَـكِن يُرِيدُ لِيُطَهِّرَكُمْ وَلِيُتِمَّ نِعْمَتَهُ عَلَيْكُمْ لَعَلَّكُمْ تَشْكُرُونَ
Believers! When you rise to pray, wash your faces and your hands as far as the elbow, and wipe your heads and wash your feet to the ankle. And if you are [ceremonially] unclean, bathe yourselves. But if you are sick or on a journey, or when, you have just relieved yourselves or had intercourse with women, you find no water, take some clean mud and wipe your faces and your hands with it. God does not wish to burden you; He seeks only to purify you and to perfect His favour to you so that you may express gratitude. (Quran 5:6)
As far as the philosophy behind these directives is concerned, following response may be helpful in this regard:
Discussion 5794 • Reply 5942