Surely this is an over simplification? It may (or may not) be true that the water is never actually destroyed (although I thought that was a claim only made by energy) but the problem could well be where it is and in what form. All of the "drinking" water may still be out there in the system but it might not fall on our hills in the same quantities. It might evaporate from our reservoirs at a higher rate. It might solidify at the poles more than in recent history.
Everything I say is a simplification, of course, otherwise I would fill the entire forum with the answer to one question. I interpret your call for 'less black and white' as a call to add more detail. Is this correct? I shall try to be succinct...
Water is not destroyed when it becomes vapour, but could be said to be destroyed if it is reacted with another material to form a compound. Probably the largest human use here is concrete - Ca3SiO5 + H2O → (CaO)·(SiO2)·(H2O)(gel) + Ca(OH)2. I guess this is dwarfed by photosynthesis and other natural hydrolysis - 2n CO2 + 4n H2O + photons → 2(CH2O)n + 2n O2 + 2n H2O . I suspect this process is roughly in balance across nature with water evolved during rotting, where the long H-C chains in carbohydrates are oxidised, but since this process involves a lot of complex chemistry going on inside bacteria it would be arduous to expand here.
So I am indeed guessing when I suggest that the volume of water remains the same on the planet throughout the ages, but it would be a hard point to come to a firm conclusion on either way. Of interest is the University of Iowa 'small comets' hypothesis, which suggests that diffuse, 'house-sized' water-ice comets strike the Earth's atmosphere very frequently, adding to our total water supply -
http://smallcomets.physics.uiowa.edu/ refers...
The issue of where our drinking water is, and in what quantities it falls is more germane to the discussion. Ice solidification at the poles (particularly the Antarctic, which has been growing a lot recently) does not really take water out of the yearly cycle, and water vapour is readily replaced into the atmosphere from the ocean surface. We, of course, are primarily concerned with the UK water provision, and we know something specific about this. Here is the measured rainfall across the UK for the last 100 years.
http://www.cru.uea.ac.uk/cru/info/ukrainfall/ You will see that rainfall has INCREASED quite distinctly during this time. You will also see that variations are quite common - of the order of 30% of the total. So our infrastructure should be designed to support 30% delivery variation, and then should have no problems coping if the population has not changed (which, of course, it has).
Once population change is factored into the equation, we need to consider what an appropriate storage figure per head of population is, given that a 30% variation in precipitation year on year is to be expected historically. I asked DEFRA this very question three weeks ago, and am still waiting for an answer. What I suspect is that OFWAT does NOT apply such a figure, leaving it up to the water companies, who are 'trimming' the margins of the infrastructure provision in order to keep making profits. If this is occurring, every time precipitation falls below average (as it does, by definition, 50% of the time) we will have problems, and this seems to be what is happening.
Does this give you a better idea of the thinking behind my recent response? Take it from me - you don't want to hear me talk about global weather patterns...