What’s on Earth is hydrology ? It is the science that studies the movements of water, from the atmosphere to the ground and into the ground and then back to the atmosphere again. This book does not aim to cover all the variety of issues and topics the movement of water causes but is mainly concerned with keeping the material simple and possibly concise. Insights are left to notes and to appropriate literature.
Everyone sees water moving on the Earth surface, maybe excluding those who lives in deserts, and it looks like the it should be easy to get a quantitative mathematical description of water movements: but this is not the case! Even the channelized flow, the simplest of the water flows, shows a great complexity, due to the turbulent nature of the flow and the variety of boundaries in which the water is “constrained”. I have put constrained among quotes “” because water is never really constrained. It goes everywhere, sometimes very slowly, compared to the subjective time of our human actions. Other flows needs to cope with the medium where water flows, for instance the porous soils, the underground, the plants vessels.
More than that, water on Earth is usually liquid, solid, in form of ice or snow, vapor and each of the phase of water interacts with and transform into the others. The complexity of phases is just one side of the multifaceted dynamics of water because the elementary physics or thermodynamics of water movements at the centimeter or smaller spatial scale (but how the small is small if, for instances, pores in soil can be micrometers and intercellular space in leaves nanometers ? ) dynamically organizes when we look at the movements at the meter scale and reorganizes at the decameters scale and reorganizes again at the hectare, and, again and again, at kilometers, up to the dimensions of continents. In this reorganization some feature of the elementary motion is loosed while some other feature emerges as fundamental. We could say that the core of contemporary hydrology is in getting a reason of these scales behavior in heterogeneous, i.e. affected by a great variety of small causes, environment. Galilean science is the light to move among the variety. However hydrologists, the scientists who study and evolve Hydrology’s understanding, rarely can setup experiments (they do, obviously, in the labs) which are really answering the questions, because simplifications and distortion, implicit in other physical sciences branches do not work when you have to deal with a transpiring plant or when the inanimate (?) but complex channel networks forms. Hydrology is mostly observed and recorded, in a sequence of single cases, each one at best slightly different from the others and often very different from the others. Therefore from where we start ? A few certainties we have: mass conservation is the first. We give it for granted but it was recognized more than two centuries ago and its status was even more uncertain in hydrology up to Dalton’s research. The second is the energy conservation, which is in fact more elusive, since the difficulty to estimate some of its parts is bigger than the one to measure mass flow. Those two statements seem so obvious when enunciated but when we come to measure the mass budget and the energy budget closure troubles begin. Historically, actually, their were not the center of scientists’ research, because this was concentrated merely on fluxes determination, like the river discharge, the evaporative transport, or groundwater flow, at most. However, let’s say, that this book is very much concerned with the closure of these budgets, than concentrating on any of the fluxes, on which actually various sub-disciplines flourished in the past.
Newtonian physics rules, when messing with hydrology, but also the explication of the second law of the dynamics is largely simplified, even in the recent treatments, because of the pitfalls it hides when dealing with simple mechanical systems but that reveals in their whole complexity when the case of water fluxes is the topic.
The nature of the measured hydrological quantities is usually presenting a degree of randomness, even if their physics is known to be deterministic. This apparent randomness is certainly due to the high number of degree of freedom that the fluid nature of the water, either liquid or gaseous, brings, but also of the uncountable histories that water encounters in its travel. Probably it is not true randomness, is maybe random-mess, because of the structure of the medium water crosses.
If then it is true that determinism is at the core of the hydrological thermodynamics (or maybe saying “thermodynamics” we are contradicting ourselves?) the analysis of the matter lives at the faint light of statistics for almost every phenomenon. Therefore, almost always hydrologists have hard times to deduce patterns from very uncertain information. All of these issues in fact are those that makes this science one of the most fascinating of the contemporary science.