Mention hydrology to a potholer and, if he thinks of anything at all, he will think of colour testing, of finding out where such and such a stream reappears. If a little more sophisticated, he may think in terms of automatic dye absorbers such as charcoal for fluorescein or mordanted cotton for rhodamine B. Although such information as is provided by colour tests and related methods is a first essential if the hydrology of a region is under consideration, it tells us little of the cave itself. A full consideration of limestone drainage is very complex and, as yet, little studied but we can, by being observant and perhaps being a little scientific from time to time, go much further than this.
To begin with, if we know how long it takes a dye to traverse a cave system and we know where the sink and resurgence are, we can judge if the rate of flow is normal, slow or fast, once we have established standards. Allowing for the average gradient from sink to rising, we can thus hazard a guess as to whether the flow is free, or ponded either in vadose or phreatic conditions.
When heavy rainfall occurs, rivers rise generally, but a definite flood wave travels along a cave. If the cave is vadose, this wave will take some considerable time to travel down the cave, whereas the same pulse will pass instantaneously through a phreatic zone as water is directly displaced. This effect may often be observed by comparing the flow of a cave with the river into which it flows, a greater delay in the rising of the spring pointing to a greater length of vadose and a smaller length of phreatic passage. This effect is often striking enough for the local farmer to have noticed it and is often worth asking about. Another marked effect is the turbidity of a resurgent stream which will be high if the stream comes from peaty moorland and which will increase on flooding. An absence of turbidity in areas where surface feeders are turbid points to pickup from percolation, condensation and local drainage off limestone. If, during the initial stages of the flooding of a spring, clear water is ejected which later becomes turbid, this indicates a system with surface feeders and a phreatic zone. This effect is caused by the flood pulse which is transmitted instantaneously through the phreatic zone whereas the major part of the turbidity is due to the arrival of actual flood water when this has passed through the phreatic zone to the resurgence.
From this point onwards the amount of information we can obtain depends on the trouble we take. For example, both the pH and the hardness of the water increase, in general, as we pass through a cave system. A sudden lowering of these quantities can indicate an inlet long after temperature changes have been absorbed. It should, however, be noted that waterfalls produce a drop in pH of as much as 0.2 for a 30' fall in flood (this change is not linear) and so care must be observed in interpreting such results. Consider a cave system of known sink and resurgence and hence of known height difference and order of length. If the overall pH across such a system is lower than normal, once again in reference to standards to be established, unless there is a local inlet we would suspect a system of many pitches rather than one having a steady fall. In this respect we would have to consider the existence, or otherwise, of a phreatic zone which tends to increase the pH considerably.
Considerably more information can be obtained by considering the fluctuations of these many quantities in time and this far more complex matter will be given a preliminary consideration in my article "The hydrology of Karst Districts" to appear in C.R.G. Transactions Vol. 6, No. 2.