It took well over 2000 years for a reasonable understanding of the science of hydrology to evolve. Not until the pioneering work of John Dalton in about 1800 were all the mechanisms of the large scale hydrological cycle properly determined. It was because of his significant contribution to the science that the EGS has named its new medal for eminence in hydrology The Dalton Medal.
Back in the times of the early Greek geographers (c.500 BC) the land mass of the Earth was thought to be surrounded by an immense sea called Oceanus. The origin of the water in the rivers was unknown, but, because of its unusual seasonal behavior, the Nile was thought to be connected directly to Oceanus. Aristotle (c.350 BC) wrote the first meteorological treatise and showed some understanding of the process of precipitation. He suggested that the origin of rivers could be rainfall and not some vast underground reservoir as was commonly believed at the time.
It took until the time of Leonardo da Vinci (1452-1519) for further significant progress to be made. Leonardo made various types of hydrological observation and developed a concept of the hydrological cycle. This included evaporation from land and water with precipitation returning water to the surface, but rivers were supplied with water from the sea via underground veins in the earth. The idea of the sea as a direct source of water to supply rivers was maintained for some time - for instance by Kircher (1602-80) who produced elegant illustrations of the underground passages and mountain storage caverns required.
Work in France in the latter half of the seventeenth century did however begin to produce evidence that precipitation alone could account for the origin of rivers, at least on the small (catchment) scale. For instance studies by Perrault (1608-80) showed that for a catchment in the headwaters of the Seine the rain and snow that fell in the area was more than adequate to keep the river flowing all year.
About the same time Edmund Halley (1656-1742) was working in England. He carried out evaporation experiments and by extrapolating figures for the estimated flow of the Thames he investigated the water balance of the Mediterranean. However, despite almost certainly knowing of the French studies, he still held that additional water supplied from vast caverns inside mountains filled by some atmospheric condensation mechanism were necessary to supplement precipitation to maintain the annual flow of rivers.
John Dalton (1760-1844) at the end of the next century finally was able to correctly describe the hydrological cycle.
He was born to a Quaker weaving family in Northern England and was lucky to receive at an early age a sound grounding in science and mathematics. At the age of twelve (!) he opened a school in his home town which proved to be quite a success, later co-founding a larger school with his brother in Kendal. As he was a Quaker the universities of Oxford and Cambridge were closed to him and his subsequent progress to the world of higher education took him to the recently opened Manchester Academy. This was specifically open to "young men of every religious denomination". In later years and after several moves it was finally established at the end of last century in Oxford, where it is now known as Harris Manchester College, a full college within the University taking mature students. John Dalton left the Academy in 1800 to concentrate on research, private teaching and a leading role in the prestigious Manchester Literary and Philosophical Society.
Dalton was a true scientist of his age, a natural philosopher, turning his mind to a wide variety of problems. He is probably best known for his work in chemistry, atomic concepts, meteorology, and colour blindness, but it was his quantitative hydrology studies, carried out while at the Manchester Academy, that have led to the naming of the EGS medal in his honour.
He was always interested in the weather - some members of the society complaining that when chairing meetings he always managed to bring the discussions round to meteorology! Over his lifetime he made and recorded over 200,000 meteorological observations which formed the basis of many of his scientific publications. During his years at the Manchester Academy he carried out the studies necessary for the calculation of the annual hydrological balance of England and Wales which culminated in his beautifully written seminal paper presented in 1799 and published in 1802 entitled:
"Experiments and Observations to determine whether the Quantity of RAIN and DEW is equal to the Quantity of Water carried off by the Rivers and raised by Evaporation; with an enquiry into the ORIGIN of SPRINGS"
In order to calculate the water balance it is necessary to quantify the three terms: precipitation input, river outflow and evaporation loss. By the late eighteenth century there were some raingauge records for a fair proportion of the counties in England. Dalton collected together one of the most comprehensive sets of data that had ever been obtained and after appropriate corrections for missing areas achieved an overall mean for England and Wales of 31" (1"=2.5cm) to which 5" was added for annual dewfall. In order to calculate the total river outflow he divided the country up into large catchment areas and by using the flow of the Thames adjusted from the estimate of Halley and by taking account of the relative sizes of the rivers he finally obtained a figure of 13". Even to this day the calculation of the annual evaporation for the whole of England and Wales is a daunting task. In those times there had been a some small-scale experiments, especially for water surfaces, but in order to obtain a better estimate for bare earth and vegetation Dalton obtained new data on the long-term water balance of a soil-filled container. He finally came up with 30" for the annual evaporation loss.
The resulting calculation for the water balance was therefore: 31" rain plus 5" dew in = 36" and 13" flow plus 30" evaporation = 43" out. The original discrepancy of 7" was accounted for by errors mostly in the evaporation which had been over-estimated due to problems with the measurements and non-representativeness of the location. He therefore commented:
"Upon the whole then I think that we can finally conclude that the rain and dew of this country are equivalent to the quantity of water carried off by evaporation and by the rivers. And as nature acts upon general laws, we ought to infer, that it must be the case in every other country until the contrary is proved"
He went on to set down a method for estimating river flows from a knowledge of precipitation and evaporation and river length and in the final section of the paper he finally lays to rest the concept of any additional mechanisms beyond that of precipitation being required to explain the origin of springs. In a subsequent paper further experiments and improvements in evaporation theory added to the quantitative understanding of hydrological processes.
The correct understanding of the hydrological cycle was established and from then to now the work in hydrology has gradually grown and improved with new techniques and the availability of more data. However in this field, vital to all our lives, we still owe a great debt to Dalton and the other pioneers who helped to form a basis for our current knowledge.
Dalton's Paper: Memoirs of the Litemry and Philosophical Society of Manchester, Volume V, Part 11 pages 346 to 372, MDCCCII.
General Background: History of Hydrology by A K Biswas. Pub North Holland, 1970.
An excellent longer review of the hydrological work of Halley and Dalton in: The Development of Hydrological Concepts in Britain and Ireland between 1674 and 1874 by J C I Dooge. Hyd Sci Bul XIX, 3 9/1974 pp 279-302.