The millennium old hydrogeology textbook “ The Extraction of Hidden Waters ” by the Persian 2 mathematician and engineer

22 We revisit and shed light on the millennium old hydrogeology textbook “The Extraction of Hidden Waters” by the 23 Persian mathematician and engineer Karaji. Despite the nature of the understanding and conceptualization of the 24 world by the people of that time, ground-breaking ideas and descriptions of hydrological and hydrogeological 25 perceptions such as components of hydrological cycle, groundwater quality and even driving factors for 26 groundwater flow were presented in the book. Although some of these ideas may have been presented elsewhere, 27 to the best of our knowledge, this is the first time that a whole book was focused on different aspects of hydrology 28 and hydrogeology. More importantly, we are impressed that the book is composed in a way that covered all aspects 29 that are related to an engineering project including technical and construction issues, guidelines for maintenance, 30 and final delivery of the project when the development and construction was over. We speculate that Karaji’s book 31 is the first of its kind to provide a construction and maintenance manual for a engineering project. 32 33 34 35


Prologue 38
The eleventh century Arabic book "Inbat al-miyah al-khafiya" (Arabic: ‫انباط‬ ‫المیاء‬ ‫الخفیه‬ ) or The Extraction of Hidden 39 Waters by Abubakr Mohammad Ebn Al-Hassan Al-Haseb Al-Karaji (Arabic:  ‫ابوبکر‬  ‫محمد‬  ‫بن‬  ‫الحسن‬  ‫الکرجی‬ ) is a 40 pioneering text on hydrogeology (Karaji, 1941). The book is in Arabic, the scholarly language of Persia in the 41 Medieval Islamic Civilization era. The book was translated from Arabic into Persian by Hoseyn Xadiv Jam in 1966 42 (Xadiv Jam, H. 1966). Karaji's book was also translated into French in 1973 (Mazaheri, 1973), Italian in 2007 43 (Ferriello, 2007), and into English in 2011 by Schade in her PhD Thesis (Schade, 2011). Schade's English 44 translation was made from the French translation. 45 In an interesting article, Nadji and Voigt (1972) presented a glimpse into the book.. They stated that, based on this 46 11 th century book, the basics of the hydrologic cycle and components of underground water quality were already 47 known by Arab and Persian scientists of that time. They mentioned that the techniques of wells and qanats digging, 48 which were developed for groundwater exploitation in the Middle East, were of such a high standard that they are 49 still in use today. Prompted by Nadji and Voigt (1972), Davis (1973) put Karaji's work in a broader scientific 50 context, explaining the lack of appreciation, value and awareness of Middle Eastern science and scientists in 51 general. 52 We believe that Karaji's contributions to hydrology and hydrogeology are significant and should be remembered 53 and revisited in this Hydrology and Earth System Sciences special issue on the 'History of Hydrology'. In this 54 essay, we revisit Karaji's book and provide an English translation of pieces from the book that crucially offer 55 pioneering ideas in hydrogeology and in general for engineering projects. The translations presented here are based 56 on the Persian translation of Karaji'sbook. We believe it is important to include quotes from Karaji to ensure 57 historical veracity and authenticity and hence a historically faithful essay. It is also fascinating to hear Karaji's 58 thoughts in his own wordsbringing his story, his motivations and his scientific contributions to life. 59 We hope this essay brings about new insights and information that were not provided in the previous written 60 accounts. We hope that it helps to contribute to a growing awareness of Karaji was about practical hydrology in this period. Although it has been mentioned that the book was written by him as 97 a means of earning a living (Nadji and Voigt, 1972), we speculate that the topic was of great practical interest in 98 the arid area of the Persian plateau. It is also very likely that this topic was of interest to Karaji personally and he 99 certainly knew it was vitally important. As will be mentioned, in the extracts of Karaji's preface to his book, he 100 notes that to provide people with guidance on how to build a good water supply would be a most beneficial work.

101
The book is considered by some to be "the oldest textbook on hydrology" (Nadji and Voigt, 1972). It is certainly 102 one of the earliest known works focussed on both hydrology and hydrogeology. Figure 1 shows a statue of Abubakr 103 Mohammad Karaji at the Water Museum of Sa'dabad Museum Complex in Tehran, Iran. 104

Qanat 105
Karaji wrote extensively on qanats in his book. Qanat or Kariz is an old system of water supply from an aquifer.

106
Qanat is an Arabic word and Kariz is in Persian, although Qanat is now also used in Persian. It consists of a gently 107 sloping underground tunnel that brings groundwater to the surface by gravity flow. The main qanat channel is hand-108 dug and just large enough to fit the person doing the digging, and a series of mother wells and vertical access shafts 109 traversing different topographies and geologies along its course (e.g., English, 1968 Qanat technology spread across the world, first westwards to the Mediterranean and Egypt, and southwards to 115 Oman and Southern Arabia.. A second major diffusion of qanat technology occurred with the early conquests by 116 Islam into Northern Africa, the peninsular Spain and the Canary Islands (Lambdon, 1989; Martínez-Santos and 117 Martínez-Alfaro, 2012). Finally, as a consequence of Spanish conquests, the technology also spread to South and 118 Central Americas, such as in Mexico, Peru, or Chile (Martínez-Santos and Martínez-Alfaro, 2012). 119 Karaji's book not only explains his understanding of hydrology at his time, but it also provides a practical manual 120 on how to construct a qanat. 121

The Extraction of Hidden Waters 122
In the preface to his book, Karaji wrote "I do not know any other profession more beneficial than extraction of 123 hidden water, as it flourishes and cultivates lands, improves people's welfare, and grants ample profits" [Translated 124 from Xadiv Jam, H. 1966]. Figure 2 illustrates the first page of Inbāt al-miyāh al-khafiya. This is from a later-125 century copy of the original book of Karaji that is kept at the University of Pennsylvania, in the Lawrence J. 126 Schoenberg Collection (Karaji, 1675 The titles of the book sections provide a fascinating insight into the wide range of topics that were covered in the 138 book. It is amazing that the book not only covers the conceptual and technical aspects as well as construction 139 guides, it also provides guidelines for maintenance and even advice on how to deliver and consign the project when 140 the development and construction is over. It even touches on important social aspects such as religious regulations. 141 The book is like a construction and maintenance manual for a modern engineering project. Lewis (2001), who 142 explored the history of surveying instruments of the Greeks and Romans, has referred to Karaji's book and his 143 contributions to the procedures and inventive instruments for levelling and sighting in surveying engineering. 144 Karaji's ideas in surveying revealed his sense of engineering concerning an understanding of accuracies and 145 awareness of essential elements of the construction and exploitation of qanats (Stiros, 2006 ]. It appears this shows an 188 understanding of recharge processes and the way in which water interacts with rocksearliest conceptions of 189 "hydrogeology"the study of water and rock. 190 Karaji provided explanations about soil and rock classifications based on their colours and characteristics and 191 described the indicators that could be used to find out where water might be available underground and in springs. gushes or rises up, unless its source is in a location that is higher than the location of gushing." [Translated from 225 Page 63: Xadiv Jam, 1966]. Concepts such as mass, force, energy, gravity field, and many other physical properties 226 and processes, which are easily comprehensible now, did not exist in eleventh century conceptualizations of the 227 universe. However, we may speculate that there are some very early insights into the modern-day concept of 228 hydraulic headnamely that groundwater flows from points of high hydraulic head to points of low hydraulic head 229 -in Karaji's descriptions of water flow. We are unaware of any other documented cases where ideas of groundwater 230 flow, from higher grounds to lower grounds, had been published any earlier than Karaji's treatment. 231 To understand how different the conceptualisation of the world was in old times, the following is a quote from da 232 Vinci (1452-1519) to explain water flow, in which he creates an analogy between water flow and blood circulation 233 in the human body: 234 "Natural heat keeps blood in the veins at the top of the man, and when the man has died this blood becomes cold 235 and is brought back into the low parts, and as the sun warms the man's head the amount of blood there increases, 236 and it grows to such an excess there with the humors as to overload the veins and frequently to cause pains in the 237 head. Humor is Latin for moisture. da Vinci, who is recognised as one of history's most brilliant minds, lived 500 years 246 after Karaji's time. We may appreciate Karaji's profound knowledge of hydrology and hydrogeology, especially 247 when considered in the context of his time. da Vinci was clearly on the incorrect path with water flowing uphill. 248 However, Karaji seems to be very close to understanding the core of the hydrologic cycle and the mechanisms of 249 water flow from higher ground levels to lower ground levels. We note that it was only in the seventeenth century 250 that a clear understanding of hydrologic cycle was finally realized (Todd and Mays, 2004). Hanifeh) who had referred to prophet Mohammad's practices and sayings. It is intriguing to note that the only 259 available and ruling law at the time in the Islamic world was strictly based on religious ideas and texts. Thus, all 260 matters relating to ownership, property and rights were based entirely on religious ideas and works. These were 261 developed, promoted, espoused and written entirely by religious scholars. They were linked to the practice of the 262 prophet Mohammad and his companions' practices. Karaji's work began to bring science, engineering, maths and 263 technology to this importantand at that time entirely religiouslegal discussion, principles and practice. 264 Next, Karaji defined protection limits based on his knowledge and consideration of differing soil types. "The 265 protection areas of qanat in hard soils is less than that for loose soils." [Translated from Page 74: Xadiv Jam, 266 1966]. Karaji understands that wells placed in the more permeable material (the loose soils) require a greater area 267 or size for the groundwater protection zone around it compared to that in the less permeable material (the hard 268 soils). Groundwater protection or buffer zones are based on the very same principle todaya principle that Karaji 269 conceived a thousand years ago. We speculate that what Karaji mentioned here is related to his intuitive 270 understanding of the ease of water flow in loose soils compared to that in hard soils. It is possible that Karaji 271 understood that water flowed more easily through loose soils than it did through hard soilsleading to a 272 concomitant increase in the size of the protection zone for a well in the more permeable material (the loose 273 materials). This may be some of the very earliest documented insights into the rates and ease of groundwater flow 274 through different geologic materialsearliest conceptions of what we would call hydraulic conductivity today. 275 They are also earliest known documented conceptions of modern-day hydrogeology. as true today as they were a thousand years ago. Karaji was a prognostic hydrologist and hydrogeologist hundreds 298 of years ahead of his time. Beyond the specific topic of Karaji's book on the extraction of hidden waters, the 299 comprehensive content, details and topics that he has covered in the book are very impressive for engineering 300 construction project management. This important point has not been noted before, to the best of our knowledge. 301 Therefore, Karaji's book is not only, according to some, "the oldest textbook on hydrology", but also among the 302 earliest known texts on engineering construction management. It is certainly one of the earliest known works 303 focussed on both hydrology and hydrogeology. 304 Like previous authors, we too assert Karaji deserves more credit in hydrologic and groundwater science and 305 engineering than has been the case to date. We hope our paper plays a part in rectifying this. We hope that it helps 306 to bring Karajithe scientist and his scienceto the attention of current and future generations of hydrologists, 307 hydrogeologists, scientists and engineers around the world. 308