A Brief History of Hydrology* \ The Robert E. Horton Lecture Rafael L. Bras Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 1. Introduction 2. From the Greeks to Horton I am grateful to the American Meteorological So- My family and I returned to the United States af- ciety for bestowing on me the honor of being the ter a year's sabbatical in 1983. A not-so-friendly im- Horton Lecturer. The award honors Robert E. Horton, migration officer in New York detained us for over 40 a truly multifaceted individual whose curiosity and minutes wondering what had we done over the past genius led him to blaze the trails in hydrology and 12 months, suspicious of the trips to China, Europe, hydrometeorology that many of us have followed. and South America. I patiently explained that I was a Horton wrote about infiltration, runoff production, hydrologist and lectured in all those places. A quizzi- river basin response, erosion and fluvial geomorphol- cal look was followed by the question, "What is a hy- ogy, evaporation, and hydrometeorology (Horton drologist?" After a carefully crafted explanation she 1932,1933, 1940,1945). Everything he did was pro- gave me an incredulous look and asked, "Why would found and thorough, setting standards, hypotheses, and anybody care about that?" theories that are still debated. But Horton was not an Many have cared about hydrology, dating back to ivory tower scientist. All his work was motivated by great civilizations in China, the Middle East, Greece, and the problems of society. For almost 30 years I have Rome. Early thinkers and philosophers did not under- followed Horton's delight in the variety of challenges stand three basic hydrologic principles (Eagleson 1970): that hydrology poses. Like him I have dabbled in prac- tically all elements of physical hydrology. And like 1) conservation of mass, him, I have had a wonderful time! Hydrology has 2) evaporation and condensation, and changed and evolved dramatically over my career. 3) infiltration. The following will be a short history of hydrology. It will also be a biased history, colored by my experi- They were worried about how water gets up to the ences, interests, and work. I make no claim of com- mountains, flows down to the sea, and fails to raise the pleteness or of neutrality. I do claim pride in being a level of the latter. Because of what may be called lim- small part of what I think have been the best years of ited spatial awareness, they could not see rainfall as a hydrologic science. sufficient source of stream flow. To account for ob- served water behavior, underground reservoirs (be- neath mountains) were hypothesized. Water was believed to be pushed up the mountains by vacuum Corresponding author address: Dr. Rafael L. Bras, Room 1-290, forces, capillary action, or "rock pressure," surfacing Department of Civil Engineering, Massachusetts Institute of Tech- as stream flow. These underground reservoirs were nology, Cambridge, MA 02139. E-mail: [email protected] replenished by the sea. In final form 12 March 1999. Vitruvius, during the first century B.C., stated that ©1999 American Meteorological Society the mountains received precipitation that then gave rise Bulletin of the American Meteorological Society 1151 Unauthenticated | Downloaded 10/08/21 08:48 AM UTC to stream flow. A filtration process by which water tion of river basin behavior. Horton's ideas on infil- percolated into soil was also acknowledged by tration, soil-moisture accounting, and runoff are still Vitruvius and later by da Vinci. recognized by present-day hydrologists. It was in the seventeenth century that Perrault proved by measurement that precipitation could ac- count for stream flow in the Seine River, France. 3. Thoughts on the last 30 years Similar quantitative studies were made by Mariotte and Halley during this historical period. At this stage, By the time I arrived on the scene, as a student in the mass balance concept was pretty well established, 1968, generations of engineering hydrologists had although questioning of it continued well into the been educated using the pioneering textbooks of twentieth century. The eighteenth century saw advances in hydraulics and the mechanics of water movement by Bernoulli, Chezy, and many others. The nineteenth century saw experimental work on water flow by people like Darcy and Manning. The above names are familiar to stu- dents of groundwater and surface-water movement. Until the 1930s hydrology remained a science filled with empiricism, qualitative descriptions, and little overall understanding of ongoing processes. At that time, people such as Sherman (1932) and Horton (1940) initiated a more theoretical, quantitative ap- proach. Sherman's unit hydrograph concept still re- mains with us as the most successful (but not necessarily the best) and most well-known explana- FIG. 2. High-resolution in situ observations of the soil-moisture field in the small (0.1 km2) Tarrawarra experimental catchment; southeastern Australia demonstrates the role of competing hydro- logic processes in defining spatial structures in the soil-moisture field (Western and Grayson 1998). (top) The volumetric moisture content within the top 30 cm of soil during a typical wet season day (27 Sep 1995). In this case the soil-moisture pattern is orga- nized by terrain elevation. The wetter zones follow elevation con- tours and drainage depressions. High hydraulic conductivity of moist soils allows significant redistribution along topographic gradient. During a typical day in the dry season (22 Feb 1996; conditions shown in the bottom panel), there is no discerned cor- respondence between topography and soil moisture. Dry soil moisture has lower hydraulic conductivity and lateral redistribu- tion is less dominant as a factor in organizing soil moisture pat- terns. The random spatial variability of soil texture may be a more critical factor in explaining any spatial variability in the relatively dry soil moisture field. Lateral redistribution in the saturated and FIG. 1. Moisture content vs depth profiles for (a) the Horton unsaturated zones, groundwater and surface-water interaction, mechanism and (b) the Dunne mechanism. Overland flow evapotranspiration, macropore flow, and soil texture heterogene- generation for (c) the Horton mechanism and (d) the Dunne ity are among the competing factors that influence the spatial or- mechanism (Freeze 1980). ganization of surface soil moisture fields. 1152 Vol. 80,, No. 6, June 1999 Unauthenticated | Downloaded 10/08/21 08:48 AM UTC Linsley et al. (1958, 1982). Their Hydrology for En- empirical results or data may change regionally or gineers was one of the few, and overwhelmingly the geographically, the fundamental principles governing dominant, hydrology textbooks in the United States. It hydrologic processes, when defined in mathematical is probably the historical best seller in the field, by far. terms, do not vary, and therefore have general appli- Ven Te Chow's (1964) encyclopedic Handbook of cation." The era of pure empiricism was over. Finally, Applied Hydrology seemed to codify a "mature" field. in 1970 Peter S. Eagleson wrote Dynamic Hydrology. To students like me, the field was lacking new ideas. A best-seller it was not, but its impact on the genera- Luckily, my impressions were wrong; a revolution tions of hydrologists to follow was enormous. was brewing and it raised its head around 1970. Eagleson's work screamed science and emphasized Crawford and Linsley's (1966) work on the Stanford that the hydrology of the land was intertwined with Watershed Model and Harley et al.'s (1970) MIT the atmospheric phenomena. Catchment Model showed that digital computers There were other key actors and other key events, but offered hydrologists the opportunity to integrate pro- to me the above were pivotal to launching the hydrologic cesses to simulate complicated behavior in a system- revolution of the last 30 years. Things have changed so atic, integrated fashion. Work at the Harvard Water much that in a recent speech (1998) I joked that it seemed Program and at Colorado State University by that all hydrology I learned at MIT was wrong. An ex- Yevjevich and students began viewing natural hydro- aggeration, but it gets the point across! Several new logic processes as random phenomena and represent- principles, realizations, concepts, tools, and method- ing them in that fashion. The International Hydrologic Decade had been on-going for five years and a wealth of new data and group efforts were becoming avail- able. One such result, The Handbook on the PRIN- CIPLES of Hydrology (capitalization added by the author), edited by Donald M. Gray (1973), stated in its foreword "Hydrology is only an 'infant' in growth in the modern-day family of sciences . our ancient forebears, if they could but see us now, would be shocked to find how lax we have been in neglecting the study of water." It further claimed that "although FIG. 4. Each of a number of different river basins is presented in the plot by two points: a circle for 1987 data and a triangle for 1988 data. The abscissa represents "model error," i.e., it represents the average runoff generated for the basin by nine land surface parameterizations (LSPs) minus the observed runoff there. The LSPs were forced with observed precipitation, radiation, and other atmospheric variables for 1987-88 as part of the Global Soil Wetness Project. The ordinate shows the density of rain gauges within the basin, a measure of the accuracy of the precipitation FIG. 3. Typical input and output of the distributed model by forcing used by the LSPs. The salient feature of the plot is the Garrote and Bras (1995a,b) illustrating the state of various significant reduction in model error as the density of rain gauges hydrologic variables at a particular time in a simulation over the increases.