Hewlett, J D

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John D. Hewlett


John D. Hewlett 1922 (Philadelphia, Pennsylvania, USA) to 2004 (Athens, Georgia, USA)


John Hewlett was raised in the Shenandoah Valley of Virginia. He entered military service in 1942 and served in the European theatre of World War II until the end of the war. He earned his bachelor’s degree in Forestry from New York State College of Forestry in Syracuse, NY, in 1949 and a Master of Science degree in Forest Ecology from Syracuse University in 1956. In 1960 he obtained a doctorate degree from Duke University, Durham, NC, with an emphasis on plant water physiology. In 1956, Hewlett joined the staff of the Coweeta Hydrologic Laboratory, Southeastern Forest Experiment Station, United States Department of Agriculture (USDA) in western North Carolina. He became Project Leader until 1964, when he joined the faculty of the School of Forest Resources at the University of Georgia in Athens (UGA), a position he held until retirement in 1984.

From 1965 to 1967 he served under the Office of Science and Technology on the Panel on Water Resources Research. This panel produced the first 10-year programme of federal water resources research. In 1965 he helped organize the First International Symposium on Forest Hydrology at Penn State University.

Hydrological Achievements[edit]

Many fundamental precepts of modern hydrology can be traced back to Hewlett’s work, including the variable source area concept, the importance of antecedent conditions on streamflow response to rainfall, and the use of stream buffers to protect water quality. Hewlett’s work continues to factor into areas of current controversy, such as the occurrence and importance of interflow processes.

While working at the USFS Coweeta Hydrologic Laboratory in the southern Appalachian mountains, Hewlett was fascinated by two apparent hydrologic paradoxes: that streamflows rose during rainfall while no overland flow was observed, and that streamflow was sustained during droughts despite the lack of significant valley aquifers. His thinking on these matters was influenced by early research at Coweeta by M.D. Hoover and C.R. Hursh, who described a dynamic form of subsurface flow that contributed to storm flow.

Hewlett took these concepts and his own theory of saturated/unsaturated drainage from the soil profile and verified them experimentally, with colleagues A.R. Hibbert and W.L. Nutter, using artificially packed soil slabs on hillslopes at Coweeta (Hewlett, 1961; Hewlett and Hibbert, 1963). These seminal experiments demonstrated the importance of unsaturated interflow in replenishing valley aquifers and maintaining streamflows, and also the potential role of translatory subsurface flow in the stormflow response of forested catchments.

The artificial hillslope experiments, along with field knowledge of the Coweeta watersheds and analysis of stream hydrographs, led Hewlett and colleagues Hibbert, Nutter, and C.A. Troendle to develop the variable source area concept that incorporated and integrated the range of hillslope flow paths which are variable in time and space (Hewlett, 1961; Hewlett and Hibbert, 1967; Hewlett and Nutter, 1970). This work set the stage for modern concepts and simulation models that are still major topics of research and application in forest hydrology. Hewlett, Troendle, and P.Y. Bernier went on to develop variable source area simulation models (VSAS1 and VSAS2), but were frustrated by the difficulty of capturing soil moisture dynamics and the effects of micro-relief in the models.

Hewlett was an effective champion of the use of small catchment experimentation as a source of sound knowledge and technology needed to guide forest management (Hewlett et al., 1969). During his tenure at Coweeta, he was instrumental in organizing the research into an interdisciplinary programme as previously outlined by C.R. Hursh (Douglass and Hoover, 1988). This approach in studying basic hydrologic processes provided a basis for mathematical modelling of forest responses to disturbance. Throughout his career, he was active in the conduct and analysis of paired-watershed experiments to evaluate hydrologic and water quality effects of silvicultural practices (e.g. Hewlett and Helvey, 1970; Bosch and Hewlett, 1982; Hewlett and Fortson, 1982; Hewlett and Doss, 1984).

In the late 1970s and early 1980s, Hewlett used climate and streamflow records to challenge engineering hydrologic orthodoxy and demonstrate that hourly rainfall intensity was a poor determinant of peak flow rates in forested basins (Hewlett et al., 1977, 1984). Hewlett et al. (1977) wrote, ‘While it is obvious that rainfall intensity dominates storm flow responses from compacted fields, parking lots, and small areas of saturated ground, it is by no means obvious that whole watersheds are highly responsive to intensity. Nevertheless, the assumption that rainfall intensity normally over-rides infiltration capacities to deliver storm flow is either explicit or implicit in most hydrologic theory and method.’

In 1969, Hewlett and his colleague Wade Nutter wrote a textbook on forest hydrology, An Outline of Forest Hydrology, which was revised in 1982 and retitled Principles of Forest Hydrology. A large part of a generation of forest hydrologists was educated using this textbook. He served on the United States Forestry Curriculum Development Program from 1969 to 1972. He also served on the education and training committee of the US International Hydrologic Decade programme. Hewlett was influential in applying the science of hydrology to the development of best management practices in the timber industry. Many of the modern recommendations on managing road and skid trail runoff and implementing riparian buffers are based partly on studies conducted by Hewlett. During John’s tenure at UGA, he took a special pride in pursuing practical approaches to applying forest hydrology. John worked closely with the industry on developing Georgia’s first version of Best Management Practices for Forestry Operations. He followed this up by working with companies on developing demonstration areas to illustrate sound road construction and maintenance practices, thus dealing with one of the largest potential sources of pollution from silvicultural operations.

Reference Material[edit]

Rhett Jackson, C., Swank, W. T., & Olszewski, R. (2005). Hydrologist: John D. Hewlett (1922–2004). Hydrological Processes, 19(10), 2093-2095.

Major Publications[edit]

Hewlett JD. 1961. Soil moisture as a source of baseflow from steep mountain watersheds. US Forest Service Southeast. Forest Experimental Station Research Paper SE 132.

Hewlett JD, Hibbert AR. 1963. Moisture and energy conditions within a sloping soil mass during drainage. Journal of Geophysical Research 68(4): 1081–1087.

Hewlett JD, Hibbert AR. 1967. Factors affecting the response of small watersheds to precipitation in humid areas. In Forest Hydrology, Sopper WE, Lull HW (eds). Pergamon Press: New York; 275–290.

Hewlett JD, Lull HW, Reinhart KG. 1969. In defense of experimental watersheds. Water Resources Research 5(1): 306–316.

Hewlett JD, Nutter WL. 1970. The varying source area of streamflow from upland basins. In Proceedings of the Symposium on Interdisciplinary Aspects of Watershed Management. American Society of Civil Engineers: New York; 65–83.

Hewlett JD, Helvey JD. 1970. Effects of forest clear-felling on the storm hydrograph. Water Resources Research 6(3): 768–782.

Hewlett JD, Fortson JC, Cunningham GB. 1977. The effect of rainfall intensity on storm flow and peak discharge from forest land.Water Resources Research 13(2): 259–266.

Bosch JM, Hewlett JD. 1982. A review of catchment experiments to determine the effect of vegetation changes on water yield and evapotranspiration. Journal of Hydrology 55: 3–23.

Hewlett JD, Fortson JC. 1982. Stream temperature under an inadequate buffer strip in the southeast Piedmont. Water Resources Bulletin18(6): 983–988.

Hewlett JD, Doss R. 1984. Forests, floods, and erosion: a watershed experiment in the southeastern Piedmont. Forest Science 30(2):424–434.

Hewlett JD, Fortson JC, Cunningham GB. 1984. Additional tests on the effect of rainfall intensity on storm flow and peak flow from wild-land basins. Water Resources Research 20(7): 985–989.