Geoscience Reference
In-Depth Information
Box 1.1 The Legacy of Robert Elmer Horton (1875-1945)
Robert Elmer Horton was one of the major figures of 20th century hydrology, the foremost
scientific hydrologist of his time in the USA and perhaps the only hydrologist to have a waterfall
named after him, near to his long-time home in Voorheesville, New York State. A more detailed
account of his life and publications has been compiled by Hall (1987). When Horton died in
1945, his scientific papers were bequeathed to the American Geophysical Union and are
now housed in the National Archives in College Park, Maryland. There are 95 boxes of them,
representing the broad range of his interests, the experimental data collected by him and his
assistants (Richard van Vliet, Howard Cook, Harry Leach and James Erwin) and his work as a
“hydraulic consulting engineer”.
As far as I know, only a small proportion of the boxes of papers have been examined since
they were archived by Walter Langbein more than 50 years ago, but they contain a range of
fascinating material concerned with infiltration processes, surface runoff, the importance of
subsurface flows and hydrological predictions (Beven, 2004a, 2004b, 2004c). They include
records of data collected in the Horton Hydrological Laboratory (in his garden) and nearby
LeGrange Catchment which was monitored using a thin plate weir. Horton emerges as an
impressively careful experimenter, demanding of both himself and his assistants (and very
demanding of others in the Geological Survey, Soil Conservation Service and National Weather
Service with whom he had to deal), though not without humour.
In many hydrological textbooks, Horton is primarily remembered for being the originator
of the concept of infiltration excess overland flow. Indeed this is very often now called “Hor-
tonian overland flow”. His final monumental paper, presenting a theory for the development
of hillslopes and drainage basins by surface erosion, is based on this concept (Horton, 1945).
In addition, his infiltration equation (see Box 5.2) has been widely used to predict effective
rainfall and surface runoff. His papers reveal, however, that his conception of hydological
processes in catchments was much more complex than this. He recognised the importance
of partial surface runoff areas, of rapid subsurface responses, and of macropores, air pressure
and surface effects in infiltration (Beven, 2004b, 2004c). In addition, his experimental data
on rainfall rates and infiltration rates suggest that the occurrence of widespread surface runoff
might have been rather rare on the LeGrange catchment (Beven, 2004a).
He made his living, however, as a hydrological consultant. Part of his work involved
analysing rainfall-runoff relationships and predicting the response of catchments, effectively
using rainfall-runoff models. The infiltration equation provided Horton with the means to
make predictions of runoff volumes (at least if the changes of infiltration capacities due to
soil, seasonal and land treatment effects could be estimated). Elsewhere, he treats the prob-
lem of allowing for depression storage and the hydraulics of overland flow and channel flow,
including transitions from laminar to turbulent flow, in routing that runoff to the basin out-
let (Horton, 1935). He realized, however, that in applying this theory he would need to deal
with the fact that there might be different infiltration capacities in different parts of a basin
and that this would not be a problem in prediction (he proposed a distributed approach based
on the division of a basin into “meshes” of different shapes and characteristics; Horton, 1938).
It would, however, be a problem in the analysis of rainfall-runoff data in a complex basin. He
realised that it would then only be possible to derive some average infiltration capacity over a
whole catchment based on the observed rainfalls and discharges.
Horton does not, however, seem to have had too many doubts about applying average infil-
tration capacities derived under one set of rainfall conditions to the prediction of runoff under
other conditions. He did observe that there were seasonal changes in infiltration capacities
derived in this way (Figure B1.1.1), and that minimum values might be more robustly esti-
mated than maximum values (which might still underestimate the potential maximum values
achievable). He suggests, however, that minimum values might be useful in the estimation of
the “maximum flood intensity” to be expected on a given area (Horton, 1937, p. 385).
