Hydropedology: Concept and Opportunities
Henry Lin
Dept. of Crop and Soil Sci., The Pennsylvania State
Univ., University Park, PA 16802, USA. (henrylin@psu.edu)
Hydropedology
has emerged in recent years as an interdisciplinary science that has been
recognized by the Soil Science Society of America, International Union of Soil
Sciences, American Geophysical Union, and European Geosciences Union. It
integrates pedology, hydrology, and geomorphology to study interactive
pedologic and hydrologic processes and landscape-soil-hydrology relationships
across space and time, aiming to understand pedologic controls on hydrologic
processes and properties and hydrologic impacts on soil formation, variability
and functions (Lin et al., 2006). It
emphasizes in situ soils in the
landscape, where distinct pedogenic features, environmental variables, and
anthropogenic impacts prevail and interact, leading to the first controls on
landscape water flux.
Landscape
water flux here encompasses the source, storage, availability, flux, pathway,
residence time, and spatio-temporal distribution of water (and the transport of
chemicals and energy by flowing water) in the variably-saturated soil
zone. While source, storage,
availability, and flux of water have been studied considerably in the past,
attention to pathway, residence time, and spatio-temporal pattern of flow and
transport has been limited.
As
illustrated in Fig. 1, hydropedology attempts to connect the pedon and
landscape paradigms through linking phenomena occurring at the microscopic
(e.g., pores and aggregates) to mesoscopic (e.g., pedons and catenas) and
macroscopic (e.g., watersheds, regional, and global) scales. Hydropedology, combined with hydrogeology,
promotes an integrated systems approach to study the interactions of water with
solid earth (soil and rock). In terms of
temporal dimension, hydropedology deals with both short- and long-term changes
of landscape-soil-hydrology interrelationships.
In essence, hydropedology seeks to answer the following are two
fundamental questions:
- How the structure and distribution of soils over
the landscape exert the first control on landscape water flux across
spatio-temporal scales in the shallow subsurface?
- How landscape water flux (and associated
transport of chemicals and energy by flowing water) impacts soil
evolution, variability, and functions?
Fig.
1 Hydropedology as an
interdisciplinary science that promotes integrated studies of interactive
hydrologic, pedologic, and geomorphic processes across spatial and temporal
scales.
Some terminologies need a clarification here:
- Pedology vs. soil
science: “Ped” is a unique term in soil science,
and “pedology” captures that uniqueness well. The natural soil “architecture” is the
essence of the soil. A crushed
sample of soil is as akin to a natural soil profile as a bulk of ground
beef is to a living cow (Lin, 2007).
Pedology has been regarded by many as the only sub-discipline of
soil science that has its own unique theory and in itself is an integrated
earth science, while the other sub-disciplines of soil science tend to be
applied math, physics, chemistry, and biology to the study of soils.
- Soil function vs. soil
formation:
Classical pedology has focused on soil genesis and classification, whereas
hydropedology attempts to focus on quantitative soil functions driven by
water and to link the “past” (i.e., soil formation as records of past
environment, often at geological time scale) to the “present and future”
(i.e., soil functions for the current and future land use, especially at
human time scale). Soils are natural integrator of multiple functions in the earth system and space
exploration (Lin, 2005), providing a central link in the integrated
interdisciplinary study of the “Critical Zone” (from the top of vegetation
down to the bottom of aquifer, upon which nearly every life-sustaining
resource and all human activities depend).
- Mr. Water vs. Mrs.
Soil: Water flux into and through soils in the
landscape is the essence of life, which resembles in a way blood
circulates in a human body (Bouma, 2006).
The interactions of soil and
water are so intimate and complex that they cannot be studied in a
piecemeal manner, but rather as a system across spatial and temporal
scales. Hydrology also can aid in quantifying soil-forming processes,
since all natural soil-forming factors (as well as human activities)
affect and are affected by hydrology.
Water is also key to quantifying soil functions, because once water
regime is characterized, soil physical, chemical, and biological functions
can be added as they strongly depend on the water regime and on
interactive processes with the soil.
- Hydropedology vs.
vadose zone hydrology (or soil hydrology): While overlaps exist between these two closely related disciplines, there
are clear distinctions between the two: 1) Hydropedology emphasizes in situ soils in the context of the
landscape, where intact pedogenic features (such as aggregation,
structure, and horizonation) and soil-landscape relationships are
essential, thus requiring attention to catena, soil mapping, soil
morphology, and soil geomorphology; 2) Hydropedology is not just about
hydrology, equally important is hydrologic feedback to pedogenesis, soil
variability, and soil functions; and 3) Hydropedology deals with
variably-saturated zone in the surface and near-surface environments,
including unsaturated zone, wetlands, and even subaqueous soils, and
encompassing both shallow root zone and deep vadose zone.
Opportunities
There are enormous opportunities for hydropedology
to contribute to the advancement of soil science, to open the doors for
interdisciplinary collaborations, and to propel for breakthroughs in sustaining
the earth’s Critical Zone. Highlighted
below are some of such examples.
- Hydropedology represents a paradigm shift in our basic thinking
and approach to ped, pedon, landscape, watershed, regional, and global
scale analysis of soil and water interactions. Besides the challenge of solving Jenny’s
state-factor equation and the need of developing a soil function theory,
other critical hydropedological issues that can stimulate the advancement
of soil science include: 1) Quantifying
the relationships between soil and hydrologic structures and functions at
different scales; 2) Linking the
Darcy-Buckingham’s law of water flow through soils with the Dokovchiav-Jenny’s
theory of soil formation and distribution; 3) Developing tools and
techniques for in situ high resolution and continuous noninvasive
mapping of the subsurface flow networks.
- Water is a unifying
theme for research on complex environmental systems. Hydrogeoscientists
are encountering a new intellectual paradigm that emphasizes connections
between the hydrosphere and other components of the earth system. While hydroclimatology, hydrogeology,
and ecohydrology are now well recognized, an important missing piece of
the puzzle is the interface between the hydrosphere and the pedosphere.
·
The integrated study of the
earth’s Critical Zone has been suggested as one of the most compelling research
areas in earth science for the 21st century (NRC, 2001). The emerging Critical Zone science provides a
stimulating platform for advancing hydropedology and soil science in
general. Example include: 1) Coupling hydropedology and biogeochemistry
for ecosystem and weathering studies; 2) Identifying “hot spots” and
“hot moments” of biogeochemical processes triggered by soil hydrologic
conditions, including biogeochemical
reaction rate as limited by transport and pathway in the field; 3) Quantifying
variation in soil microbial communities in surface and subsurface soils, and
its relation to hydropedology and preferential flow pathways.
·
A global homeland security
issue: Protecting soil and water resources should be
considered as a global “homeland security” issue if we are to sustain our home
planet and human society, because we (and all ecosystems and wildlife) depend
on them every day for food, water, air, energy, and habitat. As
·
Environmental regulations
for water and soil protection: Numerous practical applications in our daily life call
for expertise in integrated soil and water sciences, such as water quality, land degradation, land use
planning, watershed management, wetland protection, nutrient cycling,
contaminant fate, waste disposal, precision agriculture, and ecosystem
restoration. Examples of emerging
regulations include: 1) Sustainable land-use planning and proactive design,
including hydropedology as a foundation for spatial land use planning using a
“three-layer” model (Bouma, 2006); 2) Scientifically-sound and
socio-economically-feasible trading of water quality, water quantity, and
carbon at different scales, including how these trading impacts the physical
reality of soil and water resources.
For more information
Please visit hydropedology web site at www.hydropedology.psu.edu
The semi-annual Hydropedology Newsletter can
also be viewed at www.iuss.org
References
Bouma, J. 2006. Hydropedology as a powerful
tool for environmental policy research. Geoderma 131:275-286.
Hillel, D. 1991. Out of the Earth –
Civilization and the Life of the Soil. The Free Press,
Lin, H.S. 2005. Letter to the Editor on “From
the Earth’s Critical Zone to Mars Exploration: Can
Lin, H.S. 2007. Cattle vs. ground beef: What
is the difference? Soil Survey Horizons 48:9-10.
Lin, H.S., J. Bouma, L. Wilding, J.
Richardson, M. Kutilek, and D. Nielsen. 2005. Advances in hydropedology.
Advances in Agronomy 85:1-89.
Lin, H.S., J. Bouma, Y. Pachepsky, A. Western,
J. Thompson, M. Th. van Genuchten, H. Vogel, and A. Lilly. 2006. Hydropedology:
Synergistic integration of pedology and hydrology. Water Resource Research 42, W05301, doi:10.1029/2005WR004085.
National Research Council (NRC). 2001. Basic
Research Opportunities in Earth Science.