Multiscale Interdisciplinary Integration of Soil-Hydrology-Plant Interactions I: Landscape Hydropedologic Perspective
Presiding: H Lin, Pennsylvania State University; R Hooper, Consortium of Universities for the Advancement of Hydrologic Science, Inc.; R van Genuchten, George E. Brown Jr. Salinity Laboratory, USDA Agricultural Research Service
H41B-01 INVITED 08:30h
Soil as the Central Link in the Hydrological Cycle
We present an overview of the function fulfilled by the soil in the chain of processes constituting the hydrological cycle in the terrestrial domain. Considering the height of the atmosphere, the thickness of the earth's rock mantle, and the depth of the ocean, we note that the soil is amazingly thin -- typically not much more than one meter thick. Yet, with its sponge-like porosity, internal surface area, and finite capacities to absorb, store, and transmit water and solutes, the soil determines the fate of precipitation reaching the ground surface - whether it flows over the land as runoff, is detained and made available to plant roots, or seeps downward to an aquifer. Without the soil as a buffer, rainfall would produce violent floods rather than sustained stream-flow. The soil also acts as a living filter, in which dissolved and suspended constituents are retained or transmuted into nutrients for the continual regeneration of life. Energy as well as matter is in constant flux as solar radiation is absorbed by the soil and growing plants and as heat is exchanged in evaporation and condensation as well as in biochemical reactions. Plant leaves absorb carbon dioxide from the air and synthesize it with soil-derived water to form the primary compounds of life, while oxygen emitted by the leaves makes the air breathable for animals, which in turn fertilize the plants. However unique in form and function, the soil is not an isolated body but interacts with the overlying atmosphere and underlying strata, and with surface and underground bodies of water. Especially important is the interrelation between the soil and the climate, enhancing or mitigating the greenhouse effect via the soil's uptake or release of radiatively active gases (carbon dioxide, methane, and nitrous oxide). For all these reasons, it is vital that we give full consideration to the soil's role in the context of hydrology. The problem to overcome, however, is the disjunct between the characteristic spatial scales of the two disciplines. Whereas hydrology typically operates on the watershed level, traditional soil science has dealt with phenomena on the scale of a vertical profile or a restricted field. Recent efforts to define soil processes in a catenary sequence in the landscape and concurrent efforts to define the spatial variability of soil properties offer a way to close the gap and thus integrate the sister sciences.
H41B-02 08:55h
Spatial and Temporal Linkages Related to Hydrogeomorphic Processes: an Emerging Discipline
Hydrologic and geomorphic processes are intrinsically linked related to such fundamental catchment-scale processes as stormflow generation, sediment production and routing, and nutrient cycling. The emerging discipline of hydrogeomorphology focuses on these process linkages in both time and space. Many attempts to quantify and model hydrogeomorphic processes across various temporal and spatial scales have been unsatisfactory due to a poor understanding of processes linkages. From a practical viewpoint, models that yield the correct hydrogeomorphic response at the small catchment scale (5-20 ha) are often considered to accurately represent internal hydrogeomorphic processes. Nevertheless, even at this small scale, various hydrogeomorphic processes could produce similar responses. Understanding the pathways of water and sediment from the hillslope to the stream, and subsequently within stream systems is essential to evaluate effects of land use. Many of the hydrologic and sediment models that have attempted to incorporate processes still greatly simplify the complexity of hydrogeomorphic pathways. To understand the movement of sediments and water from source to sink, it is necessary to link hydrologic processes with their respective geomorphic settings and vice versa. For stormflow processes, the hydrogeomorphic paradigm of stormflow generation is proposed, whereby catchments are partitioned into landform attributes (riparian corridors, hillslopes, hollows) based on their unique hydrologic response. Sediment pathways are more obvious than hydrologic pathways, but are still poorly understood. Dynamic sediment budgets that include temporally and spatially explicit estimates of sediment sources and sediment routing (affected by hydrology) and linkages (influenced by land practices) are useful to assess the complex internal hydrogeomorphic processes that affect sediment and related nutrient fluxes.
H41B-03 09:15h
Does Variability Matter? - Virtual and Field Experiments to Understand Runoff Generation
The influence of the spatial variability of soil hydrological properties (water retention curve, saturated hydraulic conductivity, preferential pathways) and the spatial and temporal variability of precipitation and soil water content on runoff generation have been observed in many watersheds. Focusing on individual aspects, studies were able to describe and sometimes predict details of the observed variabilities. Understanding the combined responses, however, has been difficult. We present a study that combines individual variables and variabilities in space and time to systematically analyze their effect on runoff generation. First, we use field experimental investigations including tracer studies to detect and elucidate the main controls on runoff generation. Then, we combine the gained "field intelligence" with numerical simulations within the framework of virtual experiments to explore how the different spatial and temporal variabilities affect runoff generation, response time, and solute transport. In particular, spatial variability of soil depth, initial soil moisture content, and precipitation variability due to canopy interception is analyzed. Combining our conceptual understanding and field observation of runoff generation processes with virtual experiments, we can more effectively determine the influence of variability on runoff generation, organization of water in the hillslope, and isolate controlling factors.
http://faculty.forestry.ubc.ca/weiler/
H41B-04 INVITED 09:30h
Use of Time to Incipient Ponding as a Hydropedologic Index for Surface Soils
Physically-based hydrological modeling using numerical solutions to Richards equation requires considerable parameterization in order to specify the relations between the soil water pressure head and the soil water content and between the hydraulic conductivity and either the soil water content or soil water pressure head. The problem is exacerbated in catchment hydrologic applications because the physical parameters vary spatially and some even vary temporally. This difficulty of parameter acquisition has led to a widespread interest in development of pedotransfer functions to derive the requisite properties from more basic data. It has also resulted in attempts to simplify spatial data input requirements by identifying domains of sensitivity (with respect to model outputs) as functions of soil type, usually on the basis of texture. For classifying the runoff potential of soils, one important hydrologic characteristic is the time to incipient ponding. Estimations of incipient ponding can be obtained numerically using Richards equation and `like soils' grouped on the basis of pedotransfer inputs derived from the texture triangle. Alternatively, predictions can be made using quasi-analytic infiltration theory containing only two parameters of importance: the sorptivity, S, (which has a dependence on initial moisture content) and the saturated hydraulic conductivity. This considerably simplifies the grouping of `like soils' and provides one objective index for hydropedologic mapping of surface soils. In this paper we discuss the relation between Sorptivity and other hydraulic parameters and then use estimations of time to incipient ponding to map this hydropedologic index onto the USDA texture triangle.
H41B-05 09:45h
Parameterizing Root Water Uptake as a Function of Water and Salinity Stress
The extraction of water from soil by plant roots is a critical component of water and energy balances at the land surface. Root water uptake is affected by a number of hydropedologic variables, including soil salinity and/or drought. We review various approaches that may be used to model root water uptake as a function of water and salinity stress, emphasizing particularly the different forms of the sink term that may be incorporated into the Richards equation for local scale modeling, as well as the challenges posed by the need to model water uptake at local, field, and regional scales. Modeling approaches are illustrated using numerical simulations and experimental data collected on forage crops grown under combinations of water and salinity stress.