Remote Sensing and Geographic Information Systems for Design and Operation of Water Resources Systems (Proceedings of Rabat Symposium S3, April 1997). IAHS Publ. no. 242, 1997

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Soil moisture, the hydrologie interface between surface and ground waters

EDWIN T. ENGMAN Hydrological Sciences Branch, Code 974, Laboratory for Hydrospheric Processes, NASA, Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

Abstract A hypothesis is presented that many hydrologie processes display a unique signature that is detectable with microwave remote sensing. These signatures are in the form of the spatial and temporal distributions of surface soil moisture. The specific hydrologie processes that may be detected include groundwater recharge and discharge zones, storm runoff contributing areas, regions of potential and less than potential évapotranspiration (ET), and information about the hydrologie properties of soils. In basin and hillslope hydrology, soil moisture is the interface between surface and ground waters.

INTRODUCTION

The hydrologie cycle's interaction with the Earth's land surface occurs within a thin reservoir that stores and distributes (spatially and temporally) water that falls on the surface in the form of rain or melting snow. This reservoir is commonly referred to as soil moisture. Soil moisture integrates much of the land surface hydrology, acts as the interface between the land surface and the atmosphere, the land surface and the groundwater reservoir, and controls the infiltration and surface runoff processes. The production of surface water through the runoff process is to a large degree controlled by the antecedent soil moisture and groundwater recharge is also controlled by soil moisture.

Although soil moisture physically represents only the surface layer of soil, much of the time it is highly correlated with the total water in the soil profile and is an indicator of total water availability. Unfortunately, soil moisture is not a uniform variable in either a spatial or temporal sense. The very large spatial variability of soil moisture is the result of variable inputs (rain or snowmelt), land cover, highly variable soil properties and topography. The temporal patterns of soil moisture respond to the spatial distribution and variable atmospheric forcing. The net result of this is that soil moisture is a difficult variable to measure, not necessarily at one point in time, but in a temporally consistent and spatially comprehensive basis. Because it exhibits such large spatial and temporal variability; a snap shot or point measurements have had very little meaning in a hydrologie sense.

This paper reviews the current interest in soil moisture which is primarily driven by the science interests in the land-atmosphere interactions, both at the General Circulation Model (GCM) scale and at the mesoscale. Although these interests have driven the development of remote sensing of soil moisture, there are other potential applications for soil moisture in hydrology. The paper makes the case for interpreting remotely sensed soil moisture as a hydrologie signature. A hydrologie signature that can be used to identify runoff areas, recharge areas, and the general

130 Edwin T. Engman

hydrologie processes in a spatial sense. The spatial and temporal signatures of soil moisture may provide the key to realistic distributed modelling of hydrology.

LAND SURFACE HYDROLOGY

Land surface hydrology refers to the spatial and temporal storage and redistribution of rainfall and snowmelt as it falls on or enters into the soil. The various processes that occur define the hydrologie response of a given basin. These processes include infiltration, évapotranspiration (ET), interflow or rapid shallow groundwater flow, surface runoff, groundwater recharge and discharge, and soil water storage and movement (both saturated and unsaturated). To some degree or other, each of these processes is controlled by or reflected as soil moisture.

Runoff

Surface runoff is produced by two basic processes, one can be considered to be caused by saturation from above and the other by saturation from below. The first is the so-called Hortonian runoff which describes the mechanism for producing surface runoff from rainfall. Hortonian runoff (Horton, 1935) occurs when the rainfall intensity or rate exceeds the infiltration rate or saturated hydraulic conductivity for a period of time that exceeds the time to ponding. The rainfall that cannot infiltrate into the soil is referred to as rainfall excess and becomes available for surface runoff.

The second is commonly referred to as saturation overland flow or as Dunne runoff (Dunne & Black, 1970). Saturation overland flow occurs when available storage in the soil is saturated either through rainfall exceeding drainage or by rising groundwater intersecting the land surface and filling the available storage. Additional rainfall that falls on these saturated areas, plus any up slope groundwater flow that is forced to the surface because of lack of storage, becomes water available for surface runoff.

In the field it is sometimes difficult to tell which mechanism is dominating, and often, especially with intense rainfall, it is a combination of the two processes. In other cases where the extremes of climate, topography and soil properties are encountered, the processes are quite distinct.

Partial area hydrology

Hortonian runoff occurring from the entire drainage basin was challenged by Betson (1964) from field observations of surface runoff occurring over only a portion of the basin. Although Betson assumed that the process was the result of excess rainfall, this finding spawned a number of research papers all trying to better identify the sources and process of partial area runoff. About the same time, Whipkey's (1965) field experiments demonstrated that subsurface stormflow could account for a significant portion of a storm hydrograph. Hewlett & Hibbert (1967) related quick rises in stream flow to variable source areas and subsurface translatory flow which emanates from rapid displacement of soil water by new rain.

Soil moisture, the hydrologie interface between surface and ground waters 131

Since these pioneering studies there have been a number of studies and models that have furthered our understanding of the humid region runoff process. Engman & Rogowski (1974) attributed the partial area production of runoff to differences in infiltration capacity controlled by soil properties. Beven & Kirkby (1979) developed a simple contributing area model based on an analytically derived topographic parameter. Anderson & Kneale (1982) attributed the locations of saturated areas to the areal configuration of concave and convex slopes. These and other studies, attempted to understand and model the flow processes illustrated schematically in Fig. 1. Over the past decade or so there have been a number of contributing area models proposed that attempt to define the spatial distribution of runoff and the process for producing relatively rapid storm flows.

More recently, Salvucci & Entekhabi (1995) proposed a statistical-dynamic model for coupling the saturated ands unsaturated flows throughout a hillslope basin. In their model, water tables near the surface delineate areas of surface runoff, évapotranspiration at the potential rate and produce base flow to the channel. Conversely, deeper water tables indicate areas where ET will first be limited, infiltration enhanced and groundwater recharged. Duffy (1996) has proposed a conceptually similar model where saturated and unsaturated storage control the rainfall-runoff process and the topography and the distribution of saturated and unsaturated soil moisture define the flow geometry. Both of these models implicitly use soil moisture to define the location and extent of various hydrologie storages and fluxes. Thus, at least conceptually, one could identify runoff contributing areas if we could determine the location and spatial extent of high soil moisture areas.

Groundwater recharge and discharge

Water that does not runoff during a storm is stored in the soil profile or the subsurface aquifer. The residence and travel times for which water remains in the

(Tennessee Valley Authority, 1964)

Fig. 1 Schematic diagram of the partial area concept.

132 Edwin T. Engman

soil or aquifer depends upon many factors such as the soil properties, geology and topography. The water may exist as storage or move as either saturated or unsaturated flow.

Groundwater recharge can be considered to be the downward flow of infiltrated rainfall through the soil profile to the aquifer where it is temporally stored and adds to the pressure gradient that moves the groundwater mass. Recharge can occur as either saturated or unsaturated flow.

Groundwater discharge can be considered to be the lateral or upward movement of saturated water flow in the soil and rock complex that makes up the aquifer. Discharge occurs when the water is forced to the surface of the land or into a stream bed. The rate of discharge depends upon the gradient and the hydraulic characteristics of the land surface or stream bed.

The scale of the groundwater system can vary greatly and often has several shallow or perched systems superimposed upon a large regional system. In humid areas, the underlying geologic strata control the shallow groundwater flow in the weathered zone. Figure 2 illustrates how less permeable rocks force water to the surface, creating seepage zones. During the wet periods, these seepage zones may

APPALACHIAN RIDGE AND VALLEY Fig. 2 Schematic diagram of geologic influence on surface water.

Soil moisture, the hydrologie interface between surface and ground waters 133

actually flow as springs and in dry periods maintain relatively high antecedent moisture which become contributing areas during rain events. In either case the discharge zones may be able to be delineated by higher levels of soil moisture.

Land surface-atmosphere interactions

At the continental scales the land surface-atmosphere interactions are concerned with the partitioning of incoming radiative energy into sensible and latent heat fluxes. The major factor involved in determining the relative proportions of the two heat fluxes is the availability of water, generally in the form of soil moisture. There have been a number of modelling studies that have demonstrated the sensitivity of soil moisture anomalies to climate (Walker & Rowntree, 1977; Rowntree & Bolton, 1983; Delworth & Manabe, 1993). It has been generally concluded that soil moisture is the second most important forcing function, second only to the sea surface temperature in the mid latitudes, and it becomes the most important forcing function in the summer months.

The role of soil moisture is equally important at smaller scales which affect the regional and local weather. Recent studies with mesoscale atmospheric models have similarly demonstrated a sensitivity to spatial gradients of soil moisture. For example, Fast & McCorcle (1991) have shown that soil moisture gradients can induce thermally induced circulations similar to sea breezes. Chang & Wetzel (1991) have concluded that the spatial variations of vegetation and soil moisture affect the surface baroclinic structures through differential heating which in turn indicate the location and intensity of surface dynamic and thermodynamic discontinuities necessary to develop severe storms.

Two recent extreme events experienced in the midwestern United States have been scientifically explained as having direct cause-effect relationships to soil moisture or the lack of it. An analysis of the 1988 drought in the midwestern United States by Atlas et al. (1993) has shown that these conditions could be modelled accurately only when the soil moisture values were realistic. A reanalyses (Beljaars et al., 1996) of the conditions leading to the 1993 floods in the United States illustrated greatly improved precipitation forecasts, both in quantity and location, when realistic soil moisture values were used in the model.

From a meteorological perspective, one can expect ET to vary with crop type, stage of growth, land aspect and available water. However, when looked at from a small basin or hillslope context, one can conceive a situation where areas of well drained soils restrict ET because off low available moisture and areas near streams, of poorly drained soils, or regions of groundwater discharge maintain ET at potential rates. Thus, being able to map areas of high and low soil moisture in a basin would also identify those areas where ET may be at the potential rate and those areas where ET may be less than potential.

REMOTE SENSING OF SOIL MOISTURE

Recent advances in remote sensing technology have demonstrated that soil moisture can be measured by a variety of techniques. However, only microwave technology

134 Edwin T. Engman

has the ability to quantitatively measure soil moisture under a variety of weather, topographic and vegetation cover conditions so that it could be extended to routine measurements from a satellite system. A number of experiments using truck mounted sensors and aircraft and spaceborne sensors have shown that soil moisture within a thin layer of soil, on the order of 5 cm, can be accurately measured (Engman & Chauhan, 1995).

There are two basic microwave approaches that are typically used; one is passive that is based on radiometry and the other is active and uses radar. Both approaches utilize the large contrast between the dielectric constant of dry soil and water. At L-band the dielectric constant can vary from about 4 for dry soil to about 20 for wet soil which can result in a change in emissivity for passive systems from about 0.95 to 0.6 or lower and an increase in the radar backscatter approaching 10 dB. There are major technical differences between the two systems in spatial resolution, swath width, data rate and power requirements. However, almost without exception, the two systems are complementary, that is strengths in one are match by weaknesses in the other, and vice versa.

Both active and passive systems can be significantly affected by target characteristics other than the soil moisture; for example surface roughness, vegetation and topography. The two most important target properties are the surface roughness and the vegetation canopy, with roughness perhaps being more important with radar and vegetation having a larger relative effect on radiometry. Much of the current research in the microwave field is currently devoted to developing robust soil moisture algorithms and significant progress is being made. Even though the algorithms are still very much a research topic, there has been enough evidence compiled from aircraft campaigns and analysis of existing satellite data to conclude that microwave remote sensing can provide accurate estimates of the spatial and temporal distributions of soil moisture within a natural basin or landscape.

SOIL MOISTURE AS A HYDROLOGIC SIGNATURE

Hydrologie signature defined

If we are to propose a concept of a hydrologie signature it first must be defined. In any natural hydrologie setting, say a drainage basin or a hillslope, one encounters soil moisture variability in both spatial and temporal domains as well as upper and lower limits over which soil moisture can vary. The upper bound is when everything is very wet, say just after a heavy rain; and a lower bound when everything is very dry, say after an extended drought. In between these times one observes changes in soil moisture patterns that are neither uniform spatially nor temporally. The degree of nonuniformity depends upon the climatic region where the basin is, its distribution of soil properties and topography.

The hydrologie signature is thus defined as the pattern of soil moisture that exists on the landscape at times between the extremes of wet and dry. Being time dependent, the soil moisture patterns will exhibit a continuum ranging from wet to dry. The important thing to realize is that during each cycle from wet to dry, the same spatial pattern of relatively wet and dry soils will be repeated. That is the

So/7 moisture, the hydrologie interface between surface and ground waters 135

pattern is not random but is formed by the climate, soil properties and topography. Each of these three characteristics is discussed in more detail.

Factors affecting a hydrologie signature

Climate effects can be simplistically delineated between arid and humid basins. Arid basins spend most of their time history in a very dry state where the soil profile can be assumed to have a zero flux lower boundary under all but the most exceptional situations. This means that over most of the basin's area, there is no groundwater recharge. The exception to this statement is ephemeral channels. Groundwater recharge in arid regions is generally limited to channel bottoms and their adjacent flood plains. Humid basins, on the other hand, spend their time history varying between a wet and dry state but in most cases a portion of the basin will maintain a perpetual wet state, usually in valley bottoms, concave landscape features and areas near perched water tables. Groundwater recharge is possible throughout the basin (no zero flux soil boundary) and is only limited when the water table intersects the land surface.

In the somewhat antiquated catena classification, soils are classified on the basis of drainage or differences in relief. Thus soils developed from the same parent material may have significantly different properties because of their location within the landscape. Poorly drained soils are usually associated with high water tables, either the result of geologic seepage zones or low-lying land adjacent to a stream. Figure 3 is a schematic of the catena concept and how soils delineated on the basis of drainage may be more or less conterminous with recharge zones or discharge and surface water contributing areas.

One of the more obvious exceptions to this description of a hydrologie signature is encountered when the rainfall is not distributed uniformly in space and time. Extreme examples of this can be found in the southwestern United States where Osborne & Keppel (1966) point out that runoff producing rains often cover areas of one square mile or less. This is an extreme but it illustrates that a hydrologie

Fig. 3 Schematic diagram of a soil catena diagram.

136 Edwin T. Engman

signature may be defined by the storm characteristics and have nothing to do with the physical basin boundaries or distribution of soils.

The soil moisture patterns or hydrologie signature are controlled by a number of factors, including potential ET, soil properties, land cover, geology, topography, proximity to streams and groundwater, and in some cases spatially varied rainfall. There are a number of possible ways to interpret the signatures. For example, areas that dry relatively quickly can be associated with soils with higher hydraulic conductivity, low water tables, convex slopes in profile, low runoff producing areas and potential groundwater recharge areas. Conversely, areas that dry more slowly can be associated with soils with slower hydraulic conductivity, higher clay contents, high water tables, concave slopes in profile and plan, high runoff producing areas and groundwater discharge areas.

Physical basis for a hydrologie signature

With the exception of the cases where the highly variable precipitation is the reason that soil moisture exhibits a high degree of spatial variability, I contend that each time a hydrologie unit or drainage basin goes through a drying cycle, from wet to dry, the same spatial patterns will evolve each time. The rate of change may vary if the potential ET is not the same but he relative patterns will repeat themselves. These repeatable patterns are the hydrologie signature of the hydrologie unit or basin.

If this statement is true, the reason has to be tied to the soil properties and their control on the various hydrologie processes. Analysing the spatial and temporal

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Soil moisture, the hydrologie interface between surface and ground waters 137

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patterns of soil moisture from the WASHITA 92 data, Mattikalli et al. (1996) have shown that indeed one may be able to derive information about the soil physical properties from the spatial and temporal patterns of surface soil moisture. Figures 4 and 5 illustrate the relationship between the change in microwave estimated soil moisture and two soil properties, the sand/clay ratio and the saturated hydraulic conductivity. If these relationships could be established for other regions and soils, it would make a compelling argument that the soil moisture patterns are tied to the physical properties of the soils and thus control the hydrologie processes spatially. Consequently, the patterns of soil moisture observable by microwave remote sensing techniques can be a physically based hydrologie signature. This signature reflects the physical properties of the soils and their control over the hydrological processes.

Examples of hydrologie signatures

Results from aircraft campaigns and analysis of satellite data in which intensive field data have been collected have demonstrated the potential for interpreting spatial and temporal patterns of soil moisture as a hydrologie signature. Several recent examples have demonstrated the role of contributing area hydrology in MACHYDRO 90 (Engman, 1991; Wood et al, 1993) soils properties from Washita 92 and 94 (Jackson etal., 1995; Mattikalli et al, 1996) or the spatial distribution of rainfall in MONSOON 90 and 92 (Jackson et al., 1993).

138 Edwin T. Engman

One of the first examples of using microwave measurements of soil moisture in a hydrologie context was reported by Jackson et al. (1993). Using both the PBMR and the ESTAR during Monsoon 90 and 91, they showed how soil moisture patterns were related to the spatial extent and amount of rainfall. Figure 6 shows the comparison between the rainfall and the microwave derived soil moisture. In this case the hydrologie signature in the form of soil moisture spatial gradients defines the region of high and low évapotranspiration.

An even more dramatic example of hydrologically significant soil moisture was derived with the ESTAR during the WASHITA 92 (Jackson et al, 1995). From a hydrologie perspective we were able to follow a drying period from very wet to dry over a period of ten days. It had rained for 26 consecutive days in Oklahoma and initial conditions were very wet. The drying pattern as well as the spatial variability reflected by different soil properties is shown in Fig. 7. In this case the hydrologie signature is defined by the soil properties and their control over drainage and ET.

(d) Runoff (mm) - PBMR SI (e) Runoff (mm) - CREAMS SI

(DPBMR-derivedSI Fig. 6 Initial soil moisture index estimates based on the PBMR and a water balance model ((a) and (b)), the precipitation (c), and the spatial distribution of runoff for the Kineros and CREAMS models. On the bottom is the PBMR produced maps of volumetric soil moisture.

Soil moisture, the hydrologie interface between surface and ground waters 139

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CONCLUSION

In this paper, I have tried to make the case that many hydrologie processes have a signature that is detectable with remote sensing in the form of the spatial and temporal signatures of soil moisture. The level (wetness) of the signature, its location in the landscape, its rate of change can all provide information to the hydrologist about the spatial distribution of processes in a basin. The types of information that

140 Edwin T. Engman

may be detected include groundwater recharge and discharge zones, storm runoff contributing areas, regions of potential and less than potential ET, and information about the hydrologie properties of soils. Microwave remote sensing has the potential to detect these signatures within a basin. These signatures should also be the key on how and where to apply soil physical parameters in distributed models.

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