1. Introduction

Hillslope hydrological processes control the fate of rainwater by absorbing, transporting, storing, and releasing it. These processes are partly responsible for the formation of soil. The flow of water in soil is in all directions as evapotranspiration moves it upwards, drainage downwards, interflow lateral and in sloping topography, also upward in the soil profile. Water is stored in the soil horizons and in rock fractures, pockets, and bedding planes below the soil. These processes determine if and where the water exits the soil. Water can exit the soil where it flows into a wetland or river, or by evapotranspiration or it enters the groundwater. During storage and flow the water interacts with the parent material to leave a associated distribution pattern of morphological, mineralogical, chemical, and physical properties. This relationship is so strong that the morphological properties have more information than what meets the eye. Morphological soil properties are well expressed and easy to access and identify.

The basis of this relationship between hydrology and soil formation firstly lies in the dominant role of water in weathering and soil formation. Secondly, the soil that forms due to the impact of water is controlling the response of soils and hillslopes to rainwater. Thirdly, soils receiving return flow from the fractured rock layer, express a distribution pattern of morphological signatures related to the nature of the water flowpaths. It is against this background that conceptual hydrological response models are developed.

Due to the process of soil formation dominating soil properties, combinations of soil properties, and the spatial distribution of soil properties, are signatures of the long-term hydrological response of a soil. The dominant, or specific combination, of soil forming processes active in the soil, is a set of conditions controlled by the five factors of soil formation. During soil formation substances are added, removed, shifted, and generated in the soil. A soil is therefore a product of a phase of soil conditions dominated by water related processes. Several of the soil forming processes are buffered creating the opportunity to define soil type specific properties and arrange them in a soil, classification system.

Secondly, soil is a first order control of the behaviour of rainwater. Soils with similar hydrology, but with different soil morphology (due to other factors of soil formation playing a hydrologically insignificant role in soil formation) can be grouped as hydropedological soil types.

Thirdly, soil properties related to soil water regimes and processes of hydrology dissimilar to the climate and rather controlled by hillslope processes are proven indicators of the nature of flowpaths in hillslopes.

The hydrological response of land to rainfall events varies from an immediate reaction of peak flow causing floods (an event driven response) with a short baseflow, to a delayed response with a lower, longer peak and base flow that lasts up to a year or longer. These differences are expressed in a variation in residence time of the water in the hillslope. The time spent in the hillslope is controlled by the properties of the flowpaths as determined by the factors soil, lithology, topography, climate and vegetation. All playing more or less of a role in specific hydrological hillslopes in an endless variation of response curves.

All changes in land-use have an impact on some or all these factors, changing the equilibrium in the interaction, and changing the hydrological response and changing the impact of water and often soil on the ecosystem. To sustain the ecosystem, the established ratio of flow responses needed is affected and hydropedology can play a role to reduce the impact by identifying the hydrological flowpaths and construct a conceptual hydrological response model as basis. Where large areas are of importance, a soil survey of the hydrological soil types is needed to produce a hydropedological map of the area. The next step is to measure hydrological parameters in representative horizons of representative hydrological soil types. Value is then added by predicting the response of the hydropedological hillslopes.

2. Soil science: Pedology

Pedology is a subdiscipline of soil science traditionally focussing on the formation of soils – the factors controlling formation and the processes active in formation, the resultant properties, classification, and the potential of sustained use of the soil for several land uses. Weathering of rock to soil is the first step in soil formation. Horizons (layers) of soil with different morphological, physical, chemical, and biological properties form due the impact of water and biological activity from the top and parent material from the bottom. The properties of the horizons change vertically to form horizons with homogeneous morphology. Horizons of thin (<5cm) to thick (>2m) are common. The sequence of horizons is defined in soil types (forms) in South Africa. Down slope horizons change laterally to form a topographical soil sequence, or catena of soils that repeat itself on hydrological similar hillslopes. With all other factors being similar climate sequences also occur and so does biological sequences. Parent material sequences with a gradient in soil forming properties are possible but extremely scarce.

All these processes of soil formation leave signatures related to the conditions created by a combination of the chemical condition of the draining water, aeration, chemistry (Fe-, Mn-, Ca-, Na-, Mg-, N-, S-, heavy metal-salts, etc.) released by the weathering parent material, drainage rate and direction, biological activity, humus content and type, etc. These properties are established after long periods of a repetitive water condition which may be of a large range of conditions but typical and leave soil properties typical of these conditions. Many of these vary with changing conditions but most of the morphological properties are slowly reversable and used for soil classification. The process of soil formation is complex and must be interpreted taking all possible role players into account.

This complexity is a challenge for the ever-improving soil classification systems of the world that use soil morphology e.g., South African and Australian (with similar factors of soil formation). In other countries of the world have a weak association with the chemistry in some of the soil horizons. Yet it has a strong relationship with the most important physical properties. The relationship with hillslope hydrology, is probably the most reliable.

Soil genesis

The factors of soil formation individually, and in combination, play a role in the behaviour of water, and therefore the impact of individual factors on the hydrology and the signatures engraved in the soil over time, needs to be part of the knowledge of the hydropedologist. The factors of soil formation are climate, parent material, topography, biology and time. Human impact also plays a role if the land-use is changed, and the rainwater is redistributed in the hillslope.

Climate

The impact of the climate on wetness on soil formation in South African is related to precipitation but better defined by taking evapotranspiration into account. Aridity index (AI) is the yearly average precipitation (P) divided by the average yearly evapotranspiration (ET). This implies that rainfall is more effective under conditions of low ET. Evapotranspiration is relatively lower at the coastline due to high humidity and increases rapidly within a few kilometres. The small sub-humid, large semi-arid and arid inland is a result of low rainfall and high ET. High temperatures over the inland and additional dense vegetation in the sub-humid regions also increase ET and decreases the additional water available for drainage and storage (ET-excess). Rainwater reaches greater depths than is generally perceived. In arid zones, water drains through the thin soils and frequent rock outcrops and subsequently drains away in the fractured rock zone out of reach of root uptake. The border line between arid and semi-arid climates co-inside with soils having chemically weathered impermeable underlying layer. It is an indication that ET-excess is long enough for chemical weathering to dominate physical weathering in the subsoil of at least some deep subsoils. In the high rainfall areas, rainfall infiltration is high and large amounts drain from the upper and intermediate vadose zones to groundwater due to high increasing in ET-excess water.

The impact of climate in arid zones requires a rare but exceptional combination of storms, rock formations and topographical settings to support lateral flow (interflow) of hillslope hydrology. Examples of a rare combination of conditions in arid climates is where the elevated area is a highly absorbent and an effective recharge area, for example sand dunes or fractured rock in hillslope on a sloping impermeable rock or clay layer. In an exceptional high rainfall year, ET excess is exceeded long enough to create soil forming processes responsible for soil horizons and properties typical of semi-arid climate. Examples are plinthic soils around the Witsand oasis (Northern Cape province) and plinthic soils stretching north. Parallel to it is a mountain range about 80 km away. A valley bottom of impermeable rock, covered by sand which serves as water supplies of varying rates and volumes in the deep subsoil to the lowest topographical position. The plinthic soils and oasis are evidence of a combination of conditions creating soil/rock interflow and even wetland conditions.

A similar condition occurs at the Westcoast with the exception that the elevated area is sand dunes. The result is the same as the plinthic properties occur at the mouth of the catchment in the deep subsoil of sand on rock.

The impact of exceptional combinations to the climate rule also occurs in wetter climates. A good example is a well gleyed horizon in rock fractures in the granite geology of Johannesburg and gleyed clay filled rock fractures at Gods Window.

In most cases these conditions are interpreted as relict, or simply ignored as the contribution of these horizons to hydropedology was not considered.

Parent material

The lithology of the earth can be divided in sedimentary, igneous and metamorphic rocks. They differ firstly in origin, grain size, mineral and element content. Due to its origin, igneous rocks are formed in the earth and found in big, massive bodies. Some of the magma followed cracks to the earth’s surface of which some flowed out on the earth’s surface, typically in periodic deposits. After the eruptions the magma cooled down in the fissures of vertical and lateral extent. Sedimentary rocks formed in layers of loose material that has been deposited by wind or water. The mineralogy of and water deposits vary from chemical deposits, for example, limestone to physical deposits of sand as quarts grains.

The impact of parent material on hillslope hydrology is largely related to the classification of igneous rocks. Igneous rocks are divided in the rocks that formed deep in the earth, have large crystals and weathers by forming small cracks and weathers quite evenly with widespread fissures in lateral positions. Vertical fissures probably exist but are hard to identify. Physical and chemical weathering combine to form weathered pockets in an irregular distribution pattern.

Parent material with similar total chemical content distributed in different minerals, with a similar position in the landscape reacts very similar in high rainfall areas as weathering of the lithology and formation of other silicate clay minerals is of the most important processes. The chemistry of the lithology influences the weathering pattern and the formation of soils. Diverse types of lithology commonly have different types of fracture systems controlling water flow in the lower vadose zone. Layered lithology e.g., the Karoo sedimentary system, has bedding planes between layers. The bedding planes are porous zones and play an important role in interflow. Little water flows through rocks and flow depends on the fractures and bedding planes.

Topography

The soil catena can be a dominant factor in the hydrology to produce a variation in lateral (planform) and down slope (profile) conditions. This is evident in rocky hills with mountainous hillslope vegetation reacting to areas of deep storage of water in the fractured rock where ET is extremely low and water relative availability to deep rooted plants high. Soils are thin, if present, on the crests and concave/concave slopes and ET excess water funnel into rock fractures to be stored in the deep fractures supplying shrubs and trees with water between rain events and rainy seasons. Where rock layers of impermeable hydraulic conductivity occur, it is visible in a stepwise pattern of vegetation.

Biology

Biology (plants) influence soil formation and hillslope hydrology as the extractor of water from the soil influencing ET-excess. Several cases of exploitation of hillslopes with agroforestry or alien trees have been reported to dry up the flow in the rivulet. The biological impact of the surface and subsoil meso and microbiological activity also has a large impact on soil formation and hillslope hydrology, by increasing the role of organic carbon on the hydrological response of soils and hillslopes.

The interpretation of the role of biological activity on hydrological response must be considered where the soil surface was disturbed. The infiltration rate is reduced and as the biopores, largest contributors to high infiltration rates are destroyed. At the same time the structural interpores, some of which organic matter and shrink/swell clay relationships combines with organic matter to produce structure. All disturbances of the topsoil, mainly the surface, negatively influences infiltration and decreases peak flow. The accumulation of humus in soil is concentrated at the soil surface where it creates a microstructure that increases soil microstructure and water infiltration rates. Mole holes and root channels also take water down and lateral at a high flowrate.

Time

South Africa typically has old land surfaces that have matured and old soils. The soils are therefore closely related to the factors of soil formation and well developed catenas. The major contribution of well developed soils and catenas are the expression of morphology are expressed in the relationship with hillslope hydrological response. There are cases where the morphological properties are out of sync with the current climate, although this is very unique and uncommon.

Process hydrology

Soil is therefore a first order control of the processes active in hillslope hydrology, that is from crest to valley bottom. All the subdisciplines of soil science are involved but the largest component is the interaction of all processes supporting pedology and the water driven process stands out.

Spatial survey

Soil survey is expensive and with the development of hydropedology in South Africa it was clear from the beginning that the results need to be extrapolated to larger areas. Intensive surveys were done on large areas and the relationships between topographical aspects and soil types used to extrapolate soil type data. The success resulted in more research exploiting the subject. The soil map became a soil hydrological response map and the catena a hillslope response map. Hillslope response units makes up catchments.

Modelling

The accurate representation of the hydrological response of soils (hydrological soil types), hillslopes (hydrological hillslope types) and catchments (hydrologically unclassified) in hydrological models is very important. The volume and rate of flow needs to be quantified for the individual hydrological units. The quantified soil parameters are applied to suitable models to predict the response under natural circumstances using historic climate data, or a simulation can be run to better understand the impacts of the proposed land use change.

The threat of relict soil morphology

This emphasizes the importance of using soil morphology to classify soils, combining soil types into hydrological classes, develop CHRM’s and parameterise the dominant soil features and model the hydrological response of the soils and hillslopes. The quantification of hydrological parameters for parameterisation of models intercepts this threat and emphasizes the essence of using soil parameters of dominant hydropedological horizons of any stage of soil development namely young, mature or relict to model the hydrological response of dominant hydrological soil types and hillslopes including extrapolation to catchments of larger size.

The importance of application of hydropedology to classify soils and grouping into hydropedological classes that can be surveyed and mapped effectively using digital soil mapping, parameterisation and modelling is a mouthful of specialities serving the preservation of the ecosystem (both peak flow and base flow) and sustain storm water control mechanisms in urban developments.

Some perceptions

Wetlands are sources of water: Most wetlands are the water flowpath linking hillslope water (usually from the deep subsoil and fractured rock zone) where water flows slow enough to support a reduction condition in which wetland plants grow optimally. Water enterring the wetland may be in the reduced state already. Some wetlands have exits and deliver water to a river.

Hills and mountains differ in their capacity as water stores. They may however store enough water to keep wetlands wet all year long.

The catena effect: A set sequence of soils occurring down slope is due to lateral flowpaths in the soil and quite often under the soil in the fractured rock zone and return to the soil from the bottom upwards.

Hydropedology is a science build on proven relationships between water and soil morphology. The interpretation is however lacking.

Pieter le Roux