A. G. Williams

Preliminary analysis of water and solute movement beneath a coniferous hillslope in Mid-Wales, U.K.

Abstract Streams draining coniferous forests are often loaded with solutes such as hydrogen ion, sulphate, nitrate and aluminium. As a result, fish populations can be reduced and water quality may fall below recommended potable standards. The transport of ions into water-courses is governed by the movement of water. Within most temperate and tropical areas the stream discharge and chemistry, during periods of rapid runoff, is dominated by the exfiltration of water and solutes from stream-side soils. The movement of water to stream-side or ‘riparian’ areas remains, however, an enigma. This paper attempts to explain how the riparian area might be rapidly recharged during storm events. Two analytical techniques, the free-surface method and tangent-continuity method, are applied to hydrological properties monitored on a steep coniferous hillslope, during a selected storm event. Comparison of the ionic concentrations of waters within each component of the hydrological system, is used to verify the hydrological analysis. Perched water-tables developed within the basal zones of the O/Ah and Eag soil horizons of the steep podzolic hillslope, during all major storm events. Most of the rapid response within the riparian zone could be explained by lateral flow in these near-surface soil horizons, particularly in the saturated basal zones. This pathway is corroborated by the similarity of riparian zone and near-surface (or topsoil) chemistries. Relatively low concentrations of monomeric aluminium and relatively high concentrations of chloride, sodium and hydrogen ion were observed within these zones, compared with the subsoil (Bsl and B/C) horizons.

AGGREGATE STABILITY OF SOILS IN CENTRAL SPAIN AND THE ROLE OF LAND MANAGEMENT

The paper examines the relationships between soil aggregate stability, selected soil properties and land use in central Spain. Aggregate stability indices derived from three procedures were found to be significantly (p > 0.01) correlated with each other as well as with clay content, organic carbon and a range of water-soluble salts. Soils with a higher clay content have a lower aggregate stability. It appears that the presence of expandable clays has a major negative influence, although this impact is strongly modified by recent land-use history and contemporary land-management practices. Agricultural land, abandoned in the 1940s, was subsequently invaded by Cistus matorral or planted with Pinus. The most stable aggregates occur under matorral and may represent a lag of more resistant aggregates surviving past land-use-related erosional processes. Comparisons with aggregates under Pinus however suggest that hydrophobic substances from the Cistus may have increased aggregate stability. Aggregates from areas remaining in cultivation are the least resistant although the stability envelope overlaps with areas under Pinus. These differences may be related to cultivation practices whereby clay-rich subsurface horizons characterized by higher proportions of expandable clays are drawn to the surface, and to enhancement of aggregate stability under forest by fungal hyphae.

A FIELD STUDY OF THE INFLUENCE OF LAND MANAGEMENT AND SOIL PROPERTIES ON RUNOFF AND SOIL LOSS IN CENTRAL SPAIN

Since the 1950s afforestation of degraded land has been the principal means of combatting erosion in a seasonally arid area of central Spain. In the 1970s tree planting of steep hillsides and gully sides was preceded by bench terracing. Experimental sites have been established to monitor runoff and soil losses under mature Pinus forest, 12-year-old Pinus forest, and Cistus matorral. The experiment is being conducted at three scales: large gully or small watershed (c. 3.5 ha), runoff erosion plot (10–21.5 m2), and rainfall simulation plot (1 m2). Monitoring began in October 1992. Discharge was recorded continuously, while sediment loss and soil moisture content were measured on a storm basis. The paper presents summary data on runoff and soil erosion for the three scales and comments on relationships between land management, site characteristics, and these losses. We stress the crucial role of vegetation and its interrelationship with soil properties such as structure and aggregate stability. Matorral was effective in combatting water and soil loss, but we question the practice of afforesting seasonally arid, steeply sided areas that have highly erodible soils.

Quantification of pore size distribution and the movement of solutes through isolated soil blocks

Techniques to provide a quantitative description of soil structure in a reasonable time scale using equipment that is widely available are examined. Three complementary techniques for quantifying soil structure within the pore size range from 136 to 4000 μm (equivalent diameter) are evaluated for two isolated, unsaturated soil blocks (0.85 m3). Profile tracing and binary transect methods were used to examine the pore sizes >1000 μm. Pore sizes <1000 μm were quantified using 2-dimensional image analysis of resinated core samples. Prior to collection of the blocks, the technique of mole ploughing was used to generate a range of pore sizes in the heavy clay soil (Hallsworth series) by shattering. Pore size distribution was related to observed changes in solute concentration following a miscible displacement of a 250 mg l−1 Cl− solution applied at an irrigation rate of 3 mm h−1. Analysis of soil matric potential data showed that solute movement was predominately limited to pores of between 136 and 1000 μm equivalent diameter. Using a combination of Poiseuilles law and pore-size distribution data, it was inferred that the observed rate of flow in the soil could have been achieved through a range of pore-size distributions, between 300 and 1000 μm as a sufficient quantity of pore space within the range of 136 to 1000 μm (diameter) was recorded. It is concluded that the rapid movement of tracer to depth can occur within an unsaturated soil in which active porosity is limited to mesopores and this rapid movement can be described as preferential flow.

Localised impact of Sitka spruce (Picea sitchensis (Bong.) Carr.) on soil permeability.

A typical upland soil catena afforested with Sitka spruce (Picea sitchensis (Bong.) Carr.) was chosen to examine the localised effect of trees on soil permeability. A borehole permeameter was used to measure soil permeability at 0.2 m and approximately 2 m distance from the stem of 20 trees at a fixed measurement depth of 0.25 to 0.45 m. In the case of the ‘near-tree’ measurements, this corresponded to soil beneath the main root plate of each conifer. Two principal elements of the soil catena: the ferric podzol of the mid-slope and histosol soil of the foot-slope were investigated.The preliminary data set shows that within the ferric podzol element, the permeability of the soil beneath individual conifers was a factor of 5.4 less than that of the adjacent soil. In contrast, within the histosol sub-tree permeabilities could not be distinguished from those of soil 2 m away from each tree. The decrease in sub-tree permeability within the podzol may be caused by sensitivity of the Bsl horizon to consolidation by tree weight or by enhanced illuviation resulting from changes in local soil chemistry. The histosol may be less sensitive to such processes. The results of a consolidation test applied to the Rawls and Brakensiek model of soil permeability supported the possible role of consolidation in the reduction of soil permeability beneath conifers in podzolic soil. Additional data on soil bulk density, porosity and texture are required to corroborate either the consolidation or illuviation hypotheses. As the Bsl horizon of ferric podzol soil is typically slowly permeable, a further decline may (i) restrict root development and thus, increase windthrow hazard, and (ii) increase the lateral flow of water within podzolic Eag horizons and thus affect stream acidification. Deep ploughing of a site prior to afforestation may mitigate such impacts.

Computer control for investigating water and chemical transport in a large isolated soil block

Abstract This paper describes the computer controlled data acquisition systems designed to obtain fine spatial and temporal resolution data of tracer in soil water within a large undisturbed soil block. Such information is desirable to help understand and model transport mechanisms in light of increasingly strict environmental pollution limits. The soil block was 5.4 × 3.4 × 1.2 m, instrumented in nine layers with time domain reflectometry (TDR) waveguides multiplexed using a PC; pressure transducer equipped tensiometers multiplexed using a datalogger and suction samplers connected to in situ flow-injection analysis systems controlled by a PC. A rainfall simulator capable of applying tracer to the block without altering the surface flux (which could range from 1 to 30 mm h −1 ) was controlled by a datalogger. Initial conditions when the soil block was subjected to a surface flux of 10 mm h −1 are presented along with an example of a chloride breakthrough curve to illustrate the nature of the data obtained from the soil block system. It is concluded that the integrated system described is a significant advance in working technology for field and plot scale tracer experiments because the spatial and temporal resolutions achieved are possible with a single person maintaining the system.

A field-based spectrophotometric flow-injection system for automatic determination of chloride in soil water

The use of chloride as a tracer for soil water investigations is discussed. Limitations with laboratory based soil core and field based sampling are considered with respect to the poor suitability of the data for rigorous assessment of mechanistic models. Investigation of water movement in soil has been restricted by limitations in spatial and temporal sampling. Fine resolution sampling generates large numbers of samples which cause problems with post sampling laboratory analysis. This paper describes a field-based system for the analysis of chloride in soil water. There are three component parts to the system, (i) a custom sampling sub-system comprising of ceramic cup suction samplers and sample traps, (ii) a sample routing sub-system to channel sample to (iii) a sample analysis sub-system utilizing a flow injection method for sample analysis using a custom built photo-diode detector. The three sub-systems were controlled by a suitably equipped personal computer. A calibration procedure is described with a third order polynomial equation derived to convert millivolt response from the detector into chloride concentration. Field and laboratory data from a tracer experiment are presented and discussed, and it is concluded that the system is well suited to field-based applications. Finally it is noted that the photo-detector is suitable for colourimetric analysis of any tracer with suitable chemical determination.

Ecological effects of afforestation: a case study of Burrator, Dartmoor

Abstract Intensively managed coniferous afforestation schemes cause profound changes to the components of an ecosystem. This paper reviews the ecological effects of afforestation around Burrator Reservoir, Dartmoor (1916–1990). The human management decisions concerning the development of the plantations are considered, together with the ecological consequences. This research shows that the less intensive management regime for the forests at Burrator has resulted in a woodland ecosystem with a more complicated structure than might have been expected from a more intensively managed scheme. An assessment of the potential changes to forest management following water privatization is also made, together with a consideration of the possible ecological impacts.

Ecological and landscape effects of afforestation at a second rotation plantation: a case study of Fernworthy, Dartmoor

Abstract Ecological and aesthetic criticisms of commercial coniferous plantings in the early 20th century, together with growing environmental awareness of issues such as multiple‐use, sustainability and biodiversity, have prompted new policies and approaches to forest management. This paper examines the management and development of Fernworthy Plantation, Devon to illustrate the extent to which the management strategies have changed through time in response to experience, public pressure and policy review. The results of an ecological survey of the vegetation of adjacent moorland and plantation habitats are discussed to establish ecological change in relation to the forest management cycle at the site.

Near Real-Time Determination of Nitrate Leaching as an Integrated Component of a Large Soil Block Experiment for Fine Resolution Investigation of Preferential Flow

A large soil block was isolated in situ at North Wyke, Devon, UK. It was designed as the basis for fine spatial and temporal resolution investigations of water and chemical movement by preferential flow through a well structured soil. Three simultaneous tracer experiments were conducted within the block using a chloride/nitrate/deuterium cocktail. This paper consideres the nitrate experiment. The block was densely instrumented with time domain reflectometry waveguides (to measure volumetric water content), tensiometers (to measure matric potential) and suction samplers (to extract soil water and tracer). All instrumentation was automated using computers, and chloride concentration was determined using in situ flow-injection systems. To determine nitrate concentrations, samples were obtained from the samplers (54) once an hour. In the day-time some samples were analysed using test-strip technology and some stored for analysis the next day. In a 24 hour period a total of 1296 samples were available for analysis. The temporal resolution obtained, while not as fine as that for chloride, was greatly in excess of anything previously attempted in the field. The fine resolution was possible, partially because a single operator could perform analysis as sample became available without total reliance on a busy laboratory facility.