Clive Agnew

Sulphur leaching from headwater catchments in an eroded peatland, South Pennines, U.K

A detailed investigation into sulphur leaching in peatland headwater catchments in the South Pennines, UK shows that, despite significant reductions in sulphur emissions, sulphur remains a key acidifier. This sulphur can be considered as legacy atmospheric pollution, stored within the peat by processes of dissimilatory sulphate reduction and now being leached into the regions surface waters. Persistently lower water tables at gully edge locations define a thick erosional acrotelm that is vulnerable to aeration, oxidation and flushing throughout the year, and not solely confined to periods of drought. Stream discharge behaves as a two-end member system, whereby pre-event water, rich in DOC and sulphate, is diluted by event water as a result of event water flowing through fast flow pathways such as macropores and overland flow. A rapid increase in water table elevation during the storm and a decrease in elevation after the storm indicate that event water has infiltrated the peat and has then been released into the stream. Streamwaters in peat dominated upland catchments with high densities of gullying have high concentrations of sulphate and low concentrations of DOC, whereas the reverse is true for those catchments with low densities of gullying. This is consistent with the concept that high concentrations of sulphate can suppress the solubility of DOC. A significant store of sulphate exists within South Pennine peats, and continued gully erosion will enhance sulphur leaching meaning that the timescale involved for any depletion is uncertain. It is therefore important that models predicting recovery from acidification in these upland systems include an understanding of how this stored sulphur is being leached, especially with respect to gully erosion, climate change and reduced precipitation.

Ammonium release from a blanket peatland into headwater stream systems

Hydrochemical sampling of South Pennine (UK) headwater streams draining eroded upland peatlands demonstrates these systems are nitrogen saturated, with significant leaching of dissolved inorganic nitrogen (DIN), particularly ammonium, during both stormflow and baseflow conditions. DIN leaching at sub-catchment scale is controlled by geomorphological context; in catchments with low gully densities ammonium leaching dominates whereas highly gullied catchments leach ammonium and nitrate since lower water tables and increased aeration encourages nitrification. Stormflow flux calculations indicate that: approximately equivalent amounts of nitrate are deposited and exported; ammonium export significantly exceeds atmospheric inputs. This suggests two ammonium sources: high atmospheric loadings; and mineralisation of organic nitrogen stored in peat. Downstream trends indicate rapid transformation of leached ammonium into nitrate. It is important that low-order headwater streams are adequately considered when assessing impacts of atmospheric loads on the hydrochemistry of stream networks, especially with respect to erosion, climate change and reduced precipitation.

Water resources and development.

1. Water Management Best Practice in the Twenty-First Century 2. Economic Growth, Environmental Limits and Increasing Water Demand 3. Climate Change and Fresh Water Resources 4. Water Resources in Colonial and Post-Independence Agricultural Development 5. Water Supply 6. Water Demand 7. Catchments and Conflicts 8. Water Management. Conclusions

Estimating evaporation and surface resistance from a wet grassland

Abstract This paper describes the investigation into evaporation loss at a wet grassland in south-east England. The investigation has two phases. The first attempts to derive values of surface resistance, rs, over a period of soil moisture decline which, when used in the Penman Monteith equation, would enable the computation of actual evaporation loss. The second investigates why the computed values of actual evaporation exceed potential ones. Values of potential evaporation, Eo, were computed from Penman Monteith by setting rs = zero (reference conditions of a wet canopy), while values of actual evaporation, E, were derived using a soil moisture balance approach. With these values of potential and actual evaporation, the Penman Monteith formula was rearranged to find rs, following the approach by Russell (1980). The resistance values range from 8 sm−1 to 155 sm−1 which are in the same order of magnitude observed by other researchers and follow the expected trend of increase with soil moisture stress. Of the soil moisture range 28 to 19%, values of resistance are below 60 sm−1 between 28 to 22%, thereafter increasing to a value of 155 sm−1 at a soil moisture content of 19%. There were three time steps from the total eight, in which actual evaporation loss computed from the soil moisture balance approach was higher than the potential rate which was unexpected. It is discussed that these results could either be due to problems of time lag or spatial heterogeneity which could introduce errors into the soil moisture balance approach, or that the potential rate of evaporative loss from the wet grassland is being underestimated by the Penman Monteith equation.

Detecting a moorland wildfire scar in the Peak District, UK, using synthetic aperture radar from ERS-2 and Envisat ASAR

Wildfires occur annually in UK moorland environments, especially in drought years. They can be severely damaging to the ecosystem when they burn deep into the peat, killing ground-nesting birds and releasing CO2 into the atmosphere. Synthetic aperture radar (SAR) was evaluated for detecting the 18 April 2003 Bleaklow wildfire scar (7.4 km2). SAR’s ability to penetrate cloud is advantageous in this inherently overcast area. SAR can provide fire scar boundary information which is otherwise labour intensive to collect in the field using a global positioning system (GPS). This article evaluates the potential of SAR intensity and InSAR coherence to detect a large peat moorland wildfire scar in the Peak District of northern England. A time-series of pre-fire and post-fire ERS-2 and advanced synthetic aperture radar (ASAR) Single Look Complex (SLC) data were pre-processed using SARScape 4.2 to produce georeferenced greyscale images. SAR intensity and InSAR coherence values were analysed against Coordinate Information on the Environment (CORINE) land‐cover classes and precipitation data. SAR intensity detected burnt peat well after a precipitation event and for previous fire events within the CORINE peat bog class. For the 18 April 2003 fire event, intensity increased to 0.84 dB post-fire inside the fire scar for the peat bog class. InSAR coherence peaked post-fire for moors and heathland and natural grassland classes inside the fire scar, but peat bog exposed from previous fires was less responsive. Overall, SAR was found to be effective for detecting the Bleaklow moorland wildfire scar and monitoring wildfire scar persistence in a degraded peat landscape up to 71 days later. Heavy precipitation amplified the SAR fire scar signal, with precipitation after wildfires being typical in UK moorlands. Further work is required to disentangle the effects of fire size, topography, and less generalized land‐cover classes on SAR intensity and InSAR coherence for detecting fire scars in degraded peat moorlands.

Geostatistical Analysis and Numerical Simulation of West African Sahel Rainfall

The pattern of rainfall in the Sahelian zone of West Africa has been the subject of much analysis since the major droughts of the 1970s. It is widely accepted that annual rainfall has declined since that time and that it is a regional trend. The rain gauge network has changed during this recent period of desiccation. Since the rainfall of the Sahelian region is now recognised as not being homogeneous, there is some doubt about the reliability of regional rainfall aggregated statistics. This analysis uses geostatistics to investigate the impact of changes to the rain gauge network since 1931 and numerical simulations in an attempt to validate the results. This approach can include most of the rainfall records, unlike conventional methods. It seems that the configuration of the rainfall stations in the E-W direction has accounted for the spatial variation of rainfall better since the 1970s because of the eastward extension of rainfall stations. The rainfall stations in the N-S direction do not sample adequately the generally large spatial variation for the period between ca. 1945 to 1975. The only period when the distribution of rainfall stations in both directions was adequate to sample the spatial variation in rainfall was since 1970. Thus, the results suggest that the persistent downward trend in the aggregated annual rainfall since the 1970s is a return to a more accurate estimate of the rainfall in this region; prior to the 1970s the aggregated annual rainfall was overestimated. The pattern of regional annual aggregated rainfall statistics is an artifact of the annual location of stations and the spatial variation in rainfall. Regional maps of (total summer) rainfall are based on the geostatistical analysis which takes account of the spatial variation in the rainfall. These maps are believed to be more reliable than those based on conventional interpolation approaches and form the basis for a revised median annual rainfall estimate for the west African Sahel between 1931 and 1990.