The impact of land-use change on water quality at the catchment scale: theuse of export coefficient and structural models

The consequences of a decline in soil nitrogen following ploughing of perma nent pasture and land-use change are explored in terms of nitrate export at the catchment scale. The release of reserves of organic nitrogen built up in grassland soils is modelled as a first-order kinetic decay. The build up of reserves of nitrogen upon reversion to pasture is modelled both as a fi rst-order process and under the assumption that new grassland can absorb al l the nitrogen applied to it. Results show that the release and sequestrati on of nitrogen in these reservoirs shows supply-limited hysteresis, and con sequently the ploughing-up of permanent pasture has the dominant effect. Al lowing for the present land-use and the effect of rainfall, the model is co mpared to streamwater nitrate concentrations measured in the Slapton Wood c atchment, south west England. Significant overestimates are observed that s uggest that nitrogen released from ploughing up of grassland is either in a n organic form or that significant denitrification capacity is available. O ptimising the model against the data from Slapton Wood catchment confirms t here is an elastic capacity for denitirification within the catchment. At t he catchment scale the grassland reservoirs acts as a constant source of ni trogen whilst the effect of the reversion of land to permanent pasture, at the catchment scale, attains rapid equilibrium and does not continue to rem ove significant levels of nitrogen after the first year. (C) 1999 Elsevier Science B.V. All rights reserved.