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

Abstract

The consequences of a decline in soil nitrogen following ploughing of permanent 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 first-order process and under the assumption that new grassland can absorb all the nitrogen applied to it. Results show that the release and sequestration of nitrogen in these reservoirs shows supply-limited hysteresis, and consequently the ploughing-up of permanent pasture has the dominant effect. Allowing for the present land-use and the effect of rainfall, the model is compared to streamwater nitrate concentrations measured in the Slapton Wood catchment, south west England. Significant overestimates are observed that suggest that nitrogen released from ploughing up of grassland is either in an organic form or that significant denitrification capacity is available. Optimising the model against the data from Slapton Wood catchment confirms there is an elastic capacity for denitirification within the catchment. At the catchment scale the grassland reservoirs acts as a constant source of nitrogen whilst the effect of the reversion of land to permanent pasture, at the catchment scale, attains rapid equilibrium and does not continue to remove significant levels of nitrogen after the first year.