NONLINEAR RESPONSES OF SOIL-EROSION TO CLIMATE-CHANGE - A MODELING STUDY ON THE UK SOUTH-DOWNS

A modelling approach is used to estimate some effects of changed clima te upon rates of soil erosion on agricultural land on the UK South Dow ns. Previous studies have concentrated only on estimating shifts in lo ng-term mean erosion rate: these were found to be approximately linear . However such simple shifts mask changes in the underlying distributi ons of annual erosion. A first series of simulations indicated that, u nder a wetter climate, erosion rates in wet years will generally incre ase more than rates in dry years. Under a ''best guess'' rainfall scen ario with a 10% increase in winter rainfall, annual erosion increased by up to 150%. Erosion rates for individual years were shown to change in more complex nonlinear ways however, with decreases as well as inc reases occurring. These could be explained by the interaction of timin g of rainfall with changes in the rate of crop growth. Most earlier wo rk also assumed an equilibrium climate for the simulations, with clima tic parameters such as mean monthly rainfall having stabilised at some new value, usually for a 2 x CO2 atmosphere. This however leads to an ''initial conditions'' problem: how will soil characteristics have ch anged by the time of CO2 doubling? A decrease in erodibility of about 20% by the time of CO2 doubling was indicated, resulting from changed soil profile properties. However, a second series of runs employed ''t ransient'' weather sequences (i.e. with a trend imposed). For these, p resent-day soil profiles could legitimately be used.