The PESERA model: summary
PESERA is a process-based model that is designed to estimate long term average erosion rates at 1 km resolution and has, to date, been applied to most of Europe and parts of North and West Africa. . The model is built around a partition of precipitation into components for overland flow (infiltration excess, saturation excess and snowmelt), evapo-transpiration and changes in soil moisture storage. Transpiration is used to drive a generic plant growth model for biomass, constrained as necessary by land use decisions, primarily on a monthly time step. Leaf fall, with corrections for cropping, grazing etc, also drives a simple model for soil organic matter. The runoff threshold for infiltration excess overland flow depends dynamically on vegetation cover, organic matter and soil properties, varying over the year. The distribution of daily rainfall totals has been fitted to a Gamma distribution for each month, and drives overland flow and sediment transport (proportional to the sum of overland flow squared) by summing over this distribution. Total erosion is driven by erodibility, derived from soil properties, squared overland flow discharge and gradient; it is assessed at the slope base to estimate total loss from the land, and delivered to stream channels.
PESERA is attempting to make best use of advances in process understanding, while allowing application across a large region, e.g. at 1-km resolution across most of Europe. Although it is recognised that limitations of resolution in time and space, determined by ready availability of suitable data, must partially compromise the accuracy of any such forecast, an objective comparison tool of this type has proved valualbe, for example in developing the European Soil Protection Policy. The PESERA model, providing explicit dependence on climate and vegetation, both retains essential features of more detailed process models and shows a clear response, in the appropriate direction, to the components that drive other assessments, such as USLE and CORINE, combining these and other driving factors within a consistent process-based rationale.
A number of factors contribute to the risk of erosion and, in PESERA, they are combined, in a physically meaningful way, with the intention of making the best possible estimate of long-term average erosion rates. The current version of the model was developed within the structure of the PESERA project, and partly based on previous funded and un-funded research (Kirkby & Neale, 1987; de Ploey et al., 1991; Kirkby & Cox, 1995; Kirkby et al., 2000). The PESERA model combines the effect of topography, climate, vegetation cover and soil into a single integrated forecast of runoff and soil erosion. It is recognised that data for validating estimates of soil loss from erosion models are sparse, and that current models generally forecast runoff as a necessary intermediate to forecasting sediment transport. Since runoff processes are also better understood than sediment transport, particularly on hillsides, it has seemed sensible to build PESERA on a hydrological core.
Erosion by running water has been identified as the most severe hazard threatening the protection of soil in Europe (European Commission, 2006), and is one of the major forms of desertification. By removing the most fertile topsoil, erosion reduces soil productivity leading, where soils are shallow, to a progressive and ultimately irreversible loss of natural farmland, and in vulnerable areas, is one major process of desertification. Severe erosion is commonly associated with the development of temporary or permanently eroded channels or gullies which can fragment farmland. Much of the soil and runoff removed from the land during a large storm generally accumulates below the eroded areas, with some sediment spilling offsite and in severe cases blocking roadways or channels and inundating buildings. Erosion rate is sensitive to both climate and land use, as well as to detailed conservation practice at farm level, as documented by the US National Resources Conservation Service and similar organisations worldwide. In a period of rapid changes in both climate and land use, resulting from revised agricultural policies in reaction to global warming and international markets, it is valuable to be able to assess the state of soil erosion at a regional scale. This needs an objective methodology operating on standard data sets, which allows the assessment to be repeated as conditions, pressures and drivers, change, or to explore the broad scale implications of prospective global or Europe-wide changes. This provides a sound basis for estimating the overall costs attributable to erosion under present and changed conditions, and objectively suggests areas for more detailed study and possible remedial action.
The PESERA model provides such an objective harmonised estimate of current rates of soil erosion, averaged over a series of years with current climate and land use. European estimates have been made at a resolution of 1 km, and indicate the rate of loss of material from hillslopes. Sediment delivery through the river system is explicitly not taken into account, and much of the eroded material generally remains close to its source, with significant off-site effects generally confined to a local area, and strong de-coupling between slope and channel sediment transport (e.g. Trimble, 1981, Govers, 1987).
