In large parts of the Netherlands crop growth depends on the water table. If groundwater is withdrawn the water table is lowered and agricultural crop production may be reduced. Farmers in drinking water collection areas are legally compensated for these crop yield reductions. Soil maps are used to estimate crop yield reductions and hence legal compensations. We calculated the benefit of a detailed soil survey from the reduction of errors in legal compensations that can be achieved if a detailed soil map, 1:25,000, is used for estimation instead of the national soil map, 1:50,000. We compared this error reduction with the costs of the detailed soil survey. We selected 40 farms by stratified random sampling in the drinking water collection area ‘Vierlingsbeek’. At each farm soil profile descriptions were made at a total of 137 randomly selected locations. Legal compensations estimated from the 1:50,000 soil map and information from the 1:25,000 soil map were compared with legal compensations calculated from the soil profile descriptions, and errors were calculated for each farm in € ha- 1 year- 1. With an investment in detailed soil survey of €30 ha- 1 the absolute error could be reduced on average by €13.16 ha- 1 year- 1, the present value of which is €258 ha- 1 assuming an interest of 3% and yearly compensations during a period of 30 years. We conclude therefore that for this study area detailed soil survey was worth the costs. Furthermore, we conclude that insight in the spatial dependence structure of classification errors at soil maps of various scales would be very helpful prior information in deciding on the detail of soil survey needed to support decisions at farm level.
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