Qualitative and quantitative physical land evaluation : an operational approach

Abstract

Physical land evaluation methods are crucial for evaluating potentials and constraints of land for intended land use. Physical resources, such as soil, climate, hydrology, and topography are evaluated. Different technical procedures are used for physical land evaluation ranging from simple methods based on expert knowledge to more complex methods based on simulation models. The expert knowledge is derived from farmers' experiences. The methods using expert knowledge provide broad descriptive answers regarding land qualities and suitability and, therefore, they are described as qualitative evaluation methods. Qualitative physical land evaluation methods are developed and applied to screen possibilities of Dutch land for injection of slurry from animal manure, and to assess the growth potential of sugar-beet in the European Communities. Quick answers are obtained if the knowledge is captured into expert models in a computer system and when they are linked to a geographical information system.The more complex methods are based on computer models simulating transient soilwater flow and crop growth. These methods are described as quantitative because they produce specific expressions in quantitative terms, such as occurrence probabilities of soilwater deficits, average crop yields, and temporal variabilities of crop yields. Quantitative methods are elaborated and their abilities are illustrated with the assessment of growth potential of potatoes in the Netherlands, and of sugar-beet and wheat in the European Communities. The impact of some land use options on crop production is explored, such as set-aside of land. Quantitative evaluation yields more specific results than qualitative evaluation, but it is more time-consuming and requires more specific input data. Because of these higher demands, a mixed qualitative/quantitative evaluation approach is introduced. In this approach expert models are used to screen land for severe restrictions for a defined use, and, subsequently, simulation models are applied to the remaining potentially suited land. An analysis of required efforts for various evaluation approaches is presented.Finally, a quantitative physical land evaluation is elaborated to assess the effects of soil management on soil structure degradation and regeneration on farm scale. The major role of the land characteristic "soil macrostructure" is described. Several soil-structure types resulting from different soil management systems are recognized in sandy loam and clay loam soils, and characterized quantitatively in soil-morphological and soil-physical terms. The data are used as input for a soil- water flow model to calculate water-associated land qualities for land units with different soil- structure types. Differences in land qualities are interpreted as effects of soil-structure change. The modifications of the soil-water flow model to account for bypass flow and internal catchment (subsurface infiltration) are described