Some models (e.g., MAGNET, ENV-Linkages) focus solely on agricultural land and allow the total supply of land to vary depending on returns to farming. Competition for land can take place at several levels - first among broad land cover types such as crops, pasture and forestry, followed by the competition for cropland among different crop sectors. Demand for land is typically derived from a production function, in which land enters as an independent input with a certain degree of substitutability with other input factors.