In the assessment of the impact of underdraining on nineteenth-century agriculture, a measure of the area that would benefit from the improvement and of its distribution is a priority. Few attempts were made by nineteenthcentury agriculturalists to calculate the amount of land in need of draining in England. The dichotomy between heavy- and light-land farming was widely reported in the contemporary literature at both the national and local scale: as William Marshall explained in 1818, ‘the most natural division, and at the same time the best agricultural distinction, is into strong and light lands’. And many commentators reiterated the view of Leonce de Lavergne in 1855 that ‘the draining away of superabundant water, especially upon stiff soils, has always been the chief difficulty in English agriculture’. Although aware of the problem, most nineteenth-century agriculturalists relied on a qualitative judgment of the extent of the improvement's need, exemplified by that of Josiah Parkes in 1845 that ‘a most enormous and untold quantity’ of land required to be drained.

In general, those agricultural writers who added some quantification to such statements gave no indication of either the basis of calculation of the areas or their distribution: nevertheless, the results achieved currency in the contemporary literature. Thus Philip Pusey claimed in 1841 and 1842 that one-third of England, 10 million acres, required underdraining.