A method is described for calculating the water balance for a site from standard measurements of rainfall and potential evapotranspiration. This gives the potential soil moisture deficit (PSMD) for monthly or daily periods, depending on the meteorological data available; the deficits can then be accumulated during the course of a growing season to give the minimum amount of water available to plants for any time. Estimation of the water available (A_v) for plant growth in different soils is also described. Subtraction of PSMD values for a site from the available water (A_v) at different points within it enable the degree of moisture stress suffered by a growing crop to be accurately assessed. A procedure is outlined for adjusting PSMD under different crops given the same climatic conditions. The parameters of PSMD and A_v are of interest to archaeologists in the study and interpretation of crop marks, particularly those caused by buried archaeological features. At a former Iron Age farmstead in Staffordshire England (UK), crop marks were observed in 1974 and 1975 under spring barley, the nature and occurence of these marks being closely correlated with assessments of differential moisture stress over the site based on A_v and PSMD. The described techniques can therefore aid archaeologists in the search for hitherto unknown sites of antiquity and the study of existing ones. Once rainfall and potential evapotranspiration data have been obtained for a particular site and available water has been estimated, the calculations are easy and their execution promotes a better understanding of crop marks.