This paper describes the theoretical background and recent developments to the present version of the Hydrological Land Use Change (HYLUC) model which has been used to investigate the impacts of land use change, particularly changes relating to forest cover, on water resources in many countries of the world. The model makes use of the limits concept and the Penman equation to ensure parsimony in terms both of data requirements (daily rainfall, daily potential transpiration, and land cover data) and in terms of the parameters required to take account of the different evaporative responses of different vegetation types. The model is applicable both for research and land and water resource management purposes (where input data is generally very limited).