Transcending scales of space and time in impact studies of climate and climate change on agrohydrological responses

The scale "jump" in hydrology and agriculture from the small scale at which individual processes such as infiltration, soil water redistribution, evapotranspiration, soil loss or crop development/yield have been studied, to the global scale at which climate change impacts and international trade in agriculture manifest themselves, has presented agrohydrologists with conceptual as well as practical problems of scales and scaling. In this context, selected scaling issues are, therefore, identified and highlighted. The paper discusses why scaling problems arise, defines concepts and types of scales, poses what are considered key questions with regard to upscaling and downscaling, as well as to dis-aggregation to homogenous landscape response units (HLRUs) and to re-aggregation Examples from southern Africa are then given of space/time scaling approaches, ranging from country to local-scale levels, followed by an evaluation of types of errors associated with scaling. The paper concludes by identifying what, in the author's perception, some of the challenges are which relate to scaling applications of the "real world" and which hydrologists and agriculturists face in the next few years. (C) 2000 Elsevier Science B.V. All rights reserved.