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Watershed management programmes, in terms of conservation planning over a large watershed, must identify the subsets of hydrologic units that are contributing most sediment load in the watershed. It is these priority or critical hydrologic units or sub-watersheds that need preferential soil and water conservation measures to reduce maximum sediment load in the watershed, particularly in arid/semi-arid fragile environments.

The semi-quantitative method of the Sediment Yield Index (SYI) model, developed by the All India Soil and Land Use Survey of the Government of India for priority delineation in River Valley Projects, is used in India and studies have revealed that there is an overall reduction of sediment outflows of up to 60%.

The paper describes a new concept of introducing Integrated Resources Units (IRUs) in the model. An IRU is the smallest, but viable, planning unit which leads to strategies ideally suited for scientific development of a watershed. The IRUs encompass all the relevant watershed resources, generated from map-, ground- and space-based systems, and is the strategic unit for assigning the erosivity and transportability values of the detached material in the SYI model for gradation of priority classes of sub-watersheds. The IRU approach is also helpful in monitoring the dynamic aspects of the watershed and the consequent redefinement of management strategies. The paper also envisages some future studies for watershed management policies. (C) 1995 Academic Press Limited