A user agency's view of hydrologic, soil erosion and water quality modelling

The Natural Resources Conservation Service (NRCS) of the US Department of A griculture provides assistance for land management planning and the use of conservation measures on private farmland in the United States. The NRCS mu st concern itself with a broad range of issues with regard to models and th eir application to support the assessments and decision making associated w ith these activities. These issues include the basic science for the descri ption of physical processes, user issues in the practical application of th e model, and software maintenance. In recent years, a significant amount of effort has gone into implementing existing agricultural hydrology/erosion/ water quality models. There are, however, some important areas of model dev elopment that need to be addressed, including: reconciling the strengths an d weaknesses of existing models; accounting for spatial variability of prec ipitation over the catchment; rectifying weaknesses in the stochastic clima te generators currently included in some erosion models; improving the repr esentation of runoff generating processes and water flow paths; and improvi ng our understanding of ephemeral gully (thalweg) erosion and including alg orithms to describe it. New model development also needs to follow modern s tandards of software engineering to ensure code reusability and maintainabi lity. Although the NRCS is primarily a model user agency, it must be involv ed in all aspects of model development as well as model application to ensu re satisfactory results. (C) 1999 Elsevier Science B.V. All rights reserved .