Assessing the impact of farm dams on streamflows, Part I: Development of simulation tools

Farm dams play an important role in Australian agriculture. In most cases the impact of an individual farm dam is relatively small, though the cumulative impact of farm dams on streamflows can be significant. A simulation model, Tool for Estimating Dam Impacts (TEDI) has been developed with the specific intention of modelling the hydrologic effect of farm dams on downstream flows. The TEDI model has a conceptually simple structure that was developed to be commensurate with the coarse level of information that is generally available at the catchment scale. Given the simplicity of this model, it is desirable to investigate the salient assumptions to determine whether there are compelling scientific reasons to increase the level of modelling complexity. This paper describes the development and application of a detailed simulation model (called CHEAT) that was used to assess the efficacy of the simplifying assumptions used in TEDI. The CHEAT model was configured to exactly emulate TEDI, and enhancements were progressively introduced in order to simulate greater levels of hydrologic complexity. The impacts of different simplifying assumptions are assessed for three catchments, one in Victoria with low streamflow yields and a moderate density of farm dam development, and the other two are located in Victoria and South Australia, with high yields and high density of farm dam development. All three catchments are located in regions that typically support dryland farming. On the basis of the three catchments considered it is tentatively concluded that the combined effect of the simplifying assumptions in the TEDI model have a small influence on the results, and that it can be used a practical tool for the estimation of farm dam impacts.