Development of conceptual hydrological FLEX-Topo model for loess watersheds influenced by piping and tunnel erosion in Golestan Province of Iran

Document Type : Complete scientific research article


1 Corresponding Author, Professor, Dept. of Watershed Management, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran.

2 Associate Prof., Dept. of Watershed and Desert Management, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran.


More than twenty percent of Golestan Province of Iran is covered by wind-blown deposits called as loess. These deposits are silty or loamy materials with yellow to light brown colors. Special erosion phenomena such as pipe and tunnel erosion are common in loess deposits that are important in terms of studying their hydrology and the hydrologic response of the catchments influenced by these phenomena. In this paper, with briefly describing the hydrology and erosion situation of Tamer watershed located in the northeast of Iran with dominant cover of loess deposits, using the Height Above Nearest Drainage map (the topographic index HAND) and the slope map, three hydrologic landscapes were determined. For each of these three hydrologic landscapes (lowland, hillslope and plateau), a conceptual hydrologic model according to the FLEX-Topo model was designed and described. In the FLEX-Topo model, the spatial variation is taken into account by the zonation of the watershed based on topography in a semi-distributed fashion. Considering the intense presence of piping and tunnel erosion in the region and its impact on the hydrologic response of the watershed; in particular, with some changes in structure and formulation of the common structure of FLEX-Topo, a tailor-made conceptual model was suggested and its formulation, required for programming, was described. Likewise the FLEX model, in the FLEX-Topo model four reservoirs including the interception, the unsaturated, the fast response and the slow response reservoirs along with a time lag function play important roles. To consider the effect of piping in the proposed model (FLEX-Topo-P), a splitting parameter for the input rainfall and a shared fast reservoir between hillslope and plateau landscapes were suggested. The proposed model has to be implemented and tested through a methodic and robust research, and its performance has to be evaluated in comparison with flow observations and benchmarked with the FLEX-Top.
Keywords: loess deposits, piping, tunnel erosion, conceptual hydrologic FLEX-Topo model, Topographic index HAND


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