TOPMODEL - Cornell University



Comparison of TOPMODEL and STOPMODEL:

Conceptualizations and Implementations

Dawn Sedorovich, M.Todd Walter, Pierre Garard-Merchant, Tammo S. Steenhuis,

Abstract

TOPMODEL (Beven and Kirkby 1979) is a popular physically-based watershed model capable of simulating variable source area processes (Anderson et al. 1997). STOPMODEL is a refined conceptualization of TOPMODEL for systems in which the variable source area processes are driven by shallow, transient, interflow (Walter et al. 2002). Although the landscape drainage expressions are very similar for TOPMODEL and STOPMODEL, it is unclear how STOPMODEL’s refined conceptualization will alter model results. Figure 1 illustrates the conceptual differences between TOPMODEL and STOPMODEL. The following is a summary of the implementations of each model.

Model Structures

|TOPMODEL |STOPMODEL | |

|Both models perform a suite of local water balance calculations for each topographic index, λi, representing a discrete area | |

|of a watershed, Ai. These calculations are (1) – (7). | |

| |(1) |

|The first operation is to determine the local saturation deficit, Si. | |

|[pic] | |

|Calculate the root zone water deficit: |Determine the local soil water: |(2) |

|[pic] |[pic] | |

| |Update SWi using the Thronthwaite-Mather methodology. | |

|If RZ < 0, then the drainage into the unsaturated zone, DUZi = |If SWi > FCi, then there is drainage into the deep groundwater,|(3) |

|RZi and RZi is set to zero. |Di, equal to the saturated hydraulic conductivity of the | |

| |restricting layer, Ks. | |

| |Bulk drainage, Dgw = Dgw + DiAi | |

| |The soil water is updated: | |

| |[pic] | |

|Water in the unsaturated zone, UZi is updated: |If SWi > SATi, then the overland flow is: |(4) |

|[pic] |[pic] | |

|If UZi > Si then the overland flow is: |SWi is set to SATi. | |

|[pic] |The net change in soil water, ΔSW = ΔSW + ΔSW | |

| |(5) |

|Bulk overland flow is OF = OF + OFi | |

|If Si > 0, then there will be drainage to the deep saturated |For simplicity, STOPMODEL assumes all water leaving the root |(6) |

|groundwater region: |zone goes directly to the groundwater. We may want to modify | |

|[pic] |this later. | |

|Make sure no more water drains from the unsaturated zone than | | |

|is available: | | |

|If DSZi > UZc, DSZi = UZi and UZi = UZi – DSZi | | |

|The bulk drainage, DSZ = DSZ + DSZi | | |

|The root zone deficit is updated, taking into account |This was done in (1) |(7) |

|evaporation: | | |

|[pic] | | |

| | |

|The following calculations are done on a bulk, or watershed basis at the end of each time step. | |

|The drainage from the landscape, or baseflow, is calculated as:|The drainage from the landscape, or interflow, is calculated |(8) |

|[pic] |as: | |

| |[pic] | |

|The average saturation deficit is updated: |The average soil water deficit is updated: |(9) |

|[pic] |[pic]is: | |

| |The groundwater is updated: |(10) |

| |[pic] | |

| |Base flow is calculated using a linear reservoir assumption: | |

| |[pic] | |

|Streamflow is calculated: |Streamflow is calculated: |(11) |

|[pic] |[pic] | |

References

Anderson, M, N.E. Peters, and D. Walling. (eds). 1997. Special Issue: TOPMODEL. Hydrol. Proc. J. 11(9): 1069-1356.

Beven, K.J. and M.J. Kirkby. 1979. A physically based variable contributing area model of basin hydrology. Hydrol. Sci. Bull. 24(1):43-69

Walter, M.T., T.S. Steenhuis, V.K. Mehta, D. Thongs , M. Zion, E. Schneiderman. 2002. A Refined Conceptualization of TOPMODEL for Shallow-Subsurface Flows. Hydrol. Proc.16(10): 2041-2046.

Figure 1: Schematics of the conceptual design for TOPMODEL (a) and STOPMODEL (b). ST and SS indicate the physical locations where TOPMODEL and STOPMODEL, respectively, store the water that controls variable source areas. Also note the different interpretations of recharge, R.

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