At present, the Topographic Index Model （TOPMODEL） has been recommended for integration in Land Surface Models （LSMs）. But, the applicable scope of the original TOPMODEL （OTOP） is limited because the OTOP derivation relies on three fundamental but unrealistic assumptions. In this paper, several versions of a generalized TOPMODEL （GTOP）, which relax some unrealistic assumptions involved in OTOP, are presented, and the theoretical derivationsn to obtain these modifications are demonstrated in detail. Specifically, the extension for the OTOP applicability comes down to following three basic cases： （1） Give up the assumption of spatially uniform recharge rate to the groundwater and let the rate be spatially varying, （2） Keep same original exponential distribution profile of hydraulic conductivity used in OTOP but change the saturated hydraulic conductivity and effective soil depth from spatial constants in OTOP to spatially variable quantities; and （3） Extend the original exponential distribution profile of hydraulic conductivity to more general power law distribution profile of hydraulic conductivity together with spatially variable saturated hydraulic conductivity and effective soil depth. Finally, a brief numerical sensitivity study based on one version of GTOP using an exponential distribution profile for soil hydraulic conductivity is conducted. This shows the heterogeneous effects of the effective soil depth, saturated hydraulic conductivity, at ground surface and groundwater recharge rate on hydrological processes and serves as an example application of GTOP to a heterogeneous catchment.
