Subsurface water flow dynamics and availability are strongly influenced by topography, particularly in forest landscapes characterized by complex and mountainous terrain. Such topographic effects on hydrology, influencing spatio-temporal characteristics of water availability, can be modeled quasi-distributed (i.e. statistical partitioning of watersheds into hydrologically similar areas, e.g., the TOPMODEL approach of Beven and Kirkby 1979) or explicitly (i.e. simulating lateral flow between entities, e.g., the DHSVM approach of Wigmosta et al. 1994). Band et al. (1993) and Engel et al. (2002) give examples for an integration of the former approach within established ecophysiological frameworks that can be used to study spatio-temporal landscape level water cycling. Integrated ecosystem models using explicit soil water routing are still rare, although the work of Tague and Band (2001) highlights the advantage of this approach in simulating spatially distributed soil moisture patterns.
In the current iLand version landscape scale water dynamics is not explicitly simulated. Different hydrological regimes within a forest landscape, e.g. influenced by topography, are represented solely by soil characteristics (e.g., differences in soil depth and soil physics) and climate input (e.g., precipitation levels, temperature-dependent snow water storage). A future improvement of the representation of the hydrological regime at the landscape level could be the adaption of the TOPMODEL approach into iLand, as implemented by Band et al. (1993), Zierl et al. (2007). The approach of TOPMODEL, partitioning a watershed into hydrollogically homogeneous areas, would structurally correspond well with iLands model structure of resource units and sites.