LNWB Ch02 Model Processes

Overview:
Topnet-WM refers to the Water Management version of Topnet developed as a work product for the Utah State University WRIA 1 Watershed Management Project (Tarboton, 2007). This version of the model evolved from the Topnet Model developed in a collaboration between NIWA New Zealand and Utah State University (Bandaragoda et al., 2004; Ibbitt and Woods, 2004) that combines TOPMODEL concepts (Beven and Kirkby, 1979; Beven et al., 1995a) for the simulation of relatively small drainages combined with channel routing. This approach provides a modeling system that can be applied over large watersheds using smaller drainages within the large watershed as model elements.

In Topnet-WM, spatial variability is represented by subdividing the watershed domain into model elements at the scale of drainages. Within drainages, the modeling is essentially lumped but includes parameterization of some subgrid variability, notably (1) the wetness index, used to parameterize the variability of soil moisture, (2) a depletion curve, used to parameterize the variability of snow water equivalent, (3) the fraction of area that is irrigated, and (4) areas with artificial drainage. Surface runoff and baseflow can be designated as model outputs at multiple nodes within a drainage. The model may thus be classified as semi-distributed.

Topnet-WM includes many enhancements beyond the original Beven and Kirkby TOPMODEL, such as: (1) calculation of reference evapotranspiration using the ASCE standardized Penman-Monteith method (Allen et al., 2005; Jensen et al., 1990); (2) calculation of snowmelt using the Utah Energy Balance Snowmelt model (Tarboton et al., 1995a); (3) the partition of model elements into separate components representing irrigated and non-irrigated areas; (4) artificial drainage to represent the effect of ditch and tile drained areas on the runoff response; (5) the partition of the model elements into pervious and impervious areas to allow representation of urbanization; (6) options for the diversion and storage of water under different management options; and (7) components to calculate water use and implement water right rules.

The Lower Nooksack Water Budget will be estimated based on the distributed hydrologic model, Topnet-WM. The Lower Nooksack Water Budget included updating the data inputs and model calibration, which requires a thorough understanding of how the model represents physical hydrologic processes. In order to guide the development of model inputs and analysis of model outputs, the project team has edited and reviewed portions of the WRIA 1 Water Management Project Phase III Task 4.1 report (Tarboton, 2007) to include in the general description of the Topnet-WM model that follows. This chapter provides reference to the details of the model processes used by Topnet-WM to convert data inputs into model outputs.

This resource is a subset of the Lower Nooksack Water Budget (LNWB) Collection Resource.