Introduction to WATFLOOD
Overview of CHARM
CHARM is the hydrological modeling component in WATFLOOD. CHARM is supported by a number of pre and post processors in the data management system. CHARM was designed for distributed modelling using remotely sensed data, particularly from remotely sensed land cover maps and weather radar. To efficiently interface with such data, WATFLOOD/CHARM uses a Cartesian coordinate system, usually aligned with the local UTM system but equally capable of using lat-long coordinates with variable areas. It can therefore easily accommodate any georeferenced imagery, be connected to a GIS for input and output, and can be setup for any appropriate resolution.
Interception of precipitation by vegetation is calculated by using the equation by Linsley and surface storage is based on the ASCE Manual of Engineering Practice No. 37. The values for surface storage in the model were 1.25 mm was used for impervious urban areas, 2.0 mm was used for pervious urban areas and smooth cultivated land, 3.0 mm for good pasture and low vegetation and 10 mm for forest litter. All of these values are within the ASCE limits except for forest litter.
The Philip formula is used for the infiltration process in runoff calculations. Infiltrated water percolates downward into the upper zone storage (UZS). From there is exfiltrated to nearby water courses (interflow) and/or is allowed to drain downwards to the groundwater reservoir. Interflow and UZS drainage are represented by simple storage discharge relationships. When the infiltration capacity is exceeded by the water supply, and the depression storage has been satisfied, water is discharged to the channel-drainage system.
As with all hydrological processes, the melting of snow is treated separately for each land cover class. Snowcover depletion curves (SDC) are used to summarize the relation between snowcover distribution and an average snowcover property, such as depth of water equivalent, for a given area. More specifically, these curves provide the amount of snow covered area for a given depth of water equivalent for each land cover class. This is an important factor because snow can only melt where it exists, not on the bare areas.
Also included in the model is a snow redistribution algorithm. An upper limit is set on the water equivalent for each land cover class. For forests and glaciers this is very high but for barren areas and wetlands it is quite low. When falling snow increases the snow water content beyond the limit for a class, the snow is redistributed to the nearest forest.
The snow melt process is modeled by an index method, in this case, a degree-hour based heat input or loss. An accounting of the heat content of the snow pack allows re freezing.
Base flow is calculated by a two parameter, non-linear storage-discharge function. The ground water reservoir (or lower zone storage LZS) is common to all GRU's within a computational grid. The initial base flow is determined from the first value of a measured hydrograph at the basin outlet, using the non- linear storage-discharge function in reverse. This initial base flow contributed by each grid is found by prorating it to the total basin area of the gauged watershed in which the grid is located. For ungauged areas, the average initial flow per unit area of nearby streams are used. The ground water reservoir is replenished by drainage from UZS.
A two stage flow routing procedure is used in CHARM. For overland flow, an explicit method is used that is based on continuity and Manning's formula and incorporates the average surface slope for each grid. River flows are similarly based on continuity and Manning's formula but separate roughness parameters are used for channel and floodplain roughness. A geomorphologic relationship is used to relate channel cross-sectional area to the drainage area of the basin. The grid size provides the overland flow distance in each grid and the channel reach lengths and river bed elevations, obtained from topographic maps provide channel slope. The GRU approach, coupled with this hydraulic overland and channel routing method accounts for the transferability of the WATFLOOD system from one watershed to another within a physiographically similar area because all hydrological processes are tied to relevant and measurable watershed data.
The total inflow to the river system is found by adding the surface runoff contributions from the various land cover classes to the base flow. These flows are then added to the channel passing through the grid and the outflows from upstream grids are added as inflow to lower grids, where outflows from other contributing grids are added to the local flow and routed to the next downstream grid. Computed flows can be compared to measured flows wherever measured flows are available.
Three reservoir routing options are available in CHARM. In the first, the user simply enters the reservoir releases which are then routed downstream. The second method is a storage-discharge lake routing method and the third is an external method, where reach numbers are specified and contributions to these reaches are printed to a file in the format used by the NWS Dynamic Wave Operational Model (DWOPER).
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