Hydrologic Response Units

Hydrologic Response Units (HRUs) were developed for the Hare River watershed by combining land use, soil and terrain-slope information. The HRU framework enables SWAT to represent spatial variations in runoff generation, infiltration, evapotranspiration, soil-water storage, subsurface flow and erosion within each modelled subbasin.

Following completion of watershed delineation, the land-use, soil and slope datasets were reclassified into SWAT-compatible classes and overlaid. The final HRU definition retained all valid combinations of land use, soil and slope by applying zero-percent elimination thresholds.

The completed HRU framework has now been used successfully in continuous SWAT simulations for the approximately 161.95 km² Hare River watershed.

Land-Use Classification

The land-use raster was clipped to the Hare River watershed and reclassified into SWAT land-use categories. Six principal land-use classes were retained within the final watershed.

Trees represent the largest mapped land-use component of the watershed, followed by cropland and grassland. Retaining these classes separately allows SWAT to apply different vegetation, interception, evapotranspiration, infiltration, runoff and erosion characteristics.

DEM-Derived Slope Classes

Terrain slope was calculated directly from the same digital elevation model used for watershed delineation. A five-class slope system was adopted so that differences between relatively gentle terrain and the steep upper watershed could be represented explicitly within the HRUs.

Slope is particularly important in the Hare watershed because it influences surface-runoff velocity, erosion potential, infiltration opportunity and the rate at which water and sediment are transferred from the upper watershed toward the drainage network.

HRU Definition Thresholds

The Multiple HRUs option was used for the final HRU definition. Percentage-based thresholds were selected and all three elimination thresholds were set to zero.

The zero-percent thresholds were intentionally adopted so that small but potentially important land-use or terrain combinations would not be eliminated automatically.

This decision has proved useful during interpretation of the completed SWAT simulations because a very small bare-ground area emerged as one of the most important simulated erosion sources.

Highest Simulated Erosion HRU

The SWAT Error Checker identified a localized HRU with an exceptionally high simulated erosion rate.

The identification of this hotspot provides a useful target for future spatial review. The corresponding location can be examined against satellite imagery, terrain slope and field conditions to determine whether the simulated erosion is physically representative.

Connection to Sedimentation at the Hare Weir

The HRU analysis provides the source-area component of the sediment assessment, while SWAT channel routing describes how that material may be transported and deposited within the river system.

Simulation 2 produced an average upland sediment yield of approximately 58.4 Mg/ha and indicated strong net sediment deposition within the stream network.

Present-day field photographs and video of the Hare diversion weir show substantial accumulations of gravel, sand and finer sediment both upstream and downstream of the structure.

The field observations do not constitute formal numerical calibration of the SWAT sediment model. Nevertheless, the observed condition of the river provides an important physical consistency check on the modelled finding that sediment production and deposition are significant processes within the Hare watershed.