Compute hydraulic gradient per bay segment from UFA potentiometric surface contours

Usage

util_gw_grad(
  contours,
  segs = tbsubshed,
  shoreline = tbsegdetail,
  north_segs = NULL,
  buf_segs = NULL
)

Arguments

contours: sf object of Upper Floridan Aquifer contour lines produced by util_gw_getcontour.
segs: sf object of sub-watershed polygons. Defaults to tbsubshed.
shoreline: sf object of bay segment polygons used to measure distance from the watershed high point to the bay. Defaults to tbsegdetail.
north_segs: named numeric vector mapping bay segment IDs to northward extension distances (in CRS units, US Survey Feet for EPSG 6443). Segments listed here have a rectangular extension appended to the north side of their sub-watershed polygon before the contour clipping step, allowing the high-point search to reach potentiometric highs that lie north of the standard subwatershed boundary. Use this for segments such as Old Tampa Bay (segment 1) where the high point is north of the subwatershed. Names must be coercible to the integer bay segment IDs in segs. The contours passed to this function must already cover the extended area — pass the same distance (or larger) as north_dist in util_gw_getcontour. Default NULL (no extension).
buf_segs: named numeric vector mapping bay segment IDs to omnidirectional buffer distances (in CRS units, US Survey Feet for EPSG 6443). Segments listed here have their sub-watershed polygon buffered outward by the given distance, and then all bay water (shoreline) is removed from that buffer with st_difference before contour clipping. This allows the high-point search to extend beyond the subwatershed onto surrounding land without accidentally capturing potentiometric contours that pass under the open bay. Segments listed in buf_segs are removed from the default zero-gradient set and computed dynamically. Use this for segments such as Lower Tampa Bay (4), Terra Ceia Bay (6), and Manatee River (7) whose wet-season high points lie outside the subwatershed. The contours passed to this function must already cover the buffered area — pass an equivalent or larger value as north_dist in util_gw_getcontour if the buffer extends north of the watershed. Default NULL (no buffering).

Value

A data frame with columns:

bay_seg: integer, bay segment number
grad: numeric, hydraulic gradient (ft/mile); 0 for segments with no reliably computable Floridan aquifer gradient

Details

Computes the Floridan aquifer hydraulic gradient $I$ (ft/mile) for each bay segment using Darcy's Law as applied in the Tampa Bay loading model (Zarbock et al., 1994):

$$I = \frac{\text{elevation (ft)}}{\text{distance to shoreline (miles)}}$$

where elevation is the maximum UFA potentiometric surface contour value within the (optionally extended) segment watershed, and distance is the straight-line distance from that contour's representative point to the nearest bay shoreline.

The season (dry or wet) is inferred from the MONTH_YEAR field in contours ("May" = dry, "September" = wet). Segments with no reliably computable Floridan aquifer gradient receive a value of 0:

Dry season: segments 4, 5, 6, 7, 55
Wet season: segments 4, 5, 6, 7, 55 — Lower Tampa Bay, Terra Ceia Bay, and Manatee River are included by default because the subwatershed geometry does not reliably capture the correct potentiometric high point. Supply buf_segs for any of these segments to compute them dynamically using a buffered, bay-clipped search area instead.

Search area expansion: Two mechanisms are available and may be combined across different segments:

north_segs: appends a rectangular extension to the north face of the subwatershed bounding box. Best for segments (e.g., OTB) whose high point lies directly north of the subwatershed.
buf_segs: omnidirectional buffer with bay water removed. Best for segments (e.g., LTB, TCB, MR) whose high point lies east or southeast of the subwatershed. Removing the bay polygon prevents the algorithm from matching potentiometric contours that pass under open water.

Hillsborough Bay (segment 2): Segment 2 uses a weighted average of three sub-zone gradients following the original flow net analysis. Sub-zones are constructed by unioning tbdbasin drainage basins:

Polk County drainage (weight 0.4): basins 02301000, 02301500
Pasco County / Hillsborough River drainage (weight 0.3): basins 02300700, 02301300, 02301750, TBYPASS
Alafia River drainage (weight 0.3): basins 02301695, 02303000, 02303330, 02304500, 204-2, 205-2, 206-2

Examples

util_gw_grad(contdry)
#>   bay_seg     grad
#> 1       1 3.985806
#> 2       2 2.989021
#> 3       3 1.463759
#> 4       4 0.000000
#> 5       5 0.000000
#> 6       6 0.000000
#> 7       7 0.000000
#> 8      55 0.000000
util_gw_grad(contwet)
#>   bay_seg     grad
#> 1       1 4.330350
#> 2       2 3.982828
#> 3       3 2.341348
#> 4       4 0.000000
#> 5       5 0.000000
#> 6       6 0.000000
#> 7       7 0.000000
#> 8      55 0.000000