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Compute hydraulic gradient per bay segment from potentiometric surface raster

Source: R/util_gw_grad.R
util_gw_grad.Rd

Compute hydraulic gradient per bay segment from potentiometric surface raster

Usage

util_gw_grad(
  pot_rast,
  season = c("dry", "wet"),
  segs = tbsubshed,
  shoreline = tbsegdetail,
  buf_segs = NULL
)

Arguments

pot_rast

SpatRaster (or PackedSpatRaster) of Upper Floridan Aquifer potentiometric head (ft above MSL) as returned by util_gw_getcontour, or the package datasets contdry / contwet.

season

character, "dry" or "wet".

segs

sf object of sub-watershed polygons. Defaults to tbsubshed.

shoreline

sf object of bay segment polygons used to derive the bay water mask and measure distances. Defaults to tbsegdetail.

buf_segs

named numeric vector mapping bay segment IDs (as character strings) to omnidirectional buffer distances in US Survey Feet (CRS 6443). The subwatershed for each listed segment is buffered outward by the given distance and bay water is removed before the potentiometric high-point search. Listing a segment here also removes it from the default zero-gradient set so it is computed dynamically. When NULL, season-specific defaults are used (see Details).

Value

A data frame with columns bay_seg (integer) and grad (numeric, ft/mile; 0 for zero-gradient segments).

Details

Computes the Floridan Aquifer hydraulic gradient \(I\) (ft/mile) per bay segment using the Darcy's Law framework of Zarbock et al. (1994):

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

The max potentiometric head within the search area is located in the interpolated raster and the distance is measured from the nearest shoreline crossing along the bay centroid-to-head transect (see below).

Zero-gradient segments (hardcoded): The following segments always receive a gradient of 0 based on the original SAS loading analysis (Zarbock et al., 1994; GWld2224_SASCode.txt):

  • Boca Ciega Bay (segments 5 and 55), both seasons: the urbanized coastal watershed has no meaningful Floridan Aquifer recharge directed toward the bay.

  • Lower Tampa Bay (4), Terra Ceia Bay (6), Manatee River (7), dry season only: the potentiometric gradient is negligible during the dry season. These segments are computed dynamically in the wet season via the default buf_segs.

Any segment listed in buf_segs is removed from the zero set and computed dynamically.

Default buf_segs (calibrated against 2021 SAS reference values):

  • Dry season: c("1" = 100000) – Old Tampa Bay subwatershed buffered ~19 miles; captures the potentiometric high north/northeast of the standard watershed boundary.

  • Wet season: c("1" = 100000, "4" = 100000, "6" = 100000, "7" = 100000) – adds LTB, TCB, and MR (each ~19 miles) to unlock wet-season computation for those segments.

Buffer distances were tuned to produce gradients within ~15\ FDEP potentiometric surface values used in the SAS analysis.

Distance calculation: Rather than measuring from the potentiometric high to the nearest shoreline point (which can hit an extreme geographic corner), the function draws a line from the bay segment centroid to the max-head cell. The portion of that line inside the bay polygon is subtracted from the total length, giving the distance from the shoreline crossing point to the high point along a representative transect.

Hillsborough Bay (segment 2): Uses a three-zone weighted gradient (Polk County 0.4, Pasco County 0.3, Alafia River 0.3) following the original flow net analysis. Sub-zones are constructed from tbdbasin drainage basins as in the original SAS code.

Benchmark warning: After computing gradients, each non-zero segment is compared against the 2021 SAS reference values (GWld2224_SASCode.txt). A warning is issued for any segment whose computed gradient deviates by more than 50\ reference, indicating a potentially anomalous potentiometric surface or a need to revisit the buf_segs configuration.

References

Zarbock, H., A. Janicki, D. Wade, D. Heimbuch, and H. Wilson. 1994. Estimates of Total Nitrogen, Total Phosphorus, and Total Suspended Solids Loadings to Tampa Bay, Florida. Technical Publication #04-94. Prepared by Coastal Environmental, Inc. Prepared for Tampa Bay National Estuary Program. St. Petersburg, FL.

Examples

if (FALSE) { # \dontrun{
pot_dry <- util_gw_getcontour("dry", 2022)
util_gw_grad(pot_dry, season = "dry")

pot_wet <- util_gw_getcontour("wet", 2022)
util_gw_grad(pot_wet, season = "wet")
} # }