---
title: "Temporal head change and vertical hydraulic gradients"
description: "Compare monitoring events and calculate explicitly signed vertical gradients from compatible paired intervals."
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Temporal head change and vertical hydraulic gradients}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r setup}
library(potentiomap)
data("synthetic_events")
a <- subset(synthetic_events, event == "spring")
b <- subset(synthetic_events, event == "autumn")
pa <- ps_make_points(a, "x", "y", "head", "well_id", "EPSG:26916")
pb <- ps_make_points(b, "x", "y", "head", "well_id", "EPSG:26916")
```

```{r change}
change <- ps_head_change(pa, pb, "well_id", method = "IDW", grid_res = 350)
change$summary
gradient <- ps_vertical_gradient(upper_head = 10, lower_head = 12,
                                 upper_elevation = 100, lower_elevation = 80,
                                 positive = "upward")
gradient[c("signed_gradient", "direction_class", "sign_convention")]
```

Paired measured change and modeled surface change remain separate. Head change
is not storage change, and a vertical hydraulic gradient is not flux.