---
title: "Introduction to ET0RadModels"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Introduction to ET0RadModels}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r setup, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  fig.width = 7,
  fig.height = 5
)
```

## Overview

The `ET0RadModels` package provides 10 radiation-based empirical models for
estimating daily reference evapotranspiration (ET0). The FAO Penman-Monteith
method is included as the standard reference against which the radiation-based
models are compared and evaluated.

Radiation-based models require solar radiation (or sunshine hours) as the primary
input, making them suitable where radiation data are available but wind speed or
humidity data are limited.

### Radiation-Based Models

| # | Model | Function | Primary Inputs |
|---|-------|----------|----------------|
| 1 | Jensen-Haise (1963) | `et0_jensen_haise()` | Tmean, J, lat, n |
| 2 | McGuinness-Bordne (1972) | `et0_mcguinness_bordne()` | Tmean, J, lat, n |
| 3 | Irmak (2003) | `et0_irmak()` | Tmean, J, lat, n |
| 4 | Makkink (1957) | `et0_makkink()` | Tmean, J, lat, n, z |
| 5 | Turc (1961) | `et0_turc()` | Tmean, J, lat, n |
| 6 | Caprio (1974) | `et0_caprio()` | Tmean, J, lat, n |
| 7 | Abtew (1996) | `et0_abtew()` | Tmean, J, lat, n |
| 8 | Priestley-Taylor (1972) | `et0_priestley_taylor()` | Tmin, Tmax, RH, n, J, lat, z |
| 9 | Jones-Ritchie (1990) | `et0_jones_ritchie()` | Tmin, Tmax, J, lat, n |
| 10 | Hargreaves (1975) | `et0_hargreaves()` | Tmean, J, lat, n |

## Quick Start

### Loading the Package and Data

```{r load}
library(ET0RadModels)
data(jhansi_weather)
head(jhansi_weather)
```

### Computing ET0 for a Single Day

```{r single-day}
# FAO Penman-Monteith (reference)
et0_fao_pm(Tmin = 15, Tmax = 35, RH_morning = 90, RH_evening = 40,
           u2 = 1.5, n = 9, J = 172, lat = 25.43, z = 216)

# Jensen-Haise (simple radiation-based)
et0_jensen_haise(Tmean = 25, J = 172, lat = 25.43, n = 9)

# Priestley-Taylor (uses net radiation)
et0_priestley_taylor(Tmin = 15, Tmax = 35, RH_morning = 90,
                     RH_evening = 40, n = 9, J = 172,
                     lat = 25.43, z = 216)
```

### Computing All 10 Radiation-Based Models at Once

```{r all-models}
result <- et0_rad_all(
  Tmin       = jhansi_weather$Tmin[1:5],
  Tmax       = jhansi_weather$Tmax[1:5],
  RH_morning = jhansi_weather$RH_morning[1:5],
  RH_evening = jhansi_weather$RH_evening[1:5],
  u2         = jhansi_weather$WS[1:5],
  n          = jhansi_weather$SSH[1:5],
  J          = jhansi_weather$J[1:5],
  lat        = 25.43,
  z          = 216
)
round(result, 2)
```

## Evaluating Model Performance Against FAO-PM

```{r evaluation}
full_result <- et0_rad_all(
  Tmin       = jhansi_weather$Tmin,
  Tmax       = jhansi_weather$Tmax,
  RH_morning = jhansi_weather$RH_morning,
  RH_evening = jhansi_weather$RH_evening,
  u2         = jhansi_weather$WS,
  n          = jhansi_weather$SSH,
  J          = jhansi_weather$J,
  lat        = 25.43,
  z          = 216
)

obs   <- full_result$FAO_PM
preds <- full_result[, c("Jensen_Haise", "Makkink",
                          "Priestley_Taylor", "Turc")]
metrics <- evaluate_models(obs, preds)
print(metrics)
```

## Visualization

### Scatter Plot

```{r scatter, fig.width=6, fig.height=6}
plot_scatter(obs, preds,
             main = "Radiation-Based Models vs FAO-PM")
```

### Taylor Diagram

```{r taylor, fig.width=7, fig.height=7}
plot_taylor(obs, preds,
            main = "Taylor Diagram - Radiation-Based Models")
```

## References

- Allen, R.G., Pereira, L.S., Raes, D., & Smith, M. (1998). Crop
  evapotranspiration: Guidelines for computing crop water requirements.
  FAO Irrigation and Drainage Paper No. 56.
- Satpute, A.N., Barman, S., Singh, A.K., Ghosh, A., & Gupta, G. (2026).
  A multi-scale evaluation of 30 empirical models for reference
  evapotranspiration estimation in a data-scarce semi-arid region:
  A case study of Jhansi, India.
  *Journal of Hydrology: Regional Studies*.
  https://doi.org/10.1016/j.ejrh.2026.103925