Climate · Hydro–Thermal Stress

PTR – Precipitation Temperature Ratio

PTR integrates rainfall and land surface temperature to measure climate stress on vegetation. High PTR indicates favorable moisture and cooler conditions, while low PTR indicates heat–dry stress.

1. Scientific Definition

PTR relates rainfall availability to thermal conditions. It is widely used in crop-climate analysis, drought studies, and vegetation stress assessment.

Formula

PTR = Rain / LST

Interpretation

PTR high → good moisture + cooler temperatures (healthy vegetation)
PTR medium → moderate stress
PTR low → heat stress + insufficient rainfall (drought risk)

2. Required Data

Rainfall

CHIRPS Monthly or Daily

Temperature

MODIS LST_Day (converted to °C)

Suggested Palette

["#a50026","#f46d43","#fdae61","#fee08b","#66bd63","#1a9850"]

3. Google Earth Engine Code – PTR


// PTR = Rain / LST_DAY

var roi = geometry;
Map.centerObject(roi, 6);

// ---------- 1. Load Rainfall (CHIRPS Monthly)
var rain = ee.ImageCollection("UCSB-CHG/CHIRPS/Monthly")
  .filterBounds(roi)
  .filterDate("2023-01-01", "2023-12-31")
  .select("precipitation")
  .sum()
  .rename("RAIN");

// ---------- 2. Load MODIS LST Daytime
var lst = ee.ImageCollection("MODIS/061/MOD11A2")
  .filterBounds(roi)
  .filterDate("2023-01-01","2023-12-31")
  .select("LST_Day_1km")
  .mean();

// Convert to Celsius
var lstC = lst.multiply(0.02).subtract(273.15).rename("LST_DAY");

// ---------- 3. PTR = Rain / LST
var ptr = rain.divide(lstC).rename("PTR");

// ---------- 4. Visualization
var vis = {
  min: 0,
  max: 2,
  palette: ["#a50026","#f46d43","#fdae61","#fee08b","#66bd63","#1a9850"]
};

Map.addLayer(ptr.clip(roi), vis, "PTR (Precipitation / LST)");

// ---------- 5. Export
Export.image.toDrive({
  image: ptr,
  description: "PTR_export",
  fileNamePrefix: "PTR_2023",
  region: roi,
  scale: 5000,
  crs: "EPSG:4326",
  maxPixels: 1e13
});