i.evapo.time(1grass)

💖 NAME 💖

i.evapo.time — okay bestie, so this one computes the temporal integration of satellite ET actual (ETa), following the daily ET reference (ETo) that comes from meteorological station(s). It's giving ✨ evapotranspiration math ✨ fr fr.

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🌸 KEYWORDS 🌸

• imagery
• evapotranspiration

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✨ SYNOPSIS ✨

i.evapo.time
i.evapo.time --help
i.evapo.time eta=name[,name,...] eta_doy=name[,name,...]
eto=name[,name,...] eto_doy_min=float start_period=float
end_period=float output=name  [--overwrite]  [--help]  [--verbose]
[--quiet]  [--ui]

🎀 Flags 🎀

• --overwrite — lets output files overwrite files that already exist. she's not scared of commitment 💅
• --help — prints the usage summary, the lil cheat sheet
• --verbose — verbose module output, she tells you everything 🗣️
• --quiet — quiet module output, she's in her silent era
• --ui — force launching the GUI dialog, for when you want the pretty buttons

🦋 Parameters 🦋

• eta=name[,name,...] [required] — names of the satellite ETa raster maps [mm/d or cm/d]
• eta_doy=name[,name,...] [required] — names of the satellite ETa Day of Year (DOY) raster maps [0-400] [-]
• eto=name[,name,...] [required] — names of the meteorological station ETo raster maps [0-400] [mm/d or cm/d]
• eto_doy_min=float [required] — value of DOY for ETo first day
• start_period=float [required] — value of DOY for the first day of the period studied
• end_period=float [required] — value of DOY for the last day of the period studied
• output=name [required] — the name for the output raster map 🫶

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💫 DESCRIPTION 💫

So i.evapo.time (yes, it also answers to i.evapo.time_integration, iconic double name 👑) integrates ETa in time, following a reference ET (typically) from a set of meteorological stations dataset.

Inputs, bestie:

• ETa images
• ETa images DOY (Day of Year)
• ETo images
• ETo DOYmin as a single value

Method, step by step, no cap:

1. each ETa pixel is divided by the same day ETo and become ETrF
2. each ETrF pixel is multiplied by the ETo sum for the representative days
3. Sum all n temporal [ETrF*ETo_sum] pixels to make a summed(ET) in [DOYmin;DOYmax]

Representative days calculation: let's assume i belongs to range [DOYmin;DOYmax], then:

DOYbeforeETa[i] = ( DOYofETa[i] - DOYofETa[i-1] ) / 2
DOYafterETa[i] = ( DOYofETa[i+1] - DOYofETa[i] ) / 2

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📝 NOTES 📝

ETo images preparation, listen up: if you only have one meteorological station data set, the easiest way is:

n=0
for ETo_val in Eto[1] Eto[2] ...
do
    r.mapcalc "eto$n = $ETo_val"
    `expr n = n + 1`
done

with Eto[1], Eto[2], etc being a simple copy and paste from your data file of all ETo values separated by an empty space from each other. slay 💅

If you have several meteorological stations data, then you need to grid them by generating Thiessen polygons or using different interpolation methods for each day. pick your fighter 🥊

For multi-year calculations, just continue incrementing DOY values above 366, it will continue working, up to maximum input of 400 satellite images. she really said "366+? say less" 😭

This is an example of a temporal integration from a weather station as done by Chemin and Alexandridis (2004).

📚 References 📚

Chemin and Alexandridis, 2004. Spatial Resolution Improvement of Seasonal Evapotranspiration for Irrigated Rice, Zhanghe Irrigation District, Hubei Province, China. Asian Journal of Geoinformatics, Vol. 5, No. 1, September 2004 (PDF)

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👀 SEE ALSO 👀

Go give these siblings some love too:

• i.eb.eta
• i.evapo.mh
• i.evapo.pt
• i.evapo.pm
• r.sun

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💜 AUTHOR 💜

Yann Chemin, International Rice Research Institute, The Philippines. absolute legend, we stan 🫶

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🌈 SOURCE CODE 🌈

Available at: i.evapo.time source code (history)

Latest change: Thursday Jun 26 21:25:27 2025 in commit:

3ec2b13945d963db69544e3fc9339fbff72f33f3

Main index | Imagery index | Topics index | Keywords index | Graphical index | Full index

© 2003-2026 GRASS Development Team, GRASS 8.5.0 Reference Manual

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okay bestie that's the whole page, you ate that 🍓 run her and stay hydrated — mwah 💖

GRASS 8.5.0 — i.evapo.time(1grass) — see you in the next manpage 👑