> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/chapter-2-2-evapotranspiration/2-2.3-actual-evapotranspiration/2-2.3.4-evaporation-from-ponded-water.md).

# 2:2.3.4 Evaporation from Ponded Water

Evapotranspiration (ET) is estimated as the sum of plant transpiration and evaporation rates. SWAT+ inherits the SWAT model simulating potential evapotranspiration (PET) rates using the Priestley-Taylor method (Priestley and Taylor, 1972), the Penman-Monteith method (Monteith, 1965), or the Hargreaves method (Hargreaves and Samani, 1985). The estimated daily PET serves as the maximum daily actual ET amount. Thus, daily ET is assumed to be the same as PET if the sum of transpiration (EP) and soil evaporation (ES) is higher than PET. In this case, the soil evaporation rate is adjusted as ES = PET – EP. In rice paddies and wetlands where standing water exists, daily ET is calculated as the sum of EP and EVP, which can exceed the calculated daily PET. However, shades of mature rice straws can limit water evaporation (Choi et al., 2017; Sakaguchi et al., 2014b).

 $$EVP\_t=0.6\*(1-\frac {LAI}{4.0})\*PET,  ; if LAI≤4.0$$                                2:2.3.22

$$EVP\_t = 0, ; if LAI>4.0$$                                                                             2:2.3.23

where $$EVP\_t$$ is the evaporation of standing water, LAI is the dimensionless leaf area index, and PET is the potential evapotranspiration.

The SWAT soil evaporation equation substitutes Equation 2:2.3.23 if standing water is entirely depleted by transmission losses.

Table 2:2-3: SWAT+ input variables used in soil evaporation calculations..

| Variable Name | Definition                                          | File Name                                                                                             |
| ------------- | --------------------------------------------------- | ----------------------------------------------------------------------------------------------------- |
| ESCO          | $$esco$$: soil evaporation compensation coefficient | [hydrology.hyd](/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/introduction/hydrology/hydrology.hyd.md)      |
| LAI           | Dimensionless leaf area index                       | [plant.ini](/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/introduction/landuse-and-management/plant.ini.md) |
| PET           | Potential evapotranspiration                        | [codes.bsn](/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/introduction/basin-1/codes.bsn.md)                |


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://swatplus.gitbook.io/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/chapter-2-2-evapotranspiration/2-2.3-actual-evapotranspiration/2-2.3.4-evaporation-from-ponded-water.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.