> For the complete documentation index, see [llms.txt](https://water-strategy.gitbook.io/waterstrategy/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://water-strategy.gitbook.io/waterstrategy/tutorials/creating-a-reservoir-system/adding-reservoir-details/adding-monthly-evaporation-and-rainfall.md).

# Adding monthly evaporation and rainfall

In this step, we will add the the Evaporation and Precipitation rates (mm/day). Internally the reservoir multiplies the real-time Reservoir area by the Evaportion.

The Evaporation can be defined by a parameter or a scalar. For example, a time series can be used that is correlated to the flow scenario time series. However in this case, we will use a [Monthly profile parameter](https://water-strategy.gitbook.io/water-strategy/modelling-fundamentals/parameters/monthly-profile) which assigns a different value to each month in the year.

1. Select the reservoir and edit the Evaporation attribute.

<figure><img src="/files/IUYmnasnZXNIrRi3VniW" alt=""><figcaption><p>Edit the Evaporation attribute</p></figcaption></figure>

2. WaterStrategy has a Json editor for a Monthly profile parameter. To use it, In the options tab select **PYWR\_MONTHLY\_PROFILE**.

<figure><img src="/files/xMTZOssup2M9XGCByxEC" alt="" width="375"><figcaption><p>Select PYWR_MONTHLY_PROFILE</p></figcaption></figure>

3. In the **Monthly Profile tab**, enter the evaporation rates in the table below:

<table><thead><tr><th width="146"></th><th width="67">Jan</th><th width="68">Feb</th><th width="65">Mar</th><th width="63">Apr</th><th width="67">May</th><th width="64">Jun</th><th width="68">Jul</th><th width="69">Aug</th><th width="67">Sep</th><th width="66">Oct</th><th width="63">Nov</th><th width="100">Dec</th></tr></thead><tbody><tr><td>Evaporation (mm/day)</td><td>2.70</td><td>4.02</td><td>1.45</td><td>1.98</td><td>0.98</td><td>0.10</td><td>0.04</td><td>0.03</td><td>0.04</td><td>0.48</td><td>1.14</td><td>2.45</td></tr></tbody></table>

<figure><img src="/files/tmVuP3000n96MRNfievS" alt=""><figcaption><p>Enter the evaporation</p></figcaption></figure>

Then, save it.

4. Follow the same steps for the Rainfall attribute using the table below the figure.

<figure><img src="/files/3yFYFtJpmUpvu6DRwxKG" alt=""><figcaption><p>Edit the Rainfall</p></figcaption></figure>

<table><thead><tr><th width="142"></th><th width="66">Jan</th><th width="68">Feb</th><th width="64">Mar</th><th width="67">Apr</th><th width="67">May</th><th width="64">Jun</th><th width="64">Jul</th><th width="73">Aug</th><th width="67">Sep</th><th width="68">Oct</th><th width="64">Nov</th><th>Dec</th></tr></thead><tbody><tr><td>Precipitation (mm/day)</td><td>4.91</td><td>2.33</td><td>1.24</td><td>2.30</td><td>0.39</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.01</td><td>0.48</td><td>1.76</td><td>2.23</td></tr></tbody></table>

5. Finally set the **Evaporation Penalty** to -2000 and the **Evaporation Unit Conversion** on the reservoir node to 0.001.

<figure><img src="/files/GYeBqXbx5eRic1E8RMFL" alt=""><figcaption><p>Set the Evaporation Penalty and Evaporation Unit Conversion</p></figcaption></figure>

The highly negative Evaporation Penalty of -2000 is higher priority than either the reservoir and any other nodes in the system. This ensures that **evaporation outflow** is met first **before** any management rules are implemented.

The **Unit Conversion** allows the model to correctly convert the evaporation in mm/day and reservoir area in Km2 to the correct flow units in the template which are Mm3/day

6. Run this scenario and compare the simulated volume against the **'Demand with GW'** scenario.

Evaporation is shown to result in a decrease in reservoir levels during droughts. Precipitation additions do not compensate for the losses.

<figure><img src="/files/62XJUhYn6tfaikeXoYib" alt=""><figcaption><p>Reservoir volume compare</p></figcaption></figure>


---

# 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, and the optional `goal` query parameter:

```
GET https://water-strategy.gitbook.io/waterstrategy/tutorials/creating-a-reservoir-system/adding-reservoir-details/adding-monthly-evaporation-and-rainfall.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

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.