How to find out if you can sustain your trees by rain alone

tree-satisfaction-with-water-harvesting-techniques

As mentioned in our last blog post, I created a spreadsheet called Tree Satisfaction that you can view and copy it into your own library so you can use it for your own planning.

The example shows the data for Barcelona and we are going through the data based on the example. But don’t worry, I will also show you how to use it for your own location.

Prerequisites

We want to know if we can sustain a young tree through the means of catching rainwater only. To find out the answer, we need to consider a few things:

  • how much does it rain?
  • how frequently does it rain?
  • what is the area a tree can draw water from?
  • how much rainwater can be caught when it rains?
  • how much water does a tree consume?
  • how much water is lost through evaporation, transpiration or other means?

Please note that we will only take the area around the tree into account. Other means of catching rainwater such as dams, diversion channels or swales are not considered here.

Let’s study Barcelona

First, we need to get relevant historical data regarding precipitation – ncdc.noaa.gov offers free historical data from weather stations around the world. In our example, we will study the years 2015 to 2018.

Equipped with this data, we can already print some useful graphs that can tell us something about the situation:

Temperatures fluctuate between 2 and 37° throughout the year. Winters are cold but don’t go below 0° C.

Minimum and maximum temperatures in Barcelona 2015 - 2018
Minimum and maximum temperatures in Barcelona 2015 – 2018

The precipitation shows us that 2015/16 were much dryer than 2017/18. Something interesting is going on here and we should look at the data on a much longer timescale. 4 years of data are not enough to jump to conclusions.

Precipitation in Barcelona 2015 - 2018
Precipitation in Barcelona 2015 – 2018

Especially interesting is the chart of days without rain. The red triangles, that are building up mostly peak between 20 – 25 days but in the Winter of 2015/16, we had a staggering 73 days without rain.

Days without rain in Barcelona 2015 - 2018
Days without rain in Barcelona 2015 – 2018

So many days without rain

20 – 25 days without rain… It looks like we can’t rely on rain alone, right? Well, don’t underestimate how much water can be held in the soil.

You know, when it rains, the soil can stay wet days after the rain occurred. Let’s have a deeper look.

Regular soil can hold water up to a quarter of its volume. A cubic meter, therefore, can hold up to 250 liters.

Water, once in the soil, can only disappear through 3 main ways:

  • evaporation through sun and wind influences
  • water flow within the soil to other areas
  • evapotranspiration by the plants

The more we are able to reduce these factors, the longer the soil can retain the water and make it available to our plants.

Tree satisfaction

In order to combine it all together, I will use the term tree satisfaction. It’s a metric that tells us how happy trees are based on how much water they have available in the soil.

When they receive water, the satisfaction rises. When the soil is completely dry, the satisfaction is basically 0.

Let’s assume a few things…

Assumptions for tree satisfaction calculation
Assumptions for tree satisfaction calculation

A tree younger than 2 years usually requires around 80 liters a week, or 11 liters a day. We assume they can draw water from a 2 meters radius which gives us the area of rainwater a tree can collect (12.57 m²).

As a starting point, we also assume that we can collect 100% of the rain and don’t lose any water. Let’s have a look at the following chart.

Tree satisfaction in a perfect world
Tree satisfaction in a perfect world

A lot comes together in this chart, let me break it down for you.

The green line is the tree satisfaction. In a perfect world, it is constantly rising until the soil cannot hold more water.

The blue columns show the precipitation of the day. You can see that when it rains, tree satisfaction rises.

The red triangles, in the beginning, are sad tree days. Those are the days when the tree has absolutely no more water to drink from. We want to avoid them.

Finally, the orange line is the anticipated tree consumption. During winter the consumption is lower due to less direct sun. The calculation in the spreadsheets makes this assumption based on temperature.

A more realistic example

In reality, we cannot catch all the water and also lose water every day through the soil:

Water loss/day5%
Catchment Coefficient70%
A more realistic example of tree satisfaction
A more realistic example of tree satisfaction

As you can immediately see, the sad tree days increased significantly and tree satisfaction stays low most of the time. There are 3 major periods ranging from 50 to 80 days. In a Mediterranean climate as we have in Barcelona, plants usually don’t go into dormancy as temperatures stay constantly above 0° C.

In this case, we have to step in and make sure that our trees receive the water they need.

or we apply water harvesting and retention techniques

There are many things, we can do to harvest more water and to retain it over longer periods of time. Let’s assume the following improvement:

Water loss/day2%
Catchment Coefficient90%
Tree satisfaction when applying water harvesting techniques
Tree satisfaction when applying water harvesting techniques

We can see that our trees most likely stay happier throughout the years. Granted, there are still 3 major periods of sad tree days, but they seem less scary.

I think those periods can then be mitigated by installing swales and dams.

How to analyze your region

This is how you can do the analysis for your property:

  1. copy the Tree Satisfaction Spreadsheet
  2. go to NCDC Climate Data Online Search
  3. Select Weather Observation Type/Dataset: Daily Summaries
  4. Select your (rough) date range (you will select it again later)
  5. Search for your station, city, ZIP code, etc..
  6. Click Search
  7. Select your weather station: Add to cart
  8. Go to your cart
  9. Select Custom GHCN-Daily CSV
  10. Select your date range again
  11. Continue
  12. Change units from Standard to Metric (otherwise you will have to rewrite the formulas and units)
  13. Choose PRCP and open Air Temperature > select TMAX and TMIN
  14. Continue
  15. Type in your email address and submit the order
  16. You will receive a download link in your inbox
  17. Download the CSV and open it
  18. Clear columns A to F in the Tree Satisfaction Spreadsheet
  19. Copy and paste all values from your spreadsheet to the columns A to F of the Tree Satisfaction Spreadsheet

Now you can play around with the values of the yellow cells and see how the charts change.

If you have any questions, criticism or ideas, let us know in the comment section below!

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