Arizona Rainfall Roundup

What kind of rain gauge do I need to use?

Any type of simple, plastic rain gauge will do for this project. The raingauge should be able to hold at least 6 inches of rainfall. Since we are monitoring monthly total rainfall, a rain gauge with a 6 inch holding capacity may fill up and need to be emptied before the end of the month. A larger gauge will need to be checked less often. Below are pictures of several common types of home/garden rain gauges.

Wedge Type: easier to read with smaller amounts of precipitation

Simple Cylinder or Rectangular Catch: basic construction and inexpensive

Funnel Catch with overflow: Very useful for monitoring individual precipitation events under 1". Overflow may evaporate over time.

These type of rain gauges are relatively inexpensive ($10-$30) and can be found at most hardware/garden centers or online. Here are links to information on a couple of ideal rain gauges:

• Tru-Chek Rain Gauge
• All-Weather Rain Gauge

NOTE: The funnel catch type rain gauge with the overflow tube will need to be checked more often than once a month. Mineral oil placed in the central tube will overflow into the second cylinder leaving precipitation in the central tube open to evaporation. This is a great gauge for monitoring precipitation, but will require a committment to check it more frequently.

Contact me if you have questions on buying a rain gauge or trouble finding one.

How and where should I install my rain gauge?

The most critical issue in collecting good rainfall data is where your gauge is located. Site your gauge in an open area away from obstructions like trees or overhangs and also in an area protected from high winds. Mount your gauge on a post at a height of 3-5 feet with the gauge sticking up several inches beyond the post. Make sure that the top of the gauge is level. Again, make sure the post is clear of areas that will allow for rain water to indirectly splash into the gauge (sheds, equipment, carports, etc.). If you have a large yard or management area, consider installing several rain gauges.

Ideal rain gauge installation

Since we are interested in collecting monthly total rainfall, we need to make sure that precipitation in the gauge doesn't evaporate in between events. Keeping a small amount of mineral oil in the gauge will retard evaporation over the month (see note above regarding funnel catch type gauges). The mineral oil will float on top of the rainfall as it accumulates in the gauge and form a barrier to evaporation. Remember to new mineral oil each month after you empty your gauge or if the gauge overflows.

NOTE: The volume of mineral oil you add will depend on the type of gauge you are using. The surface area of the gauge opening is different depending on the size and shape of the gauge. A wedge-type gauge has increasing surface area with increasing depth of rainfall (i.e. more surface area on top of rainfall in gauge as rainfall accumulates). This requires you to add a volume of oil that will be sufficient to form a layer when the gauge is almost full. Filling an empty wedge-type gauge to "0.05 appears to be enough oil to slow evaporation. Other cylinder/rectangular gauges should have a "0.01 thick layer of oil. This may take experimentation depending on your gauge.

I suggest using mineral oil because it is non-toxic and relatively stable. It can be safely emptied with accumulated precipitation in your yard. Other oils may be toxic and not recommended. Mineral oil will breakdown in direct sunlight and at high temperatures. Keep an eye on the mineral oil amount and wash out your gauge each month to remove residues.

What do I need to do each month?

• Periodically check your gauge to make sure that it hasn't overflowed and still has a thin film of mineral oil to protect from evaporative loss. If the gauge has overflowed, note the date, empty the gauge/replace the mineral oil and communicate this to me in your monthly report.
• Note other interesting climatic events as you monitor rain fall. Did a certain plant species bloom or grow significantly with rainfall events? Was there an especially significant rainfall event (very long in duration, very intense)? Did you notice changes in local hydrological features over the month (dropping/rising lake or stockpond levels, running washes, flooding events, unusually dry conditions with a feature)? Please report these observations in the notes section of the reporting form.