If you own or run a dam—whether it’s for irrigation, livestock, or just a big water feature in your yard—you need to know how much water you’re losing to evaporation. To figure this out, you first need to know the area of your dam’s surface. This guide offers simple, practical ways to get the most accurate measurement possible.
Why Surface Area is Important
Your dam’s surface loses water. The bigger the surface area, the more water you lose. If you don’t get the surface area right, your projections for how much water would evaporate will be inaccurate. This can disrupt your water planning, especially during dry seasons.
Easy Ways to Measure Small Dams
If your dam is small (less than a couple of football fields in size), you can usually measure it yourself.
Tape Measure or Measuring Wheel
- How to do it: Walk around the edge of your dam with a long tape measure or a measuring wheel.
- Square or rectangular dams: Measure the length and width at the water’s edge. Multiply them to get the surface area.
- Round dams: Measure the circumference (distance around the edge) and use this formula:
- Surface Area = (Circumference)² / (4 × 3.14)
- (Note: The original article had an incorrect formula. This is the correct one for area from circumference.)
- Surface Area = (Circumference)² / (4 × 3.14)
- Limitations: This method can be tricky if your dam has a wavy shoreline or lots of vegetation near the edge.
Using Technology for Bigger or More Complex Dams
Technology can help with bigger dams or when you need more accuracy.
Maps and Photos from the Air
- Google Maps or Google Earth: Zoom in on your dam, use the “measure distance” tool to draw a line around the water’s edge, and the computer will calculate the area for you.
- Accuracy: Pretty good, especially if the image is clear. There may be a slight margin of error, but it’s usually good enough for most uses.
Unmanned Aerial Vehicles (UAVs)
- How it works: Fly a drone over your dam and take pictures. Special software can stitch these together into a map and calculate the surface area.
- Pros: Very precise, especially for large or oddly shaped dams.
- Cons: You might need to hire someone with a drone and the right software.
Advanced Techniques for Experts
If you manage a really big dam or need measurements that are extremely accurate, consider these advanced methods.
Surveying with GPS
- How it works: Walk around the dam with a GPS device that records your position. The device can map the shoreline and calculate the area.
- Pros: Very precise, especially for large or irregular dams.
- Cons: Requires more time and equipment.
Bathymetric Surveys
- What is it? This means mapping the bottom of the dam, not just the surface. It’s helpful if your dam has changed shape because of silt or erosion.
- How it works: Usually done with special boats and sonar equipment.
- Pros: Gives you a complete picture of your dam’s shape and volume.
- Cons: Expensive and usually only needed for large or critical dams.
How to Avoid Common Mistakes
- Irregular Shorelines: If your dam has a lot of bends or inlets, it’s easy to get the wrong dimensions. Use more points or technology to get a better result.
- Silt and Erosion: Over time, dirt can build up in your dam and change its structure. Check your measurements once a year or every two years.
- Changes in Water Level: The surface area changes when the water level goes up and down. For best results, measure at your usual water level and update if things change.
An Example from Real Life
Let’s say your dam is 100 meters long and 75 meters wide.
- Surface area: 100 × 75 = 7,500 square meters.
- Evaporation estimate: If you lose about 5 mm of water every day, that’s 7,500 × 0.005 = 37.5 cubic meters (or 37,500 litres) per day. Over a year, that’s a lot of water!
Picking the Right Method: Summary Table
| Dam Size/Shape | Best Method | Precision | Cost and Labour |
|---|---|---|---|
| Small, simple | Tape or measuring wheel | Okay | Low |
| Medium, wiggly edges | Google Earth or drone | Very good | Medium |
| Large, complex | GPS survey/bathymetry | Very good | High |
Last Tips
- Check your measurements: If you use technology, do a quick walk-around to double-check.
- Update regularly: Dams can change shape over time, so update your measurements every year or after significant storms.
- Get help: If you’re not sure, local surveyors or drone operators can help you get a precise measurement.
This guide will help you accurately measure your dam’s surface area and make better evaporation estimates for smarter water management.