Aerial NDWI Data Collection For Use in Resource Management

Katie Gilmore, PAAP Drones


 

Abstract:  

The Normalized Difference Water Index (NDWI) can be used to monitor and manage water resources.  Using drones and multispectral sensors to collect high resolution data will enable water management personnel efficiently and effectively measure and monitor changes in and around water resources.

 

Introduction:  

Monitoring of lakes, ponds, and other water resources can be time-consuming and difficult.  Traditional measurements such as airplane imagery and ground analysis are expensive and time consuming, and satellite data does not provide adequate resolution for smaller bodies of water.  The Normalized Difference Vegetation Index (NDVI) can be useful in agriculture and some water analysis, but it can also give false or incomplete data around shorelines with dense vegetation.  Alternatively, NDWI can be the right tool to make efficient and effective decisions about water resources.

 

NDWI is calculated by measuring the reflection of visible light (specifically the green channel) and near infrared (nir) light and is a measure of liquid water molecules in vegetation that interacted with the incoming solar radiation.  Vegetation with a high amount of water absorbs visible light, reflects near infrared light, and returns positive NDWI values with positive, higher values indicating vegetation with more water.   Vegetation that contains less water results in negative NDWI values.  While NDVI is typically able to measure 3 layers of leaves, NDWI has been shown to give a cumulative water content measurement of vertical plant layers, and at large NDVI values (such as healthy algae or standing water in fields) NDVI levels become saturated while NDWI remains sensitive to liquid water in vegetation.  

 

NDWI applications:

  • Shorelines: Algae tends to accumulate along shorelines and plant growth can be abundant.  NDWI can also give indications of water through shoreline vegetation.   

  • Wildfire Risk: Plant humidity

  • Drought identification: Plant humidity

  • Flood and drainage monitoring: Wet vegetation will give different values than “normal” vegetation

  • Snow Cover monitoring

 

Methodology:

A DJI Inspire 1 was flown over water body LP47 in Eagan, MN on October 11th at 2pm, at approximately 340ft above ground level.  The drone carried a Sentera Multispectral Sensor, capturing 59 images each of RGB and nir imagery.  Image post processing was completed in Pix4D utilizing the Index Calculator module.  Pix4D allows the user to enter custom equations to be applied to imagery.  The following equation was used to calculate NDWI:

 

ndwi.png

 

Typical NDWI values are typically displayed in a 5-class gradient where negative values indicate little to no water content, and increasing positive values indicate increasing amounts of water content.  

 

Figure 1 shows the aerial view of LP47 in RGB.  Surface algae is visible on the north-east and west areas of the pond.  There is significant tree and brush growth around the shoreline and the lake is bordered by a road to the east, a parking lot to the north, and a residential area to the south.

Algae Lake

Figure 1: LP47 RGB

 

Figure 2 provides an informative view of surface NDWI values for LP47.  As expected, we do see positive NDWI values across the lake.  Areas with algae buildup or dense shoreline vegetation have have lower values than water, but indications of water can be identified.   

 

NDWI Lake Algae

Figure 2: LP47 NDWI Values



 

It is useful to monitor water boundaries both in 5 gradient and 2 gradient analyses.  A 2 gradient view gives a view of shorelines that are easy to monitor and measure over time.  A comparison of NDVI and NDWI values can be seen in Figure 3.  

NDWI vs NDVI

NDVI                                                           NDWI

Figure 3:  NDVI vs NDWI


 

Figure 4 shows the difference between NDVI and NDWI measurements.  Dark blue shading shows areas where both indices indicated presence of water.  Light blue shading shows areas where NDWI finds additional water.  NDWI extends the boundaries of the shoreline to include algae-covered surfaces of water.

NDVI NDWI Lake Algae Shoreline Overlay

Figure 4: Overlay

 

Conclusion:  Collection and analysis of aerial RGB and nir data can provide efficient analysis of water monitoring by using NDWI calculations.  Aerial data collection is a cost effective, fast, and efficient use of resources to help aid our communities in management of water resources and should be one of many tools available to our environmental resource technicians.  

 

Acknowledgements:

Sentera: MultiSpectral Sensor & Technical Expertise, www.sentera.com.  

 

Contact:

Katie Gilmore, PAAP Drones. katie@paapdrones.com. 612-470-4333.

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