# Assessment of Aquatic Weed in Irrigation Channels Using UAV and Satellite Imagery
## 3 Results
### Intensive Small Area Mapping by UAV
#### 3.1.7 Assessing Change in Weed Growth  
One application of irrigation channel weed mapping is assessing change in weed growth. This could indicate potential hot spots of growth that need controlling as soon as possible. It could also provide a method to assess the efficacy of different weed control methods, for example different chemical or mechanical controls. The long-term trends of weed growth could be analysed as a multi-year image database is built.  
The four channels (Table 2) were imaged throughout the study period. The fixed training geometries for land and channel were defined for each of the channels. After classification, the area of each of the classes (water, land and weed) was computed. This could be used to indicate decline or growth of weeds. All this takes less than a minute in the GEE cloud computing environment. The results are shown in Figure 7. Note that these results are during the winter period, where there are a lot of dynamics in channels being drained and refilled, and weed control works being done. The characteristics are likely to not show as much variance during the peak irrigation season.  
\begin{table}
\begin{tabular}{l c} \hline \hline
**Image Date** & **Kappa** \\ \hline
2 July 2018 & 0.98 \\
12 July 2018 & 0.98 \\
1 August 2018 & 0.99 \\
9 August 2018 & 0.98 \\
23 August 2018 & 0.92 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Classifier accuracy for a time series of images over the untreated (U) untreated channel.  
Figure 7: Area of each of the classes over time for the four channels mapped with the UAV. (**a**) C1; (**b**) C2; (**c**) M; (**d**) U.  
The visual and classified images from 2 July, 9 August and 23 August for the C1 channel are shown in Figure 8. Figure 7a shows the corresponding change in relative areas of land, water and weed for this channel. It is surprising to see the land area decline until 9 August, then rise again. The reason becomes clear when the water depth data in Figure 4 is examined. The water level dropped due to evaporation and leaking/seepage. On 11 August, areas of dry ground were visible. The channel was then refilled to more than its starting level on the 23 August image. There was little weed visible in the channel on this date, which is also correctly shown in both the percentage area graph and the classified image.  
The other channels in Figure 7 show land area that is relatively constant, as the channels did not empty to the same extent as the C1 channel. In general, the weed area declines in the chemically and mechanically controlled channels (C1, C2 and M). In the untreated channel (U), the weed area is  
Figure 8: The (**a**) first, (**b**) fifth, and (**c**) sixth images from the time series UAV images of the C1 channel (small portion shown). The corresponding classified results for the (**d**) first, (**e**) fifth, and (**f**) sixth images are shown in the bottom row, with blue = water, red = land and green = weed.  
relatively constant (see Figure 7d). However, the 23 August image shows a lower weed area. This is for the reason discussed in Section 3.1.6--that the water level was dropped and refilled immediately prior to the image, leading to some of the weed patches becoming submerged on 23 August.  
This section has shown the usefulness of using the developed classification method over a time-series of images of irrigation channels to monitor weed growth.