## 3 Results and Discussion
Similar results are evident for the South Texas site where many areas to the west and center appear to be dominated by, again, the yellow class for the 2009 to 2010 years, and almost the entire scene for the 2010 to 2011 years. Both indicate a move towards increased drying (Figure 11b). The two threshold cutoffs of 0.5 standard deviation to determine pixels of low-level change, and 1.0 standard deviation to represent pixels of high-level change resulted in 85% and 82% of the sites with no change pixels for the 2009 to 2010 and 2010 to 2011 years respectively. High-level change is higher for the 2010 2011 years at just under 12% compared with around 9.5% for the 2009 2010 years. This indicates that a greater amount of change as well as a greater intensity of change occurred in the 2010 to 2011 years. \begin{table}
\begin{tabular}{c c c c c} \hline
** NDVI-NDMI (2009–2010)** & **Houston Area (\%)** & **Houston Area (km2)** & **S Texas Area (\%)** & **S Texas Area (km2)** \\ \hline No change & 81.9 & 18,049 & 75.5 & 17,829 \\ Low level change & 6.2 & 1372 & 9.6 & 2258 \\ High level change & 11.9 & 2612 & 15.0 & 3538 \\ \hline
**NDVI-NDMI (2010–2011)** & & & & \\ \hline No change & 78.2 & 17,230 & 73.5 & 17,372 \\ Low level change & 7.7 & 1696 & 11.2 & 2636 \\ High level change & 14.1 & 2818 & 15.3 & 3618 \\ \hline \end{tabular}
\end{table} 
Table 2: Changes for Houston and South Texas sites. Figure 9: Image differences for 2009 and 2010, and 2010 and 2011 using TCT brightness, greenness, and wetness NDVI, BI, and NDMI for **(a)** Houston, **(b)** South Texas. Figure 10: (**a**) Change vector magnitude for TCT BGW at Houston site. (**b**) Change vector direction for TCT BGW at Houston site. Figure 11: (**a**) Change vector magnitude for TCT BGW at South Texas site. (**b**) Change vector direction for TCT BGW at South Texas site. In terms of change direction masked by the high threshold cells, it is difficult to discern the proportions of the overall change types. However, individually, for the Houston site, the higher change classes are, in descending order for 2009 the 2010 years: B+ G+ W+, B+ G- W-, B- G+ W+, and B+ G+ W-. For the 2010 to 2011 years, the only substantial change classes are B+ G- W- at over 60% of the high threshold and B- G- W- at around 25%. It is interesting to note the presence of certain change classes when the expected change is known. For example, B- G+ W+ is expected during a change towards normal, wet conditions; however, the presence of B+ G- W- indicate that for significant areas there was continuing drought in 2010 from the previous year. As for B+ G+ W-, certain vegetation types may have higher brightness values than their background, suggesting that in some cases an increase in vegetation may result in an increase in brightness. For the South Texas site, B+ G- W- covered over 60% of the high threshold for the 2009 to 2010 years while it is approximately 90% for the 2010 to 2011 years. In terms of actual acreage change, agricultural land use contributed around 42% of the total changed cells for the 2009 to 2010 years at the Houston site, and over 50% for the 2010 to 2011 years. In contrast, agricultural land use at the South Texas site contributed up to 76% of the changed cells for the 2009 to 2010 years, and as high as 86% for the 2010 to 2011. Similarly, forest land cover changes are much higher for the South Texas site: 30% and 7% for the 2009 to 2010 and the 2010 to 2011 years respectively, compared to 21% and 1% across the same years for the Houston site (Table 3). Both support the notion of the droughs affecting the more rural South Texas site. Changes between land use/cover types is even more pronounced during drought conditions. Using the high-level threshold cut-off from the TCT we can measure the most prevalent changes; we call them the top three \"from-to\" land use/cover types (Table 4). While it may be difficult to discern the precise type, eight change directions emerge from our results.