**5. Results and discussion**

## **5.1 Disturbed area caused by the flood**

The summary statistics of the flood affected forests is shown in **Table 2**. According to the % disturbed area it was found that the year 2003, 2008, 2009 and 2010 were the most disturbed years caused by river flood in comparison to rest of the years. These findings are in corroboration with the rainfall data collected from Indian Meteorological Department (www.imd.gov.in). During 2001, 2002, 2004, 2005 and 2006, the disturbed area for all the selected forests were less than 10% of the total area, whereas in 2007 and 2011, only few of the selected forests like Pobitora, Orang and Kukrakata Hills could cross the disturbed area limit of 10%. During 2003 the impact of flood was more than 2010 in the forests namely Phathasil, BuraMayang and Orang. On the contrary, in case of Pobitora, Kamakhya Hills, Kukrakata, Kaziranga and Dihing mukh the devastating effect of the 2010 flood was found to supersede the effect of 2003 flood.

#### *Remote Sensing*

The disturbed area maps of the Assam forests showed (**Figure 5**) that the area under disturbance was much higher in the year of 2003, 2008 and 2010 in comparison to others, whereas it was minimum in the year of 2006. The flood intensity maps (**Figure 2**) showed that the extent of flooded area is mainly confined around the Brahmaputra and Barak River valley, but it was interesting to note that a subset of the flood pixels were marked as disturbed pixels (**Figure 5**). Hence, the temporal frequency of flood along with extent was a decisive factor whether a pixel was disturbed or not. For example, in BuraMayang around 80% of the total forest area


#### **Table 2.**

*Percent disturbed area caused by flood in selected Assam State forests.*

#### **Figure 5.**

*The disturbed area maps of Assam forests shows higher disturbances in the 2003, 2008 and 2010 and a minimum in 2006.*

*Assessment of Ecological Disturbance Caused by Flood and Fire in Assam Forests, India… DOI: http://dx.doi.org/10.5772/intechopen.94282*

was under flood during 2008 with frequency value of one, whereas during 2010 around 40% forest area was inundated by flood with frequency more than 10. Consequently, the disturbance was recorded in 9% of the total forest area in 2008, whereas it was around 24% during 2010. Similarly, in case of Pobitora the whole forest was under flood during both 2008 and 2010, but due to the difference in flood frequency the disturbed area were 2.5 and 30.1% of the total area, respectively.

The temporal dynamics of disturbed area, caused by river flood, for two selected forests, namely Kamakhya and Kukrakata Hills was shown in **Figure 6**. In case of Kamakhya Hills the year 2010 was found to the most disturbed year, followed by 2003 and 2008. Hardly any disturbance was noticed during 2005, 2006, 2009 and 2011. Similarly, in case of Kukrakata Hills also three major flood events were observed in 2010, 2008 and 2003. Though the major flood years were common in both the cases, their magnitude varies.

#### **5.2 Disturbed area caused by forest fire**

The distribution of disturbed area caused by forest fire is shown in **Table 3**. Unlike flood, the spatial extent of % disturbed area caused by forest fire was much lower as the fire is a localized phenomenon. A maximum value of 3.5% of the total forest area was disturbed due to the fire during 2010 in Barail forest. More than 2% of the forest area was affected in Sonai Rupai, Langting Mupa, Barail and Badsahitila forests during 2003. Whereas in 2010 more than 1% of the total forest area of Sonai Rupai, North Cachar and Barail was affected.

It was noteworthy that unlike flood, forest fire intensity was not in direct corroboration with disturbed area statistics (**Tables 1** and **3**). As discussed earlier, the point locations of forest fire were converted to fire intensity information using 1 km grid, to make it spatially contiguous, as the extent of forest fire information was not available. Hence, the discrepancy between the disturbed area statistics and fire frequency may be attributed to lack of spatial representation of fire extent. In addition frequent fire incidences might have hindered the process of re-generation and vegetative growth of the forest causing insignificant changes in the MGDI values in post incidence dates. For example, in case of Rifu forest the fire frequency was 52 and 36 during 2009 and 2011, respectively, but the disturbed area was 0.34 and 0.45% of the total forest area. On the other hand, 0.84% of the forest area was found to be disturbed during 2010 with only 11 fire incidences. Similar kind of observation was found in case of Badsahitila also, where the fire frequency was

**Figure 6.** *Temporal dynamics of disturbed area caused by river flood for two selected forests of Assam state.*


**Table 3.**

*Year-wise percentage disturbed forest area caused by forest fire for selected forests of Assam State.*

**Figure 7.**

more than 150 times during 2003, 2006, 2007, 2008, 2009 and 2010, but the % disturbed area were 3.73, 0.21, 0.28, 0.05, 1.39 and 0.69, respectively.

The temporal dynamics of disturbed area induced by the forest fire for two selected forests, namely Badsahitila and Sonai Rupai, were shown in **Figure 7**. In case of Badsahitila, the major disturbance due to forest fire was found during 2003, followed by the year 2009, whereas during 2001, 2002, 2005, 2006, 2007, 2008 and 2011 very less area was noted to be disturbed due to the fire incidences. On the other hand, in case of Sonai Rupai three major disturbances were noted during 2003, 2008 and 2010, with two other intermediate disturbances during 2005 and 2009. The major fire incidents were reported from moist deciduous forest and grass lands.

#### **5.3 Mapped forest areas prone to disturbance**

The spatial distribution of both the non-instantaneous and instantaneous forest disturbances maps have been generated and presented in **Figures 8** and **9** respectively. In non-instantaneous disturbance the effect is not triggered immediately in terms of changes in LST and/or EVI. In contrast post-event effect is immediately exhibited due to changes in LST and EVI, for example, in case of forest fire both the LST increases and the EVI changes drastically. Based upon the percent

*Assessment of Ecological Disturbance Caused by Flood and Fire in Assam Forests, India… DOI: http://dx.doi.org/10.5772/intechopen.94282*

**Figure 8.** *Spatial distribution of non-instantaneous disturbance categories.*

**Figure 9.** *Spatial distribution of instantaneous disturbance categories.*

change of MGDI, both the disturbance types are classified into low, medium and high category to spatially depict the disturbance regime. It was seen that most of the non-instantaneous disturbed area falling under "low" category, followed by "moderate". Patches of "highly disturbed" areas are observed in north of Tinsukia district which falls under the flood plain of Brahmaputra River. Scattered patches of "highly disturbed" areas also noticed along the northern boundary of the Assam state adjoining to Bhutan, Kokrajhar and at the border of Karbi Anglong (East) and Golaghat districts. Under instantaneous disturbance regime most of the forest areas are falling under "low" disturbance category presumably due to high threshold

value. Notably the forests of north Tinsukia district falls under "highly disturbed" category in both the instantaneous and non-instantaneous disturbance.
