Spatio-temporal changes of LST
This study examined the LST from 2001 to 2021 (Fig 3) using MODIS images. This spatio-temporal analysis indicated a concerning rise in very high temperatures, which increased by 7.16°C over two decades. In 2001, zones of very high temperatures were predominantly found in the southern regions of the Golaghat district and parts of Lakhimpur district. High-temperature zones were also identified in the northern and north-western sectors of Lakhimpur and the western and north-western portions of Dhemaji district. By 2021, however, the thermal landscape had undergone substantial changes. The very high temperature zone had expanded further into the southern segment of the valley and clusters of high temperatures were also detected in the northern, central and eastern regions.
The percentile spatio-temporal changes of maximum and minimum temperatures of the valley were analyzed in Table 1. It was found that the maximum and minimum very high temperatures increased by 19.38% and 9.03% respectively, in two decades. These may result from increasing population (as according to the last census of India, 2011, the decadal growth of population of all the districts of the study area was 96.08% from 2001 to 2011), gradual expansion of concrete surface, global warming and decrease of vegetation.
Spatio-temporal changes of LST were assessed based on changes in maximum and minimum recorded temperature of each LST zone.
Changes in vegetation
The presence of dense vegetation plays a crucial role in significantly reducing the temperature of a region over long term. However, climatic changes, particularly fluctuations in temperature, can extremely impact the health and life of plant. In this study, NDVI was applied to evaluate the changes in vegetation in response to varying temperature conditions. Observations from the NDVI maps of 2001 and 2021 (Fig 4) indicated that the alterations in the region’s temperature were reflected in the vigor and distribution of the vegetation. As temperatures shifted, the resilience and expansion of plant life were directly affected, creating a dynamic relationship between climate and ecological health.
The mean values of each NDVI class to analyse the changes in NDVI across all classes from 2001 to 2021 is presented in Fig 5. It was revealed that there was a negative change in the high and very high NDVI classes from 2001 to 2021.
Variations in crop water stress
Soil moisture is intrinsically linked to surface temperature. Areas characterized by optimal soil moisture levels experience reduced crop water stress. In contrast, soils with insufficient moisture contribute to heightened crop water stress, subsequently diminishing agricultural productivity and increasing dependency on irrigation systems. Spatio-temporal CWSI was applied in this study (Fig 6), revealing that the regions subjected to high temperatures exhibited increased water stress. In contrast, areas with significant water bodies and dense vegetation demonstrated less CWS.
Interrelationship between LST, NDVI and crop water stress
The relationship among two or more variables can be comprehensively understood by applying statistical correlation techniques. Pearson’s correlation coefficient (r) was utilized to explore the relationship between LST, NDVI and CWSI. The value of the Pearson correlation coefficient can range from +1 to -1. A value approaching +1 signifies a strong positive correlation, 0 denotes no significant relationship, while -1 indicates a strong negative correlation.
The correlation coefficient (r) between LST and NDVI of 2001 and 2021 (Fig 7) was calculated as -0.87 and -0.89, respectively. This highly negative value suggested a strong inverse relationship between surface temperature and vegetation health, meaning that as surface temperatures rise, the vegetation index tends to decline. On the other hand, the correlation coefficient (r) between LST and CWSI of 2001 and 2021 (Fig 7) was found to be 0.99 for both years, revealing a very strong positive correlation. This indicated that increases in LST are closely linked to increases in the CWSI, suggesting that higher temperatures are associated with greater water stress in soil and directly with associated crops.
Identification of hot spots
The areas having high temperature with low vegetation and high-water stress were identified as a hotspot. The LST maps of the valley easily portrayed the areas that are experiencing high temperatures. NDVI maps helped to identify areas with low vegetation. Moreover, the areas experiencing high water stress were easily identifiable through maps of CWSI. To delineate the hotspot areas, one condition was applied using the raster calculator in ArcGIS as follows:
Con ((raster layer of ‘LST’ > ‘upper limit of moderate class of LST’) and (raster layer of ‘NDVI’ < ‘upper limit of moderate class of NDVI’) and (raster layer of ‘CWSI’ > ‘upper limit of moderate class of CWSI’), 1, 0).
The resultant raster layer represented two classes. One class (represented as 1) had all the areas that were experiencing high temperature, low vegetation and high water stress conditions. On the other hand, another class consisted of all the areas which were not fulfill the above condition (represented as 0). The hotspot areas were represented in Fig 8 for 2001 and 2021.
Spatio-temporal maps of the hotspots revealed that the total area coverage of hotspots increased by 67.33% from 2001 to 2021.
Proposed strategies for sustainable land management
LST assessment revealed that the valley had experienced an increase in temperature with high zonal variability. This may be because of irregular climate; global warming is unevenly spread over all areas. Regions with low temperature and high vegetation are converting into environmentally sensitive areas due to land-use changes, deforestation and rapid urbanization. These regions are becoming hotspots having high temperature, low vegetation and high water stress. These variations seek zone-specific land management planning and mitigation approaches. Strategies for sustainable land management specifically for agricultural practices in this region need adaptation of soil retention techniques that can minimize the problems associated with high temperature, low vegetation and high CWS. Such management strategies will be beneficial for the hotspot areas, as this is a dominant agrarian region. Some proposed strategies for this region are as follows:
· Cultivation along contours (contour tilling) helps to retain soil moisture by minimizing surface water runoff and providing more infiltration time to the soil for water (He
et al.,
2017). This technique can be applied in the hotspot areas along long contours to retain soil moisture.
· Mulching technique is another technique that helps to retain organic content and moisture
(Kader et al., 2019). This method can be applied to improve the fertility of the hotspot areas in the valley. It can enhance fertility by using a mix of organic and inorganic materials, which helps suppress weeds, reduce evaporation and control soil temperature.
· Shade net is a popular agricultural technique that creates a micro-environment by reducing wind speed and increasing air-moisture. It regulates transpiration and air temperature, offering an economic method for enhancing agro-climates in hotspot areas of the valley.