Abstract
The study investigated the influence of Tropical cyclone (TCs) to the plant productivity indices along the coast of Tanzania using both field observations and change detection methods. These indices are normally designed to maximize the sensitivity of the vegetation characteristics and are very crucial in monitoring droughts intensity, yield and biomass amongst others. The study used three types of satellite imageries including the 16 days Moderate Resolution Imaging Spectroradiometer (MODIS) of 250 × 250 m resolution; 8 days Landsat 7 enhanced thematic mapper (ETM) with resolution of 30 × 30 m composites, and 5 Landsat 8 (LC8) images, to determine the patterns and the variability of the Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) and TCs impacts on vegetation. Moreover, we used Tropical Rainfall Measuring Mission (TRMM) data and the daily to monthly rainfall data from Tanzanian Meteorological Authority (TMA). The change detection between the pre and post storm (TCs) conditions was used to analyse inter annual variability of EVI over Chwaka, Rufiji and Pugu— Kazimzumbwi. The changes in NDVI and EVI and monthly rainfall at the coastal stations were calculated, plotted and analyzed. The results revealed that, highest EVI values over coastal Tanzania were observed during March and April, and minimum (low) values in November. The results for EVI changes based on pre and post storm conditions revealed that most observed stations and most TCs led to significant EVI changes which ranged from -0.05 to 0.19, and -0.3 to 0.22, for MODIS and L7 ETM data, respectively. As for the spatial changes in NDVI results revealed that, TCs (Besija and Fobane) were associated with positive NDVI changes i.e. (enhancement) of >0.51 and >0.31, and NDVI reduction (i.e. negative changes) of -0.19 for Chwaka and Rufiji, respectively. Besides the results revealed that, TCs episodes have induced a land cover changes from i.e. water covered areas changed to be vegetation covered especially over the shorelines and inter tidal areas. Indeed, these results were consistent with the analysis of rainfall patterns which indicated that low rainfall occurred in low NDVI areas and vice versa.
Highlights
Tropical cyclones and storms, like other large natural disturbances, play a vital role in regulating ecosystem structure and affecting diverse plant and animal populations and communities [1] [2]
The results revealed that, highest Enhanced Vegetation Index (EVI) values over coastal Tanzania were observed during March and April, and minimum values in November
The results for EVI changes based on pre and post storm conditions revealed that most observed stations and most TCs led to significant EVI changes which ranged from −0.05 to 0.19, and −0.3 to 0.22, for Moderate Resolution Imaging Spectroradiometer (MODIS) and L7 enhanced thematic mapper (ETM) data, respectively
Summary
Like other large natural disturbances, play a vital role in regulating ecosystem structure and affecting diverse plant and animal populations and communities [1] [2]. In most cases plant physiognomy and its spectral characteristics exhibit abrupt changes in response to atmospheric disturbances such as cyclones, tornadoes, tsunamis, drought, deforestation among others [3] [4] [5]. The change detection of pre and post event satellite images plays a vital role in assessing vegetation responses influenced by abrupt changes of weather caused by natural events such as heavy rainfall, tsunamis and TC events [5] [6] and [9]. The change in biomass and other related spectral responses varies based on type and intensity of disturbance, land cover type, and other environmental factors. There are various change detection techniques such as classification, principle component analysis of the classified images, images differencing, rationing among others [5], but no single method is best to all problems in all environment, each method is good to a specific problem and in a given environmental settings
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