Impact of land-use change on ecohydrological characteristics of Arial beel

Abstract

Land use change can have adverse impacts on aquatic habitats and biodiversity leading to change in socio-economic activities of a country. This study aims to analyze the impact of land use change on eco-hydrology of Arial beel - a depression spanning about 723 km2 between the Ganges and the Dhaleshwari rivers south of Dhaka, the capital of Bangladesh. The changes of land use were detected by studying six classified and two unclassified Landsat TM Satellite images of Arial Beel area. Analysis of the classified images from 1984 to 1999 revealed that the areas under crop production (consists of Boro and other rabi crops) remained relatively unchanged at 32% of the study area during this period. Grass production increased from 5% to 11% over the years when livestock farming become popular in the locality. The analysis of Landsat TM image of the study area for 1989 and 2010 together with data from BBS and ground truthing showed that alteration in cropping pattern (from traditional varieties to high yielding varieties) and reduction of net cultivable area are the most possible pathways of land use change in the study area between 1984 and 2010. As gradual shift of high yielding varieties of crops could lead to more intensive use of chemical fertilizers and pesticides thus degrading the quality of water, water quality was considered as the indicator for ecosystem function of Arial Beel in the current study. Chemical analysis of the water samples taken at two times, one set in premonsoon and another set in early monsoon, revealed most of the parameters, including dissolved oxygen, pH, ammonia and nitrogen were within the tolerable limits. Nitrogen concentration varied from 0.15 mg/l to 0.25 mg/l, which is within the acceptable limit (<0.3 mg/l) but phosphorus concentration ranged from 0.2 mg/l to 0.45 mg/l, which is significantly above the acceptable limit (<0.015 mg/l). The high level of phosphorous contribution is mainly attributed to extensive use of fertilizer and popularity of livestock farming in the locality. However, the concentration is likely to decrease with increased mixing of accumulated rainwater in depression areas and floodwater from the surrounding rivers. Nevertheless, the very high level of phosphorous concentration in the month of June is a concern for beel fisheries and other aquatic lives of the beel. Findings from the participatory field research and secondary literature review revealed that the fish availability of the beel has been affected and some native fish species are now endangered- substantially impacting the livelihood activities of the fishermen. Future studies need to be carried out to explore to what extent the high level of phosphorous proliferation will continue, and the likelihood of plankton or algal blooms in Arial Beel.