Assessing climate variability and to develop adaptation strategies for coastal salinity management in Indian Sundarbans through land shaping and crop diversification

Abstract

Sundarbans in West Bengal, India by virtue of its strategic location in the Eastern coast of the Bay of Bengal falls in the most vulnerable zones of abrupt climate change. Analysis of the long period rainfall data (1966-2014) indicates that the region receives very high annual rainfall (1818.5 mm) which is concentrated only over a few monsoon months; most of the rain water goes waste as runoff and creates widespread water logging of the low-lying agricultural fields. On an average 84 rainy days in a year was recorded in the region, whereas during last ten years (2005-2014), the number of rainy days was reduced to 78.8 days/year. There was 2.7 times surplus rainfall than crop evapo-transpiration during monsoon months indicating very high scope of water harvesting to tackle water logging during the monsoon season and unavailability of fresh water for irrigation during lean season. The present study assessed the effects of different land shaping models i.e., farm pond (FP), deep furrow and high ridge (RF) and paddy cum fish (PCF) system for rain water harvesting in restoring the productivity of degraded coastal soils in Sundarbans during 2012, 2013 and 2014 with an annual rainfall of 1583 (normal), 2164 (excess) and 1368 mm (deficit), respectively. A water balance was run to estimate the soil moisture, crop evapotranspiration, runoff and water depth in the reservoir. On an average the amount of runoff harvested was 3273, 1387 and 952 m3 per hectare per year in FP, RF and PCF system. The amount of runoff going out of the system was 12.2, 23.6 and 25.5 % of the annual rainfall in FP, RF and PCF system whereas in monocrop rice-fallow system the runoff was 34.6% of the annual rainfall during these three years period (2012-2014). On an average annually 1717, 1042 and 791 m3 of harvested water was used for irrigation during lean period in FP, RF and PCF system. We estimated all the three components of water footprints (WF) i.e., blue WF (WFblue), green WF (WFgreen) and gray WF (WFgray) for washing excess salt accumulation as an aggregative indicator to evaluate environmental impact of each land shaping system along with dominant rice-fallow and rice-rice system. In FP system out of total WF of 808.5 m3t-1, WFgreen was 608.1, WFblue 103.1 and WFgray was 97.3 m3t-1, respectively, whereas in case of RF system total WF was 976.2 m3t-1out of which WFgreen, WFblue and WFgray was 783.7, 75.3 and 117.2 m3t-1, respectively and in PCF system total WF, WFgreen, WFblue and WFgray was 1029.2, 836.2, 63.3, and 129.7 m3t-1 respectively. In rice fallow and rice-rice system the total WF, WFgreen, WFblue and WFgray was 3644.8, 3113.6, nil and 531.2; and 1883.5, 974.2, 702.4 and 206.9, respectively indicating copious use of ground water in rice-rice system. Large scale adaptation of different land shaping models for rain water harvesting in farmers’ field increased the cropping intensity and net farm income in the region and there was reduction in salinity during summer and water logging during rainy season and overall improvement in soil quality.