Effect of groundwater use on groundwater quality, piezometric level and boro rice productivity in the Sundarbans of West Bengal.

Abstract

Agriculture in the Sundarbans of India is mainly mono-cropped with kharif rice (aman) grown in the monsoon season. Scarcity of fresh surface water for irrigation during the post-monsoon period is the main constraint on growing boro (post monsoon/dry season) rice and other crops. Therefore, there is increasing exploitation of groundwater through shallow tube wells (STWs). At present, STWs are used to irrigate about 32% of the total irrigated area of approximately 55,300 ha in the Sundarbans. However, there is concern about the sustainability of current rates of groundwater use and its con􀆟nued expansion. Therefore a series of surveys was conducted on the status of STWs in South 24 Parganas and North 24 Parganas districts from 2000 to 2014. The depth of STWs varied from 67 to 128 m below ground level (bgl). A study throughout 2003 showed that the piezometric pressure level was highest and salinity was least after the monsoon (November-December) and that piezometric level was least and salinity highest before the onset of the monsoon (May-June). The piezometric level and water quality recovered during the 2003 monsoon. Detailed study of STWs used for irrigating boro rice in 2003, 2009 and 2014 showed that the piezometric level decreased significantly from an average of 2.1 m bgl in January to 3.9 m bgl in May while the discharge rate decreased significantly from 10.8 to 7.2 ls-1. Over the same period, groundwater salinity increased significantly from an average of 2.0 to 3.7 dS m-1. Salinity (ECe) of the topsoil (0-15cm) in boro rice fields irrigated from STWs increased significantly from an average of 3.0 dS m-1 at sowing to 5.8 dS m-1 at harvest. The grain yield of boro rice increased significantly with increasing installation depth of STWs. STW depth accounted for 70% variation in grain yield and this was at least partly due to a significant decrease in water salinity with increasing STW depth. While the piezometric level and water quality recovered during the 2003 monsoon, whether this would be the case in more intensively irrigated areas is not known. The sustainable level of groundwater use needs to be determined across the region. Measures for more judicious use of groundwater are also needed to increase productivity of this precious resource.