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Estimating Groundwater Recharge Into a Shallow Unconfined Aquifer in Bangladesh

Sajal Kumar Adhikary*
Department of Civil Engineering, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh

Tanmay Chaki
Water Resources Planning Division, Institute of Water Modelling (IWM), Dhaka, Bangladesh

Abstract/ Executive Summary:

This paper presents a data conservative approach, in which quantitative groundwater recharge estimation in a shallow unconfined aquifer is interpreted in detail by the analysis of observed precipitation and water level fluctuations records. Kushtia district in Bangladesh has been taken as a case study area based on the observed data and information. In the adopted state-of-the-art methodology used for this study, a well-known water table fluctuation technique has been modified so that groundwater recharge in the shallow aquifers can be estimated by using the least available data and information. Observed time series of precipitation and groundwater levels records at a few monitoring wells in the study area are the only data required to carry out the study. The approach illustrated in this paper can be useful in an initial level of assessment in groundwater studies. In addition, results can be applied as input data for developing a numerical groundwater model for any groundwater resource investigation in the study area or similar drainage basins in Bangladesh.

Keywords: Groundwater recharge, Kushtia, Precipitation, Unconfined aquifer, Water table fluctuation.

Location: Kushtia district in Bangladesh

Start Date:

End Date:

Program Area: Resource Development and Management

Commodity: Groundwater

Objectives:

This paper presents a data conservative modified water-table fluctuation approach, in which quantitative estimation of GW recharge in the shallow unconfined aquifer in Kushtia district of Bangladesh is interpreted by analyzing the precipitation and GW level data. It is expected that the methodology described in this paper can be applied as preliminary input parameters for developing and simulating a GW model in the numerical environment for any GW resource investigation scheme in the study area or similar catchments in Bangladesh

Methodology:

2.1 System Conceptualization and Mathematical Formulation Records of GW level fluctuations in monitoring wells are worth the cost and trouble of collecting only if they are used as a basis for hydrologic interpretations and for subsequent applications in hydrological investigations. Although water level records have been important to draw conclusions regarding the occurrence and development of GW in specific areas under concern, many such records still await interpretations. Similarly, a wealth of climatologic and other hydrologic data is in need of analysis before taking the final conclusions (Korkmaz, 1988). In the wet period of a water year, when the precipitation occurs, the first rain wets vegetation and accordingly other surfaces and then begins to infiltrate into the ground depending on the surface characteristics.

2.2 Estimation of Groundwater Recharge In this paper, a modified data conservative GW fluctuation technique is presented for estimating GW recharge in a shallow unconfined aquifer of Bangladesh based on the statistical relationship between the precipitation and water levels. The analysis is based on the original conditions, which are unaffected by heavy pumping. Observed monthly precipitation records and weekly GW levels are the only data required to carry out the study. Data have been collected from the central database of the Bangladesh Water Development Board (BWDB). For this study, daily rainfall data at a precipitation data collection station (BWDB rainfall station ID: CL41) and GW level fluctuation data at monitoring well (BWDB GW level monitoring station ID: GT5015004) are collected for the 16 years period from 1992 to 2007. However, these two representative monitoring stations located in the study area have been selected depending on the data availability for a longer period. A shallow aquifer located in the upper part of the Gorai River catchment (mainly the Kushtia district area) in Bangladesh has been selected as a case study area. The upper part of the aquifer in this region is unconfined in nature (Halcrow, 1993; Adhikary, 2009) and GW is extensively extracted from its shallow depth due to the increased irrigation activities.

By analyzing the monthly precipitation data and water level records belonging to the 16 years period from 1992 to 2007, the amount of rainfall causing a rise in water level is determined. These rainfall amounts are called “total precipitation (Rt)” causing the rise in water level. The water level rise caused by this total precipitation is termed as “water level rise due to GW recharge (H)” and can be found from the plotted graph directly. This procedure is repeated year after year for the period of total records of 16 years from 1992 to 2007 and the relationship between these two variables Rt and H is established by using simple statistical technique. However, H is a function of the precipitation amount above a threshold precipitation value, which represents the precipitation causing surface runoff, evapotranspiration, and subsurface flow or only one of them depending on the local hydrogeological conditions in the study area (Korkmaz, 1988). These values of water losses can be obtained by setting H = 0 in the regression equation, which has been schematically presented. The recharge amount for each water year is then calculated by subtracting the threshold precipitation from the total precipitation causing GW level rise in that particular year.

Source of Information: Journal of Engineering Science 04(1), 2013, 11-22

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

This study presents a modified water table fluctuation technique for estimating GW recharge by using only the precipitation and water level information for a shallow unconfined aquifer in Bangladesh. The overall goal of this study has been a synthesis of the results obtained by analyzing the GW level fluctuations and precipitation records. By following this principle, the study establishes a data conservative approach of estimating GW recharge in any shallow unconfined aquifer under concern. However, the accuracy of the results totally depends on the basis of water level monitoring data. This study reveals that the shallow GW in the study area is recharged mainly from the precipitation sources. Besides, the annual average GW recharge in the studied aquifer is found as 1413 mm for a total 16 years period from 1992 to 2007, which is about 74 percent of the average annual precipitation. However, the study concludes that the proposed data conservative methodology presented in this study provides appropriate mechanisms for indirect estimation of the water losses such as evapotranspiration and surface runoff, which will make this method useful in future hydrological computation for GW resource assessment and management in the field of water resources engineering. Finally, the present study conclusively proves that the proposed methodology described in this study will open a window of opportunity for any sort of initial level assessment of GW resources in a shallow unconfined aquifer in Bangladesh, where the shortest possible data and information are available.

Knowledge Management: Journal