Simple method for determining available groundwater on remote islands
The Institute for Rural Engineering, NARO has developed a method to estimate the permeability coefficient which indicates the ease of passage of groundwater in geological formation by utilizing the change of the groundwater level due to the strength of natural tides. In this method, it is possible to estimate the permeability coefficient of the stratum at the distance where the tidal fluctuation is transmitted based on a single pair of groundwater level data, and to survey a wide range of strata from a few observation points. With this method, available amount of groundwater resources in remote islands including water for agricultural use can be easily determined.
Overview
- In remote islands such as the Nansei Islands and the Pacific Islands in which the grounds consist of water-permeable strata such as limestone, groundwater is an important water resource because no rivers are formed. In such areas, groundwater is the major source of freshwater, and proper use of groundwater is necessary to maintain and develop agriculture which is the main industry. Recently, it has been pointed out that sea water may enter the inland basement due to the rising sea level and groundwater resources may decrease. Therefore, countermeasures and technologies for proper utilization of groundwater resources is indispensable.
- The Institute for Rural Engineering, NARO has developed a method to easily estimate the "permeability coefficient" based on cyclical changes of the groundwater level due to fluctuation in tide level in the coastal areas, and data derived from the continuous observation of groundwater level. The permeability coefficient, which indicates the ease of passage of groundwater, is an index necessary for determining an appropriate pumping rate in well that does not draw sea water inside the well, as it affects the mixing of sea water and fresh water entering underground, and the speed of groundwater flowing in the strata.
- So far, in estimating the hydraulic conductivity, a "pumping test" is commonly used to analyze how the groundwater level dropped by pumping groundwater to the ground with a pump installed in a test well. However, in order to investigate a wide range of areas, it is necessary to conduct the test at several points, which is expensive as it requires installation of test wells, transportation of equipment and materials such as large capacity pumps, and securing workers and engineers especially in remote islands.
- The method developed this time uses data obtained by observation of groundwater level for 40 days by installing commercially available automatic observation instruments at two places near the coast and distant places, and estimating the average permeability coefficient of the stratum between these points. The permeability coefficient can be easily calculated using a common spreadsheet software.
- With this method, it is possible to reduce the cost of surveying the properties of a wide range of strata and helps when arranging a new well in a remote island area and trying to increase the use of groundwater throughout the area, or conversely trying to adjust the amount of usage so that the groundwater resources are not exhausted.
References
Shirahata et al. 2016. Digital filters to eliminate or separate tidal components in groundwater observation time-series data. JARQ 50(3):241-252
Shirahata et al. 2017. Improvements in a simple harmonic analysis of groundwater time series based on error analysis on simulated data of specified lengths. Paddy and Water Environment 15(1):19-36
For Inquiries
Dr. Tokuji YAMAMOTO
Director-General, Institute for Rural Engineering, NARO
Mr. Katsushi SHIRAHATA
Senior Researcher, Groundwater Resources Unit
Division of Facilities and Geotechnical Engineering, Institute for Rural Engineering, NARO?
Dr. Kazuko ENDO
Communications Planner, Institute for Rural Engineering, NARO
Contact:
http://www.naro.affrc.go.jp/english/inquiry/index.html
