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On the spatial nature of the groundwater pumping externality
Resource and Energy Economics, 2010
Most existing economic analyses of optimal groundwater management use single-cell aquifer models, which assume that an aquifer responds uniformly and instantly to groundwater pumping. This paper demonstrates how spatially explicit aquifer response equations from the water resources engineering literature may be embedded in a general economic framework. Calibration of our theoretical model to published economic studies of specific aquifers demonstrates that, by averaging basin drawdown across the entire resource, existing studies generally understate the magnitude of the groundwater pumping externality relative to spatially explicit models. For the aquifers studied, the drawdown predicted by single-cell models may be orders of magnitude less than that predicted by a spatially explicit model, even at large distances from a pumping well. Our results suggest that single-cell models may be appropriate for analyses of the welfare effects of groundwater management policies either in small aquifers or in larger aquifers where average well spacings are tens of miles or more. However, in extensive aquifers where well spacings are on the order of a few miles or less, such as many of those of concern to groundwater managers and policy makers, use of single-cell models may result in misleading policy implications due to understatement of the magnitude and spatial nature of the groundwater externality.
Journal of African Earth Sciences, 2009
The fact that groundwater exploitation has largely increased since 1997 in the Dar-es-Salaam aquifer, calls for a directed attention towards possible problems of aquifer overexploitation that may arise in the near future. Hydraulic parameters are important for developing local and regional water plans as well as developing numerical groundwater flow models to predict the future availability of the water resource. The determination of aquifer parameters through pumping tests has become a standard step in the evaluation of groundwater resource potential. The pumping tests in the study area were conducted in August 2004 and August 2005, where 39 boreholes were tested out of 400 visited. In the study area there are over 1300 recorded boreholes drilled by Drilling and Dam Construction Agency (DDCA) by the year 2005. Total groundwater exploitation in the study area was estimated at 8.59 Â 10 6 m 3 /year, based on yield data collected during the 2004-2005 field campaigns. The pumping tests included singlewell tests and tests with measurements on the pumping well and at least one observation well. The tests were conducted for 6 h and 30 min. The pump was shut down after 6 h of pumping and the remaining 30 min were used for recovery measurements. The pumping test analysis methods used include: Neuman type curve matching and Walton type curve matching, checked by specific well capacity assessment and Thiem-Dupuit/Thiem's method. The curve-matching results from the aquifer tests show the following parameters: an average transmissivity and hydraulic conductivity of 34 m 2 /d and 1.58 m/d, respectively for the unconfined aquifer; the semi-confined aquifer has an average value of 63 m 2 /d and 2.14 m/d for transmissivity and hydraulic conductivity, respectively. For the case of the storativity, the unconfined aquifer has an average elastic early-time storativity of 0.01, while the lower aquifer has an average storativity of 3 Â 10 À4 . Specific well capacity method and Thiem-Dupuit/Thiem's method confirm results for transmissivity and hydraulic conductivity of the semi-confined aquifer, while values for the unconfined aquifer are somewhat larger (by a factor of 2-3). The hydraulic parameters calculated appear to reasonably agree with the geological formation of the aquifers, as deduced from borehole descriptions.
Land subsidence due to groundwater withdrawal combined with a global sea level rise creates a serious environmental problem in the coastal region. Groundwater withdrawal results in fluid pressure change in the layers. The pressure change in the layers induces both elastic and inelastic land compaction. The elastic compaction can be recovered if the water level rises again and inelastic compaction becomes permanent. Groundwater response to barometric pressure change is used to estimate the elastic compaction in this study. The storativity, specific storage and other layer and hydrological information are used to estimate the inelastic compaction of the layers due to fluid withdrawal. The discussed methods are applied to estimate and predict the subsidence potentials resulting from overdrafting of the groundwater in the southern New Jersey. The estimated subsidence is about 2-3 cm near the location of monitoring wells in Atlantic, Camden, Cumberland and Cape May Counties over the past 20 years. If the current trend of water-level drop continues, the average subsidence in southern New Jersey in the vicinity of some monitoring wells will be about 3 cm in the next 20 years. The rise of global sea level is about 2 mm/year on average. Because of the very gentle slope in southern NJ, the combination of subsidence and sea level rise will translate into a potentially substantial amount of land loss in the coastal region in each 20 year period. This combination will also accelerate the coastal flooding frequency and the erosion rate of the New Jersey coastal plain, and pose a serious threat to the coastal economy.
One dimensional land subsidence with variable total stress
The variation of total stress is observed in many cases where the overburden load decreases or increases due to changes on the ground surface or water table fluctuations. In this study, starting from three dimensional coupled equations of mass and equilibrium developed by and introducing elastic stress-strain relations with variable total stress, we obtain a (vertical) one-dimensional model for surface settlement due to water table lowering in a vertical soil column. The model follows a set of solid particles, F2 = 0, which at some initial time coincides with the phreatic surface, F-^ ~ °-Numerical results are presented graphically at the spatial and temporal points of interest for a value of the resistivity of the fixed semipervious lower boundary of the aquifer.