Technical Note/ An Improved Approach for Assigning Pumping Rates to Heterogeneous Aquifer Models by Adan Pinales 1 , Adolfo Chavez 2 , Geronimo Llerar 3 , Luisa Manzanares 4 , and Arturo Keer 5 Abstract A common assumption of ground water models formulated using a block-centered finite-difference method is that a well is located at the center of a cell regardless of its actual location. Due to this assumption, errors are introduced in the spatial distribution of simulated heads. This paper presents an alternative approach for assigning the pumping rates of wells that are located off cell centers. This approach consists of assigning the pumping rate not only to the cell in which the well is located but also to adjacent cells, taking into account the length of the well screen, the hydraulic conductivity, and the distance from the well to the center of its cell. The advantage of this alternative approach over the conventional one is illustrated with a test problem of a synthetic aquifer. Statistical measures of error indicate a much better model fit when pumping rates of wells are distributed over several cells. Introduction Ground water modeling is now an essential tool in water resource management. As computer power becomes more readily available, more complex models are devel- oped in order to improve the accuracy of the simulation results. Most models are based on finite-element, differ- ence or volume techniques that involve spatial discretiza- tion of the simulation domain. The level of discretization has a strong effect on the accuracy of the results; however, a compromise must always be made between accuracy and speed/computer resource usage: the finer the mesh, the greater the accuracy, but the calculations take longer and require more computer resources. The accuracy of the simulation depends on the as- sumptions of the models. For instance, a common assump- tion is that a well is located at the center of a cell, regardless of its actual location. Therefore, a pumping well located in the corner of a cell will produce identical results as a pumping well located at any other place in the same cell. Pinales et al. (2003) showed that this conventional approach introduces errors in the spatial distribution of simulated heads, increasing with the distance from the well to the cell center and the cell dimension. These errors affect parameter estimation and model prediction. Pinales et al. (2003) proposed an area-based recharge/discharge rate distribution approach for assigning well discharge or recharge rates in block-centered finite-difference models. This alternative approach reduced errors in comparison with the conventional approach. In this paper, the area-based rate distribution (ABRD) approach proposed by Pinales et al. (2003) has been im- proved by taking into consideration the heterogeneous nature of aquifers. The paper briefly describes both the con- ventional approach, in which wells are all concentrated at the cell centers, and the original ABRD approach to highlight the improved method. Finally, a test problem involving a synthetic aquifer is used to show the level of improvement attained by this new method when 1 Centro de Investigacio ´n en Materiales Avanzados, S.C., Miguel de Cervantes 120, 31109 Chihuahua, Chih., Mexico. adan. pinales@cimav.edu.mx 2 Corresponding author: Facultad de Ingenieria, Universidad Autonoma de Chihuahua, Apdo. Postal 1528-C, 31160 Chihuahua, Chih., Mexico. achavezr@avantel.net 3 Facultad de Ingeneria, Universidad Autonoma de Chihuahua, Apdo. Postal 1528-C, 31160 Chihuahua, Chih. Mexico; gllerar@ uach.mx 4 Centro de Investigacio ´n en Materiales Avanzados S.C., Miguel de Cervantes 120, 31109 Chihuahua, Chih., Mexico; luisa. manzanares@cimav.edu.mx 5 Centro de Investigacio ´n en Materiales Avanzados, S.C., Miguel de Cervantes 120, 31109 Chihuahua, Chih., Mexico; arturo. keer@cimav.edu.mx Received December 2003, accepted June 2004. Copyright Ó 2005 National Ground Water Association. 274 Vol. 43, No. 2—GROUND WATER—March–April 2005 (pages 274–279)