Short noteAn areal recharge and discharge simulating method for MODFLOW
Introduction
As the most widely-used groundwater flow model in the world, MODFLOW can simulate external flow stresses such as wells, areal recharge, evapotranspiration, drains and rivers with a set of stress packages (Harbaugh et al., 2000, Harbaugh, 2005). According to the different characteristics, these packages can be classified into three categories, point, line and area features. The well (WEL) package can simulate specified recharge or discharge point feature such as wells. The general head boundary (GHB) package, the river (RIV) package and the drain (DRN) package usually represent line features of inflows and outflows to a groundwater system. Area features such as precipitation, plant transpiration and direct evaporation can be simulated with the recharge (RCH) package and the evapotranspiration (ET) package, respectively.
The area-feature packages do not provide a general method to process areally distributed recharge/discharge to groundwater, compared to the point-feature package and the line-feature package. The RCH package is designed to simulate areally distributed recharge, or discharge if negative recharge rates are specified. However, RCH does not allow for recharge or discharge to occur simultaneously at multiple depths in the same vertical column, because natural recharge enters the groundwater system only at its top (McDonald and Harbaugh, 1988). A general areal recharge and discharge (ARD) method can enhance the capability of MODFLOW to process inflows and outflows distributed areally, and facilitate the development of large groundwater models.
Areally distributed recharge from precipitation would be one of the most obvious sources of areal recharge or discharge in a groundwater flow model. However, there are other types of areal recharge and discharge when modeling regional groundwater system, especially large basins with developed agricultural pumping and irrigation systems. In a vast area of a regional groundwater system, even in a model discretized with a relatively high resolution, a single cell could still include an area of about 1–10 km2. Such an area generally contains at least one pumping well in most agricultural regions. Thus, each cell of the agricultural region, likely occupying great parts of the whole basin, will have at least one pumping well in the model. Therefore, the agricultural groundwater pumping in this situation could be considered as areally distributed discharge according to different rates. Likewise, irrigation infiltration return to the groundwater system could be considered as areally distributed recharge to the groundwater system.
Compared to the traditional method using the WEL package to simulate agricultural pumping discharge and using net recharge to represent precipitation recharge and irrigation return recharge, the ARD method is more convenient for the development and results analysis of models. Users do not need to take care of a large number of wells, setting different screened intervals and pumping rates series for each well in multiple layers. In addition, with the ARD method, it is easy to classify discharge into agricultural, industrial and domestic usage when calculating the water budget. Another method, the farm process package (Schmid et al., 2006, Schmid and Hanson, 2009), can dynamically simulate the integrated supply-and-demand components of irrigated agricultural and thus consider the relations between extractions and extra recharge. Given enough information of farm activities, the FMP can represent more complete hydrologic components. Since most agricultural areas do not have metered pumpage, even do not have the numbers and locations of pumping wells, groundwater pumpage are estimated indirectly from electrical power records or land-use maps. In this situation, the ARD method can be a simple and powerful approach.
Section snippets
Conceptualization and implementation
There are two options available for developing a general areal recharge and discharge tool for MODFLOW: programming a brand new package or using the existing MODFLOW capabilities of the RCH package. A new package is more elegant, but requires more development work and a new input instruction for users. The other solution is to make the best use of existing MODFLOW capabilities and develop an areal recharge and discharge tool based on the RCH package of MODFLOW. This is the best way to preserve
Application
In order to show the practical applicability of the ARD method, a case study is presented of regional groundwater flow in the Pinggu basin, Beijing, China. The study area is surrounded by mountains, including about 459 km2. Large groundwater reservoirs are present, consisting of mainly Quaternary-aged water-bearing formations. The basin, comprised of one unconfined and three confined aquifers according to the hydrogeological data, has a good groundwater supply capacity due to large precipitation
Summary and Conclusions
By reusing the code of the RCH package, a simple and powerful tool, the ARD method, has been provided for MODFLOW to simulate areally distributed recharge and discharge. The ARD method maintains the integrity of MODFLOW and its input follows the standard RCH convention.
In an application of a regional groundwater flow model in the Pinggu basin, it is demonstrated that the ARD method can efficiently simulate areally distributed precipitation recharge, multilayer pumping wells discharge and
Software Availability
The software may be requested via e-mail from the authors ([email protected], [email protected]).
Acknowledgments
We are grateful for review comments and suggestions from Dr. Richard B Winston, Dr. Willem Jan Zaadnoordijk and the anonymous reviewer that greatly improved the manuscript. This research was supported by the National Program on Key Basic Research Project of China (Grant no. 2010CB428801) and the National Natural Science Foundation of China (Grant no. 41072187).
References (7)
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- Harbaugh A.W. 2005. MODFLOW-2005, the U.S. Geological Survey Modular Ground-Water Model—The Ground-Water Flow Process....
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