Decomposition technique for contributions to groundwater heads from inside and outside of an arbitrary boundary: application to Guantao County, North China Plain

Abstract

Abstract. To assess the efficiency of the groundwater management of an administrative unit, we propose decomposing the groundwater head changes within an administrative unit into inside and outside contributions by using numerical models. Guantao County of Hebei Province, China, serves as an example to demonstrate the decomposition technique. The groundwater flow model of Guantao was constructed using observed heads as prescribed head boundary conditions. The model was coupled with Hydrus 1-D to calculate the groundwater recharge distribution in time reflecting the delay and damping effects of the soil column on seepage at the surface. The model was calibrated by adjusting parameters such as hydraulic conductivities, recharge infiltration ratios and specific yields. The calibrated parameters are then used in a large model with a boundary at a large distance from Guantao administrative boundary to determine the groundwater head changes due to inside drivers. The differences of the two models on the Guantao boundary serve as the specified head values on the boundary for a small-scale model, which is used to calculate the groundwater head imposed by outside drivers. To eliminate inconsistencies caused by the different types of boundary conditions of large and small models, the groundwater head changes due to inside drivers must be updated. The results indicate that the groundwater head changes in the center and south of Guantao County are influenced equally by both inside and outside contributions, while in the north outside contributions have the stronger impact. On average, 48.5 % of groundwater head changes in the whole of Guantao County is influenced by inside contributions, while 51.5 % is due to outside contributions. The sensitivity analysis shows that the groundwater head changes and their decomposition are much more sensitive to infiltration ratios than to the aquifer parameters. The parameters within Guantao have a certain influence on the net groundwater head changes, while the parameters outside of Guantao only have an influence on the decomposition.