"Development and application of a modified Water-Table Fluctuation (WTF) method to estimate groundwater recharge"A large number of methods for estimating groundwater recharge have been presented. Many methods focus on hydrological processes on the land surface and in the unsaturated zone. Few methods consider changes in groundwater level directly. However, groundwater level changes are the only directly-measurable response to recharge. Therefore, to estimate recharge directly from groundwater level time series, should be straightforward and less sensitive to the influence of uncertain processes in the unsaturated zone. Within the framework of the GLOWA-Danube project (www.glowa-danube.de), a modified Water Table Fluctuation method (HEALY and COOK 2002) based on the analysis of groundwater level fluctuations for selected locations in the Upper Danube Catchment (UDC) has been applied to estimate groundwater recharge. This method uses a hypothetical groundwater level rise in response to recharge multiplied by the specific yield (Sy). Several previously published approaches for the calculation of the hypothetical groundwater level rise were compared with an enhanced approach (the Groundwater Level – Decline Relation Approach). This approach assumes that in the case of no recharge, a maximum possible decline for a given groundwater level exists. A groundwater level–decline relation is determined for each observation well to estimate hypothetical groundwater rises. The calculated recharge was compared with the percolation data calculated by the physically-based soil-water balance model PROMET of the GLOWA-Danube project (MAUSER and BACH 2009). PROMET calculates daily percolation for the whole UDC with a cell size of 1 km².
Results show that the recharge calculated from the Groundwater Level – Decline Relation Approach is strongly correlated with the percolation data from PROMET. Because the Groundwater Level – Decline Relation Approach is directly linked to the measured groundwater fluctuations, it can be used to validate the results of soil-water balance models. The calculated recharge from the approach developed in this study, when compared with the resulting recharge from other WTF-based approaches, shows that the method developed in this paper is much more sensitive to groundwater level changes.
Uncertainty in the calculated recharge can mainly be attributed to the limited possibilities for determining reliable specific yield values. It should be noted that the WTF Method is only applicable to shallow water tables with equivalent groundwater level time series taken on a daily or weekly basis.
Within the GLOWA-Danube project, several options for cross-validating recharge estimates are available and for a calibration of Sy. Using the assumption that percolation and recharge are similar for shallow, unconfined aquifers, Sy can be calculated by assuming recharge is known. It shows, for example, that observation wells in wetlands dominated by direct evaporation are not suitable for the WTF method. The same holds for wells close to rivers controlled by groundwater-surface water interactions. Another cross-validation option is to use the calculated recharge values within a regional groundwater flow model of the UDC (BARTHEL et al. 2005) and to compare results from this approach to the percolation results from PROMET. It can be shown that root zone percolation and recharge are not identical for larger depth to the groundwater.
BARTHEL, R., ROJANSCHI, V. et al. (2005): Large-scale water resources management within the framework of GLOWA-Danube. Part A: The groundwater model. Physics and Chemistry of the Earth, 30(6-7), 372-382
HEALY, R. & COOK, P. (2002): Using groundwater levels to estimate recharge, Hydrogeology Journal 10(1), 91-109.
MAUSER, W., BACH, H. (2009): PROMET – a Physical Hydrological Model to Study the Impact of Climate Change on the Water Flows of Medium Sized Mountain Watersheds. Journal of Hydrology, 376(3-4), 362-377