Integration of unreported irrigation withdrawals in time-series models of groundwater levels (Seewinkel, Austria)

Groundwater monitoring networks are often particularly well established in areas where groundwater resources are exploited. Groundwater withdrawals in such areas potentially affect the observed groundwater levels. Models that are aimed at simulating groundwater levels therefore need to account for temporarily varying groundwater abstraction rates. Unfortunately, such information is rarely available, particularly in agricultural areas where multiple users extract groundwater for irrigation, depending on the seasonally varying crop water requirements. The objective of this work is to develop and test an approach for incorporating unreported irrigation withdrawals into data-driven time-series models of groundwater levels. To this end, irrigation is integrated into a lumped-parameter model coupling a root-zone model to a linear storage representing the groundwater body. While the irrigation demand is determined from the root-zone model, the groundwater abstraction needed to meet this demand is considered in the linear storage. The model is tested for the case of the shallow Seewinkel aquifer (Austria), which is almost exclusively used for irrigation. The model calibration yields a time-varying rate of groundwater abstraction. When compared with available irrigation estimates, the average groundwater abstraction obtained from the model is reasonable. The model suggests that the depletion of groundwater levels resulting from the groundwater abstraction for irrigation varies depending on the hydrological conditions. For example, an observation well where the depletion amounted to around one metre in wet years exhibited a depletion of around two metres in the years following the European drought 2003.