Impact of groundwater abstraction on physical habitat of brown trout (Salmo trutta) in a small Danish stream

The purpose of this study was to assess the impact of groundwater abstraction on stream discharge and physical habitat conditions for brown trout ( Salmo trutta ) in a small Danish stream. Stream discharge was simulated using a lumped hydrological model (NAM) and a scenario was set up for stream discharge reference conditions. Stream physical habitat conditions (WUA) were simulated for four life stages of trout using a hydraulic habitat model (RHYHABSIM). The impact of groundwater abstraction on WUA for trout was assessed by combined simulations from the NAM model and the RHYHABSIM model. The model predicted that groundwater abstraction reduced median annual discharge by 37% and mean annual 90th percentile discharge by 82%. Summer discharge was relatively most affected by groundwater abstraction and WUA was therefore particularly affected by groundwater abstraction during summer. WUA for adult trout was mainly controlled by suitable water depths (>40 cm) even under conditions without abstraction. On an annual basis WUA for fry and juvenile trout was most affected by abstraction. Future modelling should consider improving simulation of low discharges and preferably not use general hydrological models.