FORECASTING SUMMER LOW-FLOWS IN WESTERN DENMARK AND THE SEASONAL MEMORY OF A HYDROGEOLOGICAL SYSTEM

Summer low-flows are critical for the ecosystems of groundwater fed rivers. Therefore permissions of groundwater extraction to irrigation are related to a maximum allowed reduction in summer minimum flows of 5-10 %, depending on the ecological state of the river. Predictions (forecasts) of the available groundwater for extraction without reaching the critical summer low-flows would be valuable for the farmer before the growing season in order to optimize the crop plans. In the present study, we evaluate the hydrological processes important for controlling the summer low-flows of a river system dominated by groundwater. For a six-month-ahead prognosis we test the influence of precipitation during the forecast period and the influence of initial groundwater conditions on the summer low flow. This is important for the perspectives of a seasonal forecast, hence, if precipitation dominates the occurrence of critical low flows, then the hydrological forecast has to entirely rely on the quality of the metrological long term prognoses (six month). In the study we apply a correlation analysis between pre-season groundwater heads, the time where the first forecast is intended to be, and summer flows. Furthermore we analyze the correlation between spring-summer precipitation and summer low-flows which consistently occurs in July, August or September. The analysis is based on results from a physically based distributed hydrological model (MIKE SHE), set up for a 1050 km² catchment in the western part of Denmark where sandy glacial deposits dominate the regional hydrogeology. From the study we illustrate that the level of groundwater head observed in the late winter defines the probability for critical low flows during the summer. The analyzed 22 year dataset furthermore illustrate a surprisingly low correlation between low flows and precipitation in the months leading up to the low flow period.