Sweden - Lake Mälaren

Lake Mälaren, the study site in Sweden, is the third largest lake in Sweden with a surface area of 1140 km2 and a watershed area of 22 603 km2. It is one of the most important water supplies in the country providing drinking water to over 2 000 000 consumers. As part of MEWS project [https://mews-water.com/] we developed watershed hydrology and water quality models that can predict the inputs of water and nutrients to the lake that are associated with extreme hydroclimatic events under historical and future climate conditions. Here we provide an archive of the meteorological data used to force the GWLF model used here, and the measured discharge and nutrient data used to calibrate and verify the model.

Meteorological Data

The meteorological inputs to GWLF were daily sum precipitation (pr) and mean air temperature (tas). These data were obtained from the ISIMIP project [https://www.isimip.org/] For model testing the ISIMIP3a data set was used. Specifically the GSWP3-W5E5 reanalysis data were used [https://www.isimip.org/gettingstarted/input-data-bias-adjustment/details/110/]. These data are supplied on a 0.5 degree global grid. Data for the period between 1961 and 2019 within the geographic range of 58.5 - 60.5 N latitude and 14.0-18.5 E longitude were collected (Fig 1). Meteorological inputs for each of the watersheds in figure 1 were calculated as the weighted mean value of the grid cells intersecting the watershed boundaries with the weighting factors being the proportion of a grid cell that fell within the watershed area. A continuous daily data record between 1961-01-01 to 2019-12-31 was downloaded.

![[Monitored_sub-basins.png]] Figure 1 Lake Mälaren watershed sub-basins where discharge data are available. The grids lines show the ISIMIP3a grid that was used to calculate the weighted mean meteorological inputs to each sub-basin

Measured Discharge Data

Measured discharge data were used to calibrate the GWLF hydrologic model. These data were collected by the Swedish Meteorological and Hydrological Institute (SMHI), and were downloaded from https://www.smhi.se/data/hitta-data-for-en-plats/ladda-ner-observationer-fran-sjoar-och-vattendrag/waterdischargeDaily/90075

Not all stations in this data set have data that continues to present, some stations have been discontinued. However, all stations have at least 10 years of data in the 1961-2019 time period, and this was judged to be sufficient for model calibration. All stream discharge data are in units of cm/day These data are saved in excel spreadsheets that can be read by the GWLF model. In addition to the the measured total streamflow three other discharge values are calculated in the spreadsheet and used for calibration of the hydrologic model: runoff and baseflow calculated using the baseflow separation method of Arnold etal. (1995) and a 20th percentile value of the estimated baseflow.