Numerical modeling for the temporal variations of the water interchange between groundwater and surface water in a regional great lake (Poyang Lake, China)

•Variability of GW-SW exchange was dominantly affected by lake level and rainfall.•Monthly groundwater discharge is inversely related to lake level and rainfall.•GW-SW exchange increases while net groundwater discharge decreases in rainy year.•Groundwater is more likely to discharge into rivers during a rainy year. For areas possessing both regional-scale floodplain lakes and complex river system, the allocation pattern of water interchange between the groundwater and the lakes and rivers, as well as its temporal variations under year-scale climate changes, are important issues in regard to water resources and ecology environment but are rarely revealed. This study presented a numerical groundwater modeling of the Poyang Lake area to reveal the difference of the temporal variation patterns of the groundwater-lake water interchange and that of the groundwater-river water interchange, and to assess the potential effects of year-scale climate on the temporal-spatial variability of water interchange and on the allocation pattern of the net groundwater discharges into rivers and lake. It is found that the monthly groundwater discharge into surface water exhibits significant temporal variability, which reveals an inverse correlation between monthly groundwater discharge and lake water levels and precipitation amounts. Rainy months can lead to groundwater recharge from the Poyang Lake. Our simulated results reveal that, in the dry year of 2018, the variable monthly groundwater discharge into Poyang Lake and the monthly groundwater recharge from Poyang Lake were 0.97–9.67 × 108 m3/month and 0.07–2.54 × 108 m3/month, respectively. Additionally, the annual water interchange amount between groundwater and the Poyang Lake was 9.44 × 108 m3/year, and the annual net groundwater discharge into Poyang Lake was 6.76 × 108 m3/year. However, the hydraulic interaction between groundwater and the five rivers only features groundwater discharge into rivers with variable monthly groundwater discharge into the five rivers of 0.20–0.72 × 108 m3/month and an annual total groundwater discharge amount of 5.32 × 108 m3/year in 2018. Additionally, our water interchange results of the rainy year of 2010 indicate that the annual water interchange amount between groundwater and the Poyang Lake was 22.74 × 108 m3/year, the annual net groundwater discharge into the Poyang Lake was 1.26 × 108 m3/year, and the annual groundwater discharge into the five rivers was 8.10 × 108 m3/year. These com