Sequential coupling of HBV-light and MODFLOW models to assess water table variations under the future climate in agricultural peatlands

Abstract

Understanding water table (WT) variation is essential in peatland agriculture as the WT position is a major determinant of cultivation conditions and greenhouse gas emissions. To simulate past and future WT, we developed a sequential modelling approach by coupling two well-known models: HBV-light for cold climate hydrologic processes and MODFLOW for saturated subsurface flow conditions. We tested this approach in an agricultural peatland on Norway's west coast under two drainage configurations: a traditional pipe-drained field and an adjacent inverted peatland with a complex drainage system. In peat inversion, mineral soil excavated from below the peat is subsequently placed on top of the peat to form a protective cover layer aimed at reducing peat decomposition. Our modelling approach captured the daily WT dynamics of the fields between July 2022 and January 2025. The modelling results indicate that lateral flow dominates over the vertical flow in both drainage systems. Under future climate conditions between 2025 and 2100, the buried peat in the inverted peatland is expected to remain saturated for 25% of the time, whereas the peat in the pipe-drained field will be waterlogged rarely. Our modelling approach to simulate WT in managed peatlands can be set up relatively easily.