Using jointly calibrated fine-scale drain models across Denmark to assess the influence of physical variables on spatial drain flow patterns

Study region:
Denmark 

Study focus:
Tile drainage, widespread across agricultural areas in Denmark, significantly impacts the hydrological cycle. Tile drain flow dynamics and their spatial patterns are crucial for water managers to address water quality and quantity issues. However, these processes are challenging to simulate accurately. In this study, we developed 10 m resolution drain models in MIKE SHE for 26 drain sites across Denmark to predict drain flow spatial and temporal dynamics. Joint calibration of all drain models was conducted by evaluating PBIAS and KGE of simulated and observed drain flow. Subsequently, we performed a correlation analysis between physical parameters and spatial patterns of simulated drain fraction (DF, ratio of drain flow to recharge per grid cell). 

New hydrological insights:
The jointly calibrated models achieved average PBIAS and KGE of − 6.7% and 0.53 respectively, for drain flow predictions across 26 drain sites. The correlation of DF with topographical variables was highest on a national scale and on most of the drain sites, including Gyldenholm1–4, Norsminde1–11, and other Jylland drain sites. Lillebæk drain sites showed a high correlation with average clay content in 0–30 cm layer of soil. Thus, in addition to developing scalable fine-resolution drain models for Denmark, this study also identifies the control parameters for spatial patterns of drain flows across Denmark.