DRAINMOD Simulation of macropore flow at subsurface drained agricultural fields: Model modification and field testing

[Display omitted] •DRAINMOD model was modified to simulate macropore flow using an empirical approach.•Modeling macropore flow improved daily drainage prediction for a drained field with cracking soil.•Predicted macropore flow contributed about 15% of annual subsurface drainage.•Modeling macropore flow is key for modeling phosphorus dynamics in drained cropland. Macropores are critical pathways through which water and pollutants can bypass the soil matrix and be rapidly transported to subsurface drains and freshwater bodies. We modified the DRAINMOD model to simulate macropore flow using a simple approach as part of developing the DRAINMOD-P model to simulate phosphorus dynamics in artificially drained agricultural lands. The Hagen-Poiseuille’s law was used to estimate the flow capacity of macropores. When ponding depths on the soil surface are greater than Kirkham’s depth, water is assumed to flow through macropores directly to tile drains without interaction with the soil matrix. In the modified model, macropore size is adjusted based on wet or dry conditions while connectivity is altered by tillage. The model was tested using a 4-year data set from a subsurface drained field in northwest Ohio. The soils at the field are classified as very poorly drained and are prone to desiccation cracking. The modified model predicted the daily and monthly subsurface drainage with average Nash-Sutcliffe efficiency (NSE) values of 0.48 and 0.59, respectively. The cumulative drainage over the 4-year simulation period was under-predicted by 8%. The new macropore component was able to capture about 75% of 60 peak drainage flow events. However, surface runoff was over-predicted for the entire study period. Annual water budgets using measured data (precipitation, subsurface drainage, and surface runoff) and model predictions (evapotranspiration, vertical seepage, and change in storage) were not balanced with an average annual imbalance of 6.4 cm. The lack of closure in the water balance suggests that errors may have occurred in field measurements, particularly, surface runoff. Overall, incorporating macropore flow into DRAINMOD improved predictions of daily drainage peaks and enabled the model to predict subsurface drainage flux contributed by macropore flow, which is critical for expanding DRAINMOD to simulate phosphorus transport in subsurface drained agricultural land.