Spatial and temporal variability in discharge and nitrate in Iowa subsurface drains

Area of Science:

• Agricultural hydrology
• Water quality management
• Environmental science

Background:

• Agricultural subsurface drainage systems are crucial for managing soil water but can transport significant nitrate loads to surface waters.
• Understanding nitrate export variability is essential for effective water quality management in agricultural watersheds.
• The South Fork Iowa River watershed is characterized by intensive agriculture and extensive subsurface drainage networks.

Purpose of the Study:

• To investigate the spatial and temporal variability of nitrate concentrations and loads from agricultural subsurface drains into a small Iowa stream.
• To identify the contribution of different subsurface drain sizes to overall nitrate export.
• To determine the influence of precipitation patterns on nitrate delivery via subsurface drainage.

Main Methods:

• Sampling of 93 subsurface drains up to 5 times between 2006-2008.
• Intensive monitoring of 2 subsurface drains and the receiving stream during growing seasons (2007-2008).
• Analysis of nitrate concentrations and water discharge in relation to drain size, spatial location, and precipitation history.

Main Results:

• No distinct spatial patterns in nitrate concentrations were observed across the watershed.
• Median nitrate concentrations did not significantly differ among various subsurface drain diameters.
• Eight large subsurface drains contributed 70-87% of total water and nitrate loads, exhibiting less concentration variability than smaller drains.
• Medium-sized drains became more significant contributors during high-discharge events.

Conclusions:

• Nitrate delivery to streams from agricultural subsurface drainage in this landscape is spatially homogeneous within a season.
• Large-diameter public drainage networks are dominant pathways for nitrate export.
• Precipitation history is a key driver of temporal variations in nitrate discharge and loads.