Natural and anthropogenic controls on the frequency of preferential flow occurrence in a wastewater spray irrigation field

- Irrigation proved to be far more influential than land cover on preferential flow occurrence and flow patterns.
- Overall averaged frequencies of preferential flow at the non-irrigated sites were about 25%, while that at the irrigated sites were around 45%.
- Soils at the irrigated sites have been altered by decades of irrigation to cope with large volumes of water.
- Preferential flow rates during natural precipitation events at the irrigated sites were 58% higher than that at the non-irrigated sites.
- Of the 20 possible controls on preferential flow occurrence examined in this study, the most influential ones were related to water inputs, especially the total input and peak input intensity.

To quantify the frequency of preferential flow (PF) occurrence and its controls, and to examine the interactions of soils and irrigation on water flow dynamics, soil moisture was monitored at six depths every two minutes at four sites in a wastewater spray irrigation field from 2009 to 2011. Two irrigated sites - one cropped and another forested - were compared with two corresponding non-irrigated sites. Activation of preferential flow pathways was determined from the sequences of soil moisture responses at various depths for a total of 633 water input events, including 82 irrigation events. The results showed that the overall averaged frequencies of PF at the non-irrigated sites were 24.9% at the cropped site and 24.7% at the forested site. By contrast, the averaged frequencies at the irrigated sites were 47.1% at the cropped site and 45.0% at the forested site. The temporal stability of PF frequency was evaluated, showing that the overall frequency of PF stabilized after 70 events at non-irrigated sites, while the irrigated sites took as many as 105 events (24 of which were irrigations). Associations between 20 possible controls related to water input, moisture response, and season were quantified using the Student's t-test between each parameter and PF detection. The most influential factors were water inputs, especially the total input and peak input intensity. Antecedent soil moisture and response slope (maximum 2-min increase in moisture content during the response) were also significant in some but not all cases. Soil horizonation influenced soil water storage and dynamics by restricting flow and causing perched water tables. Irrigated sites experienced saturated conditions for as much as 38% of the monitoring time at some depths, compared to a maximum of 3% at non-irrigated sites. Preferential flow was not only more likely to occur during irrigation events, but also during natural events at the irrigated sites, suggesting that the irrigated soils have physically adapted to accommodate large volumes of water, after decades of spray irrigation. The results of this study have implications for understanding hydrology and contaminant fate in anthropogenically-altered landscapes as well as implementing sustainable management practices.