Preferential Flow and Soil Hydrology Dynamics in Wastewater Irrigated Land.

Soil moisture at six depths was measured every two minutes from 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. Preferential flow (PF) rates were determined from the sequences of soil moisture response at various depths for a total of 633 events, including 82 irrigation events. The overall averaged frequency 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 non-irrigated sites reached stable frequency values after 70 events, while the irrigated sites took as many as 105 events. A total of 22 parameters related to water input, response, and drainage were determined for each event, and correlations were calculated between each parameter and PF occurrence frequency. Irrigation was among the strongest correlations for individual sites (R = 0.193, p = 0.005 at the cropped site and R = 0.229, p = 0.001 at the forested site). Across all four sites, total water input (R = 0.313, p = 0.001) and peak water input intensity (R = 0.318, p = 0.001) were the most strongly correlated with PF. Soil horizonation played an important role in soil water storage and dynamics by restricting flow and causing perched water tables. These phenomena were more frequent at the irrigated sites, which received several times the volume of water inputs. Preferential flow was not only more likely to occur during irrigation events, but also during natural events at these irrigated sites, suggesting that changes have occurred in the irrigated soils after decades of spray irrigation.