Soil water, ground water, and wetland seepage within an effluent-irrigated hillslope

Treated municipal wastewater from Rotorua, New Zealand, has been spray irrigated onto the soils of nearby Whakarewarewa forest at rates averaging 70 mm/week since 1991. Herein we describe soil water, ground water and wetland seepage responses to irrigation within a hillslope at this land treatment system, which was instrumented with small weirs to monitor seepage flows, and a grid of piezometers to monitor ground water response. Time domain reflectometry and tensiometers were used to monitor soil-water dynamic of an irrigated profile. Ground water levels and seepage discharge increased after onset of irrigation and peaked after about 30 hours. Seepage discharge was commonly more than 1 L/s, and, after winter irrigations, peaked at about 3 L/s. Peak seepage discharge could be predicted from antecedent profile water storage and irrigation loading, suggesting that irrigation could be managed to control seepage flows. Ground water responses were affected by layering of the volcanic soils and proximity to sprinklers. A gravel lapilli deposit appeared to be a preferential pathway for unsaturated movement of water downslope. Soil water dynamics were dominated by gravity drainage, and soils were usually at water potentials greater than field capacity. Upward movement of soil water from 0.52 m depth in response to evaporative demand was found only during summer and at least 3-4 days after irrigation. While the volcanic soils at this site can accept heavy irrigation loadings, the additional water influences soil water dynamics and hillslope seepage in ways that may be important to performance of the system.