Soil hydrology at the Living Filter: Analysis of preferential flow and moisture-temperature dynamics under wastewater irrigation
Open Access
- Author:
- Hopkins, Isaac E
- Graduate Program:
- Soil Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- January 29, 2016
- Committee Members:
- Hangsheng Lin, Thesis Advisor/Co-Advisor
- Keywords:
- VWC
soil temperature
preferential flow
thermal
diffusivity
living filter
soil hydrology
hydrology
soil physics
soil - Abstract:
- Soils are very complex systems with emergent properties which are inherently difficult to directly observe without destroying the object of study. Recent technological progress has enabled field monitoring of soils that can achieve unprecedented duration, temporal precision, and accuracy. Soil moisture lies at the heart of many soil dynamics, so this thesis explores two relationships involving soil moisture. First, it uses preferential flow patterns to examine the interplay between water movement and soil structure. Second, it looks at aspects of soil climate (soil moisture and soil temperature) on two time-scales: trends over several years and daily dynamics. To quantify the rate of occurrence of preferential flow (PF), and its controls, and to examine the influence of land cover and irrigation on water flow dynamics, soil moisture was monitored at six depths every two minutes from four sites in or near a wastewater spray irrigation field from 2009 through 2011. Two irrigated sites – one cropped and another forested - were compared with two corresponding non-irrigated sites. The activation of preferential flow pathways was determined from the sequences of soil moisture responses at various depths, with a total of 633 events across four sites, including 82 irrigation events. The results showed that the overall averaged rates of occurrence 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 rates at the irrigated sites were 47.1% at the cropped site and 45.0% at the forested site. The temporal stability of PF occurrence was evaluated, showing that the overall rate of occurrence 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 controlling parameters related to water input, moisture response, and season were quantified using the Student’s t-tests between each parameter and PF detection. The most influential factors were water inputs, specially the total input and peak input intensity, with three of the four sites indicating these as strong (p < 0.05) controls over PF occurrence. Antecedent soil moisture and response slope (Maximum 2-minute increase in moisture content during the response) were also significant at some depths at some sites. Soil horizonation influenced soil water storage and dynamics by restricting flow and causing perched water tables. Irrigated sites experienced satiated 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 these irrigated sites, suggesting that the irrigated soils have physically adapted to accommodate large volumes of water, as a result of 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. Soil moisture and soil temperature have a complex relationship that has been rarely studied explicitly in field conditions over long periods of time. Soil moisture data was accompanied by soil temperature data collected from six depths from 5 cm to 100 cm over 4.5 years (2008 through 2012), allowing us to study the effects of land cover and wastewater irrigation on soil moisture, soil temperature, and their relationship, at four sites at Pennsylvania State University’s “Living Filter.” Exploration of long-term trends showed negative correlations (linear R2 as high as 0.92 at 5 cm) between soil moisture and temperature at most depths and sites. Land cover effects were largely overshadowed by the strong influence of irrigation, despite clear effects of forest cover on temperature. Soil moisture and temperature experienced their greatest variability in alternating seasons (temperature in Spring/Fall, soil moisture in Winter/Summer), highlighting the complexity of their relationship. The long-term correlations between soil moisture and temperature were dependent on season, with winter and spring often causing small positive correlations. Daily soil thermal parameters, damping depth and thermal diffusivity, were calculated for each probe (5 cm to 40 cm), and depth-dependent damping depth curves were proposed to improve on the simple profile-wide damping depth parameter used in most modeling. Daily thermal diffusivity values were compared to mean daily soil moisture, but the expected non-linear relationship was not found. Finally, a single zoomed plot was used to illustrate soil water acting as a thermal buffer.