Soil Moisture Measurements and Modeling at a Geomorphically Reclaimed Coal Mine in New Mexico
Geomorphic reclamation is a relatively new technique in which landforms are built to be similar to those that eventually form in nature given the particular conditions of a location, making them self-sustaining and functional. The near surface water balance will have a large impact on the establishment and health of vegetation which in turn is important for maintaining a stable landform. This study developed a one-dimensional water balance model for a geomorphically reclaimed site and used the model to investigate seed germination and plant survival for a range of slope aspects and climate conditions. Disturbed and undisturbed samples of reclaimed soil were collected for laboratory tests from the mine site located in northwest New Mexico. Saturated and unsaturated hydraulic properties were determined from falling head tests, hanging column tests, pressure plate tests, and dew point potentiometer measurements. In addition, hydraulic properties were estimated from field tension infiltrometer tests. Laboratory and field tests accounted for the variable soil density in the near surface. Periodic soil moisture measurements at the reclaimed site were made from August 2012 to March 2014. Simulated and measured water contents were compared by statistical analyses to validate the numerical model, and indicated that a reasonable comparison was attained between predicted and observed soil water contents. The water balance simulations revealed that it is necessary to consider near surface variability in soil hydraulic properties to produce reasonable predictions of soil moisture. The numerical model was then used to predict soil water potential and temperature for different climates (one wet year and one dry year) and aspect conditions. A population based hydrothermal time model was used to predict the seed response to environmental condition available at soil surface. Germination potential of a native plant bottlebrush squirreltail (Elymus elymoides), and an invasive species cheatgrass (Bromus tectorum) was determined in terms of cumulative progress towards germination (PTG). Survivability of existing plant blue grama grass (Bouteloua gracilis) and cheatgrass was also predicted by comparing soil water potential at different depths with the wilting points of the respective plants. The results show that establishment of vegetation in geomorphically reclaimed soil is dependent on climate and aspect conditions. The study of germination potential indicates that for a range of climates studied, cheatgrass has higher germination potential than bottlebrush squirreltail. Germination potential and survivability is greater on the northern aspect than the southern aspect. Wet conditions provide favorable conditions for germination and survival for all species. Potential for invasion by cheatgrass is greater on the northern aspect in dry climate conditions when bottlebrush squirreltail is seeded on reclaimed soil. This study reveals that the method of placement of soil and watershed design followed in geomorphic reclamation provide favorable environment for vegetation during the wet climate conditions. The dependence of plant germination and survivability on climate and aspect in this study suggest that geomorphic reclamation - which results in numerous small watersheds with slopes with varying aspects - may result in a healthier overall ecosystem compared to the larger, more uniform aspects associated with traditional reclamation. Comparing the predictions of seed germination and plant survival with long term field measurements and observation of vegetation cover is suggested to validate the approach described in this study.
Geomorphic reclamation, HYDRUS 1D, Soil moisture modeling, soil hydraulic properties, tension infiltrometer, germination potential, plant survival
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