|NOUWAKPO, SAYJRO - University Of Reno|
|SANDERSON, MATT - Retired ARS Employee|
Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2018
Publication Date: 7/1/2019
Citation: Nouwakpo, S., Toledo, D.N., Sanderson, M., Weltz, M.A. 2019. Understanding the effects of grazing and prescribed fire on hydrology of kentucky bluegrass dominated rangelands in the Northern Great Plains. Journal of Soil and Water Conservation. Vol 74:4. https://doi.org/10.2489/jswc.74.4.360.
Interpretive Summary: Kentucky bluegrass is in over 85% of the areas sampled in the U.S. northern Great Plains. This grass can develop a dense thatch layer and root mat near the soil surface. This can affect how water infiltrates and runs off of a site. We used rainfall simulators and water droplet infiltration tests to determine whether Kentucky bluegrass affected water infiltration and runoff in Kentucky bluegrass dominated areas. Rainfall simulation on dry soils showed that runoff occurred sooner, and more runoff occurred as the amount of Kentucky bluegrass increased. When dry, Kentucky bluegrass litter tends to repel water , and is more repellant than thatch, root mat or mineral soils. However, bluegrass litter is less water repellant after it has been wetted. Results are useful to ranchers and grassland managers in making decisions to reduce accumulation of Kentucky bluegrass litter to make better use of rainfall.
Technical Abstract: According to National Resource Inventory data, Kentucky bluegrass is now present in over 85% of the areas sampled in the US. This invasive, perennial, cool season grass can serve to stabilize soils and increase site stability; however, it also alters nutrient flows, soil structure, and plant community composition ultimately degrading biotic integrity. In addition, Kentucky bluegrass alters the hydrologic function of an area by changing root structure and the way in which water flow is captured and released back into the ecosystem. To clarify the effect of Kentucky bluegrass on hydrological characteristics of invaded sites, rainfall simulation experiments and hydrophobicity measurements with water drop infiltration time and molarity of ethanol droplet tests were conducted at three locations all within the same ecological site in the northern Great Plains. Rainfall simulation experiments were performed on 24 large plots (6 m x 2 m) at 63.5 mm/hr and 127 mm/hr intensities and on 16 small plots of size 0.7 m x 0.7 m at 63.5 mm/hr and 103 mm/hr. Rainfall was maintained on the large plots until 10 minutes of steady-state runoff was measured or for a maximum of 30 minutes in the absence of runoff while on the small plots, rainfall duration was set at 25 minutes. The soil layer was divided into 4 strata (litter, thatch, root mat and mineral soil) which were physically separated for their hydrophobic behavior in laboratory and field water drop penetration tests and molarity analyses. Our results indicate that on dry soil strata, water drop penetration time increased by 20 seconds on litter and 3 seconds on thatch for every percentage point increase in Kentucky blue grass in the vegetation, confirming the close association between this grass species and the development of soil hydrophobicity. Rainfall simulation on dry soils (less than 20% volumetric water content) also revealed that the time needed to initiate runoff was shortened by 5 minutes and the runoff ratio increased by 0.004 for every percentage point increase of Kentucky blue grass in the vegetation. Hydrophobicity dramatically declined in the thatch layer by a factor of 4 and was completely absent from the litter layer after wetting. In contrast to the rainfall simulations on dry soils, wet runs (volumetric water content = 20%) showed a beneficial effect of Kentucky bluegrass on hydrologic response with delayed runoff and reduced runoff ratios. Prescribed fire increased litter hydrophobicity, but this did not adversely affect hydrologic response. This study highlights the need for further research contrasting detrimental effects of Kentucky bluegrass on hydrologic response in dry soil conditions with the beneficial effect of this grass on infiltration under wet conditions to better predict the overall ecohydrological outcome of an invasion by this grass species.