Soil water plays an important role in ecosystem processes and functions. Soil water dynamics are affected by the interaction of climate, precipitation, vegetation and soil types. In the southern Great Plains, encroachment of eastern redcedar (Juniperus virginiana L.) has resulted in rapid changes in ecosystem function and structure. However, it is unclear how these changes interact with soil types to affect the soil water dynamics and hydrological processes at a watershed scale. To understand the interactive effects of vegetation and soil types on the temporal dynamics of soil moisture at different depths, we continuously measured volumetric soil water content at depths of 5, 20, 45 and 80 cm for two soil types (coarse and fine) on grassland watersheds and watersheds heavily encroached by eastern redcedar. Soil water content was measured using EC-5 soil water content sensors (Decagon Devices, Pullman, WA). Measurements started in March 2010 at the Cross Timbers Experimental Range of OSU, Stillwater, OK. The effect of vegetation, soil types and time on weekly and monthly means of soil water content were analyzed as a completely randomized design with repeated measurements using the PROC MIXED procedure of SAS (v. 9.2; SAS Inst. Inc., Cary, NC).
The preliminary results indicated that vegetation type affected the weekly and monthly means of soil water content at depths of 5 and 20 cm significantly. Soil type affected weekly means of water content at depths of 20 cm depending on vegetation, and vegetation type affected weekly means of water content at the depths of 45 cm. The annual means of soil water depth were higher on grassland watersheds than eastern redcedar encroached watersheds. The implication of the interactive effects on surface runoff/runon processes and streamflow generation potential are discussed.