Modeling the Hydrologic Impact of Arundo Donax on the Headwaters of the Nueces River Using the Swat Model

The invasive species Arundo donax (hereafter Arundo), has invaded the riparian zones of the Rio Grande River and the rivers of the Texas Hill Country over the last two decades. Arundo, also known as the giant cane, is a robust herbaceous plant that can grow in many different climatic conditions. Arundo was first observed along the Nueces River in 1994 by the Nueces River Authority (NRA). It then spread rapidly downstream due to its high growth rate and/or stream flow and completely displaced the native vegetation, primarily P. virgatum (hereafter switchgrass), in the riparian zone wherever it got established. An eradication program was started in 2010 by the NRA to remove Arundo from the Nueces River. The objective of this research project was to (1) develop an algorithm to simulate the propagation of Arundo, (2) study changes in streamflow patterns during pre- and post- Arundo invasion periods, (3) calibrate and validate the Soil Water Assessment Tool (SWAT) for the Nueces River Headwater (HUC 12110101) watershed in central Texas, and (4) assess the effects of the invasion of Arundo on the watershed hydrology by comparing it to the native grass species switchgrass (Panicum virgatum) that used to be the dominant species in the watershed. Arundo parameters appropriate for the Nueces River were added to create a new crop category in the SWAT database. Calibration and validation of SWAT were based on measured streamflow data available at the USGS gage (USGS 08910000) on the Nueces River for the period 1960 to 1994.

Switchgrass, the native vegetation, was chosen as the plant to compare Arundo with so that the difference in hydrology could be understood. The results revealed that accumulated evapotranspiration was not statistically different between Arundo and switchgrass for the period of 16 years (1995-2010). There was also no difference in the water yields of Arundo and switchgrass. In conclusion it appears that Arundo in the Nueces River has not caused any changes in water uptake compared to the native grass, switchgrass, that previously dominated the headwaters.