Assessing surface runoff dynamics of a woody plant encroached rangeland

Conversion of grasslands and open savannas into woodlands (i.e. woody plant encroachment) is often associated with increased evapotranspiration and commonly believed to reduce groundwater recharge and spring flow. However, it has also been argued that woody plants might improve infiltration rates and reduce overland flow, which could ultimately increase spring flow, especially in regions where groundwater is not accessible to plants and where water percolates quickly through the bedrock (e.g. karst regions). To test this hypothesis, we compared surface plot scale runoff on areas with and without juniper shrubs (Juniperus sp.) at a site located in the Edwards Plateau, a major karstic region of Texas. We installed and monitored 8 paired runoff plots (with and without shrub) of areas ranging between 1 m² and 6 m². Plots with shrubs had a mix of intercanopy and canopy cover, with shrubs located at the downslope part of the plots and covering 50% or less of the plot area. Intercanopy cover was mostly composed of a mix of short and tall grasses and rocks. Additionally, we monitored soil moisture sensors installed vertically at 0-20cm depth both under the canopies and on the intercanopy areas adjacent to 5 shrubs. Our results show that, on average, plots with shrubs yield approximately three times less surface runoff than intercanopy plots (4.4% vs 11.8%). However, the difference seems to be less accentuated during high-intensity storms. For instance, for a 63mm event with 5-minute intensities of up to 160mm/h, shrub plots yielded 25mm of runoff while intercanopy plots yielded 33mm. We believe that the lower runoff rates at shrub plots are mainly due to highly infiltrable soils under the canopies, which capture portions of hortonian overland flow generated at the intercanopy portion of the plot (runoff/runon redistribution). Further support to this interpretation is provided by soil moisture data, which shows that under canopies there is often a steeper increase in volumetric water content a few minutes after the beginning of storms. Our results provide support to the hypothesis that juniper encroachment has improved the infiltration capacity of these previously degraded rangelands, which has likely contributed to the increasing baseflow of many rivers in the region.