PS 6-63 - Microhydrology of a Periodically-Patterned Arid Grassland: Surface Water Dynamics, Infiltration, and Soil Water Accumulation in a Simulated Rainfall Experiment

Background/Question/Methods

Periodically patterned landscapes, including alternating bands of vegetation and bare soil along gentle slopes, are observed in semi-arid and arid ecosystems worldwide. The patterns are likely a result of a positive feedback where the vegetated patches intercept sheet flow of rainwater and allow it to infiltrate at faster rates, enhancing vegetation growth. However, the spatial and temporal movement of water across these banded landscapes is still poorly understood. We examined water and sediment transport within the patterned system by carrying out simulated rainfall experiments in eight plots in a shortgrass steppe ecosystem in south-central Colorado, dominated by blue grama grass that receives an average 406mm of annual rainfall. Each plot contained the following zones from upslope to downslope: (1) zone of dying sparse vegetation, (2) bare soil zone and (3) downslope vegetated patch (containing hummocks of blue gramma interspersed in a matrix of bare soil).

Results/Conclusions

During watering, we observed the formation of consistent sheet flow that carried water and sediment from the interpatch zones downslope into the vegetated patch zones. Initially, the sheetflow was fully absorbed by infiltration in the vegetated zones. However, after approximately 3 minutes, water accumulated in pools in the grass interstitial zones of the vegetated patch and remained 20-30 minutes after watering ended. In several plots a pool also formed in the bare zone of the interpatch but did not persist for long after rainfall ended. Surface water was deepest, 1.2-2.9cm, in the vegetated patch interstitial zones, where the pools formed. Pooling in the interstitial zone of the vegetated patch seems to play an important role in water accumulation within blue grama patches. We found that percolation depths were statistically significantly greater in vegetated patch zones than interpatch zones (8 experiments: F(3,28)=18.01-45.95, p< 0.0001). Total volume of soil water and soil water percentages followed the same trend. Our results highlight how microtopographic and microhydrologic mechanisms are coupled in the runoff-runon system of patterned landscapes. This coupling likely facilitates water-use efficiency within the vegetated patch, promoting ecosystem resiliency in the face of increasing aridity.