Redistribution of Runoff Among Vegetation Patch Types: On Ecohydrological Optimality of Herbaceous Capture of Run-On

Abstract

A central tenant of ecohydrology in drylands is that runoff redistribution from bare to vegetated patches concentrates the key limiting resource of water, which can then enhance vegetation growth and biomass. Conversely, a reduction in vegetation patches, particularly those associated with herbaceous plants, can lead to a threshold-like response in which bare patches become highly interconnected, triggering a large increase in hillslope runoff and associated erosion. However, generally lacking is an assessment of how maximization of run-on to herbaceous patches relates to minimization of hillslope-scale runoff. To illustrate how runoff redistribution potentially changes in response to conversion of herbaceous patches to bare ones, we used a spatially distributed model, SPLASH (Simulator for Processes at the Landscape Surface-Subsurface Hydrology), with an example of a semiarid piñ on-juniper woodland hillslope with seven combinations of bare and herbaceous patch cover, culminating in complete loss of herbaceous patches, for a 1-yr design storm. As expected, the amount of hillslope runoff increased curvilinearly with reductions in herbaceous cover as runoff per cell increased from bare patches and run-on per cell increased for herbaceous patches. Notably, the total amount of run-on to all herbaceous patches was greatest when the amount of bare cover was intermediate, highlighting a trade-off between the source area for generating runoff and the sink area for capturing run-on. The specific nature of patch-hillslope runoff redistribution responses certainly depends on several site-specific conditions, but the general nature of the response exhibited in our example simulation may be indicative of a general type of response applicable to many rangelands. We suggest that a more robust suite of such relationships could be valuable for managing rangelands by enabling explicit accounting for optimality and trade-offs in biomass per herbaceous patch, total herbaceous cover, and prevention of hillslope-scale connectivity of bare patches that triggers a large increase in runoff and associated erosion.

Soil indicators to assess the effectiveness of restoration strategies in dryland ecosystems

“…Puigdefábregas et al (1999) suggested that the ratio between sink and source areas in functional ecosystems remains within an optimum range that maximizes functionality. Urgeghe et al (2010) reported that the collection of runoff by herbaceous patches in a dryland pinyon-juniper ecosystem in southwest USA was maximum when both interpatch bare soil and herbaceous cover were intermediate, suggesting a trade-off between source and sink areas at the finer scale and the existence of herbaceous cover thresholds at the broader catchment scale. Some properties of the sink/source pattern, such as the upslope length and the size of the source area, have been successfully related to the performance of planted seedlings in drylands' restoration actions (Urgeghe and Bautista, 2015).…”

A review of concentrated flow erosion processes on rangelands: Fundamental understanding and knowledge gaps

“…The same landscape with uniform disturbance may experience significantly more runoff and soil loss from a similar runoff event due to increased connectivity of bare soils and formation of well-organized concentrated flowpaths. These organized flowpaths rapidly accelerate runoff velocity and the ability of water to erode and transport sediment downslope (Davenport et al, 1998;Urgeghe, Breshears, Martens & Beeson, 2010;Wilcox, Davenport, Pitlick & Allen, 1996). Tongway, and Ludwig (1997) found for example that on degraded tussock grasslands, overland flow was concentrated in long straight paths between the grasses.…”

Rainfall intensity switches ecohydrological runoff/runon redistribution patterns in dryland vegetation patches

“…The direction and intensity of water redistribution depend on a complex relation between vegetation attributes and soil properties, in combination with rainfall characteristics (e.g., event size or intensity; Cerda`1997, Davenport et al 1998, Puigdefabregas et al 1999. Runoff/runon redistribution can enhance NPP by concentrating water in vegetated patches, potentially reducing direct evaporation and increasing water available for plant transpiration (Bhark and Small 2003, Ludwig et al 2005, Urgeghe et al 2010.…”

“…Two main challenges that livestock production faces in the region are the lack of fresh surface or groundwater sources for drinking supply, and the low fraction of the NPP that is allocated to forage (as opposed to non-forage woody tissues) in the rangelands (Baldi and Jobba´gy 2012). While the lack of fresh water is resolved by runoff harvesting in human-made impoundments (Magliano et al 2015b), the availability of forage here as well as elsewhere depends on the redistribution of surface water via runoff/runon to vegetation patches (Ludwig et al 2005, Urgeghe et al 2010. Both issues are highly dependent on horizontal water redistribution at the patch to landscape scales (Breshears et al 1997, Davenport et al 1998, and can be affected intentionally or non-intentionally by the management of livestock grazing and associated trampling and soil compaction (George et al 2004).…”